So, you made it through calving season with good success, and your calves appear to be thriving. At weaning, you vaccinate all your calves using appropriate products and follow label instructions. But soon after, you discover a severe outbreak of respiratory disease racing through your herd and several calves die.

Something obviously went wrong, but what? A South Dakota rancher, who prefers to remain anonymous, asked himself that very question back in October 1999. He had just vaccinated his 175 calves with a 4-way modified-live viral vaccine and a 7-way Clostridial/Haemophilus combination product. He vaccinated his heifer calves two days after weaning and his steer calves at the same time, returning the steers to pasture with their mothers for later weaning.
Twelve days after vaccination, a heifer calf did not come to the bunk at feeding time, subsequently leading to treatment for pneumonia with long-acting tetracycline. The calf did not recover and died two days later. Over the next two weeks a total of seven heifer and steer calves got sick and died.

A natural inclination at that point might have been to assume the vaccines did not work, or even that a vaccine caused the disease.

Fortunately, however, the rancher's contact with his veterinarian and his vaccine supplier led to an extensive investigation into the true source of the problem, aimed at developing a long-term, comprehensive herd-health strategy.

Veterinarians Jerry Woodruff and Jim Hall, both Technical Services managers with Fort Dodge Animal Health, conducted the investigation and reported the results in a case study. They found that several complex factors were involved in the outbreak, leading back to issues of biosecurity, nutrition, herd monitoring and diagnostics.

A trail of evidence
In most mystery novels, at least one character emerges early as an obvious suspect. But once the persistent investigators follow the trail of evidence down every avenue of possibility, they pin the crime on someone the reader never suspects.

In this case, veterinarians investigating the outbreak began with the calves that died.

Diagnostic laboratory evaluation of blood and tissue samples determined that the Bovine Viral Diarrhea Virus (BVDV) was causing the sickness and death loss in this herd. Continued evaluation identified persistently infected (PI) animals within the herd. Confirming the PI status of these animals was critical to identifying the cause of BVDV within the herd.

Next, the veterinarians extended their investigation to include the past history of the herd in question. The cowherd, it turned out, had not been included in a continuous vaccination program. The rancher had, however, vaccinated replacement heifers before adding them to the breeding herd, one year with Modified Live Virus (MLV) vaccine, the rest of the time with two doses of a killed virus vaccine at or around weaning time.

Six of the seven dead calves were from one herd subset of 3-year-old cows. The other dead calf was from a share-basis cow that pastured with the 3-year old cows during the 1998 breeding season. Although the rancher generally retained heifer calves for breeding purposes, he had purchased the group of 3-year-old cows as bred heifers through an auction barn. Some share-basis cows were also part of this herd.

A sneaky thief
University of Nebraska Veterinarian Bruce Brodersen explains that persistently infected calves result from a non-cytopathic strain of BVDV infecting the fetus while inside the cow's uterus.

New cattle brought in from the outside usually introduce the virus to the herd. The virus can then spread among non-vaccinated animals, and Dr. Brodersen notes that infected cows often show no sign of the disease.

Timing of the pregnant mother's BVDV exposure plays a role in determining whether the fetus succumbs to or survives the infection, or whether the fetus can mount an immune response to the foreign invader. PI calves typically result from exposure during a critical time from about day 60 to 120 or 140 days of gestation, before the fetus develops its own immune system. The fetus, and eventually the calf, fails to recognize the virus as a pathogen, mounts no immune response, and spreads the virus for as long as it lives.

Some persistently infected (PI) calves are aborted or die at or near birth, but some can live for years, exposing their herd mates and subsequent calf crops to the disease. When a persistently infected animal becomes exposed to a cytopathic strain of BVDV, either through mutation of the virus in its system or from exposure to other sources, it can develop the mucosal form of the disease.

Dr. Brodersen notes that the "trigger" for PI animals developing mucosal disease can come from several sources, and on rare occasions even from vaccines. If, he says, a cytopathic strain of BVD virus used in a modified-live vaccine is closely related to the non-cytopathic strain infecting the PI calf, mucosal BVD can result. He is quick to point out, however, that the vaccine is not the source of the problem. The real culprit is the persistent infection, which resulted in part from a failure to vaccinate breeding animals.

While the loss of a calf to respiratory disease is serious, Dr. Woodruff notes that the greatest risk to the herd is the exposure of cows during and shortly after the next breeding season. The PI-BVD calf is an excellent source of virus, potentially exposing the upcoming calf crop and creating another generation of PI calves. All of this, he says, can occur in a herd that appears completely normal.

Accessory to the crime
Dr. Woodruff notes that veterinary research links trace mineral deficiencies with impaired immune system response. In this case, analysis of liver tissue from the dead calves showed 2 parts per million (ppm) copper, far below the 30 ppm level veterinarians consider minimal. He points out that efforts to address trace mineral deficiencies and imbalances prior to fall vaccination are a critical component in comprehensive health programs that maximize immunity and vaccine response.

In this case, cooperation between the rancher, the herd veterinarian and the vaccine manufacturer helped identify the source of the problem. Once the veterinarians completed testing the herd, they confirmed that five additional animals were persistently infected with BVDV.

A vaccine-responsible failure did not occur in this instance, Dr. Woodruff says. Rather, the high exposure level to natural, virulent BVDV overcame any established immunity. He adds that the level of immunity within the herd probably was low due to severe copper deficiency. Without the vaccination program, he says, the consequences could have been much worse.

As the 2000 calving season began, veterinarians found continued but diminished signs of PI calves in the herd. Dr. Woodruff stresses that herd surveillance and strict biosecurity will be necessary for several years to assure this problem is under control.