A review of zoonotic chlamydiae species in Africa: assessing their burden and potential impact through One Health perspective

Abstract

Chlamydia species, a group of obligate intracellular Gram-negative bacteria, affect humans, livestock, companion animals, and wildlife, with infections ranging from asymptomatic to severe depending on host species and strain. Diagnosis can be difficult due to mild lesions or co-infections. Because Chlamydiaceae infect multiple hosts, a One Health approach, integrating human, animal, and environmental health is essential for effective control and prevention. C. trachomatis remains endemic in many regions, while Chlamydia pneumoniae is implicated in community-acquired pneumonia. C. abortus threatens livestock and people in pastoralist communities. Other species, including C. caviae, C. felis, C. muridarum, C. pecorum, and C. psittaci, cause high morbidity in animals, and many are zoonotic, posing risks to humans through cross-species transmission. Closely related Chlamydia-like bacteria also pose emerging threats in both human and animal populations. In Africa, diverse ecosystems facilitate frequent cross-species contacts that can drive disease emergence. Rapid urbanization, population growth, and widespread poverty increase transmission, while political instability and food insecurity reduce public health responses. As the continent faces a disproportionate burden of emerging and re-emerging infections, strengthening surveillance and targeted interventions is crucial. This review examines current knowledge on the transmission dynamics and public health implications of Chlamydiaceae species in African settings.

Introduction

Chlamydiae, a diverse phylum of obligate intracellular, Gram-negative bacteria share a unique biphasic developmental cycle enabling their replication within eukaryotic hosts [1]. This conserved biphasic developmental cycle alternates between two forms: the reticulate body, responsible for replication and the elementary body, which facilitates infection [2]. Owing to their intracellular lifestyle, chlamydiae rely on host metabolites, which significantly impacts diagnostics as they require cell culture for their isolation [3].

Species like C. trachomatis and C. pneumoniae are well-known causes of human diseases such as genital tract infections and respiratory illnesses [4, 5], respectively. Other species within the Chlamydiaceae family primarily infect animals (Tables 1 and 2), causing substantial economic losses by affecting livestock, while others pose zoonotic threats to humans [5]. Advances in molecular techniques have shown an even broader diversity within this family, including Chlamydia-like bacteria (CLB) [3]. Notably, Waddlia chondrophila, associated with cattle abortion and elevated risks of adverse pregnancy outcomes in women who have contact with infected bovines [6]. Frequently detected in water sources such as lakes and wells, often via amoebae [7] this organism shows the broader implications of cross-host transmission for both human and animal health.

Table 1.

Overview of the Chlamydiaceae family members, the diseases they cause in their primary hosts, and the zoonotic potential of animal chlamydia species

Species	Host	Disease	Zoonotic Potential*
C. trachomatis(8)(9) 3 Biovars; Trachoma, Genital tract, Lymphogranuloma venereum (LGV)	Humans	Inclusion conjunctivitis in newborns; trachoma. Infantile pneumonia, urethritis in men; epididymitis; urethral syndrome in women; cervicitis; endometritis; salpingitis; tubal factor infertility; proctitis; perihepatitis; peritonitis; and endocarditis.	-
C. pneumoniae (3)(10)(11)	Human, koalas, marsupials, horses	Humans: Lymphogranuloma venereum, Pharyngitis; bronchitis; pneumonia; atherosclerosis Animals: Rhinitis, pneumonia, conjunctivitis	Probable zoonosis
C. abortus (11)(12)	Small ruminants	Abortion in late gestation or deliver weak/dead fetus	Zoonotic
C.avium (11)(13)	Birds	Enteritis and respiratory disease	Putative zoonosis
C. psitaci (3) (14)	Birds	Conjunctivitis, pneumonia, enteritis, hepatitis	Zoonotic
C. caviae (11)(15)	Guinea pigs	Conjunctivitis and urogenital tract infections	Probable Zoonotic
C. suis (3) (16)	Swine	Conjunctivitis, enteritis, polyarthritis, pneumonia	Putative Zoonosis
C. felis (3)(17)	Cats	Conjunctivitis and upper reproductive tract infections	Probable Zoonosis
C. pecorum (3)(18)	Koalas & small ruminants	Encephalitis, polyarthritis, pneumonia, enteritis, vaginitis, and endometritis in sheep and cattle; polyarthritis, serositis, enteritis, and pneumonia in swine; keratoconjunctivitis, vaginitis, ovarian cysts, and infertility in koalas.	Unknown
C. gallinacean (19)(13)	Birds	Reduced body weight	Putative Zoonosis
C. muridarum (3)(20)	Rodents	Cervicovaginal infection and oviduct occlusion	Unknown
C. serpentis (21)(22)	Snakes	Unknown	Unknown
C. poikilothermis (11)	Snakes	Unknown	Unknown
Ca. C. sanzinia (11)(23)	Snakes	Unknown	Unknown
Ca. C. ibidis (11)(24)	Ibis	Unknown	Unknown
Ca. C. corallus (23)	snakes	Unknown	Unknown

* Zoonotic = confirmed animal-to-human disease transmission; Probable zoonosis = molecular or serological match plus compatible illness, but epidemiological proof incomplete; Putative zoonosis = cross-species infection documented, yet human disease unproven; Unknown = no reliable evidence of cross-species infection or transmission to date. Explained in Table 4.

