Herd certification is loosely defined on this web page: any official program designed to classify herds according to the probability that individual cows in those herds are infected (or not infected) with M. paratuberculosis.
The primary aim of certification programs is two-fold:
| ||1. ||Provide a simple system to communicate to cattle buyers the risk of buying an M. paratuberculosis-infected animal. |
| || |
| ||2. ||Stop the spread of this infection to non-infected herds. Herd certification is the foundation of paratuberculosis prevention. |
| || |
programs apply laboratory
tests to herds in order to classify them 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 they can use 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: apply a herd test to sort the
herds, and prevent movement of infected cattle into the non-infected
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 using annual testing which, when consistently negative, graduates herds to higher levels of certainty that they are free of the infection. The reason herd tests must be repeated regularly is that diagnostic tests, for Johnes disease or any other disease, are not 100% sensitive and specific. This means that occasionally infected herds are not detected (false-negative herd test) or non-infected herds are incorrectly classified as infected when consistantly negative (false-positive herd test). Programs are designed to minimize such errors and an 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 if to use if they think the test results are incorrect. The programs incorporate confirmatory tests to settle disputes. The adjacent graphic depicts the situation resulting from these imperfect tests. Annual herd testing and herd classification for Johnes disease is very similar to what was done to control and eventually eradicate bovine tuberculosis (TB) and brucellosis (Bangs disease) from many countries around the world. Tests for those diseases are not perfect either.
testing of a herd that either never introduces new animals from
other herds, or introduces only those animals of the same certification
status, raises 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, The graph to the right illustrates this concept.
illustration in this chart assumes a region (state or country) has 30,000 herds
and that 25% of them are infected with M. paratuberculosis. The majority
of herds are detected after the first round of herd testing. However, among the
test-negative herds there remains a residual population of infected herds. These
are typically low prevalence herds that were missed by the first round of testing
because they have few infected animals and those that are infected have not yet
progressed to the stage when they can be detected by diagnostic tests. The good
news is that in this scenario the probability that a randomly selected one-time
test-negative herd is NOT infected is 91.6%. The probability an individual cow
from such a herd is NOT infected is even higher and therefore a buyer would be
quite confident they would purchase a cow free of Johnes disease from such
a herd. After the second round of testing, a year later, a few more infected herds
will be detected among the test-negative herds during the first round. These herds
are "removed" from the program, or have their classification changed,
making the remaining cows in the now twice test-negative herds even less likely
to be carrying this infection. Herds found to have infected cows can follow the
recommendations described in this website for Johnes disease control
and enter the program as soon as they become test-negative.
model was developed to help clarify this concept of the improved
probability of having truly non-infected herds among herds that
were test-negative one, two, three or four times using any desired
test of any specified accuracy. The model also calculates the cost
to the herd owner and state (if they picked up the cost) of pursuing
such a program. This model was used in the formulation of herd certification
programs for the Netherlands and the USA. Descriptions of these
programs appear below. As with most models, it is not meant to precisely
predict reality but rather to help participants understand the critical
variables in attaining a sufficiently high level of confidence that
cattle are NOT infected at the least cost. (For readers interested
in the particulars of a model to predict the probability a herd
is not infected, the spreadsheet is described on pages 66-75 of
the Proceedings of the Sixth International Colloquium on Paratuberculosis,
held in Melbourne Australia February 14-18, 1999. The book can be
ordered from the International
Association for Paratuberculosis.
The model illustrates what seems intuitively obvious:
| ||1 - ||the more cows in a herd you test, the greater the confidence the herd is correctly classified as either infected or not infected (the model assumes a 100% specific test, i.e. no false-positives. This is achievable by using fecal culture or ELISA with fecal culture follow-up for the ELISA-positive cows). |
| || |
| ||2 - ||the more times you test a herd, the greater the confidence (probability) the test-negative herd is NOT infected. |
| || |
model was designed solely to express the probability that a
herd is NOT infected. It cannot be used to predict the success
of a control program for infected herds. The model also illustrates
something that is not intuitively obvious: the first herd test
is the most cost-effective in determining whether the herd is
or is not infected. It buys you the greatest gain in confidence
about the non-infected status of test-negative herds. You could
call this maximum gain in "certainty" or, in lay terms,
the most "bang for the buck". The model also supports
the idea that testing as few as 30 animals older than 3 years
old in a dairy herd can give a high level of confidence in the
true infection status of the herd. Visit the Prevention
web page under the Dairy topic to see a table contrasting
the specific probabilities of buying an infected cow from a
random untested herd compared to a herd that is one-time test-negative.
| || |
Australia has created Johnes
disease certification programs for cattle, 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 Johnes
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 Johnes Working Group (NJWG) formulated a model program for herd certification. It is titled Voluntary Johnes Disease Herd Status Program (VJDHSP). This program was approved by the NJWG and then the Johnes 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 level by doing a limited amount of testing. Details of the VJDHSP are described both on the USDA website and at the website maintained by the Minnesota Board of Animal Health. This latter site has schematic diagrams illustrating the program.
The VJDHSP is a model program designed to guide 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. 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 Johnes 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 Johnes disease.
has long been innovative in design and implementation of programs
and regulations to help control Johnes 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 nfection. 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 Johnes Disease" by default. Herds attaining
the A classification are encouraged to enroll in the VJDHSP described
above. A brochure describing the program is available as a "pdf"
the news section of this website will update visitors on the number
of Wisconsin herds in each classification. Sign up on the mailing
list and you will automatically be notified of news and updates
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, both 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
Health Service in The Netherlands. This
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 International Dairy Federation and International Association for Paratuberculosis this useful goal can be reached in the not too distant future.