Dairy herd owners and managers have two choices: 1) keep Johne's disease out by practicing good herd biosecurity, or
2) know you will buy infected animals regularly and have excellent animal husbandry systems to control the infection in your herd. This page concerns choice #1, the more cost-effective choice.

	1.	Johne’s disease is infectious. Animal health records in many countries show that the disease was once rare and is now very common. Based on surveys in several countries, a conservative estimate is that 25% of dairy herds are infected with M. paratuberculosis internationally. The infection rate may be lower in other animal species, but the pattern of a spreading infection is most likely the same. In the absence of efforts to limit the chance of moving M. paratuberculosis-infected cattle into non-infected herds, the infection will continue to spread.

	2.	Owners of non-infected herds should do everything possible to keep their herds from becoming infected. In fact, such owners could profit from the added value that being free of the infection gives their animals. If they can document their herd is not infected, they can charge a higher price for the animals being sold.
	3.	Use of existing diagnostic tests and herd certification programs can significantly limit the risk of buying M. paratuberculosis-infected animals. On the individual animal level, the diagnostic accuracy of tests for M. paratuberculosis infection may not be very impressive. However, when tests are applied to whole herds, they are quite efficient at detecting herds that are infected. The secret to success is to buy replacements based on HERD test results, not individual animal test results.
	4.	Logic (backed by probability theory) helps you manage the risk of bringing Johne’s disease into your herd. If you, the herd owner, understand the risks of buying M. paratuberculosis-infected dairy replacements you can make informed decisions about whether or not to buy an animal. Knowing the serious consequences of this infection to your herd is also vital. The presence of Johne’s disease should influence the market place: animals with the lowest risk of being infected should be of higher market value. When this happens, Johne’s disease control programs can be driven by market forces instead of government regulations.

	1.	Keep a closed herd, or buy as few animals as possible. As in other realms in today’s world, abstinence is the safest practice for limiting the chance of getting an infectious disease.
	2.	Buy based on objective information i.e., laboratory tests. At any given point in time, most (>90%) of M. paratuberculosis-infected animals appear healthy. Herd owner knowledge about Johne’s disease is often limited and there is a clear conflict of interest for owners to tell prospective buyers about a possible herd infection. Laboratory test results are best for presenting objective data about a herd’s infection status.
	3.	Buy based on herd tests, not individual animal test results. The animals of interest for purchase are often bred heifers that are too young to reliably be tested due to the biology of M. paratuberculosis infection. The probability that an individual animal will develop Johne’s disease is directly related to the probability the herd is infected and the extent to which it is infected. Therefore, test the adult cattle in a herd to estimate the probability the herd, and thus the heifers, are infected. The minimum number to test from the adult herd is 30. For more information on testing see the Diagnosis section of this website.
	4.	Herds in an official herd "certification program" (called different things in different countries) with annual negative herd tests have the very lowest risk of being infected regardless of which test is used or the number of animals tested. The probability a herd is NOT infected goes up each year a herd stays in a herd certification program with negative test results. Herds with the highest status or level in the program are therefore the best source for replacements (from the Johne’s disease perspective). For more on herd certification programs visit the Certification section of this website.

The probability of buying at least one M. paratuberculosis-infected animal using different buying strategies is shown in Table 1 below. For illustration purposes, dairy cattle are used as the model with a "worst case scenario": a high herd infection rate and a low diagnostic sensitivity for routinely used Johne’s disease tests. (This means that the probability of buying at least one infected animal in the example below is likely to be lower than the figures shown here for most states or countries.)

Assumptions for calculations in the Table 1:

• Percent of herds infected in the country = 50%
• Percent of cows infected in the infected herds = 10%
• Sensitivity of diagnostic test = 30% on adults
  (> 2 years old) and 5% on heifers (1-2 years old)
• Specificity of the diagnostic test = 98%

Calculations are based on standard principles of clinical epidemiology (Sackett et al. 1991) and new concepts described by Collins (Collins, 1999).

Table 1. Probability of buying at least one M. paratuberculosis-infected animal

	Random collections of cattle; unknown herd source				Program herds; known source herds
Number of animals purchased	No pre-purchase testing	Buy & test adults: keep test-negatives	Buy & test heifers: keep test- negatives		Buy any animal from any one-time test- negative (TN-1) herd

Prevention is the most cost-effective way to manage Johne’s disease. It is far less expensive to avoid introducing Johne’s disease into a herd than it is to control or eradicate the infection once it creeps in and starts to spread. Risk management is the foundation of any good business and the risk of becoming infected by purchasing infected animals is manageable. Smart buyers of cattle will apply the concepts of risk management and require diagnostic test results for the herds from which they purchase dairy replacements to limit their risk as much as possible. If sellers refuse to allow their herds to be tested before you purchase their animals, there is a good likelihood their herd may be infected. Find another herd to buy from.

If herd managers believe that the cattle market makes it impossible to buy from herds of known test status, then they must assume that they are buying M. paratuberculosis-infected cattle on a regular basis and have heifer-rearing systems in place to try to control the further transmission of the infection.

Other routes by which Johne’s disease may be introduced to herds exist, but they are of much lower risk (although data on the measurment of these risks is limited). These other routes include use of semen or embryos, spreading manure from potentially infected farms on land the herd owner uses for grazing or forage production, or from water run-off from adjacent farms. These routes are theoretically important, but the risk of acquiring the infection from them is likely much lower than through purchase of cattle.

1. Collins, M.T. 1999. Spreadsheet model for estimating the probability herds are free of paratuberculosis after successive serial tests. in Proceedings of the 6th International Colloquium on Paratuberculosis, E.J.B. Manning and M.T. Collins, ed., Melbourne, February 14-18, pages 66-75.

2. Sackett, D.L. Haynes, R.B., Guyatt, G.H., and Tugwell, P. 1991. The interpretation of diagnostic data. in Clinical Epidemiology - A Basic Science for Clinical Medicine, 2nd editioni. Little, Brown and Company, Boston. pages 69-152.