Backyard poultry: exploring non-intensive production systems

Abstract

The concept of backyard poultry historically encompassed “food-producing animals.” Nevertheless, a recent shift in livestock production paradigms within developed countries is evident, as backyard poultry owners now raise their birds for purposes beyond self-consumption, raising animals in a familiar way, and fostering emotional bonds with them. Because backyard animals are frequently privately owned, and the resulting products are typically not marketed, very little information is available about the demographic profile of backyard owners and information on flocks’ characteristics, husbandry, and welfare. Thus, this review aims to clarify the characteristics of backyard poultry, highlighting the prevalent infectious diseases and the zoonotic risk to which farmers are exposed. According to the FAO, there are different types of poultry production systems: intensive, sub-intensive, and extensive. The system conditions, requirements, and the resulting performance differ extensively due to the type of breed, feeding practices, prevalence of disease, prevention and control of diseases, flock management, and the interactions among all these factors. The presence and transmission of infectious diseases in avian species is a problem that affects both the animals themselves and public health. Bacterial (Escherichia coli, Salmonella, Campylobacter, and Mycoplasma), parasitic (helminths, louses, and mites), and viral (Avian influenza, Newcastle, Marek, Infectious Bronchitis, Gumboro, Infectious Laringotracheitis, and Fowlpox) are the most important pathogens involved in backyard poultry health. In addition, Avian influenza, Salmonella, Campylobacter, and E. coli, could be a risk for backyard farmers and/or backyard-derived products consumers. Thus, proper biosecurity implementation measures are mandatory to control them. While the principles and practices of on-farm biosecurity may be well-versed among commercial farmers, hobbyists, and backyard farmers might not be familiar with the necessary steps to protect their flocks from infectious diseases and curb their transmission. This sector represents the fourth category of poultry farming, characterized by the lowest biosecurity standards. Consequently, it is imperative to address the legal status of backyard poultry, educate owners about biosecurity measures, and promote proper veterinary care and disease control.

INTRODUCTION

Historically, the human activity of breeding animals in backyards for self-consumption has been widespread worldwide. However, over time, there has been industrialization and intensification of livestock systems to meet the growing demand for meat or eggs in the market. Nowadays, these intensive systems are facing numerous criticisms in several countries, creating intense pressure for the transformation of animal production into a less intensive model, such as backyard farming (Pollock et al., 2012). But which characteristics are considered for a “backyard flock”? The World Organisation for Animal Health (WOAH) points out that there is no globally accepted definition (Smith and Dunipace, 2011). The concept of backyard poultry traditionally encompassed “food-producing animals.” However, a recent shift in livestock production paradigms is evident in several developed countries, as numerous families decide to become backyard poultry owners, raising their birds for purposes beyond self-consumption and fostering emotional bonds with them (Pollock et al., 2012). In line, different surveys aim to comprehend why society is inclined toward transitioning to backyard farming systems. In an Australian survey, animals’ ownership was positively associated with social contact, interaction, and perceptions of neighborhood friendliness. Hypothetically, backyard owners within the same neighborhood may develop a heightened sense of community and belong through shared discussions about their birds (Pollock et al., 2012). In another survey conducted in 2014 in the USA, poultry owners completed a questionnaire regarding flock characteristics, healthcare, owner attitudes, and demographics. The results showed that 57.4% of the respondents raised backyard poultry as domestic animals, and around 35% did so for hobbies, fun, education, or therapeutic purposes (Elkhoraibi et al., 2014). These findings align with another survey conducted by the United States Department of Agriculture (USDA) (Pollock et al., 2012). On the contrary, in a similar survey in New Zealand, about 30% of the owners answered that they kept the poultry as a pet, 15% as a hobby, and 50% as a self-consumption (Lockhart et al., 2010). However, the legislation does not recognize the status of pet animals for those animals intended for human consumption, backyard flocks must continue to be subject to regulations on welfare, prescribing, banned procedures, disposal of carcasses, feeding bans, notifiable diseases, and disease surveillance in those countries there is specific legislation in place for them, such as the United Kingdom (Whitehead and Roberts, 2014).

There is much variability between countries, but many jurisdictions already allow raising animals such as rabbits, goats, ducks, and geese in urban backyards. For example, North American cities including Portland, Oregon, Seattle, Washington and Vancouver, and British Columbia allow the keeping of poultry in urban backyards. However, other cities continue to debate or ban urban poultry keeping for reasons such as noise, odors, and pests (Pollock et al., 2012).

Because backyard animals are privately owned, and the resulting products (eggs and meat) are typically not marketed to the public, very little information is available about the demographic profile of backyard owners and information on flocks’ characteristics, husbandry, and welfare (Elkhoraibi et al., 2014). In addition, the lack of knowledge of the noncommercial sector of the domestic poultry population is of concern to both the commercial poultry industry and animal health authorities. Firstly, because infectious and parasitic diseases are more likely to be introduced into backyard flocks due to the inevitably varying levels of biosecurity. Secondly, in the event of an infectious disease outbreak, backyard flocks may pose a risk, not only to human health but also to neighboring commercial poultry farms (Lockhart et al., 2010).

Thus, this review aims to clarify the characteristics of the backyard poultry-rearing system, highlighting the prevalent infectious diseases and the zoonotic risk to which farmers are exposed.

Backyard Flock Characteristics

According to the Food and Agriculture Organisation (FAO), there are different types of poultry production systems: intensive, sub-intensive, and extensive. The system conditions, requirements, and the resulting performance differ extensively due to the type of breed, feeding practices, prevalence of disease, prevention and control of diseases, flock management, and the interactions among all these factors (FAO, 2014). Table 1 summarises the main differences among different poultry breeding systems (FAO, 2014; Ahmed et al., 2021).

Table 1.

Type of poultry farming systems and their salient features (FAO, 2014; Ahmed et al., 2021).

