Animal health matters: Monitoring health and reproduction can help fend off Bovine Viral Diarrhea

Russ Daly
South Dakota State University Extension
Columnist

The west river rancher had never had so tough a spring. 

His family had raised cattle in the rough country of southwestern South Dakota for decades. This spring’s calves were like none he’d ever seen before. Depending on the day, it could be argued the poor-doers outnumbered the normal ones. Some were clearly stricken with pneumonia, others just scrawny and tired looking. And those were the survivors: many others had been found dead in the expansive pasture over the past weeks. 

The ranch only had veterinarians on the place for bangs vaccinations in the fall, but this situation called for special attention. Rounding up the calves for vaccines, minerals and vitamins, along with antibiotics for the ill, seemed to help some for a while but didn’t turn the group around.

Post-mortem examinations of dead calves didn’t show a clear pattern.

Then, lab results came back from the diagnostic laboratory, revealing a significant smoking gun.  Bovine Viral Diarrhea (BVD) virus was detected in multiple body organs. 

This finding started to make sense to the veterinarian.  Bovine Viral Diarrhea virus infections cause a lot of problems, besides diarrhea.

The virus causes a profound suppression of the immune system. Problems such as pneumonia, navel infections, pinkeye and abnormal bleeding result – illnesses that calves with healthy immune systems can slough off just fine. Reproductive trouble is in store for infected cows, including infertility, miscarriages and birth of defective calves – things the ranch couldn’t assess since no pregnancy checking was ever done. 

BVD virus doesn’t survive for very long outside the animal (in manure or saliva, for example).  Most calves or cows, as described above, are only temporarily infected. So how was BVD virus maintaining itself in this herd?    

The answer lies in what happens when BVD virus infects a pregnant cow. Should the virus get past the cow’s immune defenses and infect the developing fetus during the stage where its own immune system is developing — 40 to 120 days of gestation, the virus establishes itself in the fetus. After the calf is born, it spews out tremendous amounts of BVD virus every day of its life. This condition is referred to as persistent infection, or “PI.”  The virus from PI calves infects others with the immunosuppressive BVD virus, causing a smorgasbord of ailments and — to perpetuate itself in the herd — it infects cows, causing the birth of more PI calves. 

With advice from the veterinarian, the rancher embarked upon a plan to identify any PI calves in the herd. Luckily, this is not difficult nor expensive — an ear notch from each calf, sent to SDSU’s diagnostic lab, would determine whether the calf was PI or not.    

The severity of the ranch’s situation was evident when the BVD results came in. More than 40% of the tested calves were determined to be PI and needed to be euthanized.  Bovine Viral Diarrhea had taken an extreme toll on the ranch and its caretakers, to an extent I’d never previously encountered. 

What factors contributed to this wreck? How did this virus first enter the herd? Probably not through the purchase of a PI animal. This was a closed herd that only brought in a few replacement bulls each season. A cow’s fenceline encounter with a neighboring infected animal is a possibility. Additionally, the cow herd had not been vaccinated for BVD or other infectious reproductive illnesses.

While even a good vaccine program can be overwhelmed by several PI’s in the herd, had the cows been vaccinated, the chance of a cow becoming infected to create a PI calf would have been much lower. 

A BVD problem this severe was likely years in the making. Pregnancy-checking cows might have identified subtle changes in reproductive success that could have tipped off a potential problem. More timely examination of dead and sick calves could have identified BVD sooner as well. 

With BVD — and most other cattle illness outbreaks — it’s easy to be an armchair quarterback after the fact.  But even if you haven’t encountered such a problem yet, realize that identifying a problem early on  through timely reproductive exams and calf post-mortems can go a long way toward nipping a real wreck in the bud. 

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University.  He can be reached via e-mail at russell.daly@sdstate.edu or at (605)688-5171.