Diagnosis of natural exposure to bovine viral diarrhea in a vaccinated herd by measuring extended antibody titers against bovine viral diarrhea virus

Abstract

Two abortions occurred in a 150-head commercial cow-calf herd. Bovine viral diarrhea was suspected and confirmed by measuring extended titers against bovine viral diarrhea virus (BVDV) in a sample of 15 breeding females. Fifteen were sero-positive and 11 had significantly high titers (1:972–1:8748), likely due to natural exposure to cattle persistently infected with BVDV.

On December 2, the field service of the Western College of Veterinary Medicine received a call from a local beef cow/calf client regarding 2 aborted fetuses, discovered that day. The client operates a well-managed, 150-head, Limousin X commercial herd. The herd's calving season is in March/April and natural breeding is used. The herd is grazed privately without the use of community pastures. The producer had vaccinated the herd 3 wk prior to breeding with modified live virus strains of infectious bovine rhinotracheitis virus, bovine viral diarrhea virus, and parainfluenza-3 virus (Bovishield 3; Pfizer Canada, London, Ontario).

Blood was taken from the 2 heifers that aborted and was submitted to Prairie Diagnostic Services to evaluate serum titers against bovine viral diarrhea virus (BVDV) and Neospora caninum. Unfortunately, only a partial fetus was available for pathologic examination due to scavenging by coyotes. This was submitted to Prairie Diagnostic Services for gross and histological evaluation, as well as immunohistochemical study. The partial fetus consisted of an intact skull, including skin; several cervical vertebra; and a short length of esophagus and trachea. The fetus was estimated to be of approximately 4 mo gestational age, based on cranial observations. Brain, skin, thymus, thyroid gland, and skeletal muscle were sectioned for microscopic examination.

The aborted first calf heifers had elevated antibody titers against BVDV, 1 with a titer as high as 1:2916. Neither animal had serological evidence of N. caninum infection. The gross and histologic pathology revealed no abnormal findings. The skin from the fetus was negative on immunohistochemical analysis, ruling out the possibility that the fetus was persistently infected (PI) with BVDV.

The increased antibody titers of the 2 aborted females led to the suspicion of their having had recent natural exposure to BVDV from a PI animal. To investigate this possibility, the investigating veterinarian took blood samples from 15 first calf heifers on December 14 for serological evaluation at Prairie Diagnostic Services, and the heifers were pregnancy tested.

All of the bred heifers had significantly elevated antibody titers against BVDV, and 11 of these had titers ranging from 1:972–1:8448 (Figure 1). These high titers were unlikely due to vaccination, which suggested that exposure to BVDV had occurred in the herd at some point, possibly by it being naturally exposed recently to a PI animal.

Figure 1. The graph shows the distribution of titers against bovine viral diarrhea virus (BVDV) in the replacement heifers. Note the distribution to the right and the absence of low titers to the left.

The herd has no previous history of BVD-related problems. The producer has not introduced any new animals into the herd, he does not attend any livestock exhibitions, and the herd has been well vaccinated with modified live BVDV vaccine. Therefore, the likelihood that there is a PI animal within the herd is low.

The suspicion is that the exposure of this herd to BVDV was fence-line exposure to a neighboring herd with a suspected endemic BVD problem. The producer's annual rotational grazing protocol includes a period of approximately 2 to 4 wk when the cattle (90–150 d gestation) were grazing the pasture with neighboring fence-line contact. During this period, 6 calves from the neighbor's herd escaped and resided with the producer's herd. Two of these calves were found dead on the producer's pasture; the other 4 were sorted out and returned to the neighbor's herd. It was assumed that this event caused the exposure of the producer's herd to BVDV through 1 or more of these calves being PI with BVDV, although this was not confirmed.

Bovine viral diarrhea is becoming one of the most significant diseases affecting bovine health today. Bovine viral diarrhea virus is present in most cattle producing countries and is responsible for a variety of syndromes, including abortions, respiratory disease, congenital abnormalities, PI cattle, mucosal disease, and acute infections (1). Lately, there is speculation that BVDV may be a major predisposing agent for other diseases in the feedlot, such as bovine respiratory disease. The prevalence of BVDV infection in a population of feedlot calves in western Canada was 27%, based on ELISA serology, and it varied from 0% to 63% (5). The prevalence of PI calves in that group was < 0.1% (5). Out of 66 herds tested for BVDV in the United States, 87% were seropositive and 1.7% were PI (2).