	Intensive	Sub-intensive/commercial	Extensive/backyard
Biosecurity	High	Moderate–low	Low–none
Location	Rural areas	Suburban/rural areas	Anywhere
Number of birds	High	High/moderate	Low
Shed type	Closed	Closed/open	Open
Birds kept	Indoor	Indoor/outdoor	Outdoor
Temperature control	Fans/cooling pads	Fans/windows	Natural
Feeding system	Automated	Manual	Kitchen scraps
Veterinary service	Personal veterinarian	Hired	None
Breed	Commercial	Commercial	Native/ornamental
Contact with wild animal/birds	None	Yes	Yes
Trained caretakers	Yes	Yes/no	No
Output	High	High	Low/none
Consumer biosafety	High	Moderate/low	Low/none

Backyard poultry is mainly characterized by an economic low-input or no-input activity with minimum health care practices (Kebede-Tsegay, 2017; Rajkumar et al., 2021) and its bird-derived products are dedicated mainly to home self-consumption (Chaiban et al., 2020).

The breeding system adopted by backyard poultry owners is not homogeneous; certainly, the breeds, housing types, and management are highly dependent on economic conditions. The economic situation in the different countries significantly influences the type of poultry farming and biosecurity measures applied (Chlebicz and Śliżewska, 2018). In several developing countries, poultry farming is mainly linked to sociocultural factors, such as religion or festivities (Conan et al., 2012), but also socio-economic, as it represents a source of income (Chaiban et al., 2020), the aim of which is not the quantity of meat or eggs sold, but rather the quality of the product. A survey in the USA found that owners think that eggs/meat from their reared animals are more nutritious, safer to consume, and tastier than store-bought products and that the health and welfare of them are better than those on commercial farms (Elkhoraibi et al., 2014). Furthermore, they are often sold at a higher price because they are considered “organic” or “ecological,” although no regulations define products from these small farms as such (FAO, 2014) However, the quality of eggs is influenced, among other factors, by the type of feeding (Pollock et al., 2012).

An important role in less developed countries is played by women (FAO, 2014; Ahmed et al., 2021; Rajkumar et al., 2021; Rehman et al., 2022) because they can manage the animals independently without complying with the rules of the “householder.” This may include activities such as feeding the birds, collecting eggs, cleaning the cages, and selling the by-products, thus contributing to food production and the family economy (Alders et al., 2018; Rajkumar et al., 2021). It also enhances their position within the family and community, especially when they are vaccinators or poultry advisers (FAO, 2014).

In general, poultry backyard is kept by their owners, who are usually women, for between less than 1 and 11 yr. In addition, the flock size is usually small (5–50 chickens or more), depending on the country. There is no predominant breed used in backyard systems, although there are usually combinations of commercial breeds, crossbred chickens (crosses between indigenous and exotic breeds), and indigenous chickens mixed with other species such as ducks and/or turkey (Chaiban et al., 2020). The most common backyard breeds are Rhode Island Red, Plymouth Rock, Ameraucana, Wyandotte, Fayoumi, Aseel, Naked Neck, Desi, and Black Australorp. Feeding, on the other hand, is mainly with kitchen scraps or commercial feed bought from the feed store. The breeding system is usually free-range during the day and closed inside at night (Lockhart et al., 2010; Pollock et al., 2012; Elkhoraibi et al., 2014) to protect birds from predators and adverse weather conditions (Rajkumar et al., 2021). Regarding temperature and light, they are usually kept in natural conditions (no artificial lighting systems or ventilation) (Pollock et al., 2012; Elkhoraibi et al., 2014; Rajkumar et al., 2021).

Major Infectious Diseases

The presence and transmission of infectious diseases in avian species is a problem that affects both the animals themselves and public health. The growth of the commercial poultry industry has successfully reduced or eliminated many of the main infectious diseases. However, in recent years, backyard poultry farming has again increased concern about the re-emergence of many diseases (Cadmus et al., 2019). Poultry pathogens can be transmitted through several routes (Singleton et al., 2021), such as direct contact with wild animals, the presence of predators, mixed species within a flock (Ahmed et al., 2021), purchase and sale of birds, ornamental poultry exhibitions and movement of birds through markets (Singleton et al., 2021). Furthermore, the transmission of pathogens is influenced by the virulence of the strains (Lockhart et al., 2010).

Commercial farms control many infectious diseases through strict vaccination protocols and stringent biosecurity measures (Cadmus et al., 2019). These procedures are often very different from backyard production, where owners are unaware of the dangers to their birds from infectious and parasitic diseases present in the environment, as well as the key role their birds play in the spread of these diseases. Moreover, they may be unaware of the need to vaccinate their animals against these diseases (Whitehead and Roberts, 2014; Cadmus et al., 2019; Abtin et al., 2022). Most owners claim not to detect clinical symptoms in their birds (Elkhoraibi et al., 2014; Singleton et al., 2021). However, owners declare that the most reported clinical sign is “wasting,” associated with lethargy/weakness and reduced appetite. The manifestation of clinical symptomatology can vary greatly due to the characteristics of the pathogen, the infectious dose, or the pathogen virulence, which can often result in subclinical symptoms, going unnoticed by owners (Singleton et al., 2021; Abtin et al., 2022). In addition, animal characteristics such as age, genetic line, immune status, or the level of maternal immunity have an influence on the disease symptoms presented (Mete et al., 2016). Susceptibility to infectious diseases is also related to genetic selection, as commercial animals are usually genetically selected to be more resistant to certain infectious diseases, compared to ornamental chickens usually kept in backyard flocks, where selection focuses on phenotypic traits of standard breeds (Mescolini et al., 2019).

