Herd certification is loosely defined on this web page as any official program designed to classify herds according to the probability that individual cows in those herds are infected with M. paratuberculosis.
The primary aim of certification programs is two-fold:
should be considered a herd,
not just an individual animal, health problem. Certification programs
apply laboratory tests to classify herds by levels of infection ranging
from not infected at all to very likely infected. Often such programs
have regulations that participating herd owners must follow regarding
the sources for replacement cattle. These rules are designed to help herd
owners avoid bringing M. paratuberculosis-infected cattle into
their herds without compromising their ability to do business. The schematic
diagram on the right illustrate these two basic concepts: 1) apply a herd
test to sort the herds and 2) prevent movement of infected cattle into
the non-infected herds.
Some programs, such as the one in Wisconsin USA, classify herds by the percentage of the herd that is infected (prevalence). Others are directed only at the non-infected herds. Annual testing is used to graduate herds to higher levels of certainty that they are free of the infection. The program in The Netherlands combines these elements into a single 10-level program. The reason herd tests must be repeated regularly is that diagnostic tests, for Johne's disease or any other disease, are not 100% sensitive and specific. This means that on occasion infected herds are not detected (false-negative herd test) or non-infected herds are incorrectly classified as infected (false-positive herd test). Programs are designed to minimize such errors and a program epidemiologist is charged with monitoring test results on participating herds to be sure the tests are properly applied and interpreted. Most programs have an appeal process for the owners to use if they think the test results are incorrect. The programs also incorporate confirmatory tests to settle disputes. Annual herd testing and herd classification for Johne's disease is very similar to what was done to control and eventually eradicate bovine tuberculosis (TB) and brucellosis (Bang's disease) from many countries around the world using tests of comparable accuracy.
Repeated testing of a herd that never introduces new animals from other herds or introduces only those animals of the same certification status, improves the probability the herd is not infected. This is called serial or sequential testing. It is possible, over time and with annual herd testing, to eliminate infected herds from the population of test-negative herds. This is accomplished by insuring that herds found to be infected in a given geographic area are not permitted to sell animals into the test-negative, presumably non-infected herds.
news for the beef cow-calf industry in the U.S. is that only a small percentage
of herds are infected. Based on a 1997 USDA blood test survey, more than
90% of cow-calf herds should test 100% ELISA-negative for Johne's disease
if adult cattle are randomly chosen for testing. These are encouraging
odds. Download a story about the USDA survey here.
models, practical experience, and logic all support the same premise about
herd certification programs:
Australia has created Johne's disease certification programs for cattle (including both beef and dairy), sheep, goats, and alpaca. They call the program for cattle the Cattle Market Assurance Program (CattleMAP). They were the first country in the world to formally adopt such a program for Johne's disease. The program is administered by Animal Health Australia, a novel non-profit corporation with joint funding from animal health industries and the state and federal governments. The program has three certification levels attained by annual herd testing. Herd owners can elect to stay at any level they reach by doing a "maintenance test" which is less demanding than the testing required to go to a higher program level. The test most commonly used is the blood test known as the ELISA. For a full description, readers are recommended to download the pdf file available on the Animal Health Australia website.
A subcommittee of the National Johne's Working Group (NJWG) formulated a model program for herd certification. It is titled the Voluntary Johne's Disease Herd Status Program (VJDHSP). This program was approved by the NJWG and then the Johne's Disease Committee of the U.S. Animal Health Association in 1998. Like the CattleMAP in Australia, the program is designed to identify those herds least likely to be M. paratuberculosis-infected. It has four herd status levels and uses a combination of blood testing by ELISA and fecal culture. Like the Australian program, herd owners can elect to remain at any certification level by doing a limited amount of testing. The VJDHSP is a model program designed to guide USA states wanting to develop a program of their own for classifying test-negative herds. Each state is free to adopt it as is or to make modifications. Details of the VJDHSP are described both on the USDA website and at the website maintained by the Minnesota Board of Animal Health.
As of February 1, 2001, Minnesota has come closest to full adoption of the VJDHSP. Other states have elected to make minor changes. The links page in this website lists websites for several U.S. states describing their Johne's disease programs in detail. An article in the January 2001 issue of Bovine Veterinarian gives an update on what each of the states in the U.S. are doing regarding Johne's disease.
Wisconsin has long been innovative in design and implementation of programs and regulations to help control Johne's disease. The latest new regulations are designed to extend herd classification to those herds that are infected. By grading herds based on the percentage of herd that is test-positive, the state hopes to provide an incentive for owners of infected herds to make progress toward control of the infection. Herds are classified A, B, C, or D with class A herds having no test-positive animals and class D herds having more than 15% of the herd testing positive. This herd classification must be disclosed to a buyer at the time of sale of any animal from the herd. Any herd that is not tested, or does not annually renew the herd classification by testing, becomes classified "Maximum Risk for Johne's Disease" by default. Herds attaining the A classification are encouraged to enroll in the VJDHSP described above.
A Dutch program for herd classification of cattle was first created in 1997. It continues to evolve as experience with the program is gained. The Dutch program encompasses all herds, infected, non-infected, and not tested in a single scheme. Herds are classified on a 1 to 10 system with 10 being the highest attainable level in the program. Herds attaining level 6 have had at least one negative herd test. Like the U.S. and Australian programs, the Dutch program allows herd owners to maintain a herd level with only a limited amount of testing each year. For a detailed description, visit the website of the Animal Health Service in The Netherlands. (This website is written in Dutch.)
Harmonization of herd certification / classification programs would greatly benefit animal agriculture and help promote trade of animals within and among countries with limited risk of M. paratuberculosis transmission. With leadership from international organizations such as the O.I.E. and the International Association for Paratuberculosis this useful goal can be reached in the not too distant future.