What if you could classify Johne’s-positive animals according to their risk for spreading infection in your herd? That way, you could get rid of the riskiest animals, and then hold onto the less-risky animals for another lactation, thus minimizing culling cost. 

It is now possible, using the “likelihood ratios” technique developed by Mike Collins, veterinary researcher and Johne’s expert at the University of Wisconsin.

ELISA blood tests for Johne’s yield a specific number known as the S/P value. The higher the S/P value, the higher the number of antibodies in the blood sample, which means there is a higher likelihood the animal is infected.

Collins’ technique classifies these S/P values into five levels — negative, suspect, weak positive, positive or strong positive — thus offering the producer different management options.

Find out if your diagnostic lab is using the ELISA test kit manufactured by IDEXX. If it is, ask if you can obtain the exact S/P values. With the SP values in hand, you can tap into Collins’ classification system at this Web site: www.johnes.org

Once there, go to the left-hand menu. Use your mouse to choose “Presentations.” Then select “Diagnosis” from the next menu that appears. This will bring up a Power Point presentation called: “A new way to use ELISAs” which explains how to use likelihood ratios.

Animals with the highest S/P values, classified as “strong positives,” represent the highest infection risk. Animals on the other end of the scale may make it through another lactation ¾ with relatively little risk to the herd ¾ if managed properly. If labeled with a leg band or other device, lower-risk animals can be monitored, allowed to calve in a separate area, and their colostrum discarded.   

The classification system offers management tips for animals in each of the five levels.

Mike Collins, University of Wisconsin