Data on infectious diseases in backyard poultry is limited, as the literature focuses more on commercial chickens. Some researchers have presented data on the leading causes of poultry mortality. Among the main causes of mortality are infectious diseases (bacterial, viral, and parasitic) and noninfectious diseases (mainly due to poor animal management or nutrition-related disorders) (Cadmus et al., 2019). However, the prevalence of pathogens varies considerably depending on the geographical and economic situation of the farmer, which affects the housing conditions of the birds (Pollock et al., 2012), but also of the method of diagnosis utilized.

Bacterial Diseases

Bacterial diseases have a significant impact on the health and productivity of poultry (Alders et al., 2018). Figure 1 illustrates the reported rates of bacterial diseases in poultry backyards in relation to their respective countries.

Figure 1.

Bacterial infection rates in backyard poultry across various countries. MG/MS: Mycoplasma gallisepticum/Mycoplasma synoviae. Abbreviation: APEC, Avian pathogenic E. coli. *Data obtained for MG/MS concern ELISA analysis; APEC were obtained with end-point PCR analysis in Italy, Egypt, and Saudi Arabia, while in Ethiopia with isolation with standard bacteriological methods; Salmonella were obtained with isolation with standard bacteriological method and end-point PCR, respectively in Bosnia Herzegovina and Australia and finally, Campylobacter were obtained with isolation with the standard bacteriological method in New Zealand and Egypt, while in Australia with end-point PCR. Ecuador (Hernandez-Divers et al., 2006), Belgium (Haesendonck et al., 2014), West Indies (Bolfa et al., 2019), Ethiopia (Sarba et al., 2019), Egypt (El-Tras et al., 2015; Ibrahim et al., 2019), Italy (Sgariglia et al., 2019), Sudi Arabia (Al-Marri et al., 2021), Bosnia Herzegovina (Koro et al., 2022), Australia (Keerthirathne et al., 2022), and New Zealand (Anderson et al., 2012).

Some of the most common bacteria causing disease in poultry are commensal bacteria that are also opportunist pathogens that take advantage of any change in the animal's condition to infect or co-infect along with other pathogens, such as Escherichia coli, Salmonella, or Campylobacter (Ahmed et al., 2021). At the same time, Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS) represent the most important avian Mycoplasma species worldwide affecting chickens (Felice et al., 2020). There are several studies that discuss the prevalence of bacterial pathogens in backyard poultry. A study carried out on causes of mortality in backyard poultry in the USA, showed that out of 2,687 birds submitted for autopsy, 1,135 birds (42.2%) were affected by bacterial diseases. The main pathogens found were E. coli (34%, 386/1,135), followed by MG and/or MS (26%, 296/1,135). Other less prevalent pathogens found were Gallibacterium anatis, Pasteurella multocida, Listeria monocytogenes, Campylobacter jejuni, and paratyphoid Salmonella enterica (Cadmus et al., 2019).

E. coli is a bacterium of the Enterobacteriaceae family (Al-Marri et al., 2021). According to the pathogeneses, E. coli can be classified into three main groups: extraintestinal pathogenetic E. coli (ExPEC), intestinal pathogenetic E. coli (InEC), and commensal strains (Ovi et al., 2023). In poultry, avian pathogenic E. coli (APEC) is found in the gut microbiota of healthy birds, but according to their health condition, it can be responsible for extraintestinal infections, causing colibacillosis (Al-Marri et al., 2021). In a study conducted in 30 backyard farms, 86 different E. coli strains were isolated and 33% were avian pathogenic E. coli (Al-Marri et al., 2021). Similar results have been found in other studies conducted in Egypt (34%) (Ibrahim et al., 2019) and Ethiopia (32.5%) (Sarba et al., 2019).

As for the genus Mycoplasma, avian mycoplasmosis, especially in chickens, is predominantly attributed to MG and MS. Both are listed as respiratory pathogens by the WOAH. MG is known to induce chronic respiratory disease in chickens and infectious sinusitis in turkeys. On the other hand, MS leads to synovitis and airsacculitis in various bird species (Yadav et al., 2022), and both can be transmitted by horizontal and vertical routes (Mugunthan et al., 2023). In a study conducted in Italy between 2016 and 2017 in different areas (north, central and southern Italy), 11 backyard flocks were tested and the results showed that 45.45% were positive for both MG and MS, at 27.27% and 27.27%, respectively (Felice et al., 2020) Another study carried out in Ontario (Canada) that studied the prevalence of infectious pathogens in backyard birds detected a similar seroprevalence as in the study cited above. On a total of 151 individual chickens, 23% and 36% of the samples submitted were positive for MG and MS, respectively (Brochu et al., 2019).

Regarding Campylobacter spp. and Salmonella spp., they that can colonize the mucosa of infected chickens’ caecum and cloaca crypts, but may also be present in the spleen, blood, and liver. These two pathogens are commonly studied, as they are the two most important zoonotic agents by number of cases of foodborne infections in the European Union each year (EFSA, 2022). The importance of these bacteria is well-known in commercial poultry flocks. However, different studies have shown a higher prevalence of Campylobacter spp. than for the Salmonella spp. in backyard poultry flocks. For example, in a study conducted in Canterbury (New Zealand), a prevalence of 86% of Campylobacter spp. was found in the 35-backyard poultry flock studied (Anderson et al., 2012), vs. the 2% prevalence of Salmonella spp. observed in 53 flocks sampled in Massachusetts (McDonagh et al., 2019). In line with these results, the prevalence observed in the study mentioned above, carried out in Ontario (Canada), showed that 35% of the flocks were positive for Campylobacter, and only 3% were positive for Salmonella (Brochu et al., 2019). According to these results, another study also conducted in Australia, where a composite fecal sample was taken from each flock, observed a prevalence for Campylobacter (10%) higher than that observed for Salmonella (4%) in the 82 backyard poultry flocks studied (Keerthirathne et al., 2022)

Viral Diseases

Viral diseases are important causes of mortality in poultry, impacting their health and productivity (Alders et al., 2018). Figure 2 illustrates the reported serological rates of viral diseases in poultry backyards in relation to their respective countries. Some of the most important ones are described in the following paragraphs.

