Diagnosis and Control of Johne's Disease (2003)

Because most cases of Johne’s disease (JD) are subclinical and prevalence data are lacking or imprecise, it is difficult to assess the economic consequences of this disease for individual cows, herds, or the nation. In the United States, several estimates of economic losses have been made, including an estimate of $15.4 million annually in New England (Chiodini et al., 1984), $54 million in Wisconsin (Arnoldi and Hurley, 1983), $5.4 million in Pennsylvania (Whitlock et al., 1985), and from $200 million to $250 million annually in U.S. dairy herds (Garry et al., 1999). By comparison, economic losses in Australia were estimated to be the equivalent of U.S. $2.1 million annually (Gill, 1989).

Economic consequences at the herd level were estimated as part of the 1996 National Animal Health Monitoring System (NAHMS) survey of U.S. dairy herds. For JD test-positive herds with fewer than 10 percent of cull cows showing clinical signs consistent JD, the adjusted annual value of production was $40 per cow lower than for JD test-negative herds that had no reported cull cows with clinical signs. For JD test-positive herds with 10 percent or more cull cows showing clinical signs, the economic impact was $227 per cow, and the majority of the loss was attributable to reduced milk production with additional losses from higher cow replacement costs and lower cull cow revenues (NAHMS, 1997a). Benedictus and colleagues (1987) apportioned JD losses into decreased production (32 percent), veterinary and treatment costs (2 percent),

reduced value at slaughter (20 percent), loss due to idle production facilities (3 percent), and unrealized future income based on age at culling and prior production (43 percent). Groenendaal and Galligan (1999) used simulation models to predict average losses of $35 per cow per year, increasing to $52 and $71 per cow per year after 10 and 20 years, respectively. Finally, Ott and colleagues (1999) calculated average annual losses of $22–$27 per cow. The variations in these estimates are partly the result of differences in prevalence, herd size, management practices, and the varying assumptions that underlie the calculations. In all estimates of the economic impact of JD, however, the losses are significant.

JD is spread most effectively through the movement of infected animals that contaminate a new environment, thus setting the stage for widespread exposure of more animals. Map also can be isolated from semen and embryos from affected breeding stock, although the effectiveness of this route of transmission is unknown. Consequently, JD has serious implications for domestic and international trade of live animals and germplasm. The identification of Map in animal products also can affect trade, especially if a link between Map and Crohn’s disease is demonstrated conclusively.

There are currently no federal limitations on intrastate movement of animals between premises. Participants in voluntary JD status programs can agree to the limitations on introduction of new animals as a requirement for maintaining existing status in state programs. Current control programs allow only animals from herds of equal or higher JD status to enter participating herds without conditions.

Interstate movement requires that animals be accompanied by a certificate of veterinary inspection stating that they show no clinical signs of any disease (U.S. Department of Agriculture [USDA], 2001b), including JD. Rule changes promulgated by USDA’s Animal and Plant Health Inspection Service-Veterinary Services (USDA/APHIS/VS) in 2000 also prohibit interstate movement of animals with a positive result to an official JD test, except movement directly to slaughter. Official JD tests detect the Map organism and must be completed by a laboratory approved by USDA/APHIS/VS/National Veterinary Services Laboratories. However, the rule changes refer only to animals with known test results and do not require JD testing of domesticated animals before interstate movement. Some states broadly prohibit entry of animals known to be affected by or exposed to any infectious or contagious communicable disease. Kentucky specifically defines communicable disease to include JD (Kentucky, 2002). However, no state currently requires Map testing or any specific certification as to the Map exposure status of the animal or its herd or flock of origin.

International trade of animals has contributed to the global dissemination of JD. Infected animals of European origin are cited as the original source of JD in several countries. Concern over the potential for spread of JD and other diseases has led to the development of international guidelines for testing and certification of animals destined for export. The veterinary administrations of 158 nations comprise the International Office of Epizootics (OIE), which has developed the International Animal Health Code, a compendium of specific recommendations concerning the international movement of animals and animal products (OIE, 2001a). OIE also promulgates the Manual of Diagnostic Tests and Vaccines, which provides standards for the application of diagnostic tests and quality assurance for vaccines (OIE, 2000).

