A frequent practice is to retest test-positive cows with another test to make sure the first test is correct. Unfortunately, often the results of two different tests do not match. Confusion and frustration set in and people wonder which test to believe.
Instead, simply consider her positive ELISA result as “more information” about the cow and add it to other information you know about her to decide on how to proceed. While cows with positive ELISA present a higher risk of transmitting the disease than other cows, they do not need to be immediately moved to the top of your cull list. Having said that, cows with a very high positive milk or serum ELISA (optical density or S/P score >1.0) are likely to be currently shedding the bacterium in their manure. These high-positive ELISA cows should not be rebred and should go to the top of your cull list. Fortunately, cows with very high scores are usually quite rare. In Ontario, Canada, where the prevalence of JD seems to be low so far, only about 230 cows of about 150,000 dairy cows tested (0.15%) have had high titer values.
Similarly, if the cow has a positive direct test (fecal PCR or culture) and you are serious about JD control, then that cow should go immediately to the top of your cull list. The test tells you that she is already shedding in detectable levels and sharing it with the rest of the herd. The longer she stays, the more time she has to contaminate the environment (including the calving pen) and the higher the probability that she'll have a chance to infect other animals.
In conclusion, ask about the JD herd status of the source herd before bringing cows onto your farm and recognize that not every ELISA positive cow should be culled. You need to use all the information given to you to make an informed decision and remove cows that are known shedders to reduce environmental contamination and infection risk to aid in JD control on your farm. Regardless of the test used, avoid calving cows with a positive test result in the calving area and do not feed their colostrum or milk to your replacement heifers. Please talk to your herd veterinarian to select the right testing regime for your herd and to assist with the interpretation of test results.