Figure 2.

Viral serology infection rates in backyard poultry across various countries. Abbreviations: AI, Avian influenza; ND, Newcastle disease; IB, infectious bronchitis; IBD, infectious bursal disease; ILTD, infectious laryngotracheitis disease; MD, Marek's disease; APD, Avian pox disease; USA, United States of America. *Data obtained for MD concern necroscopic analysis, while the data regarding ADP were obtained with end-point PCR. In countries where serology for some diseases was 0% it is because no data is available, not because the prevalence of those diseases is 0%. USA (Mete et al., 2013; Cadmus et al., 2019), Finland (Pohjola et al., 2017), Ecuador (Hernandez-Divers et al., 2006) West Indies (Bolfa et al., 2019) Belgium (Haesendonck et al., 2014), Mexico (Gutierrez-Ruiz et al., 2000), Oman (Alsahami et al., 2018), Monzambique (Pinto et al., 2022), and Bangladesh (Mili et al., 2022).

Avian influenza (AI), it is an important viral disease associated with high mortality in poultry and zoonotic risk. Backyard poultry are considered a risk for infection with the AI virus (Song et al., 2017), but their role in the spread of the virus is controversial. Interestingly, a Dutch study concluded that they played a marginal role in the 2004 H7N7 HPAI epidemic in the Netherlands due to the relative susceptibility of backyard flocks was low (Bavinck et al., 2009). However, another study of the almost simultaneous 2004 H5N1 HPAI epidemic in Thailand noted the important role of backyard chickens and free-range ducks in the epidemic but also noted that several festivals associated with the rearing, sale, and transport of poultry occurred around the outbreak (Tiensin et al., 2005). Finally, a more recent study of the 2016–2017 H5N8 epidemic in southern France showed that backyard flocks played a minor role in the dynamics of disease transmission. However, backyard flocks belonging to commercial poultry farmers showed a significantly higher risk of infection than flocks with no links to commercial farms. Similarly, backyard flocks with ducks were more likely to be AIV-positive than those with chickens only (Souvestre et al., 2019). Currently, the USDA reported in October 2023, 19 cases of backyard flocks were positive for HPAI (USDA, 2023). These different conclusions may be explained by the virus strains involved, the bird species involved, the different husbandry systems, or the different implementation of biosecurity measures, but the impact of human activities in both commercial and backyard flocks must be also considered as a main factor.

Newcastle disease (ND) is one of the most prevalent diseases in backyard flocks in countries where it is endemic. ND is considered to be a major constraint to village chicken production in certain developing countries. A study of seroprevalence and risk factors for ND in Sudanese backyard flocks concluded that flocks that experienced mortality due to infectious disease were more likely (OR 40.40 95% CI 18.78–86.82) to develop haemagglutination inhibition titers of log₂ 4 and above for ND than flocks with no reported mortality due to infectious disease. In addition, having access to the outdoors rather than being confined (OR 1.39 95% CI 0.51–3.79), contact with neighboring poultry (OR 1.44 95% CI 0.56–3.67), presence of birds other than chickens on the farm (OR 1.69 95% CI 0.27–1.71) and contact with wild birds (OR 1.20 95% CI 0.49–2.86) were also reported as risk factors. On the other hand, there was no increased risk for seroprevalence of ND for the source of chickens (market or breeder) or if they were kept for income or family consumption (Hussein et al., 2022). Even though many of the confidence intervals for the calculated odd ratios were less than 1, and some bias due to the use of ND vaccines must be considered, it must be emphasized that most of the identified factor risks describe common husbandry practices for village poultry flocks in developing countries. Due to this situation, ND control initiatives have been introduced to improve the performance of these flocks. One of the proposed strategies is the use of ND vaccines strains such as NDV4-HR and I-2 ND. They have the advantage of being heat resistance, so they are more suitable for its use in challenging rural environments where the cold chain infrastructure may be unreliable (Alders and Copland, 2005).

Marek's disease (MD) is a highly contagious worldwide-distributed viral disease that affects birds, mainly chickens under intensive systems (Mescolini et al., 2019). Typically, MD is associated with serotype 1 (MDV-1) due to its oncogenic properties. different studies have identified MD as a cause of mortality in backyard birds (Elkhoraibi et al., 2014; Mete et al., 2016; Cadmus et al., 2019). One of the main reasons could be the lack of MD vaccination or the incorrect vaccination of day-old chicks, as MD vaccines are mostly administered in commercial hatcheries. Another disease that can be misidentified as MD is the avian leucosis/sarcoma (L/S). This is a diverse group of benign and malignant transmissible tumors in chickens caused by members of the family Retroviridae of the genus Alpharetroviridae (Fadly, 1997; Payne, 1998). They are transmitted horizontally in the form of infectious viral particles called exogenous viruses but also vertically to the progeny. Breeding companies have successfully implemented initiatives to eradicate L/S viruses from their poultry pedigree and multiplier flocks (Payne and Nair, 2012). However, this task has never been done in native or ornamental poultry flocks. As a consequence, they still have a high prevalence of the disease and can act as a reservoir (Freick et al., 2022).

Avian infectious bronchitis (IB) is a highly contagious respiratory disease caused by the infectious bronchitis virus (IBV). Different seroprevalence studies have shown that IBV is widespread in backyard poultry. However, it does not correlate with a high level of clinical signs in flocks. In a study conducted in California, IBV played a primary or synergistic role in the mortality of chickens dying from other infectious diseases, with the most common lesions being found in the trachea and kidneys. Most of the IBV strains in this study matched the vaccine or known field virus, but some still did not have a clear match to any available reference strains. These findings suggest that backyard flocks may play a role as reservoirs of IBV, but also in the evolution of new variants (Gonzales-Viera et al., 2021).