Table 2.

Chlamydia-like bacteria (CLB)

Species	Host	Disease	Zoonotic Potential
Waddlia Chondrophila (3)(25)	Cattle	Abortion	Probable Zoonosis
Parachlamydia acanthamoeba (3)(26)	Cattle, sheep, goats	Abortion, pneumonia	Putative Zoonosis

Chlamydial infections in animals range from asymptomatic to severe, life-threatening illnesses, depending on the host species and the chlamydia species involved [3]. Diagnoses can be hampered by mild lesions or co-infections with more common pathogens, aside from hallmark cases such as chlamydial abortion in ruminants and avian chlamydiosis [3, 4, 11]. Additionally, several Chlamydiaceae species are either known or suspected to be zoonotic, meaning they can be naturally transmitted between animal species and humans [27].

Since several Chlamydiae species have zoonotic potential and CLBs such as Waddlia chondrophila have been detected in the environment, it is crucial to examine these organisms from a One Health (OH) perspective [27–29]. One Health recognizes the interconnectedness of humans, animals, and ecosystems, and emphasizes systematic, coordinated, and cross-sectoral strategies to mitigate global and transnational health risks [27](Fig. 1).

Fig. 1.

The One Health concept, where human health, animal health, and environmental health overlap to produce shared benefits [30]

This interconnectedness is especially relevant in Africa’s diverse ecosystems, where interactions among species are frequent. Spanning about 30 million square kilometers and home to roughly 18.3% of the world’s population, Africa hosts some of the planet’s most intact natural habitats and rich biodiversity [28, 31, 32]. Yet, poverty, food insecurity, and disease remain widespread, and many nations are classified as having “Low Human Development” by the United Nations [33].

In recent decades, the global incidence of emerging and re-emerging infectious diseases has risen, disproportionately affecting Africa [33]. The continent reports more epidemics, disasters, and potential public health emergencies than any other [30]. While enhanced surveillance partly explains these figures, drivers such as expanding cross-border travel, rapid urbanization, population growth, political instability, and informal settlements also fuel disease spread [14].

This narrative review aims to highlight current scientific findings to identify and show gaps in our understanding of their transmission dynamics, and public health implications of Chlamydiaceae species in African settings. Our goal is to emphasize the significance of Chlamydia species and highlight their impact on both veterinary and human health across Africa. By raising awareness among policymakers, we aim to support informed decision-making and promote collaborative efforts to address these challenges.

Methodology

This study applied an integrative narrative review with to map the knowledge of published evidence on zoonotic and potentially zoonotic Chlamydia species and CLB on the African continent. The study followed a SANRA checklist for narrative reviews [34].

Data sources and search strategy

Species-specific search

Between 1 January 1990 and 30 Jan 2025 we queried PubMed, Google Scholar and African Journals Online. The core syntax combined taxonomic, zoonotic and geographical terms. Language was restricted to French and English articles. Grey literature was captured through Google scholar. Queries for every species of interest were run (Tables 1 and 2). For each run the generic pathogen block with the species epithet (plus common abbreviations and recognized synonyms) was substituted while keeping (i) the identical African geographic string and (ii) the same date range, language policy and database set. For example, the string used for Chlamydia abortus was:

(“Chlamydia abortus“[tiab] OR “Chlamydophila abortus“[tiab] OR “C. abortus“[tiab] OR “enzootic abortion“[tiab] OR EAE[tiab]) AND (“Animals“[Mesh] OR “Livestock“[Mesh] OR “Wildlife“[Mesh] OR animal*[tiab] OR livestock[tiab] OR wildlife[tiab] OR ruminant*[tiab] OR sheep[tiab] OR goat*[tiab] OR cattle[tiab]) AND (“Zoonoses“[Mesh] OR zoonos*[tiab] OR “cross-species“[tiab] OR spillover[tiab]) AND (“Africa“[Mesh] OR “Africa South of the Sahara“[Mesh] OR “Africa, Northern“[Mesh] OR Africa[tiab] OR African[tiab])

Eligibility criteria

Supplementary non-African evidence

Initial review showed major data gaps from Africa; therefore, rigorously screened non-African studies were incorporated to support biological inference and risk contextualising. Non-African records were included only when (i) the same pathogen species was studied, (ii) the host or ecological context was relevant to African settings, and (iii) all other inclusion criteria were met (Table 3).

Table 3.

Eligibility criteria of the study

Inclusion criteria	Exclusion criteria
Studies conducted in Africa; plus non-African studies that investigated the same Chlamydiaceae/CLB species and host types and satisfied all other criteria (see § 2.3.2).	Commentaries, editorials, letters, conference abstracts without full data, narrative reviews without primary data.
Published or publicly released between 1 January 1990 and 30 Jan 2025.	In-vitro-only or cell-culture studies lacking host context.
Observational studies (cross-sectional, case–control, cohort), experimental infection studies, outbreak/case-series reports and systematic reviews with primary data extractable.	Studies using unvalidated or unspecified diagnostic methods.
Primary data describing occurrence, clinical impact or molecular characterisation of Chlamydiaceae or CLB in humans, domestic animals, wildlife or environmental samples.

