Johne’s disease remains one of dairy’s biggest herd health challenges.
By Phil Durst & Dan Grooms
Top scientists from around the country shared Johne’s disease research results at a three-day conference at Michigan State University earlier this year. In addition, a single-day educational conference, “New Horizons in Johne’s Disease Control: Integrating Cutting Edge Research into On-Farm Practice” was held to review practical on-farm applications. Following are summaries of some of the presentations.
Best Management Practices for Control of Johne’s Disease, by Scott Wells and C. Ferrouillet, University of Minnesota.
To assess how commercial dairy producers implemented recommended Johne’s disease control practices, the authors asked producers:
• “How effective are these practices for the control of Johne’s disease?”
• “What are the best control strategies for use in dairy herds?”
They found that while testing and culling of Johne’s-positive cattle and, in some states, vaccination, have their place, management to protect vulnerable calves from exposure to Johne’s-causing bacteria is the most important tool to control the disease.
As in Michigan, Minnesota has been involved with a herd demonstration project for five years. Six Minnesota herds completed a risk assessment at the start of the project, and herd owners determined which control practices they would implement. Annual testing, using both serum ELISA and fecal culture, measured progress. Over the five years of the study, the prevalence of Johne’s disease within the herds decreased from 8% down to 3.1% of cows (using serum ELISA) and from 10% down to 5.6% of cows (using fecal culture).
In addition, surveys of Minnesota producers enrolled in the Minnesota Voluntary Johne’s Disease Control Program showed these producers effectively reduced their Johne’s risk scores (years 1 & 2) and prevalence of cattle testing Johne’s positive (years 1-4) by changing management practices, but risk scores and prevalence of infected animals leveled off thereafter. They concluded that implementing risk control practices (see box for a list of some recommended practices) reduces the rate of Johne’s disease within herds by reducing the number of new infections.
However, even over a five-year period, the disease was not eradicated in any of the herds. It will take more time and a continued commitment to maintain tight Johne’s disease control practices, including ceasing purchase of cattle from herds of unknown Johne’s status, to eliminate the disease.
Consensus Recommendations on Diagnostic Testing for Johne’s Disease (paratuberculosis) in the U.S., by Michael Collins, University of Wisconsin.
A team of experienced Johne’s disease veterinarians set out to define the best course of action regarding Johne’s disease testing in dairy and beef herds based on business type, infection status and prevalence.
The team acknowledged that producers should focus on management as the primary means of Johne’s disease control, with testing used as a secondary tool.
They looked at available testing options based on the accepted sensitivity and specificity of each test, as well as the purpose of the test, including: classifying a herd as infected; controlling the disease; surveillance; eradication; and confirming the disease in herds with no prior confirmed Johne’s disease cases and in herds that are known to be infected. Because each test has strengths and weaknesses, test selection should be based on the objectives of the testing. Considerations included:
• Only low-cost tests are sellable to producers.
• Speed of test results is over-rated. More important is having the results at the time when the producer needs to make a decision about culling or classifying an animal as “do not breed.”
• Look at every test result quantitatively, considering the state of disease progression may be determined by response to a test.
The authors prepared a table with recommendations for commercial dairy herds, seedstock dairy herds, cow-calf beef herds and seedstock beef herds. For instance, the recommended test to classify a commercial dairy herd as infected is a pooled environmental culture, whereas the recommended test to control disease in the same herd is the ELISA. To confirm a clinical diagnosis of Johne’s disease in a herd with no prior confirmed cases, they recommend a necropsy of animals that die or can be sacrificed, or fecal culture or polymerase chain reaction testing of others.
The full table and explanation is included in the proceedings at www.jdip.org.
MAP Super-shedders: A New Paradigm Shift in Johne’s Disease, by Robert Whitlock, University of Pennsylvania.
Having previously defined Johne’s diseaseinfected cows that are “super shedders” of the bacteria, and knowing that the risk of exposure to bacteria by herdmates on the farm is high, a team working with Dr. Whitlock tested the hypothesis that some cows may ingest the bacteria Mycobacterium avium paratuberculosis (MAP) and subsequently excrete it without becoming infected. By excreting the bacteria as pass-through bacteria, they risk being identified as infected animals when they, according to the hypothesis, are only passive shedders of the organism.
To test their hypothesis, they tested almost 2,300 fecal cultures from 556 cows in three herds in three states. Seventy-eight of those cows were culture positive, for an apparent prevalence rate of 3.5%. Bacteria in cultures from manure samples are measured as colony forming units (cfu) of MAP per gram (gm) of fecal material. Low shedders may have 5 cfu/ gm of manure; moderate shedders may have 50 cfu/gm; and heavy shedders may have 500 cfu/gm. Super-shedders are defined as shedding 10,000 to 10 million MAP bacteria per gram of manure.
In these three herds with a fairly low apparent prevalence of Johne’s disease, 15 cows (some from each herd) of the 78 culture positive cows (19%) were defined as super shedders. A single super-shedder cow puts out bacteria approximately equivalent to seven clinical cases or 160 heavy shedders, or over 2,000 moderate shedders or almost 24,000 low shedders.
These cows account for tremendous numbers of bacteria in the animal’s environment and, although few in number, put many other animals at risk of infection. Therefore, identifying and eliminating super shedders is important in controlling the transmission of Johne’s disease.
Diagnostics and Strain Differentiation of Mycobacterium avium paratuberculosis: Current Tools and Challenges, by Srinand Sreevatsan. University of Minnesota.
The current tests for Johne’s disease are valuable, but limited in sensitivity, the ability to detect subclinically infected animals, determining the stage of the disease and to specifically identify differences in the strain of the disease-causing bacteria.
However, scientific advances, including the recent characterization of the complete genome sequence of a cattle isolate of MAP allow researchers to continue to seek ways to overcome those limitations with new tests and new means of identification.
The immune reaction within an animal in response to an infection has a progression that is well-defined. Research is being conducted to identify bio-markers to help pinpoint the state of the disease.
Bio-markers also may in the development of therapeutics, determining response to therapy and predicting prognosis.