Enzootic pneumonia is a common respiratory disease in pigs worldwide and is caused primarily by Mycoplasma hyopneumoniae. M hyopneumoniae also frequently leads to subclinical infection, causing lung lesions that can be detected at postmortem examination. Clinical signs, if present, are often a dry cough and decreased growth. Diagnosis is based on clinical signs, characteristic lesions, and confirmation by PCR assay. Partial disease control can be achieved via improved management practices, antimicrobial treatment, and vaccination.
Enzootic pneumonia is a chronic, infectious pneumonia of pigs that is typically associated with mild clinical signs. The disease is characterized by a persistent dry cough, impaired growth, occasional flares of overt respiratory distress, and a high incidence of lung lesions in slaughtered pigs. It tends to become endemic in infected herds and occurs worldwide.
Clinical outbreaks of enzootic pneumonia in pigs can impair growth rate and feed conversion. The effects of the disease are uneven and unpredictable, and they place limits on the efficiency and flexibility of large production operations. In swine units with good disease control measures, enzootic pneumonia can remain largely subclinical. However, total disease control seems to be achieved only by pathogen eradication.
Etiology of Enzootic Pneumonia in Pigs
Mycoplasma hyopneumoniae is the primary causative agent of enzootic pneumonia in pigs. It is recognized as part of the porcine respiratory disease complex.
M hyopneumoniae is a host-specific pleomorphic organism that lacks a cell wall and is fastidious and smaller than most bacteria. It can be cultured in specially prepared media, but isolation from field cases is difficult. It is rapidly inactivated in the environment and by disinfectants; however, it can survive longer in cold weather and on some surfaces.
Enzootic pneumonia in pigs is also frequently complicated by other mycoplasmas, bacteria, and viruses, which affect the severity of the disease.
Immunity to enzootic pneumonia develops slowly in pigs, followed by the regression of lung lesions. Older growing and mature pigs might recover from the disease completely; however, the bacterium can persist in the respiratory tract of infected animals for up to 7 months.
Pneumonic pasteurellosis occurs in pigs most commonly as a complication of enzootic pneumonia.
Epidemiology of Enzootic Pneumonia in Pigs
Pigs of all ages are susceptible to M hyopneumoniae infection; within a herd, however, pigs are colonized in the first few weeks of life through infection either from their dam or from other young pigs after mixing. The infection can be transmitted to suckling piglets by sows of any parity, but it is most prevalent in first-parity (gilt) litters.
The prevalence of M hyopneumoniae at weaning age has been suggested as an indicator of disease in finishing pigs.
The onset of enzootic pneumonia tends to be most evident in the finishing stage at the age of approximately 18–20 weeks. The incidence of lung lesions is highest in pigs 3–5 months old.
Clinical Findings of Enzootic Pneumonia in Pigs
Nonproductive coughing is the most common clinical sign of enzootic pneumonia and is most obvious when pigs are roused. In endemically infected herds, the morbidity rate is high; however, clinical signs might be minimal, and the mortality rate is low. Average daily weight gain and number of days to market weight are common production parameters negatively affected.
Individual pigs or groups sporadically develop severe pneumonia. Common predisposing factors are seasonal and other stresses, such as transient viral infections, parasitic migration, and mixing of pigs. The disease is usually more severe when it first enters a naive herd.
Lesions of Enzootic Pneumonia in Pigs
On gross examination, areas of lungs affected by enzootic pneumonia in pigs are gray or purple (see ), most commonly in the apical and cardiac lobes, and consolidated (see ). Old lesions become clearly demarcated and eventually heal, leaving visible scars. The associated lymph nodes might be enlarged.
Necropsy photograph showing gross lesions of Mycoplasma hyopneumoniae infection in lung tissue from a pig. Note the atelectasis and lobar discoloration.
Courtesy of Dr. Louise Bauck.
Necropsy photograph of lung tissue from a pig with Mycoplasma hyopneumoniae infection. Note the cranioventral consolidation, which is characteristic of enzootic pneumonia in pigs.
Courtesy of the Department of Pathobiology, University of Guelph.
Histological examination reveals inflammatory cells in the bronchioles, along with perivascular and peribronchiolar cuffing and extensive lymphoid hyperplasia.