Study selection

All studies were imported into Mendeley Reference Manager and deduplicated automatically and manually. Two reviewers screened titles and abstracts independently. Full texts were retrieved for any citation retained by either reviewer. Disagreements after full-text appraisal were resolved by discussion with a third reviewer.

Operational definition of “zoonosis”

To avoid ambiguity, this applied a tiered definition of zoonotic status before study screening. Each Chlamydiaceae species was assigned to one category; disagreements were resolved by consensus. Only agents in the first two categories are described as “zoonotic” (Table 4). Species in the third category are treated separately and explicitly flagged as “putative”. The different species are given the labels in Table 1.

Table 4.

Operational criteria used to classify zoonotic status of Chlamydiaceae and CLB

Category	Minimum evidence required (criteria must be met)	Label used in text/tables
Confirmed zoonosis	(i) Isolation and/or molecular detection of the same strain in humans and animals or (ii) an epidemiological link demonstrating human disease temporally and spatially associated with animal exposure	“Zoonotic”
Probable zoonosis	(i) Molecular or serological match between animal and human isolates or (ii) compatible clinical syndrome in humans, but epidemiological design does not establish directionality or full causation.	“Probable zoonosis”
Putative zoonosis / spill-over	(i) Documented cross-species infection (experimental or natural) involving either humans or animals, but no study yet links infection to human disease.	“Putative zoonosis”
Unknown	No published data on cross-species infection or evidence is insufficient to meet any of the above tiers	“Unknown”

Genital chlamydia trachomatis

C.trachomatis is a major human bacterial pathogen that infects mucosal epithelial tissues, causing a spectrum of diseases [35]. Among its various serovars, serovars D to K are responsible for the most prevalent bacterial sexually transmitted infections (STIs) worldwide [8]. Despite being easily treatable with antibiotics, C.trachomatis infections often remain asymptomatic, leading to delayed diagnosis and treatment [9]. If left untreated, these infections can result in severe reproductive health complications, including pelvic inflammatory disease (PID), ectopic pregnancy, and tubal infertility in women, as well as epididymitis in men [35].

Emerging evidence suggests that interactions between C.trachomatis and CLBs from environmental and zoonotic sources may exacerbate reproductive health risks [9]. Animal studies indicate that repeated exposure to multiple C.trachomatis species, including those found in poultry, could impact female reproductive health, although this remains underexplored in human populations [36]. Additionally, the high prevalence of chlamydia infections in poultry raises concerns about potential zoonotic transmission pathways [13].

Given the interplay between human, animal, and environmental health, a One Health approach [30] is needed to understand the exposure to Chlamydiales and how it may influence reproductive outcomes. Smallholder poultry farming, which is widespread across sub-Saharan Africa (Fig. 3) serves as a key livelihood source for rural populations, particularly for women and youth [37]. It contributes to food security, economic empowerment, and sustainable agricultural practices. However, the proximity of humans to poultry in small-scale farming systems increases the likelihood of cross-species bacterial transmission, warranting further investigation into its implications for reproductive health [35, 37].

Fig. 3.

Data on the distribution of cattle (red), sheep (green), goats (blue), and pigs (purple) in Africa, highlighting their susceptibility to C. abortus infection [45]

Ocular chlamydia trachomatis

Serovars A, B, Ba, and C of CT cause chronic conjunctivitis, which can progress to trachoma—the leading cause of infectious blindness worldwide. Recognized as one of the oldest diseases known to humanity, mentioned in the Ebers Papyrus and found in the eyelids of Egyptian mummies [36]. Trachoma remains a significant public health problem in 41 countries worldwide. It is responsible for the blindness or visual impairment of approximately 1.9 million people, with 72% of the most severe cases occurring in sub-Saharan Africa [36, 38].

Trachoma mainly affects impoverished, rural, and remote communities (Fig. 2) where clean water, sanitation, and healthcare are scarce [38].Its transmission is fueled by poor hygiene, direct contact, and eye-seeking flies (Musca sorbens) [39]. Consequently, the disease is consistently found among the most disadvantaged populations. Overcrowded sleeping conditions further heighten infection risk, sharing a bedroom with someone who has active trachoma doubles an individual’s likelihood of infection [39]. Moreover, those affected tend to be poorer than age- and gender-matched peers without the disease and face reduced economic productivity, even after accounting for visual impairment [39].

Fig. 2.

Sub-national prevalence of trachomatous inflammation- follicular (TF) among children aged 1–9 years in Africa (data current to 2024). Trachoma results from repeated infection with ocular C.trachomatis [36, 43]

Among the affected countries, Ethiopia and South Sudan together have the highest prevalence of active trachoma [40]. In some regions within these two countries, more than half of children aged 1–9 years have active trachoma, and trichiasis affects over 10% of adults aged 15 years and older [40].

The WHO’s SAFE strategy [41] —Surgery for trichiasis, Antibiotics (mass azithromycin), Facial cleanliness, and Environmental improvements, addresses trachoma’s multiple transmission routes, including control of the fly vector Musca sorbens [40, 41]. This integrated approach has led to steady progress in eliminating trachoma [36]. As of May 2023, Benin and Mali became the fifth and sixth African countries validated by the WHO for eliminating trachoma as a public health problem, showcasing the progress of a One Health approach [42].

