Enzootic pneumonia (EP) (Pleurisy)
Occurrence: Worldwide, some herds free.
Age affected: All ages.
Causes: Mycoplasma hyopneumoniae.
Effects: Cough, poor growth, rarely death.
Enzootic pneumonia is caused by infection with Mycoplasma hyopneumoniae, a small, shapeless, delicate bacterium, which is difficult to grow in culture in the laboratory. The cell surface bears fimbriae (filaments) but there is no cell wall (present in most bacteria), and M. hyopneumoniae is generally resistant to penicillin and is sensitive to drying, although it can survive for up to 17 days in rainwater.
A number of strains exist, but all appear to produce a similar lactate dehydrogenase enzyme and to react in a similar, but not exactly the same, way to antisera, and all can be distinguished from related mycoplasmas. The organism grows on the cells lining the trachea, bronchi and bronchioli (rigid airways of the lung) of non-immune pigs, attaching to the cilia (actively beating hairs), paralysing them, causing their loss and resulting in the accumulation of respiratory tract secretions in the lowest parts of the lung and in pneumonia.
It is found in the blood, but does not cross the placenta. Inflammatory cells and antibody-producing cells surround the affected airways and are common in the pneumonic lung. Immunity develops and the pneumonia eventually clears after a period of months, although antibody can be detected in the blood and the mycoplasmas persist in the airways in recovered pigs.
Severe pneumonia in SEW pigs that were coinfected with PCV2 and M. hyo.
Photo credit: Iowa State University (Alex Ramirez).
Mode of transmission
M. hyopneumoniae is shed in droplets or as an aerosol from the respiratory tract of infected pigs. Transmission between pigs is usually by inhalation of these aerosols but can be by direct contact between pigs or from contaminated water or freshly contaminated pen furniture such as toys or ropes. Transmission is facilitated by high levels of humidity, low airspeeds and the absence of turbulence, absence of direct sunlight and cool temperatures. Within pens and single airspaces, these factors are of less importance than direct exposure to infected pigs, but between buildings and farms they become an important influence on transmission of infection. Transmission between buildings and farms is chiefly by movement of infected pigs, but contaminated clothing and vehicles can result in the introduction of infection. Under favourable atmospheric conditions (dull, cool, damp days or nights with little wind) infection can travel for up to 9 km downwind from a point source (such as an infected farm, particularly where there is forced ventilation, or from a vehicle transporting infected pigs) in the absence of hedges or other causes of turbulence.
Few clinical signs of infection occur in chronically infected herds. Infection at 3-10 weeks of age is followed after 10-16 days by transient diarrhoea and a dry cough. A low fever may occur. A barking cough is common in finishing pigs, particularly when the animals are disturbed. Coughing spreads gradually to most members of the group within 3-14 weeks and lasts for about 50 days.
Animals gradually develop uneven size. Growth rate from 5-85 kg may be depressed by up to 15.9% and feed conversion ratios increase by up to 13.8%. Dust, chilling, high humidity and high ammonia levels exacerbate these effects on growth which appear to be proportional to the volume of lung affected. Severe pneumonia and coughing occur in pigs 2-4 months old (and at 18-20 weeks of age in the US) when other infections are present. In SPF herd breakdowns, pigs of all ages may be affected by inappetence, fever to 41.7°C (105°F), respiratory distress and coughing. Some adults and up to 50% of piglets die, often before coughing is apparent. Boars may not work and extended weaning to service intervals occur. Infection of some SPF herds has been inapparent.
Enzootic pneumonia should be suspected when coughing occurs in a herd, particularly in fattening pigs, associated with uneven growth rate and poor feed conversion efficiency but with no mortality. In an SPF herd, the disease should be suspected if fever, coughing, respiratory distress and mortality occur. Serological testing for antibody (using a commercial ELISA test) confirms the presence of past infection in an animal and the presence of the infection in a herd. The age at which infection occurs in a herd or in a building, can be determined by serum antibody testing (serum profiling).
Pleuritic lesions in the antero-ventral portion of a lung due to bronchopneumonia.
Photo credit: Merck/MSD Animal Health.
These are an important feature aid to diagnosis and monitoring infection in a pig herd. Pneumonia is present in the lungs of affected pigs as plum-coloured or pink changes in the portions which hang down in life (the dependent parts of the cranial lobes) and microscopic changes of the disease are present. These changes are still present at slaughter in many herds and can be used to identify the introduction of infection and its extent where infection is enzootic. As the lung lesions can be infected by other respiratory pathogens such as Pasteurella multocida, confirmation of infection by laboratory tests is essential for diagnosis, especially in monitoring schemes. Infection can be confirmed by the culture of M. hyopneumoniae from affected lung tissue in specialist laboratories or by its demonstration in the airways or lung using fluorescent antibody, immunoperoxidase staining, other serological tests for the organism or the Polymerase Chain Reaction (PCR) which specifically detects M. hyopneumoniae DNA.
Treatment and prevention
Enzootic pneumonia can be treated using valnemulin, tiamulin, lincomycin, aivlosin or tylosin and enrofloxacin or marbofloxacin. Tetracyclines prevent the disease. Tiamulin and valnemulin can eliminate M. hyopneumoniae from animals. Lesions do not regress immediately. Control is based on husbandry methods to reduce disease such as all-in, all-out management of accommodation or sites, Isowean, treatment of the group on entering an airspace or use of an effective therapeutic at a low level for a period (where registered). A number of effective killed vaccines can be used to prevent disease. These are generally given at 1 and 3 weeks of age and protect from 4-5 weeks, reduce the number and extent of the lung lesions and improve growth rates and profitability.
Enzootic pneumonia-free breeding stock should be vaccinated in advance of movement to infected farms. The disease can be eradicated by depopulation followed by restocking with enzootic pneumonia-free pigs. It has been eradicated from smaller farms by partial depopulation and treating breeding stock with tiamulin and valnemulin. Serological monitoring confirms freedom from infection. Enzootic pneumonia-free farms must use enzootic pneumonia free breeding stock, must be sited at least 3.2 km from any other pig farm (the organism can spread by aerosol), must maintain a barrier and ensure that visitors have not been in recent contact with pigs .Nucleus herds free from enzootic pneumonia are founded by hysterectomy derivation of the piglets and subsequent rearing on a clean site. This process eliminates many pathogens, but is cumbersome and expensive and is rarely performed.
Remnants of lung tissue are visible in the thoracic cavity of this carcass and also parts of the pleura that are attached to the thoracic wall. This could have been caused by App or M. hyo.
Photo credit: Merck-MSD Animal Health.