A producer notices there have been quite a few abortions in heifers that are due to calve in 8 to 10 weeks. Out of 50 heifers, 15 have lost their calves (30% abortion rate). The year before, there were only 2 or 3 abortions (~5% abortion rate). According to the records, the heifers are getting adequate nutrition/supplementation and were vaccinated several months prior. The veterinarian says the heifers are in good body condition and sends a fetus and placenta to the NDSU VDL for abortion work-up.
On external examination, a portion of the intestines protrude through the umbilicus. Otherwise, the coat is wet and full and the crown to rump length is 60 cm, compatible with an early third trimester fetus.
Upon opening the carcass, the liver is rounded and enlarged, and, on sectioning, the liver is friable and soft. Lung lobes are wet with prominent lobulation. On closer inspection, there are frequent, irregularly round, pinpoint, tan foci in the liver and lungs, indicating inflammation and necrosis. The placenta is unremarkable.
The changes noted grossly are indicative of systemic inflammation. A soft, friable liver is consistent with liver necrosis. Main causes of liver necrosis in a fetus are bacterial, such as Listeria, Yersinia, and Salmonella infection, or viral. Laboratory testing of tissues, most notably the brain, kidney, liver, lung, and abomasal fluid, is required to determine the diagnosis.
Although there are several viruses that can cause bovine abortion, in fetuses that are aborted during the second half of gestation, bovine herpesvirus type 1, the causative agent of infectious bovine rhinotracheitis (IBR), is common.
In this case, given the liver and lung necrosis, the age of gestation (>5 months), and the history of vaccination, IBR abortion was suspected. Indeed, there were microscopic lesions consistent with IBR. In addition, the IBR PCR was positive.
After submission of the fetus to the NDSU VDL, the producer realized that the pregnant heifers were vaccinated with a modified live IBR vaccine rather than a killed vaccine. Fetal loss caused by modified live vaccination is a common cause of IBR abortion. However, natural infections can also occur and may result in abortion storms of up to 60% in unvaccinated herds.
Bottom line, it is recommended to vaccinate pregnant animals with killed rather than modified live vaccines. In addition, an abortion rate of >5%, and especially any abortion storm, should be worked-up by a veterinarian to determine the underlying cause.
Besides infectious agents, nutritional, environmental, and genetic factors can be at fault for high abortion rates. During an abortion work-up, all factors should be conveyed to the veterinarian and laboratory to get the best results.
