Bovine Viral Diarrhea in new World Camelids

Bovine Viral Diarrhea in new World Camelids

Bovine viral diarrhea virus (BDVD) continues to plague the cattle industry, with most severe and economically devastating cases occurring in young cattle (ages 6-24 months). However, cases of BVDV infection in new world camelids (NWC) have recently emerged with clinical signs similar to those in cattle. With the increasing popularity and value of NWC, practitioners and owners should be aware of their susceptibility to Bovine viral diarrhea virus.

BVDV is an enveloped, single-stranded, positive-sense RNA virus in the genus Pestivirus and family flaviviridae. Other pestiviruses include Classical Swine Fever virus and Border Disease virus. With genetic sequencing, isolates can be grouped into genotype 1 or genotype II; each genotype has at least two subgenotypes. The genotypes can be further classified as a cytopathic biotype or noncytopathic biotype, based on their effects in infected cell culture.

In cattle, clinical manifestations of BVDV infections include subclinical disease, diarrhea, respiratory infections, ill thrift, infertility, abortion, congenital defects and gastrointestinal ulcerations, mainly in the abomasum and ileal Peyer's patches (mucosal disease). Birth defects include cerebellar hypoplasia, hydrocephalus, porencephaly, ocular anomalies, and cutaneous and skeletal defects. Persistent BVDV infection can occur in a fetus infected with noncytopathic BVDV infection in utero at less than four months gestation. Persistently infected (PI) calves shed large amounts of viral antigen and are at increased risk to the herd. If PI calves are infected with a cytopathic BVDV biotype, or their non-cytopathic biotype mutates to a cytopathic biotype, they are susceptible to fatal mucosal disease.

BVDV infected camelids exhibit clinical signs similar to those in cattle; clinical manifestations include abortions, early pregnancy loss, stillbirths, PI crias, diarrhea, respiratory disease, and ill thrift. In one reported case, a PI alpaca cria had a thin fleece with long hair-like fibers (from primary follicles) that extended past the woolly fibers of secondary follicles. These clinical findings were similar to fleece abnormalities in PI lambs infected with Border disease. Reportedly, in utero BVDV infection in NWC results in PI crias; and, like PI calves, PI crias are viremic and shed large amounts of viral antigen. However, the stage of gestation during which a camelid fetus is persistently infected is unknown.

A study involving more than 12,000 alpacas in North America was conducted to determine the most prevalent subgenotype of BVDV in PI alpacas. RT-PCR identified 46 isolates which were non-cytopathic biotype, type 1b subgenotype. Only one of the 46 isolates was phylogenetically indistinct from cattle isolates; 45 of the 46 isolates had over 99% identity to cattle isolates.

A separate study of 63 alpaca herds from the US identified 25.4% herds with a seropositive cria and 6.4% with a PI cria. In some cases, seropositivity was associated with administration of bovine colostrums. Farms with PI crias reported economic losses associated with abortions, treatment of weak crias, diagnostic testing, and lost sales.

Some diagnostic tests for BVDV in cattle are valid in NWC. Viral antigens can be detected by virus isolation, PCR, or immunohistochemistry (IHC). BVDV can be isolated from lymph nodes, whole blood, fetal tissue and placenta. Serology is also available to detect antibodies. Detection of virus or viral antigen without antibody can indicate a PI animal or an animal in the acute viremic stage of infection. Detection of antibodies alone indicates previous exposure to the virus.

Because PI crias are believed to be the main source of infection, efforts should be made to identify these animals. If current diagnostic methods are used to identify PI crias, virus should be detected in serum or buffy coat cells twice (21 days apart). Virus can be isolated from buffy coat cells, genome can be detected via PCR of buffy coat cells, and antigen can be detected in tissue by IHC. IHC of cutaneous biopsies can react positively to germinal cells of the epidermis because hair follicles can retain BVDV antigen in a low percentage of non-PI animals. If IHC is positive, this test should be followed with a separate diagnostic test 30 days later. Detection of an antibody titer on an animal that is also positive for antigen does not rule out persistent infection. PI animals can have antibodies to heterologous vaccines or natural infections; however, titers are usually low. In cattle, it is recommended to test all new calves for 8-9 months after a PI animal is detected and removed. Because the gestation period of NWC is longer than in cattle, it is recommended to test all crias born 11 months after removal of a PI animal. Seroprevalence studies show that there are few seropositive NWC; these findings indicate a large naïve population of NWC. Biosecurity is critical for keeping herds free of BVDV. Naïve animals, especially pregnant females, should not come into contact with infected animals. In addition to other NWC, cattle, goats and sheep are also primary carriers of the virus. Currently, vaccinating NWC with bovine vaccines for BVDV is not recommended. A bovine vaccine could interfere with diagnostic testing and the efficacy or safety of these products is not understood in camelids.

Bovine viral diarrhea virus (BVDV) should be considered in the differential diagnosis in NWC that have ill thrift, diarrhea, respiratory infections, abortion, and stillbirths. Introduction of BVDV into a NWC herd can lead to devastating economic losses. New additions to the herd should be screened for BVDV and biosecurity measures should be in place to prevent introduction of this disease.

-by Erica Twitchell, PUSVM Class of 2010

-edited by Dr. Tiffany Reed, ADDL Graduate Student

References:

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