Bovine viral diarrhea is a complicated disease, with multiple presentations and, globally, three species of virus associated with “BVD.” During a meeting of the NCBA Beef Herd Security/BVD Working Group at the Cattle Industry Summer Conference this week, Julia Ridpath, PhD, a research microbiologist with the USDA’s Animal Disease Research Center in Ames, Ia., outlined some current thinking on BVD, and particularly “HoBi-like” viruses, which are members of the Pestivirus genus along with BVDV1 and BVDV2.
HoBi-like viruses have not been detected in the U.S. or North America, but have been isolated in water buffalo and cattle in Brazil, Thailand and Italy. Clinical signs following acute infections look the same as those from BVDV, and the virus is difficult to detect using current diagnostic tests. Current BVDV vaccines will not protect against HoBi-like viruses.
Ridpath and her group at the Animal Disease Research Center have conducted several studies on HoBi-like viruses, looking at disease presentation, vaccine efficacy, persistently infected (PI) calves and surveillance tools.
Acute disease from HoBi-like virus is clinically indistinguishable from typical-virulence BVDV, she says, with increased temperature, mild respiratory disease and leucopenia resulting in immunosuppression. The group also observed a reduction in thymus size in a high percentage of calves at necropsy, with the reduction ranging from 20 to 70 percentsuggesting long-term reduction in immunity.
In an in vitro vaccine test, researchers collected serum from 25 calves after two doses of a killed BVDV type 1 and type 2 vaccine and 25 calves vaccinated with one dose of a modified live BVDV vaccine. In vitro neutralization of HoBi-like virus in this test was poor.
The team also infected seven pregnant heifers with HoBi-like virus at 75 days of gestation to generate PI calves. One heifer aborted and two calves were born but died within 24 hours. Four PI calves were born and appeared fairly normal at birth. Detection of the virus in the PI calves with standard tests was difficult in early weeks after birth.
The researchers used PI calves in a transmission test, co-mingling them with other animals, They also inoculated a group of calves with two strains of Ho-Bi-like virus. In their test with cattle, they observed a 100 percent transmission rate among the five calves in each group. In their test with sheep, transmission rate was 100 percent in the inoculated group and 75 percent in the comingled group. In goats the transmission rate was 50 percent in both groups. The HoBi virus also is transmissible to swine, and this test resulted in a 50 percent transmission rate in the inoculated group and 25 percent in the comingled group.