Last December, I wrote an article about looking for the unlikely or unexpected. The saying to remember is “when you hear hoof beats, look for the horse, not the zebra.” But we must also remember to not forget the zebras. I was reminded of this with a recent case which highlighted a number of important principles.
The herd case involved the acute death of a two-week-old calf. The herd had not been having any significant health issues with its calves. And, on necropsy evaluation, the calf did not have any significant lesions suggesting pneumonia or other diseases. We debated whether to send any samples in for further diagnostic testing, given there had not been any other history to suggest that we might be dealing with a significant issue or herd outbreak. After some debate, I decided that if we don’t look, we won’t find a potential cause, so the samples went off to the lab.
A few days later, various test results started to roll in. Nothing was very remarkable until I saw a BVD (bovine viral diarrhea) result, which was positive. My first thought was this was a false positive result, given the herd history, but the result was reconfirmed as positive.
How did BVD enter this herd?
In this case, I certainly found the “zebra,” or the unexpected result.
This herd had no recent history of expansion and when purchased additions were brought in years ago, we had followed a strict BVD testing protocol. A solid vaccination program exists, which includes both young stock and adults. The herd has a high pregnancy rate and low incidence of abortions. Pre-weaning calf morbidity and mortality are also better than industry standards. Nothing in this herd suggested a disease that can cause unthrifty calves, abortions, lower conception and unexplained fevers.
As we conducted more herd testing and started to investigate the possible route of entry of BVD into the herd, a number of pieces of the puzzle started to come together. There was an obvious point exposure of the herd to the virus. Only one group of heifers was likely exposed and, unfortunately, they were the most susceptible to the creation of a PI (persistently infected) calf.
Finally, we realized that our “closed” herd was not really closed.
On two separate occasions, a group of rogue steers that had escaped from another farm paid a visit to the bred heifer free-stall. Given it was summertime and the feed alley doors were open, these visitors entered the barn and spent a considerable part of a day eating and defecating in the feed alley. Our herd was no longer closed. We can’t prove that these steers were infected with BVD, but the timing of all of the events highly suggests that this was the point of exposure.