HERD HEALTH
PIH-82
pork industry handbook
PURDUE UNIVERSITY COOPERATIVE EXTENSION SERVICE • WEST LAFAYETTE, INDIANA
Porcine Infectious Pleuropneumonia
(Actinobacillus pleuropneumoniae, App)
Author:
Brad Fenwick, Kansas State University
Reviewers:
Doug Hoefling, Galesburg, Illinois
James McKean, Iowa State University
David Reeves, University of Georgia
Douglas Stine, Lenexa, Kansas
Infectious porcine pleuropneumonia, caused by the bacteria Actinobacillus pleuropneumoniae (App), occurs throughout the world and in some cases can be a significant barrier to profitable pork production. On a herd basis, infection with at least one serotype of App is common and sporadic outbreaks of overt clinical disease, including frequent deaths, occur under the correct circumstances. App has been responsible for the depopulation of numerous herds, and its control was one of the motivations behind the development of early weaning and segregated rearing systems. Pigs (particularly sows and boars) are the major reservoirs of App within a herd. Introduction of clinically healthy infected replacement stock colonized with App are responsible for most of the transmission between herds.

Overt disease outbreaks of App may cause substantial economic losses because of deaths, treatment costs, and loss of performance (reduce daily gains and feed conversion rates). Vaccines are generally of limited value in the prevention and control of App. Preventing the introduction of App and the reliable control of disease outbreaks in infected herds requires the development and application of a well-planned and faithfully implemented health management system. To be effective, this approach must be based on an understanding of the various risk factors that contribute to the introduction and maintenance of App within herd, its transmission between various production stages, as well as the health and environmental factors that contribute to the occurrence of clinical disease.

Epidemiology and Mechanism of Disease
There are gaps in understanding how App causes disease as well as what combination of factors determine whether infection results in clinically apparent disease or merely colonization. On the other hand, there are a number of things that are well
understood that are of value in the diagnosis, treatment, and prevention of App infection and disease.

Experimental evidence as well as characteristics of some disease outbreaks demonstrates that the ability of App to cause disease is dependent on the number of microorganisms the pig inhales, which under production conditions is related to the concentration of App in the environment/airspace. This dramatic dose-response relation between the number of App and the severity of disease means that a pig can become colonized, develop an immune response, and remain clinically healthy. A slightly higher level of exposure will result in severe clinical disease and mortality rates that can be as high as 50%.

The threshold exposure dose necessary to cause disease is reduced by concurrent infectious and stressful environmental and management conditions. The environmental load of App is influenced by the number of infected pigs that are actively shedding App which in turn relates to stocking densities, management, and environmental conditions such as humidity, air-changes, and temperature. Disease outbreaks may occur suddenly following a stressor such as rapid changes in environmental conditions or the movement or mixing of pigs. These factors are often the reason why the occurrence of App disease tends to take place at a specific stage of production or a specific location (problem barn or area within a barn). For the same reason, herds that have not suffered from clinical App disease and that are believed to be free of App, but are actually subclinically infected, can suddenly break with clinical disease following changes in production practices, environmental conditions, introduction of other diseases, or a decline in the level of App immunity within the herd.

App is unique in that it is a highly host-adapted primary respiratory pathogen that does not require defects in normal pulmonary defenses for it to cause pneumonia. This is reflected in the pathology where pulmonary lesions caused by App are
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