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1 Introduction Johne’s disease (JD), sometimes called paratuberculosis, is a chronic, progressive intestinal disease caused by infection with Mycobacterium avium subsp. paratuberculosis. A variety of animals can be infected, but the primary hosts are ruminants—cattle, sheep, goats, bison, deer, and other non-domesticated hoofstock. The clinical syndrome was recognized in the early 1800s, but it was not until 1895 that Johne and Frothingham demonstrated the presence of mycobacteria in the intestines of affected cattle in Germany. By the early 1900s, the disease had been found in several European countries and in the United States. Since then, it has spread throughout the world and is now found in virtually every country where there is a significant livestock industry. The causative agent has undergone taxonomic reclassification and name changes over the years. There is still some debate, but in 1990 a proposal was made to change the name of the agent from Mycobacterium paratuberculosis to Mycobacterium avium subsp. paratuberculosis (which will be abbreviated as Map in this report) in recognition of its close genetic relationship to M. avium. Throughout this report, the Committee on the Diagnosis and Control of Johne’s Disease uses the taxonomy name, although M. paratuberculosis is still in common usage. The proposed taxonomic reorganization placed Map in the group of mycobacterial agents known as the Mycobacterium avium Complex, or MAC. As a result, there are several places
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in the body of the report in which Map as a disease agent is discussed in the larger context of its position in the MAC. Control of JD on farms has been difficult for several reasons. The disease has a long subclinical phase, during which animals can spread the infection without themselves exhibiting signs of illness. Current diagnostic tests are relatively insensitive at detecting this phase of the disease. It thus is a simple matter for a producer to unknowingly purchase a Map-infected animal, which can then spread the infection throughout a herd. Subsequent detection of JD in a herd can be delayed because of a lack of producer awareness, poor sensitivity of diagnostic tests when applied to individual animals, and the fact that JD is clinically similar to other common ruminant diseases. Finally, the lack of an efficacious vaccine means that control is undertaken primarily through management practices that are designed to interrupt transmission of the agent. Producers often are slow to adopt those new methods, however, because they often fail to recognize JD as a problem of sufficient economic importance to warrant dramatic management changes. This reluctance could limit the success of control programs that focus exclusively on Map as the disease agent. Historically, control of JD in the United States has been left to the discretion of individual states. Some states have no program at all, others have chosen from a variety of voluntary options. The United States Animal Health Association has attempted to provide more coordination through the creation of its National Johne’s Disease Working Group (NJWG), which promulgated a series of recommendations, including minimum standards for administering voluntary state herd JD status programs. NJWG also has developed educational programs to increase awareness among producers and an accreditation program for laboratories that test for JD. Despite significant strides, the lack of perceived priority or cost by the industry and veterinary profession, lack of market incentives, and lack of a nationally coordinated effort and supporting infrastructure have resulted in haphazard implementation of some of NJWG’s important recommendations. In recent decades, there has been growing concern over the lack of effective JD control; the apparent increase in global prevalence; and the implications for accompanying animal health, economies, and trade. The recognition that the Map host range includes ruminant and nonruminant wildlife also is alarming. The spread of JD from domesticated animals to wildlife could significantly alter some wildlife populations, and the development of wildlife reservoirs could hamper the ability to control or eradicate JD in domesticated livestock. Finally, there is increasing concern over the human health implications of JD in livestock. Although considerable controversy surrounds this issue, there is some evidence that Map plays a role in some cases of Crohn’s disease in humans. There also is concern that Map is becoming more widespread in the environment and in the food chain (possibly through milk and meat), resulting in greater risk of human exposure. If Map were determined to be a human pathogen, JD would be transformed from an animal health and economic issue into a serious public health concern.
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THE COMMITTEE’S TASK The current status of JD research has been reviewed at national and international meetings and strategies to control the spread of Map have been presented. As part of these endeavors, in July 2000 the U.S. Department of Agriculture requested that the Board on Agriculture and Natural Resources of the National Academies convene a committee on the diagnosis and control of JD. Specifically, the committee was instructed to conduct a thorough review, evaluation, and compilation of all scientific research related to JD in domesticated and wild ruminants. The committee’s task was to: (1) review and synthesize current information regarding diagnostic techniques, mode of transmission, clinical expression, global prevalence, and potential animal and human health implications associated with JD in domesticated and wild ruminants; (2) evaluate current programs for controlling and preventing JD in ruminants; (3) provide policy recommendations for identification, monitoring, and management strategies applicable to U.S. livestock herds; (4) conduct an objective, critical assessment and summary of the state of knowledge regarding the relationship of JD in ruminants and Crohn’s disease in humans; and (5) provide recommendations on future research priorities and potential mechanisms to facilitate prevention and control of the disease. Much of the emphasis in this report is on knowledge and control of JD in dairy cattle. This emphasis is difficult to avoid, because most of the published research, diagnostic test development, epidemiology, and control efforts have focused on dairy cattle. This could be the result of the higher prevalence of JD in dairy cattle, the potentially greater economic consequences of JD for the dairy industry, or the fact that some dairy management practices facilitate the introduction and spread of the agent within a herd. This report mirrors the bias toward dairy cattle, but the committee strove to address significant gaps in knowledge, research needs, and control measures for other species of domesticated livestock and wildlife. The committee anticipates that progress with other species will accelerate as JD control in dairy cattle advances. In addition to discussions and recommendations on specific topics, the committee presents a broad outline of the steps that should be taken to control the disease, reduce the spread of Map, and minimize effects of the disease in animals. The committee also makes recommendations on research that should be undertaken to enhance control programs, to develop improved diagnostic tests and an efficacious vaccine, and to clarify the role of Map in the pathogenesis of Crohn’s disease.
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