Infectious bursal disease (IBD), also known as Gumboro's disease, is a highly contagious immunosuppressive disease affecting mostly young chickens. It is caused by the infectious bursal disease virus (IBDV). In a study from backyard poultry flocks submitted to CAHFS (California) in 2009–2017, the 21 isolated viruses were phylogenetically classified into 3 genogroups using RT-rtPCR for a 579-bp fragment of the IBDV hvVP2 gene. Fourteen samples were placed in the G3 group (vvIBDV), 6 samples in the G2 group (similar to variant strains) and only 1 sample contained a G1 virus (Lukert strain). Similar strains had already been isolated in commercial flocks, but the concern was that backyard flocks could act as a source of infection for commercial flocks or play a role in the new virus reassortment (Stoute et al., 2019). In terms of risk factors for IBD in backyard flocks, a study from Bangladesh found that flocks composed of cross-bred chickens were more likely to develop antibodies for IBDV than indigenous chickens (OR 6.5 CI 1.5–27.5). Hygienic conditions also play a critical role, as flocks kept in poor conditions were much more likely to develop antibodies than those kept in good (OR 31.3 CI 9.3–105.2). Interestingly, Housing types did not show similar results (Mili et al., 2022).

Infectious laryngotracheitis (ILT) is an important highly infectious viral respiratory disease caused by the infectious laryngotracheitis virus (ILTV). The source of infection in backyard flocks can be wild strains, but also vaccine strains from industrial flocks using live vaccines (Brochu et al., 2019), as these can revert to virulence when recirculated to unvaccinated birds (García, 2017). In a study conducted in California, of 15 strains isolated from cases in backyard flocks, 14 were found to be closely related to a Chicken Embryo Origen vaccine strain by sequencing the ICP4 gene (Blakey et al., 2019).

Fowlpox is a common viral disease in poultry, capable of infecting numerous avian species. In fact, it is a virus that adapts to each species, and the name is based on the host species (e.g., turkey smallpox, Turkeypox) (Abdallah and Hassanin, 2013). The avian pox disease (APD) is caused by an Avipoxvirus avian pox virus (APV). This infection is caused by mechanical transmission of the virus to injured or lacerated skin. Therefore, the presence of ectoparasites in birds has a major impact on the epidemiology of the disease. This is a differential factor in backyard poultry, as the exposure to ectoparasites in backyard poultry is usually higher than in commercial poultry. In a study carried out in Rio de Janeiro (Brazil), it was shown that 15% of mosquitoes (genus Culex) trapped in properties from rural and peri-urban areas with backyard poultry. were positive for APV by PCR. In addition, there was a higher concentration of APV-positive mosquitoes in certain rural areas, probably related to previous outbreaks (van der Meer et al., 2022).

Parasitic and Ectoparasitic Diseases

The backyard breeding system, in which animals are usually free-ranging, allows them to acquire many more types of parasites and ectoparasites (Murillo and Mullens, 2016). However, the difference in parasitic and ectoparasite prevalence is highly influenced by the topographic and climatic conditions, season, farm management, and the breed of birds (Kebede-Tsegay, 2017). A survey carried out in the USA, where 1,482 backyard poultry flock owners were asked, showed that 11.4% had external parasite issues (Elkhoraibi et al., 2014); a similar survey was also conducted on about 3,000 backyard poultry owners in Turkey, showing 14.3% of external parasites and 9.7% internal parasites (Özdemir, 2020).

Helminths are intestinal worms capable of causing direct damage to the host, inducing the breakdown of the gastrointestinal barrier, and indirectly through increased susceptibility to secondary infectious diseases. Among helminths, nematodes are the most studied species (90%), followed by cestodes (66%), and flukes (10%) (Shifaw et al., 2021). The most common nematodes are Ascaridia galli, Heterakis gallinarum, and Capillaria spp. (Shifaw et al., 2021). In a study conducted on 64 backyard poultry farms in the state of Alabama, a prevalence of 20.3% of Ascaridia galli and Heterakis gallinarum and 26.6% of Capillaria was observed (Carrisosa et al., 2021), similar to that reported in a study in Colombia, where birds were mainly affected by Capillaria (45.6%) (Montes-Vergara et al., 2021) or in West Indies where they found highly prevalence of Capillaria spp. (63%), followed by Heterakis gallinarum (36%) and Ascaridia galli (12%) (Bolfa et al., 2019). In contrast, the most common cestodes are Raillietina tetragona, Raillietina echinobothrida, and Raillietina cesticillus. In addition, the latter require an intermediate host such as domestic flies or beetles for their transmission, and their lower prevalence may have been linked to fewer transmission opportunities than nematodes having a direct cycle. Similarly, flukes require 2 to 4 intermediate hosts to complete their life cycle and eggs hatch only in water (Murillo and Mullens, 2016).

Instead, the problem of external parasites is that they compete for feed or cause distress to the birds and hence cause anemia, reduce growth, and may lead to death (Kebede-Tsegay, 2017). Among the most common ectoparasites are lice (Phthiraptera), followed by fleas (Siphonaptera), mites (Murillo and Mullens, 2016; Kebede-Tsegay, 2017), and avian tick. In a study conducted in the south of the Etipoia found that out of 322 backyard poultry, 56.5% were infested with one or more species of ectoparasites, of which 30.43% were fleas, followed by lice (21.73%) and tick (4.34%) (Endale et al., 2023). The exposure of backyard chickens to ectoparasites was further confirmed in a study where they compared this breeding system with an intensive system, revealing the presence of various ectoparasites (lice 65.1%, fleas 12.8%, tick 6.8%, and mite 5.9%) in the backyard system due to poor biosecurity measures (Mekuria and Gezahegn, 2010). Figure 3 represents the ectoparasite disease rates reported in poultry backyards in relation to their respective countries.