C. abortus

In sub-Saharan Africa, livestock play a pivotal role in the livelihoods of smallholder farmers, with nearly every family owning some animals [44]. Livestock production contributes approximately 35% to the agricultural gross domestic product in the region, significantly enhancing food supply, nutritional security, income generation, and employment opportunities. For many smallholder farmers, livestock are the primary assets that can be converted into cash, serving as indispensable sources of economic opportunity [44]. The distribution of livestock in Africa is illustrated in Fig. 3. Generations of pastoralist producers have demonstrated that livestock farming is the most viable economic activity in these territories due to their ability to adapt to and exploit unpredictable environments [44].

C. abortus poses a major threat to both animal and human health, particularly in small ruminants such as sheep and goats, where it remains the leading cause of ovine enzootic abortion worldwide [3, 12]. Although it less frequently affects cattle, pigs, horses, wild ruminants, and yaks, its impact can be severe. Infected animals often undergo late-term abortions, stillbirths, or deliver weak offspring, and non-pregnant ewes may carry latent infections that reactivate during pregnancy [46]. After an abortion, the bacterium is shed into the environment via placentas, vaginal discharges, and fetal remains, with resilient elementary bodies remaining infectious for days.

Human health risks are especially significant for pregnant women, who can contract C. abortus through aerosol inhalation or direct contact with infected materials [46]. Exposure during pregnancy may result in abortion, stillbirth, or septicemia, while non-pregnant women can experience pelvic inflammatory disease [12].

Studies in Kenya [47] and Zimbabwe [48] show widespread C. abortus antibodies in both livestock and wildlife, suggesting a wildlife reservoir capable of transmitting the pathogen to domesticated herds. This risk is heightened by communal animal husbandry practices common in African pastoralist communities, where livestock share grazing land, water sources, and breeding bulls without closed herd systems [49, 50]. Children frequently manage animals from a young age, further increasing their exposure [50]. Traditional Boma or kraal village layouts, central to pastoralist cultures in central, southern, and eastern Africa, also foster close contact among animals, inadvertently facilitating zoonotic disease spread [51].

Prevalence studies across Africa demonstrate considerable variation (Table 5). In Zimbabwe’s Great Limpopo Trans frontier Conservation Area, C. abortus prevalence reached 47.7% in buffaloes, 43.8% in impalas, and 32.7% in cattle [47] In Algeria, one study reported 35% prevalence in sheep and goats [50], while others found lower rates of 2.5% in camels and 7.2% in sheep [52, 53].

Table 5.

Prevalence of C. abortus across various African countries, animal species and testing methods

Citation	Country	Study population	Testing method	C. abortus prevalence
Isman., (2018) (48)	Kajiadoi, Kenya	Sheep (n = 148) Goats (n = 199)	PCR (In-house)	Sheep (20.3%) Goats (28.1%)
Tesfaye et al., (2022) (54)	Borana, Ethiopia	Sheep (n = 293) Goats (n = 213)	Commercial indirect ELISA	Sheep (7.9%) Goats (11.26%)
Merdja et al., (2015) (55)	Ksar El- Boukhari in Algeria	Sheep & Goats (n = 144)	Commercial indirect iELISA (ID screen)	Sheep & Goats (35%)
Benaissa et al., (2020) (52)	Eastern Algeria	Camels (n = 865)	Commercial indirect iELISA (ID Screen)	2.5%
Hireche et al., (2016) (53)	Northeastern Algeria	Sheep (n = 552)	Commercial indirect iELISA (ID Screen)	7.2%
Selim et al., (2021) (56)	Northern Egypt	Sheep (n = 675)	Commercial IDEXX Chlamydiosis total Ab test (IDEXX laboratories)	13.7%
Barkallah et al., (2018) (57)	Eastern Tunisia	Cattle (n = 214) Sheep (n = 164)	PCR (In-house)	Cattle (12.9%) Sheep (8.7%)
Samkange et al., (2010) (58)	Otavi district in Namibia	Goats (n = 3245)	Commercial CHEKIT®-ELISA (IDEXX laboratories)	28.2%
Bhandi et al., (2019) (59)	South Eastern Zimbawe	Goats (n = 599)	Complement fixation test for antibodies	22%
Ndengu et al., (2018)(47)	South Eastern Zimbawe	Cattle (n = 1011) Buffaloes(n = 111) Impalas (n = 32) Kudus (n = 18)	Complement fixation test for antibodies	Cattle (32.7%) Buffaloes (47.7%) Impalas (n = 43.8%) Kudus (0%)
Loureiro et al., (2017)(60)	Equatorial Guinea	Sheep (n = 1002)	Commercial indirect ELISA	19.9%
Adesiyun et al., (2020) (61)	Farms around Kruger National Park in South Africa	Cattle (n = 184)	Commercial IgG indirect ELISA (IDEXX laboratories)	23.6%
Pospischil et al., (2012) (62)	Serengeti in Tanzania	African buffaloes (n = 5) Spotted hyena (n = 7)	PCR (In-house)	African buffaloes (80%) Spotted hyena (14%)

Avian chlamydiosis (C. psittaci, C. avium, C. gallinacea and

candidatus C. ibidis)

Avian chlamydiosis is a bacterial infectious disease in birds, historically attributed solely to C. psittaci. However, recent discoveries have expanded its causative agents to include the newly identified species C. avium, C. gallinacean and C.ibidis [63]. Moreover, the genome sequence of C. psittaci strain 84/2334 was recently analyzed and led to its reclassification as C. abortus [64].