The OIE considers JD (generally called paratuberculosis in official OIE documents) a disease of major global importance. JD is categorized as a List B transmissible disease, considered to be of socio-economic and/or public health importance within countries and significant in the international trade of animals and animal products (OIE, 2001a). Specific guidance on JD in all species is provided by International Animal Health Code, Chapter 2.2.6. Current code recommendations are limited to live ruminants and address individual animal certifications and testing as follows:

• showed no clinical sign of paratuberculosis (JD) on the day of shipment;

• were kept in a herd in which no clinical sign of paratuberculosis (JD) was officially reported during the 5 years prior to shipment;

• were subjected to diagnostic tests for paratuberculosis (JD) with negative results during the 30 days prior to shipment.

Current OIE recommendations provide only a limited protection against importation of Map-infected animals, because few affected animals show clinical signs, official reporting systems are not widely enacted or enforced, and no specific recommendations are provided for the choice of diagnostic tests or their interpretation.

Adoption of the sanitary and phytosanitary (SPS) provisions of the General Agreement on Tariffs and Trade in 1994 (World Trade Organization [WTO], 1994) and the dramatic increase in international trade in animals, germplasm, and animal products over the past 25 years have pushed OIE to revisit several current guidelines. The SPS trade agreements identified OIE as the international standards-setting agency for animal health issues with regard to trade. Therefore, the OIE code recommendations represent the de facto baseline for import requirements promulgated by national veterinary administrations. OIE recommendations cannot be challenged in international trade disputes

before WTO. Countries have the right to require more stringent animal health import measures than those recommended by OIE, but SPS mandates that additional requirements are to be science based and justified by risk assessment. Additional requirements that exceed the code guidelines can be challenged on the basis of scientific validity.

Chapter 2.2.6 of the Manual of Standards, Diagnostic Tests, and Vaccines 2000 (OIE, 2000) could be reconsidered by OIE in the near future. OIE recently commissioned a supporting document on JD that provides a comprehensive review of current knowledge focusing on controlling the spread of JD via trade. Supporting documents are usually the precursor to the revision of specific code chapters. Proposed revisions to the International Animal Health Code and Manual arise from recommendations of member countries and then proceed through several committees’ review. Final disposition of proposed revisions is decided at the OIE annual meeting of the International Committee, a committee of the whole that consists of one vote for each member country. The OIE code tends to reflect broad consensus and to recognize the needs of developed and developing nations alike.

JD is likely to become more of a trade issue in the near future, both here and abroad. The implementation of various state JD control programs sets the stage for differing import requirements by individual states. International import restrictions also could increase as more countries embark on JD control and eradication programs. Australia has identified several of its states as JD-free zones, restricting the movement of animals into those areas (Kennedy and Allworth, 1999). The implementation of a national control and assurance program that incorporates internal movement control empowers Australia to require similar assurances of JD status from its global trading partners. Similar import requirements that go beyond the OIE international standard could be implemented within the legal context of the SPS by other countries that have JD control and eradication programs in place. If it is established that Map can cause Crohn’s disease, more restrictions on international movement of animals, animal products, and germplasm can be expected.

There are few restrictions on the interstate movement of JD-exposed or Map-infected animals other than those that show clinical signs of disease and those known to be organism-test positive. Because it is difficult to effectively and economically screen animals for Map, livestock is often bought and sold without consideration of Map status. Most buyers who seek large numbers of cattle, sheep, or goats currently have no practical options for ensuring JD-negative status for their purchases. Consequently, producers of rapidly expanding herds or flocks are at a very high risk of buying Map-infected animals. Current international standards for the safe movement of animals recommend JD testing, combined with clinical examination and herd history, although no specific guidance is provided for testing protocols or interpretation of results. Increased global concern about JD will encourage development of further trade barriers related specifically to JD and Map. In the absence of

coordinated JD control programs, imports and exports will be affected. Restrictions on the importation of animals infected with Map will be limited by OIE and SPS requirements. Furthermore, export opportunities will be lost unless larger populations of JD-free or low-risk animals can be certified by government agencies.