Figure 3.

Ectoparasite infection rates in backyard poultry across various countries. Abbreviation: USA, United States of America. Czech Republic (Sychra et al., 2008), USA (Murillo and Mullens, 2016), Zimbabwe (Permin et al., 2002), Bangladesh (Shanta et al., 2006), and Ethiopia (Kebede-Tsegay, 2017).

Pathogens of Zoonotic Importance

Zoonoses are diseases transmitted between animals and humans due to direct contact, indirect environmental contact or through food consumption (Chlebicz and Śliżewska, 2018). According to the WHO report published in 2015, almost 600 million cases of illness caused by contaminated food were reported in 2010, of which almost 350 million were caused by pathogenic bacteria (Chlebicz and Śliżewska, 2018). Poultry meat and eggs, as well as consumption, have been demonstrated to be an important source of infection. In addition, swine represents an intermediate host for adapting avian viruses (Li et al., 2019; Rehman et al., 2022).

The increase in backyard poultry breeding has a good impact on society, but could also mean more exposure to pathogens of great concern for public health (Pollock et al., 2012), such as AI, ND, Salmonella (Salmonella Enteritidis , and Salmonella Typhimurium), Campylobacter spp. and E. coli, which may be clinically undetectable in poultry but cause serious disease in humans (Alders et al., 2018; Cadmus et al., 2019).

Historically, AI viruses have always been important in human health because the virus that causes human influenza is also an influenza A virus (CDC 2023a). Although the AI virus is not adapted to humans, it can play an important role in the evolution of human influenza viruses under certain conditions of co-infection in third species (especially swine). Subtype H5N1 has particular significance and posed serious risks to public and poultry health (Rehman et al., 2022). Figure 4 summarizes the distribution of confirmed human cases of AI (H5N1) virus. Moreover, other new subtypes also with the capacity to infect humans directly have emerged, such as H9N2 or H7N9 (Li et al., 2019). In recent years, new outbreaks of AI—HPAI A (H5N1) clade 2.3.4.4b, have been reported in China and in Ecuador and clade 2.3.2.1c in Cambodia, following exposure of sick or dead backyard poultry (Adlhoch et al., 2023). Despite this, the risk of infection of the influenza virus currently circulating in Europe has been assessed as low for the general population in the EU/EEA and low to moderate for workers exposed (Adlhoch et al., 2023).

Figure 4.

Distribution of confirmed human cases of avian influenza A (H5N1) virus infection by year of onset and country, 2003–2023 (updated on 2 March 2023, n = 873). Data was obtained from the European Centre for Disease Prevention and Control (ECDC) and (Adlhoch et al., 2023).

Although ND can also be considered a zoonotic disease, it causes only mild, self-limiting conjunctivitis (WOAH, 2023). The first report on human infection with Newcastle disease virus (NDV) was documented in 1942 in Australia and from 1942 until now, a total of 485 human cases have been reported collectively in 20 investigations conducted in Israel, the USA, the United Kingdom, Netherland, Canada, Australia and Pakistan. The clinical manifestations may include fever, headache, eye itching, redness, lacrimation, mucopurulent discharge, chills, sore throat, depressed appetite, pain, malaise, minor photophobia, pharyngitis, slight unproductive cough and marked insomnia with general apathy (Ul-Rahman et al., 2022). Regarding ND, backyard farmers should be aware of the risks they are exposed to (Sato, 2022; CDC, 2023b). A survey carried out in the USA, where 1,482 backyard poultry flock owners were asked, showed that around 71% of respondents were familiar with AI and only 30% with ND (Elkhoraibi et al., 2014).

Among bacterial zoonoses, campylobacteriosis and salmonellosis are commonly reported gastrointestinal infections worldwide (EFSA, 2022). Regarding Campylobacter spp., Campylobacter jejuni (95% of cases of zoonoses) and Campylobacter coli (5% of infections) are the most prevalent species related to human infection, and nearly 30% of all cases of infection were caused by poultry consumption. The symptoms are watery diarrhoea to bloody stool, fever, abdominal pain, vomiting, and dehydration. Similar results have been observed in farm-level studies, as seen in backyard poultry flocks in Ecuador, where C. jejuni (32.5%) was the most frequent specie isolated, followed by C. coli (9.2%) (Ochoa et al., 2016). However, not many cases have been reported where the source of infection is backyard poultry. In a study conducted in Egypt, addressed Campylobacter infections in 103 households, where children were exposed to infected backyard poultry. Of the 379 poultry sampled, 23.5% were positive for C. jejuni and 5.5% for C. coli, in contrast to 12.3% of children positive for C. jejuni and 3.86% for C. coli out of 106 children sampled (El-Tras et al., 2015). Similar results have been observed in different geographical locations, such as in southern Ecuador, where 127 healthy children were sampled, of which 7.1% harboured C. jejuni and 6.3% harboured C. coli (Toledo et al., 2017). These results show that Campylobacter-infected backyard poultry may pose a risk in the transmission of this bacterium to exposed children.

