Dominique Comeau  

Animal Health Laboratory, University of Guelph, Guelph, ON.

AHL Newsletter 2026;30(2):11.

A Holstein calf and associated amnion were submitted to the Animal Health Laboratory as part of an investigation into an abortion storm. The farm had multiple abortions as well as multiple live calves which were born several weeks premature. On postmortem examination, there were minimal abnormalities in the fetus which had a crown rump length and other fetal characteristics consistent with a gestation age of approximately 8 months (full term). The amnion was diffusely edematous, with patchy pale yellow to brown discoloration consistent with meconium staining. Meconium staining is a nonspecific finding related to fetal stress.

Histologically, there was extensive effacement of the cardiac muscle by fibrosis with lymphocytic inflammation primarily focused along the epicardium (Fig 1). This inflammation extended into the myocardium and also involved the endocardium. There was also moderate lymphoid atrophy throughout the thymus (Fig 2). These histologic lesions are supportive of bovine viral diarrhea virus (BVDV) infection. Testing of the fetal tissues detected BVDV by PCR with a Ct value between 30 and 32.

BVDV is a well-known cause of bovine abortion, infertility, and early embryonic loss, as well as severe systemic disease presentations such as mucosal disease. However, detection of this agent has been uncommon in abortion submissions to the AHL. In review of abortion submissions in the last ten years (approximately 1079 cases), there were only seven cases in which the cause was diagnosed as BVDV (0.65 %). One case was identified in 2016, two cases from the same abortion outbreak on one farm in 2017, one case in 2018, and two cases in 2026. For 2026, in addition to the above-described case, another farm with a history of multiple abortions had a submitted fetus test positive for BVDV. There were no supportive histologic lesions noted in this second case, although detection of BVDV from fetal tissues by PCR was still considered significant.

In North America, control of BVDV has primarily focused on vaccination. Vaccination of heifers and cows against BVDV reduces the risk of fetal malformations and abortions, as well as reducing the risk of having persistently infected calves born on farm. These PI animals are immunotolerant to BVDV and persistently shed virus into the environment, acting as a major source of infection on farm. Therefore, prevention of PI animals is a large part of control by vaccination. In this case, the affected herd was not vaccinated for BVDV. This highlights that despite the low prevalence of confirmed cases of BVDV, vaccination against this agent remains an important part of bovine herd health and disease prevention.

Figure 1: A photomicrograph of the fetal heart showing areas of fibrosis (arrows) and lymphocytic inflammation in the epicardium (bottom left). H&E stain, 400X.

Figure 2: A photomicrograph of the fetal thymus showing significant reduction in the lymphocyte population (blue areas) with a relative increase in the medullary tissue (pink areas). H&E stain, 200X.

Reference

1. Newcomer BW, et al. Efficacy of bovine viral diarrhea virus vaccination to prevent reproductive disease: A meta-analysis. Theriogenology. 2015;83(3):360-365.  https://doi.org/10.1016/j.theriogenology.2014.09.028 [1]