A group of 1,200 stocker calves is purchased from different locations, processed together and sent to different pastures, up to 90 miles apart. After two weeks on grass, roughly 10% of them start developing severe eye lesions. The catch is most lack typical clinical signs of common diseases like pinkeye.
On a recent episode of “Herd Health, Bovine Science with BCI,” Kansas State University veterinarians Dr. Bob Larson and Dr. Brad White discuss this puzzling case.
Larson explains, “Common causes are the most common things to look for.”
Pinkeye typically fits for a case like this or in stocker calves, herpes virus or Infectious Bovine Rhinotracheitis (IBR) could be suspected of an outbreak of lesions like this. But this case had a high number of calves affected at a severe rate, with clinical signs not matching common causes.
Larson describes the lesions to include cloudy corneas, redness and swelling in the eyelids and even some pus in the interior chamber of the eye. Few calves had ulcers, which usually are related to pinkeye.
He explains in this situation the cases were not spread out in time, but in space, moving pinkeye and IBR lower on the list of possible causes. At first, Larson says his differential list would include pinkeye highest, then IBR and mycoplasma. But as the case became more confusing, he explains that other less-common causes were added to the list.
Evaluating Case History
“This is really where the case became interesting,” Larson explains.
These 1,200 calves had been purchased from many locations, brought to one central location for processing and then sent to 10 to 12 different pastures, ranging up to 90 miles apart. As Larson dives deeper into the case history, he explains the eye lesions started appearing about two weeks after the calves had been turned out on grass. There were cases in every pasture, all at similar rates.
There were cattle from many different sources, but in this case, the pattern didn’t seem to be coming from those sources. There was also no pattern from each pasture they were dispersed to. The only thing that was common was the time and location of processing.
Larson and White both agree with this fact — pinkeye drops way down on the list. Pinkeye is considered a propagating epidemic, where you are dealing with it for weeks at a time, with cases piling up.
Understanding Disease Types
Larson explains this situation seemed to be more point source oriented because all cases started at about the same time but in different places, meaning they were all exposed to the same thing at one point.
When comparing different disease timelines, it is expected endemic diseases for an operation like this, would see cases that are stable when compared year to year. Larson explains in those situations, you are not investigating something that changed recently since the infections are part of the disease pattern.
With an epidemic, he says something has changed recently in the host, environment or pathogen.
He explains, “A lot of my history questions and evaluation of the farm and premises were all about what’s changed in the last few days or weeks, and what happened last year is really not important to me because the problem is close in time.”
Larson goes on to confirm this situation as point source. He defines that as “feed or water exposure or something that all animals were exposed to at one point in time.”
He adds there have also been cases where it has to do with processing. Examples of this are dirty equipment or contamination of products.
Conducting the Investigation
When starting to investigate this case further, someone suggested malignant catarrhal fever (MCF), but these cattle had no exposure to sheep or wildebeests that anyone was aware of. Because these cattle were purchased and their full history was unknown, it was included on the rule-out list, even though it is rare.
Larson says they brought in an ophthalmologist because “it was at this point that his knowledge of eye lesions had reached its limit.”
Three calves were euthanized, and then their eyes were taken to a diagnostic lab for histopaths. The ophthalmologist mentioned these cases having an autoimmune cause.
Results showed the lesions were atypical lesions for cattle, but no infectious agents were identified. Bloodwork also showed no pathogens. This could be because the pathogen that caused the lesions was absent by the time tests were ran.
At this point, it is still unsure what caused this point source outbreak. The lesions peaked about two weeks after processing, and then within another two weeks, most cases had subsided.
Reflecting on the Case
Larson expresses, “frustrating cases are frustrating.” In situations like this, it is important to expand your list of causes and bring in other experts. He adds there is value in identifying if the problem is contagious or point source so you can better handle the situation.
