Respiratory diseases of pigs can be classified into two broad categories based on the extent and duration of overt disease: those that affect large numbers of pigs and may 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, but the losses are limited rather than ongoing. They include swine influenza (see Swine Influenza), classical swine fever (see Classical Swine Fever), the pneumonic forms of pseudorabies (see Pseudorabies), porcine circovirus-associated disease (see Porcine Circovirus Diseases), and porcine reproductive and respiratory syndrome (see Porcine Reproductive and Respiratory Syndrome). The causal viruses may persist in a herd, but outbreaks of overt disease tend to be self-limiting.
The most important syndromes in the second category are mycoplasmal pneumonia and pleuropneumonia (see below). Atrophic rhinitis, once considered to be a significant cause of respiratory disease in swine, has declined substantially as a result of eradication programs. Salmonellosis and Haemophilus parasuis infections may be significant problems in some herds. Moderate levels of atrophic rhinitis caused by Bordetella bronchiseptica alone may not be too significant but, when coupled with toxigenic strains of Pasteurella, are an important cause of economic loss due to decreased rate of growth and reduced feed conversion in young pigs. Enzootic pneumonia, when caused by mycoplasma alone, is of little consequence; however, when it is combined with secondary infection, eg, Pasteurella multocida, the resulting condition may be severe. Actinobacillus pleuropneumoniae may be associated with considerable losses in some herds. Migrating worm larvae or the infections listed in the first category often lead to severe problems when they occur with the infections in the second category.
The severity and economic importance of diseases in the second category also are related to population density and to the type and size of herd. They may be of little importance in weanling pig operations but become of major importance in high-density feeder-pig units. Although mortality usually is low, economic damage results from an adverse and uneven effect on growth rate, decreased feed efficiency, and additional costs of drugs, particularly medicated feed. However, when stress can be avoided by proper management, such diseases may result in only minimal losses.
Finally, it must be stressed that respiratory disease problems in pigs are frequently the result of multiple agents (co-infection) and rarely due to the effects of a single pathogen.
It is possible to set up herds free of diseases in the second category by techniques such as SPF repopulation or segregated early weaning, or by buying pigs from a pneumonia-free herd. The latter method is the least expensive, but because the etiology of diseases in the second category is complex, all the pigs should be purchased from one source. This is also true when purchasing weaned pigs for feeder-pig units.
It is difficult to keep herds free of respiratory diseases. Aerosols have been suspected as sources of pathogen entry onto naive farms. Organisms such as Mycoplasma hyopneumoniae have been postulated to be transmitted over distances of as far as 2 miles, depending on climate, terrain, and density of pigs in the locality; however, this assumption is based on speculation and use of mathematical models rather than on experimental data.
Closed herds, ie, buying in no live animals (using artificial insemination or embryo transfer to bring in new genetic material), help establish immunity to present organisms and avoid introduction of new infections, strains, or serotypes. 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.
Respiratory disease is endemic in many herds. The main control factors are stress management, stocking density, ventilation, temperature control, and freedom from mixing and moving. Multiple site production or “all-in/all-out” and closed-herd management practices greatly decrease the need for preventive and therapeutic medication.