In the Netherlands, this strain was implicated in a family cluster of community-acquired pneumonia, likely introduced via bird droppings. Evidence suggests human-to-human transmission in at least two of the cases [64]. Retrospective analyses indicate this avian C. abortus strain has been circulating since at least 2010, based on its first documented human infection among ten patients in the country [65]. This reflects the growing genetic complexity within the Chlamydia family and underscores the importance of ongoing genomic surveillance.

C.psittaci, a major source of zoonotic psittacosis (parrot fever) infects more than 500 avian species including domestic, companion, and wild birds, and can also infect 32 mammalian species [3, 66]. Within a bird population, C. psittaci is transmitted via contaminated aerosols but also vertically through eggs and via horizontal trans-eggshell transmission [67]. Humans contract the infection via inhalation of contaminated feathers or droppings, or handling infected birds or carcasses [11]. High-risk groups include poultry workers, veterinarians, and bird owners. In birds, symptoms range from lethargy to severe organ damage, whereas human infections vary from mild, flu-like illness to life-threatening, multi-organ complications [3]. Notably, C.psittaci has been detected in trachoma patients, sometimes alongside C. trachomatis [11].

Newly recognized avian Chlamydia species, including C. gallinacea, C. avium, and C. ibidis, are increasingly important [68]. C. gallinacea often infects chickens, remaining endemic in flocks with few clinical signs, though reduced body weight has been observed [3]. Its zoonotic potential gained attention following an atypical pneumonia outbreak among French poultry slaughterhouse workers, and concurrent detection with C.psittaci in healthy cattle in China suggests possible cross-species transmission [19]. C. avium has been found in pigeons and parrots [13], but its pathogenic and zoonotic profiles remain unclear. ‘Candidatus Chlamydia ibidis’ was discovered in African sacred ibis in Western France [24], yet despite this bird being widely found across sub-Saharan Africa, little is known about the virulence of Ca.C. Ibidis, its origin, or potential for cross-species infection [69].

Bird migration (Fig. 4) significantly raises the risk of Chlamydia spread, with Africa serving as a critical hub in the seasonal migration cycle between Europe and Africa [70]. Each year, millions of birds travel south for winter and return north to breed, carrying pathogens including Chlamydia, across continents and exposing diverse environments and species [71].

Fig. 4.

After breeding, many bird species migrate south to winter in Africa, following flyways across the Mediterranean, the Sahara, or the Red Sea and Nile Valley. This migration could facilitate the spread of Avian Chlamydiae, yet no studies have specifically investigated this potential transmission pathway [71]

At the same time, smallholder poultry production underpins livelihoods and nutrition in sub-Saharan Africa, especially for women and young farmers [72]. Poultry products supply vital protein and micronutrients where meat consumption is low, and family poultry makes up to 80% of poultry stocks [72]. However, dense poultry keeping in regions such as coastal West Africa, Ethiopia, and Southern Africa also exacerbates zoonotic disease risks, including avian chlamydiosis, avian influenza, and salmonella [73].

Despite these threats, avian chlamydia surveillance remains limited. In Egypt, infection rates in psittacine birds reached 52.5%, particularly among cockatiels, Fischer’s lovebirds, and rosy-faced lovebirds [74]. Risk factors included bird age, location, housing, and season. Zoonotic transmission was suspected, as 6% of the 70 swabs collected from bird handlers tested positive for C. psittaci. Positive cases were predominantly among older individuals who exhibited respiratory symptoms and regularly handled birds in pet markets [73].

Chlamydia suis

C. suis is the most common chlamydial species affecting pigs [11], with clinical presentations ranging from asymptomatic infection to respiratory disease, enteritis, and various reproductive disorders. Although pigs are the only known natural hosts, C. suis DNA has been detected in conjunctival swabs from Nepalese trachoma patients [72] and Belgian slaughterhouse workers [73], and live organisms have been isolated from Belgian farmers, though these humans showed no symptoms [3, 75].

While pigs are currently the only known natural hosts of C. suis, evidence suggests potential zoonotic transmission to humans. C. suis DNA has been detected in conjunctival swabs from Nepalese trachoma patients [76] and Belgian slaughterhouse workers [77].Uniquely among chlamydial species, C. suis harbors the tetracycline-resistance gene Tet(C), likely driven by extensive antibiotic usage in pig farming [78, 79]. This raises concerns about potential gene transfer to other chlamydial pathogens [11]. To date, no studies have searched for C. suis in African contexts.

Chlamydia felis

C. felis causes feline pneumonitis, primarily conjunctivitis, rhinitis, and pneumonitis in cats under one year of age [80]. Infection can persist for up to two months, with ocular and respiratory secretions serving as the primary shedding sources [3, 11]. Some cats remain asymptomatic carriers, posing a potential zoonotic risk. While transmission mainly occurs through direct contact. Transmission between cats occurs through direct, close contact because C. felis does not survive well in the environment, making group settings like shelters more susceptible to outbreaks [11].

