Overview of Respiratory Diseases of Pigs

Respiratory disease problems in pigs are usually the result of multiple agents (coinfection) versus a single pathogen. The term "porcine respiratory disease complex" is used to describe multifactorial pneumonia syndromes characterized by the involvement of multiple contributing factors, both infectious and noninfectious.

Primary pathogens that are part of the porcine respiratory disease complex include porcine circovirus type 2, porcine respiratory coronavirus, influenza virus, and porcine reproductive and respiratory syndrome (PRRS) virus.

Some bacterial agents—such as Mycoplasma hyopneumoniae (the primary causative agent of enzootic pneumonia), Actinobacillus pleuropneumoniae (the primary causative agent of porcine pleuropneumonia), and Pasteurella multocida (the primary causative agent of pneumonic pasteurellosis and of progressive atrophic rhinitis)—can act as either primary pathogens or secondary pathogens that complicate a primary infection.

Various other bacteria and parasites can be part of the porcine respiratory disease complex as secondary pathogens.

Respiratory diseases of pigs can be broadly categorized by the extent and duration of overt disease as follows: those that affect large numbers of pigs and can be serious but of limited duration, and those that persist in a large number of pigs for indefinite periods.

• Diseases in the first category can be costly; however, the losses are limited rather than ongoing. These diseases include influenza, classical swine fever, the pneumonic forms of pseudorabies, porcine circovirus-associated disease, and porcine reproductive and respiratory syndrome (PRRS). The causative pathogens might persist in a herd; however, outbreaks of overt disease tend to be self-limiting. (For complete discussion of PRRS, see the chapter Porcine Reproductive and Respiratory Syndrome.)

• The most important syndromes in the second category are enzootic pneumonia and porcine pleuropneumonia.

Glaesserella parasuis (formerly Haemophilus parasuis) infections can be a problem in some swine herds (see Glässer Disease in Pigs), particularly those infected with PRRS virus.

Moderate levels of nonprogressive atrophic rhinitis caused by Bordetella bronchiseptica alone are typically of low consequence. However, progressive atrophic rhinitis (caused by infection with toxigenic strains of Pasteurella spp) causes economic loss as a result of decreased growth rate and decreased feed conversion in young pigs. Progressive atrophic rhinitis, once considered an important cause of respiratory disease in swine, has declined substantially as a result of eradication programs.

Enzootic pneumonia in pigs, when caused by uncomplicated M hyopneumoniae infection, is associated with only mild clinical signs. When it is combined with secondary infection (eg, pneumonic pasteurellosis), however, the resulting condition can be severe.

Porcine pleuropneumonia, caused by the bacterium A pleuropneumoniae, can be associated with considerable losses.

The severity and economic importance of diseases in the second category also are related to population density and to the type and size of a herd. Although mortality rates usually are low, economic damage results from an adverse and uneven effect on growth rate, decreased feed efficiency, and additional costs of drugs. Viral coinfections, such as highly virulent strains of PRRS, can considerably increase deaths and costs.

Herds free of respiratory diseases in the second category can be established by using techniques such as specific-pathogen-free repopulation or segregated early weaning, or by buying pigs from a pneumonia-free herd. The latter method is the least expensive; however, the etiology of diseases in the second category is complex, so all pigs should be purchased from the same source.

In the absence of air filtration, it is difficult to keep swine herds free of respiratory diseases. Aerosols have been suspected as sources of pathogen entry onto naive farms. Organisms such as M hyopneumoniaecan be transported long distances via aerosols, depending on climate, terrain, and density of pigs in the locality; one study documented airborne transport 9.2 km from the herd (1).

Multiple-site production or an all-in all-out policy, in which the entire barn or air space is emptied before refilling, can very effectively minimize the potential effect of chronic pneumonia in pigs.