A system response to an outbreak of enzootic pneumonia in grow/finish pigs

Abstract

A Mycoplasma hyopneumoniae-negative commercial swine production system broke with enzootic pneumonia at their grow/finish site in southern Manitoba in October, 2003. System responses included feed medication, depopulation, delayed shipment of pigs to the infected site, vaccination of at risk sow herds, and disinfection when grow/finish site depopulation was completed.

In October of 2003, enzootic pneumonia was suspected to have been transmitted into the previously mycoplasma-negative grow/finish site of a commercial production system via windborne dust or moisture particles carrying Mycoplasma hyopneumoniae. Several mycoplasma-positive barns owned by different production systems are situated northwest of the grow/finish site, with a geographic separation of less than 5 km (Figure 1, numbers 29, 55, 61, 62). In the affected production system, 2, 3000-sow barns (Figure 1, numbers 29, 78) supply early weaned piglets to several nursery barns situated at a 2nd location (Figure 1, number 58). These pigs then enter the grow/finish stage of production at a 3rd location, where 3, 2100-head barns are situated, (2 high throughput, all in/all out barns; 1 slower filling, quality control barn [Figure 1, number 30]). This production system is historically negative for mycoplasma and positive for vaccine strain porcine reproductive and respiratory syndrome virus (PRRSV). A commercial mycoplasma vaccine (RespiSure; Pfizer, Kirkland, Quebec) is used routinely in the sow barn, due to the close proximity of mycoplasma-positive competitor barns. The system also has periodic outbreaks of Haemophilus parasuis and utilizes an autogenous vaccine to control disease outbreaks.

Figure 1.

Global positioning system (GPS) locations of system and competitor barns from system barn map.

O = system barns

29, 78 — at risk sow barns

30 — infected grow/finish barns

58 — nursery barns

Δ= competitor barns

29, 55, 56, 57, 61, 62 — mycoplasma positive

During the 3rd week of October, veterinarians were alerted to a decreased growth rate and increase in coughing in 1 of the high throughput grow/finish barns. These changes had started 2 wk previously and had increased in severity. Two pigs that died suddenly and a 3rd pig that was euthanized were necropsied. Differential diagnoses for respiratory disease considered at this time included those caused by M. hyopneumoniae, H. parasuis, and Actinobacillus pleuropneumoniae, as well as swine influenza and PRRSV. Postmortem examination revealed dark, reddish-purple cranioventral lung consolidation in 2 of the 3 animals, most consistent with mycoplasma pneumonia. Tarry feces were present in the intestinal tracts of all 3 animals, possibly indicating gastric ulceration, secondary to anorexia. Lung tissue was submitted to the diagnostic laboratory at the Veterinary Services Branch of Manitoba Agriculture and Food for virus isolation, histologic examination, and bacteriologic study. Polymerase chain reaction (PCR) demonstrated M. hyopneumoniae, PRRSV, and porcine circovirus. Broncho-interstitial pneumonia, inflammatory debris, and lymphocytic cuffing of small vessels were observed on microscopic examination, consistent with porcine respiratory complex and likely of mixed bacterial and viral etiology. The isolation on PCR of mycoplasma, combined with clinical signs and gross lesions, led to the diagnosis of enzootic pneumonia in this herd.

As an immediate response to the laboratory diagnosis of enzootic pneumonia, therapeutic levels of chlortetracycline hydrochloride (Chlor 50; Bio Agri Mix, Mitchell, Ontario) were added to the feed at a concentration of 660 g/tonne, and a target dose of 20 mg/kg bodyweight (BW). Sow, nursery, and grow/finish barn managers met and developed a response plan for the control, eradication, and prevention of reinfection that included all 3 stages of production within the system. Control of clinical disease centered around strategic medication of infected and susceptible pigs in the grow/finish barns. Chlortetracycline was effective in suppressing signs of clinical disease, ensuring that pigs could be marketed from the grow/finish barns without delay. Disease eradication strategies involved both the nursery and grow/finish barns. Shipment of 50 lb (22.7 kg) pigs to the grow/finish site was halted as part of the grow/finish depopulation strategy, with the nursery barns becoming deliberately overcrowded. This permitted the grow/finish site to empty all their barns completely and to clean and disinfect them before repopulating with healthy, mycoplasma-negative pigs. The risk of infection of the 2 sow herds was determined to be high, based on geographic proximity to the infected grow/finish barns. Routine sow herd vaccination was continued, along with standard biosecurity protocols. Gilts were vaccinated and received 1 booster vaccination before entry to the sow barns and all animals were vaccinated 2 wk prior to farrowing. Despite a relative lack of scientific research concerning sow vaccination, the vaccination protocol was based on the risk of infection and the relative cost of vaccine versus the cost of a mycoplasma outbreak in a naïve herd. Control or eradication of enzootic pneumonia in the 2 sow barns carried with it a much greater cost than was associated with feeder barns. As well, a pneumonia outbreak within the sow base would negatively impact the total efficiency of production for the entire system.