The only means to control or eradicate BVD is to identify and eliminate PI animals. To do this, the herds that contain PI animals must be identified. Calves infected in the first trimester of pregnancy develop immunotolerance against BVDV and are later born persistently infected with the virus (1). There is ample evidence that PI animals are the primary source of infection for other animals (2,5,6,8). These animals shed virus continuously, and herds with PI animals often have a high prevalence (> 90%) of seropositive animals (1). Therefore, a high prevalence of seropositive animals is indirect evidence of the presence of PI animals (1). In this case, the samples selected for testing were from the young (< 2 y), bred heifer group, which avoided using mature cattle that may have high titers due to previous exposure and multiple vaccinations. One hundred percent of the cattle tested had high titers; such a high percentage of seropositive animals, even in the young group, suggests a recent acute infection in the herd.

This infection was likely due to the natural exposure of PI calves from the neighbouring herd. The predominant means of transmission from one herd to another includes the introduction of PI animals into a herd, or direct or indirect contact with PI animals (2). The most effective mode of transmission is direct contact with bodily fluids of PI cattle (6). Exposure of animals through fence-line contact, communal pastures, and livestock exhibitions may be important modes of herd to herd transmission (6). Indirect contact is possible through bloodsucking parasites, fomites, and other mechanical vectors (5,6). Persistently infected animals spread the infection to practically all other animals in the herd over distances of 20 to 40 m (7). Horizontal transmission at breeding is also possible (5,6). Introduction of a PI animal into a herd can result in rapid dissemination of the virus among the majority of susceptible cattle in less than 6 mo (6).

Serological testing alone provided evidence that this herd had been exposed to a PI animal. The titers achieved were remarkable. With vaccination, such a proportion of high titers would not be expected. However, it may be difficult in some situations to differentiate between titers due to vaccination and titers due to natural infection. More research needs to be done in this area, particularly with beef breeding herds. The distribution of titers is typical, however, of natural exposure (Figure 1). This was confirmed by a skewed distribution of high titers and the complete seroconversion. There is usually a low percentage of animals with lower titers in vaccinated herds that have not been exposed to a PI animal (1). In a herd vaccinated with BVD vaccine but unexposed to PI animals, the distribution of titers is usually normal or Gaussian and includes a larger proportion of animals with low or moderate titers. It is possible to achieve a titer near 1:1000 (Prairie Diagnostic Services Virology Laboratory, personal communication) in an animal that has been vaccinated with current modified live vaccines, but this is uncommon and, on the herd level, the majority will have more moderate titers. Therefore, herds with a high percentage of animals with high antibody against BVDV and with nearly 100% seroconversion can confidently be labelled as a herd recently exposed naturally to BVDV. Dairy herds have been diagnosed as having had exposure to PI animals by testing as few as 5 animals from the herd (1). The use of a small screening sample for determination of antibody titers against BVDV would be an easy and inexpensive way of obtaining a herd diagnosis for the presence of PI animals (1). This technique has been validated only in dairy herds with the use of killed vaccine, where it was possible to categorize high titers as being > 1:128 and low titers < 1:64, and PI status could be determined by using a sample of only 5 young animals. In another study that examined herds in which killed vaccines were used, a positive serological evaluation was defined as a herd with at least 3 of 5 heifers with antibody titers against BVDV > 1:128 (8). Conversely, a negative serological evaluation was defined as a herd with at least 3 to 5 heifers with antibody titers against BVDV < 1:64 (8). It is important to realize that serum neutralization titers can vary significantly from laboratory to laboratory (6). Vaccination and natural exposure titers should be compared by using the same laboratory. There are no studies that have validated this approach in herds vaccinated with modified live viruses (MLVs). It is recommended that if MLVs are used, the sample size should be increased, and the cut-off point for high titers should be raised.

Because there is a good chance that this herd was exposed to BVDV during the susceptible window of gestation, efforts must be made to manage calves that may be born persistently infected; newborn calves that are persistently infected must be identified and removed, in order to prevent the vertical cycle of BVD.

Persistently infected animals can be identified by using laboratory methods on samples from suspected animals. Whole blood can be submitted from newborn calves and tested for antigens with ELISA, or the mononuclear cells from that sample can be separated and tested for the virus with methods of virus isolation, or both (2). Probably the most convenient and accurate test available today is immunohistochemical testing of a formalin-fixed skin biopsy. Immunohistochemical staining of haired skin is an easy, accurate, less expensive antemortem diagnostic test for detection of PI cattle when compared with virus isolation (3).

Once diagnosed, a decision must be made with regards to culling or disposal of the PI calves. The most important action is to quarantine or isolate the PI calves from the gestating herd.

Other important factors to consider are strict hygiene and the lowest stocking density during calving to reduce the BVDV load on the herd. Also, the different age groups should be segregated to protect young breeding replacements (2). In this herd, the consequences of the exposure have not yet been fully realized.