Although primarily carried by cats, dogs have also been reported as important reservoirs [11]. The ubiquity of cats and dogs and their interactions with humans may facilitate the dissemination of C. felis to people, although evidence linking it to severe human diseases is ambiguous [11]. To date, only six cases of C. felis-associated follicular conjunctivitis in humans have been documented [11, 81].

A recent meta-analysis of feline-to-human zoonotic transmission in North Africa did not mention C. felis in its findings [82]. This omission may stem from limited awareness of C. felis or the need for PCR diagnostics rather than conventional culture methods [82]. Given its potential zoonotic relevance, future research in Africa should consider C. felis investigations.

Chlamydia caviae

Guinea pigs (Cavia porcellus), though native to South America, are increasingly being adopted as micro-livestock in Africa due to their adaptability to local ecosystems [83]. Their meat is highly nutritious, containing 21% protein, they have a short gestation period of 58–72 days, mature early, are easy to manage, and require relatively little capital to raise [83]. Their growing use could address protein deficits in regions where animal protein consumption lags behind WHO recommendations.

However, C. caviae, a pathogen causing urogenital infections in guinea pigs that resemble human C. trachomatis diseases presents potential health risks [11, 84]. Repeated eye infections can lead to chronic inflammation resembling human trachoma. The pathogen spreads rapidly through close contact, sexual transmission, and from mother to offspring [11].DNA of C. caviae has also been found in cats, dogs, and rabbits, suggesting a broader host range [3]. Human cases remain rare but include conjunctivitis and pneumonia following exposure to infected guinea pigs [11]. This expanding livestock role for guinea pigs in Africa warrants attention to C. caviae’s zoonotic potential [84].

Chlamydia pecorum

C. pecroum significantly affects Koalas, contributing to population declines in Australia due to severe diseases like blindness, infertility, and urinary tract infections [3]. While Koalas are not native to Africa, C. pecorum is also associated with a wide range of animals, including sheep, goats, cattle, water buffalo, pigs, and alpine chamois [85]. In these animals, it causes clinical manifestations such as ocular infections, pneumonia, enteritis, and urinary and reproductive tract infections. Complications can include polyarthritis, sporadic bovine encephalomyelitis, infertility, and a possible association with abortion in ruminants [1, 85].

Transmission typically involves direct contact or consuming contaminated feed and water, and mother-offspring transmission also plays a role in some species [11, 85]. Though its zoonotic risk remains unknown, C. pecorum has occasionally appeared alongside C. trachomatis in human ocular infections [3]. A recent study in Ghana’s Mt. Afadjato foothills found C. pecorum in wild birds (24.1% prevalence), sheep (40.0%), goats (43.3%), and chickens (26.9%), with identical ompA gene sequences indicating cross-species transmission [86]. These findings highlight the potential for broader circulation where wildlife and livestock mingle.

Chlamydia pneumoniae

1. C. pneumoniae is a non-zoonotic pathogen primarily associated with respiratory tract infections, though it can occasionally manifest as extra pulmonary disease [87]. While infections are often mild or asymptomatic, they can be severe in immunocompromised individuals. Beyond acute respiratory illnesses, C. pneumoniae plays a role in the pathogenesis of chronic respiratory conditions such as asthma and chronic bronchitis, where patients often exhibit a strong immune response to the organism [3, 88]. Recent evidence also links C. pneumoniae to non-respiratory diseases, including atherosclerosis and coronary artery disease.

2. C. pneumoniae is nearly ubiquitous in humans, with seropositivity rates reaching 70–80% in older populations [89], indicating that most people experience infection at some point in their lives. In Africa, multiple studies have identified C. pneumoniae across various populations [88]. For instance, in the Gambia, 3.1% of young infants showed the presence of C. pneumoniae whereas in the older children 15% were positive for C. pneumoniae [89]. Additionally, the pathogen has also been commonly detected in two refugee camps in Kenya, and another study identified C. pneumoniae among Somali refugees in Djibouti [90].

Although humans are the primary reservoir, C. pneumoniae has been identified in animals spanning from koalas, horses, bandicoots, and a wide range of reptiles including a species of African Clawed Frogs (Xenopus tropicalis) [91]. Nearly all animal isolates of C. pneumoniae harbour a 7.5-kb plasmid also seen in C. trachomatis and C. muridarum, but is absent in human isolates [92]. This has led to the suggestion that human C. pneumoniae strains may have originated from animal strains that gradually adapted to human hosts through the loss of certain genes and plasmids, ultimately bypassing the need for animal reservoirs.

Chlamydia muridarum

C. muridarum is a murine pathogen closely related to the human pathogen C. trachomatis, often used in laboratory mouse models due to its similarities to human cervicovaginal infections, oviduct occlusion, and hydrosalpinx [11, 93]. In female mice, acute infection typically clears within 30 days, but chronic complications like hydrosalpinx can emerge months later. Post-recovery, mice develop immunity to reinfection with the same strain [11, 93]. In male mice, the infection causes urethritis without affecting fertility. The zoonotic potential of C. muridarum is unknown.