On the other hand, regarding Salmonella spp., the two most important zoonotic species related to human outbreaks are Salmonella Enteritidis and Salmonella Typhimurium (EFSA, 2022). Pathogenic Salmonella bacteria can cause three types of salmonellosis in humans: noninvasive and nontyphoid, invasive and nontyphoid, and typhoid fever caused by the serotype S. Typhimurium. The growing number of backyard poultry flocks worldwide has raised awareness in the possible risk they may pose in the transmission of this zoonotic pathogen. In 1955, the first outbreak of Salmonella from exposure to noncommercial poultry, caused by S. Typhimurium, was reported in the USA (Anderson et al., 1955) and since 1990–2015, 45 Salmonella outbreaks have been reported in the USA. The largest one was caused by S. Typhimurium with 356 confirmed cases, 26% of them hospitalized, across 39 states (Tobin et al., 2015). Therefore, different studies have been carried out to stablish the prevalence of this bacteria in backyard poultry and human-related cases, such as a study conducted in Iran, where a prevalence of 8.5% of Salmonella were found in the 35 flocks studied, of which 5.7% were S. Typhimurium and 2.8% were S. Enteritidis (Jafari et al., 2007). In another study in Finland, out of 51 backyard flocks studied, only one was positive in cloacal and boot sock samples (Pohjola et al., 2016). In Australia, 10.4% of the 115 samples taken from 30 backyard flocks were also positive (Manning et al., 2015). Recently, in 2023, the USA CDC reported a multistate salmonellosis outbreak where 1,072 people from 48 states and Puerto Rico were infected with Salmonella spp., which was linked to backyard poultry (CDC, 2023c).

Regarding E. coli, it is ubiquitous in the gut of humans, animals, and birds as part of the gut microbiota (Al-Marri et al., 2021). Poultry meat is known to have not only the highest levels of E. coli contamination but also high levels of antimicrobial resistance, which poses a problem in the control of this pathogen (Mellata, 2013). In a study in which 30 backyard herds were sampled, 86 E. coli were recovered, and the isolates were recovered significantly more from diseased animals than from healthy animals (Al-Marri et al., 2021). Furthermore, it has been suggested that poultry are a potential reservoir of E. coli causing extraintestinal disease as they share genotypic and phenotypic characteristics between human ExPEC and E. coli isolated from poultry (Borges et al., 2019). However, the importance of these bacteria also lies in its ability to spread AMR gens. Borges et al. (2019) compared the AMR profiles of E. coli isolated from different sources: humans, commercial poultry, and backyard poultry. Commercial poultry flocks showed the highest AMR levels (95%), when compared to backyard poultry flocks (55%) and humans (65%) (Borges et al., 2019).

Legislation

Regulation is important in disease prevention, minimized nuisance factors, and in ensuring animal welfare (Whitehead and Roberts, 2014). Each country lays down rules, regulations, or recommendations that backyard chicken owners must follow. As mentioned above, backyard poultry are not considered pets; for this reason, farmers should be better instructed in animal management, carcass disposal, prescribing of medicines, notifiable diseases, and disease surveillance (Whitehead and Roberts, 2014). In this regard, there needs to be more clarity about it; more frequently, we can find guidelines or websites the owner can follow. For example, the Animal and Plant Health Inspection Service (APHIS) of the USDA has an educational campaign that offers many resources to help protect the health of all poultry flocks (USDA, 2020), or the National Poultry Improvement Plan (NPIP), designed to help diminish the spread of Pullorum Disease, caused by Salmonella Pullorum. Currently, the program also includes monitoring for other poultry diseases (NPIP 2023).

In Vancouver (Canada), there is a guideline establishing the licensing of birds for better risk management because it will enable trace-back to flock owners in the event of outbreaks of highly contagious infectious diseases. Prohibiting chicks will decrease the risks of Salmonella, as many chicks carry this bacteria in their feces and children are more likely to have close contact with them; pertinent measures include limiting the number of hens allowed, providing specifications on coop construction, waste management, and food storage. Moreover, economic incentives for flock owners, including free licensing of birds, could help ensure that birds are properly registered and cared for (Pollock et al., 2012).

There are different rules in Europe that each Member State must follow in terms of animal welfare and the management of commercial poultry farms. After that, each country issues its own regulations. For example, in Italy, reference is made to Legislative Decree, 2006, n. 158 on the rearing of poultry for self-consumption, which also states the need to register the company at the offices of the local health authority (ASL), as well as the purpose of breeding (generally below 250 animals is not considered productive breeding), for which a farmer code will be issued. Regarding bird registration in Spain, Real Decreto, 2021 lays down the basic rules for the management of poultry farms, including backyard poultry, in which registration of the farm is also provided for even if the products are intended for own consumption.

Likewise, regarding bird numbers, each country has specific criteria: for example, the Dutch Ministry of Agriculture, Nature and Food Quality defines a backyard flock as consisting of fewer than 500 birds or as not having a unique farm number; the National Animal Health Monitoring System “Poultry 04” study in the USA defined backyard flocks as residences with fewer than 1,000 birds other than pet birds; the Canadian Food Inspection Agency defines backyard flocks as flocks that are smaller than 1,000 birds that are not registered as commercial poultry (Smith and Dunipace, 2011); California defines poultry with fewer than 1,000 animals (Cadmus et al., 2019); according to United Kingdom governmental legislation, only flock sizes of 50 or over are required to be registered with the Animal and Plant Health Agency (Singleton et al., 2021); flocks of lower numbers may be voluntarily registered. This also applies if the holdings are composed of a mixture of species (Whitehead and Roberts, 2014). In one report it was seen that between 2011 and 2013 a total of 37,086 premises were registered in Great Britain, with 17,259 premises voluntarily registered as having fewer than fifty birds (Singleton et al., 2021).

It should also be noted that chickens, especially in urban areas, are a source of nuisance for the noise and odors they emit from feces, so individual municipalities must decide whether to allow poultry rearing (Pollock et al., 2012).

Biosecurity

FAO and WOAH define biosecurity as implementing measures to reduce the risk of introducing and spreading pathogens. Although ways of classifying these measures may vary, they all refer to the same basic principles of bio-exclusion (preventing infectious agents from entering the farm) and biocontainment (preventing infectious agents from leaving) (Conan et al., 2012). Whereas the principles and practices of on-farm biosecurity may be familiar to commercial farmers, hobbyists, and backyard farmers may not know the steps required to keep infectious diseases out of their flocks and prevent their spread (Pollock et al., 2012). In fact, according to the FAO backyard farming is the fourth sector of poultry farming, with the lowest biosecurity level (Ahmed et al., 2021).

