Abstract
Johne’s disease or paratuberculosis is a transmissible, fatal disease of hoofstock. The primary route of transmission of the etiologic agent (Mycobacterium avium subsp paratuberculosis, Map) is thought to be the fecal-oral route.1 Young animals are thought to be more susceptible to infection than adults.1 Infected animals may appear clinically healthy for many years but are still capable of shedding bacteria and infecting other animals during this subclinical stage of the disease.2 Clinically infected animals have non-specific signs that include weight loss and diarrhea.2 Diagnosis of the disease is challenging due to the lack of antibody production until late in the infection, sporadic shedding of detectable numbers of bacteria, the difficulty of culturing the slow-growing organism, and minimal gross pathology in subclinical disease in some species.3
Not all animals exposed to Map develop clinical disease. It is not known whether this is because infection is never established or because the immune response controls or eliminates infection. Establishment of infection requires that large numbers of bacteria are taken up by macrophages and are able to evade the macrophages’ bacteriocidal mechanisms. While the factors that influence the establishment of Map infection are not fully understood, it appears that dose, route of infection, strain variations, environmental factors, immune status, and age of the host influence the development of disease.4
Historically, Johne’s disease has been primarily a concern due to its economic impact. In the United States it is considered to be one of the most economically significant diseases in dairy cattle.5 More recently there has been increasing concern about the disease as a possible zoonosis. Mycobacterium avium subsp paratuberculosis is hypothesized to be the cause of some cases of Crohn’s disease, an inflammatory bowel disease of humans. There is currently insufficient evidence to determine whether there is a causal relationship between human Map infections and Crohn’s disease.6
The Map host range includes both ruminant7 and non-ruminant8 wildlife species, and the spread of Johne’s disease from domestic livestock to wildlife could significantly affect wildlife ecology. In addition, the establishment of Johne’s disease in wildlife reservoirs could be detrimental to efforts to control or eradicate Johne’s disease in domestic livestock.
In 1998 a workshop was held on the diagnosis, prevention, and control of Johne’s disease in non-domestic hoofstock.9 One of the recommendations from this workshop was for zoos to establish a control or monitoring program according to risk category (low risk, high risk, or unknown Johne’s disease status). It was recommended that Johne’s management units (JMUs) be identified by each facility as part of the Johne’s disease management plan. The workshop concluded that the prevalence of Johne’s disease in a given JMU was the most valuable diagnostic information and that JMU surveillance should be the highest priority for all institutions with regard to management of Johne’s disease.
In 2001 Manning and Ziccardi reported the results of a survey of all AZA accredited institutions with hoofstock species.10 This survey found that one-third of the 133 respondents do not test for Map infection. Another third of the respondents test single animals only pre- or post-shipment. The authors conclude that there is insufficient surveillance for Johne’s disease in up to 66% of the facilities participating in the survey. Human-animal contact areas were present in 80% of the responding institutions and domestic livestock were common in these exhibits.
The National Research Council recently published their report on the Diagnosis and Control of Johne’s Disease.11 This report was in response to increasing national concern about the apparent increase in prevalence of Johne’s disease worldwide, a lack of national coordination of control programs, and public health concerns about the possible zoonotic risk of Map. The study concluded that Johne’s disease represents a significant threat to animal health and warrants implementation of control programs. The study suggests that control programs for the dairy industry are the highest priority. The study also suggests that control programs for zoo animals and wildlife should be monitored to assure that a non-domestic animal reservoir does not compromise control efforts. The potential risk of Johne’s disease to Crohn’s disease increases the perceived importance of a Johne’s disease control program.
In light of the increased concern about Johne’s disease and its implications for animal health, the economy, and possibly public health, a re-examination of current surveillance, and control programs for Johne’s disease in zoological institutions is warranted.
Literature Cited
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11. National Research Council. Diagnosis and Control of Johne’s Disease. National Academies Press, Washington, DC. 2003.