In the context of Africa, where rodent populations are abundant and diverse, C. muridarum presents a potential but underexplored public health concern. Rodents are common in both rural and urban settings across the continent, often living in close proximity to humans and livestock [93]. This close contact raises the possibility of zoonotic transmission of chlamydial pathogens from rodents to humans and domestic animals.

Serpentes chlamydia (C. serpentis, C. poikilothermis, candidatus C. sanzinia & candidatus C. corallus)

Africa’s vast snake diversity, encompassing approximately 600 species provides a reservoir for potential novel pathogens [94]. Recent culture-independent genome sequencing have led to significant discoveries of chlamydial species in snakes [23], including Ca. C. sanzinia from a healthy Madagascar tree boa (Sanzinia madagascariensis volontany) [3, 23] and ‘Ca. C. corallus in an asymptomatic Amazon Basin emerald tree boa (Corallus batesii) [3, 11].

Notably, although chromosomal sequences of chlamydiae were unresolved in other samples, plasmid sequences were obtained. This suggests the existence of additional, unexplored diversity within the Chlamydia genus, particularly in African snake populations. Currently, C. serpentis, C. poikilothermis, Ca. C. corallus, and Ca. C. sanzinia are the known chlamydial organisms found in captive snakes [11]. Their host range remains undefined, with snakes being the only documented carriers. Little is understood about their pathogenic potential, as no associated diseases have been described in animals or humans to date. Given Africa’s extensive snake fauna, further research may reveal more species relevant to wildlife health and possible zoonotic transmission.

Chlamydia-like bacteria (CLB)

In recent years, at least 13 new Candidatus species of CLB have been identified in various hosts, including reptiles, amphibians, birds, fish, and protozoa [3]. Notably, CLB from the families Waddliaceae and Parachlamydiaceae have been implicated in ruminant abortions and human miscarriages, having been detected in the placentas of women with adverse pregnancy outcomes [23].

In Africa, bovine abortions of unknown infectious origin remain a significant economic concern. In Tunisia, Waddlia chondrophila DNA was detected in vaginal swabs and placental tissues from cows that had aborted and were co-infected with Listeria monocytogenes [27]. However, subsequent experimental infections failed to establish a definitive link between W. chondrophila infection and cattle abortions. Only one out of nine heifers intravenously infected with W. chondrophila during pregnancy developed inflammatory lesions of the chorioallantois without resulting in abortion, suggesting that W. chondrophila may act as an opportunistic pathogen [27].

Parachlamydia acanthamoebae has also been associated with bovine abortions and, to a lesser extent, caprine and ovine abortions [3]. Infected cases exhibited necrotizing or purulent placentitis, often accompanied by vasculitis [95]. A Tunisian study examining aborting dairy cattle detected Brucella spp. (31.3%), Chlamydiaceae (4.66%), W. chondrophila (8%), P. acanthamoebae (5.33%), Listeria monocytogenes (4.66%), and Salmonella spp. (3.33%) [26]. Crucially, this is the first report of P. acanthamoebae in African bovine abortions, underscoring the need for further research on its interactions with other abortigenic pathogens.

Zoonosis and One-Health

Chlamydiae, as detailed above, are a group of pathogens capable of infecting a wide range of hosts, including humans, domestic animals, and wildlife [35]. Several chlamydial species, such as C. abortus, C. suis, and various avian chlamydiae, readily transmit to humans either through direct contact with infected animals or indirectly via contaminated environments [3, 11]. While ocular C. trachomatis may spread indirectly through vectors like the bazar fly (Musca sorbens) [39]. These transmission routes (Fig. 5) illustrate the adaptability of chlamydiae and highlight the importance of considering different ecosystems and human–animal interactions across Africa.

Fig. 5.

Possible routes of transmission of Chlamydiae to humans [29, 96] Created in https://BioRender.com

Domestic animals serve as key reservoirs and amplifiers for chlamydiae. Cattle, sheep, goats, horses, pigs, dogs, and cats all harbor various chlamydial species that can spill over to humans, especially in pastoral communities [97]. As African countries become more urbanized and economically affluent, there is a growing trend toward pet ownership [98]. Companion animals, including dogs, cats, guinea pigs, and exotic bird species are increasingly common in urban households. While this shift offers social benefits, it also raises the risk of chlamydial transmission to humans from pets or their environments [96].

With Africa’s population expected to double from 1.2 billion to over 2 billion by 2050, food insecurity remains a growing concern, especially as global demand for animal-derived protein is projected to rise at the same rate [99]. This increasing demand exacerbates the risk of undernourishment in sub-Saharan Africa, where zoonotic foodborne pathogens already pose significant health challenges.

While C. trachomatis is primarily recognized as a sexually transmitted pathogen [14], its interactions with Chlamydiales species in poultry warrant further investigation within a One Health framework. Additionally, C. caviae, a pathogen known to infect guinea pigs, presents potential foodborne risks as guinea pigs are increasingly being adopted as micro-livestock for consumption in parts of Africa. Understanding these zoonotic connections is essential for mitigating future public health threats.