Enzootic (mycoplasmal) pneumonia, caused by M. hyopneumoniae, is a significant problem in modern swine production systems, with the prevalence and severity of enzootic pneumonia increasing in recent years as a result of specific pathogen-free swine production (1). In one estimate, the annual cost to pork producers totals over US $1 billion, making the control of this disease a serious economic necessity (1). There are 3 primary methods of disease eradication in herds. Typical plans to eliminate M. hyopneumoniae from farms can utilize 2 of these 3 forms (2). Eradication without total depopulation relies on changing pig flow and reducing susceptibility to disease. This can include a break in farrowing, vaccination, treatment of susceptible and infected animals with antibiotics, and removal of all animals younger than 10 mo from the farm (2,3). Alternatively, complete depopulation, disinfection, and repopulation with healthy animals can be employed (2). In mycoplasma-positive swine herds, vaccination can be utilized at different stages of production, depending on when animals are most likely to come into contact with the organism (4). Vaccination typically does not prevent the spread of disease. However, it does reduce or prevent the occurrence of clinical pneumonia and reduces lung lesions found at slaughter. It has also been found to decrease mortality, increase feed conversion ratios, and improve overall financial gains in mycoplasma-positive barns (5). Similarly, the use of antibiotics in controlling enzootic pneumonia is not curative. Instead, antibiotics suppress the signs of clinical disease and prevent secondary infections from occurring.

Deciding which of these strategies, or a combination thereof, to employ during an outbreak of mycoplasmal pneumonia depends on the individual characteristics and needs of each production system or individual barn. In areas endemically infected with enzootic pneumonia, maintaining a mycoplasma-negative health status may be extremely costly and difficult. In the situation described here, strategic use of depopulation, changed pig flow, vaccination, and antibiotics was determined to be the most cost effective means of managing clinical disease and preventing the spread of mycoplasma. In areas without endemic disease, or in barns with a high stake in maintaining a negative mycoplasma status, such as purebred or breeding stock barns, the cost and effort associated with eradication of enzootic pneumonia may be profitable. Both of these factors were involved in deciding to eradicate M. hyopneunmoniae from the grow/finish site and, therefore, from the entire system.

The primary clinical sign of uncomplicated enzootic pneumonia in grow/finish pigs is coughing that is not associated with decreased performance (6). Lungs affected by M. hyopneumoniae typically show grey to purple areas of consolidation located cranioventrally. However, M. hyopneumoniae infection occurs much more commonly as part of a disease complex, such as porcine respiratory disease complex. In these cases, infection with other pathogens, such as PRRSV, Pasturella multocida, swine influenza virus, or a combination of these, occurs concurrently. With secondary infection, lung lesions are potentiated with a significant increase in severity of clinical disease (7). Enzootic pneumonia, especially as part of porcine respiratory complex, has been associated with decreased average daily gain and decreased feed efficiencies in growing pigs. This results in slow growing pigs taking longer to reach market weight. Straw et al (6) estimated in 1989 that a 37-gram loss of average daily weight gain occurred with every 10% of lung tissue affected by enzootic pneumonia. Other studies have estimated the increase in cost of production during a mycoplasma outbreak to be well over a $1/pig (8).

Transmission of enzootic pneumonia has been demonstrated to occur both through direct contact between animals and through aerosols. Close proximity of susceptible farms with infected farms has been shown to be a major risk factor in incurring an outbreak of mycoplasma (9). In addition, there is a seasonal increase in the rate of aerosol transmission between barns, with peak transmission occurring between March and November (10).

The production system in this study utilized components of 2 strategies to eliminate mycoplasma from the grow/finish site and prevent the spread of disease vertically through the system. Control measures instituted by the veterinarians and system managers to eradicate enzootic pneumonia from this production system were successful. Furthermore, careful attention to pig flow and biosecurity prevented the dissemination of disease vertically throughout the system, and both the nursery and sow barns have remained mycoplasma negative.