Enzootic pneumonia (EP) (Pleurisy)

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).

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.

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.