The presence of chlamydiae in other animals, including cats, dogs, and rabbits, suggests a broader host range and potential foodborne risks that require further study. Africa boasts some of the world’s most biodiverse and intact ecosystems, yet these habitats are under increasing pressure from deforestation, climate change, habitat loss, and rapid human population growth [29–31]. Such disruptions alter the ecological balance and may facilitate the emergence or re-emergence of zoonoses, including infections caused by chlamydiae [96]. Wildlife, including mammals, reptiles, birds, fish, and amphibians can act as reservoirs for these pathogens, passing them to domestic animals or humans under the right conditions. Factors such as wildlife farming, the hunting and consumption of bushmeat, and limited biosafety measures during the handling or transport of wild animals further exacerbate the risk [100].

Despite their substantial impact, many chlamydial infections in Africa remain severely underreported. This neglect is partly due the perception that they pose minimal global spread risk, and limited diagnostic resources in low-income regions [96]. Nevertheless, chlamydiae infections have the potential to cause significant morbidity and mortality, especially in marginalized communities. Because chlamydial infections often persist at the human–animal interface, focusing on their control and prevention can be both cost-effective and beneficial for vulnerable populations, making them a prime example of “neglected zoonoses” [37].

Effective control of chlamydiae and other zoonotic pathogens hinges on coordinated surveillance at local, national, and international levels [101]. Such monitoring can detect early infections, identify reservoirs and vectors, and locate high-risk “hotspots,” informing adaptable control strategies. There are four main surveillance types, which include; pathogen, serological, syndrome-based, and risk surveillance. These provide complementary tools to track and manage emerging or re-emerging threats [96, 102]. However, success depends on well-equipped laboratories, robust diagnostic capacity, skilled personnel, and sustainable funding.

Because chlamydial infections typically span multiple sectors—human health, veterinary health, and environmental sciences—a multisectoral “One Health” [28] framework is crucial for long-term prevention and control. Under this approach, physicians, veterinarians, ecologists, microbiologists, epidemiologists, and other experts collaborate to address the health challenges facing humans, domestic animals, and wildlife [27].

Limitations

This review is unavoidably constrained by the lack of primary literature. A majority of the eligible studies included in this paper originated from outside Africa, forcing us to supplement with rigorously screened non-African based work to avoid leaving entire host-pathogen pairs unaddressed; these extra-continental data dilute the continental specificity of the study. Even where African records exist, most document infection or spill-over rather than well-designed demonstrations of disease causation and onward human transmission. As a result, our use of the term zoonotic spans a spectrum, from agents with proven zoonoses to those with only putative cross-species potential. Study designs, diagnostics and reporting standards varied widely, precluding meta-analysis and limiting comparability, and publication, temporal biases almost certainly left additional data, especially from francophone and lusophone Africa undetected.

Conclusion

Chlamydia species have the potential to significantly impact human and animal health across Africa, leading to serious public health and economic consequences. Trachoma caused by C. trachomatis remains endemic in impoverished regions across the continent [14]. Concurrently, C. abortus endangers livestock and humans, especially in pastoralist communities where traditional practices facilitate its spread [12]. The emergence of avian chlamydiosis driven by species like C. psittaci, C. gallinacea, and C. avium is exacerbated by migratory birds disseminating pathogens [3, 11]. Additionally, under recognized zoonotic threats such as C. suis in pigs, C. felis in cats and dogs, C. caviae in guinea pigs, and novel chlamydiae in reptiles pose significant risks [3, 11, 17, 21]. These threats are amplified in impoverished and rural settings, where limited sanitation, close contact with livestock, and potential exposure to contaminated water sources heighten infection risks [39].

Africa harbors exceptional ecological diversity and an abundance of animal reservoirs; the absence of published data should not be mistaken for absence of risk. The continent’s knowledge gaps mirror a broader under-representation of African research in many disciplines, let al.one in chlamydial biology, considering high-quality studies from other regions allows us to illustrate plausible transmission scenarios, benchmark methodological approaches and identify priority questions. Moreover, a One Health approach offers the most promising path forward, applying the expertise of human health, veterinary, and environmental sectors (104). Policy measures should include incorporating One Health principles into medical and veterinary curricula, raising public awareness of transmission routes, and robust inter-sectoral communication. By taking these strategies into account, African nations can better safeguard animal health, support community livelihoods, and protect human populations from emerging chlamydial threats.

Abbreviations

CLB

Chlamydia like bacteria

OH

One Health

Author contributions

A.S.: A.S.: conceptualization; methodology; validation; writing—original draft; writing—review and editing. F.M.: conceptualization; writing-review and editing; formal analysis. D.V.: con-ceptualization; validation; supervision; writing-review and editing; formal analysis. V.O.O.: conceptualization; writing-review and editing; formal analysis. P.P.M.T.: conceptualization; methodology; supervision; writing—review and editing; formal analysis. S.A.M.: conceptual-ization; validation; supervision; writing- review and editing; formal analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This study received no external funding.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

This study is a review of previously published data. As such, it did not involve the collection of primary data from human participants and does not require ethical approval or informed consent. All data analyzed were derived from publicly available peer-reviewed articles that had obtained appropriate ethical clearance from their respective institutional review boards or ethics committees.

Consent for publication

Not applicable. This study did not involve individual participant data, images, or any other personal details requiring consent for publication.

Competing interests

The authors declare no competing interests.

Clinical trial

Not applicable.