The main tools owners have available to protect their animals are good hygiene practices, vaccination (Alders et al., 2018), and periodic mass de-worming. Table 2 reports the main hygiene and biosecurity measures to be applied in a poultry backyard farm.

Table 2.

Lists of hygiene and biosecurity measures (Conan et al., 2012; Whitehead and Roberts, 2014).

Biosecurity and hygiene measures
• Avoid mixing birds from different sources
• Quarantine new birds
• Dispose of old litter before introducing new birds
• Change clothes and footwear before and after visiting other poultry sites, shows, or sales
• Do not share equipment with other poultry owners
• Restrict access of visitors to your birds
• Wash hands before and after handling poultry
• Minimising contact with wild birds
• Feed nutritionally balanced feed (and do not feed kitchen scraps) and freshwater with feeders and drinkers free from droppings
• Ensure well-ventilated, draught-free housing with appropriate space for the number of birds
• Properly dispose of dead birds
• Ensure adequate protection against predators
• Vaccinate
• Use of antibiotics taking care not to overdo it
• Perform periodic de-worming
• Bird health monitoring and reporting in case of illness

It is important to highlight, that the incidence of viral disease in backyard poultry is not that common compared to bacterial and parasitic diseases as backyard chickens are reared in small numbers if they are completely isolated. In developing countries has been shown that indigenous chickens mostly reared in the backyard are continuously exposed to bacterial infections and parasitic infestation but rarely to viral disease as these birds are not vaccinated against viral diseases (live vaccines) and there are fewer or no chance for the virus to be present and evolve as virulent strain. Therefore, in developing countries, the focus must be on the prevention and control of bacterial and parasitic diseases in backyard poultry rather than viral diseases (Cadmus et al., 2019).

In order to reduce external parasitic infestation from lice, flies, fleas, bugs, and mites, it is important to have moist dry floors and good ventilation in poultry houses (Rajkumar et al., 2021). Despite the tools that the breeders have at their disposal, roughly 41.8% of owners in the USA do not vaccinate their birds against MD (Elkhoraibi et al., 2014). In the United Kingdom, backyard poultry are usually vaccinated for the main infectious diseases (MD, ND, and IB) (Whitehead and Roberts, 2014), but disproportionate use of enrofloxacin has been detected, which, although authorized, is not intended for food-producing animals (Whitehead and Roberts, 2014). In addition, a prescription has been detected for fipronil, an unauthorized ectoparasiticide for animals (Singleton et al., 2021). Other common biosecurity deficits that can be detected in poultry farms include mixing birds from different sources, not quarantining new or returning (e.g., from shows) birds, not restricting visitor access to birds, no footwear precautions, not washing hands before or after handling the birds, poor rodent control, no control of access of wild birds to the poultry area (many owners even feed wild birds near their poultry) (Whitehead and Roberts, 2014) and incorrect disposal of carcasses (Tenzin et al., 2017).

A survey conducted in southern Asia demonstrated that poultry keepers know the importance of hand washing with soap and water and undertake washing after handling poultry products and cleaning the poultry shed. However, a knowledge gap and practice were found among poultry keepers, such as wearing protective hand gloves and face masks while handling poultry or poultry litter (Tenzin et al., 2017).

In general, there are few documents published on the impact and efficiency of biosecurity measures in backyard poultry flocks, most of which have been issued since 2004, following the pandemic threat posed by H5N1 virus infection in humans and birds (Conan et al., 2012). Most backyard chicken owners inquire through social networks and magazines, without consulting a specialized veterinarian (Elkhoraibi et al., 2014). Some vets seem to have a relatively low competence and/or interest in backyard poultry. If this is perceived by owners, they may be discouraged from consulting vets (Whitehead and Roberts, 2014). A survey by Tenzin et al. (2017) detected that most poultry owners do not know how to seek veterinary assistance in the event of illness in their chickens or are unaware of any legal obligation to report any unusual mortality or sickness in their flocks to the veterinary authorities (Tenzin et al., 2017). However, some countries provide educational materials, such as the Canadian Food Inspection Agency, which has created educational material targeting backyard flock biosecurity (Pollock et al., 2012). Pakistan's government has implemented several initiatives to foster backyard poultry farming, including “Provision of Rural Poultry Breeds,” intended to provide improved poultry breeds at a moderate cost, and the “Backyard Poultry Initiative,” focused on the distribution of vaccinated chickens, as well as “Developing Rural Poultry Models to Support Rural Economy” (Ahmed et al., 2021). In Africa, in 1997 a network designated “Developing family poultry through networking and information sharing” (INFPD) was set up to consolidate knowledge and coordinate the development of family poultry production, to serve as a forum for the exchange of ideas, methods, resource, and results, as well as disseminating information and similar activities. Currently, the INFPD has over 1,000 members from 105 countries (FAO, 2014).

CONCLUSION

More data currently needs to be available on the poultry backyard farming system, which makes it difficult to conduct an in-depth analysis of its characteristics and potential public health risks. An analysis of the literature has shown that most farms worldwide are positive for various infectious viral, bacterial, and parasitic pathogens of animal and human health importance, usually absent or controlled in commercial farms. While the principles and practices of on-farm biosecurity may be well-versed among commercial farmers, hobbyists, and backyard farmers might not be familiar with the necessary steps to protect their flocks from infectious diseases and curb their transmission. Thus, this sector represents the fourth category of poultry farming, characterized by the lowest biosecurity standards. Consequently, it is imperative to address the legal status of backyard poultry, educate owners about biosecurity measures, and promote proper veterinary care and disease control.