Diagnosis of Enzootic Pneumonia in Pigs
Clinical signs
Histopathological findings
Herd epidemiological indicators
PCR assay
Clinical signs, histopathological findings, and herd epidemiological findings are usually suggestive of enzootic pneumonia in pigs.
M hyopneumoniae can be demonstrated in impression smears of the cut surface of affected lung tissue, identified by fluorescent antibody technique or by in situ hybridization, and isolated and identified in culture. However, these laboratory tests are not routinely performed by practicing veterinarians.
Diagnosis of M hyopneumoniae infection in live pigs can be challenging.
Bacterial isolation is seldom considered for routine diagnosis of enzootic pneumonia in pigs, because this test has low sensitivity for detection, given that M hyopneumoniae is difficult to grow.
Serological tests—principally, commercially available ELISAs—are used for enzootic pneumonia in pigs on a herd basis; however, results can be difficult to interpret because of slow seroconversion, lack of differentiation between antibodies generated by infection or by vaccination, and the frequent identification of unexpected positive reactors.
PCR assays have become the most commonly used tool for detecting M hyopneumoniae in various sample types. In live pigs, the sensitivity of PCR assay increases with greater proximity of clinical specimens to the lower respiratory tract. PCR assays on samples collected from the trachea, larynx, or nasal or oral cavity show different sensitivities; tracheal samples are associated with greater detection rates.
When postmortem sample collection is feasible, bronchial swabs or lung tissue specimens containing airways are preferred, because they are associated with the greatest probability of accurate pathogen detection.
Control of Enzootic Pneumonia in Pigs
Good management practices, particularly all-in all-out management
Antimicrobials
Vaccination to decrease clinical signs
The economic effects of enzootic pneumonia in pigs can be decreased by improvements in housing and husbandry, particularly ventilation and space allowance. All-in all-out management of pigs from birth to market is extremely effective at decreasing the negative effects of disease. Adhering to this practice improves growth performance and decreases lung lesions.
Commercial inactivated mycoplasmal bacterins consist of adjuvanted whole cell preparations. Bacterins induce protection against the development of gross lesions and markedly decrease clinical signs (coughing) in growing pigs. However, vaccination does not prevent infection.
When enzootic pneumonia first enters a swine herd, mass treatment with antimicrobials effective against Mycoplasma spp helps to control the severity of clinical signs. When disease increases in endemic herds, treatment of individual pigs with antimicrobials usually results in remission, presumably by controlling secondary bacteria.
Data indicate that prefarrowing management, vaccination with M hyopneumoniae vaccines, or antimicrobial treatment of dams markedly decreases the colonization of suckling piglets, leading to fewer respiratory disease problems in downstream flows (1).
Gilt acclimation practices aimed at uniform and controlled exposure of M hyopneumoniae in replacement females at least 8 months before their first farrowing have become common in North America. Gilt acclimation is a key component of eradication programs that rely on the known end of bacterial persistence. Gilt acclimation is also used to provide disease control in endemically infected farms.
Starting with foundation stock free of enzootic pneumonia and adopting strict precautions against direct and indirect contact with pigs from other herds is advisable. In the US and parts of Europe, most herds free of enzootic pneumonia were established by repopulation or by partial depopulation.
Swine herds free of M hyopneumoniae have been established by segregated early weaning, herd closure, and medication, or by whole herd medication. Reinfections are potentially attributable to airborne transmission, breaches in biosecurity, and, to a larger extent, lack of eradication by the original program.
Diagnostic monitoring of swine herds presumably free of enzootic pneumonia can be difficult. However, achieving a low prevalence of M hyopneumoniae, even if it is not eliminated, is a substantial production advantage in large swine units.
Key Points
Enzootic pneumonia is the most important bacterial respiratory disease in pigs.
Clinical outbreaks, subclinical infections, and endemic disease within herds are common.
Partial disease control can be achieved with improved management, vaccination, and medication.
Disease eradication programs are common in North America.
For More Information
Sample Type for the Detection of Mycoplasma hyponeumoniae [sic] Page. College of Veterinary Medicine, University of Minnesota.
References
Fano E, Pijoan C, Dee S, Deen J. Effect of Mycoplasma hyopnuemoniae prevalence at weaning on disease severity in growing pigs. Can J Vet Res. 2007;71(3):195-200. https://pmc.ncbi.nlm.nih.gov/articles/PMC1899865
