Calves were vaccinated intranasally (IN) or intravenously (IV) with a thymidine kinase-negative (tk-) BHV-1 mutant. Vaccinated calves developed neutralizing antibodies but did not show clinical signs of infectious bovine rhinotracheitis (IBR). Vaccination also prevented clinical signs of IBR disease following IN challenge exposure of the calves to parental Los Angeles (LA) and USDA Cooper strains of tk+ BHV-1. Nasal swabs were collected for 10 days after the vaccination and the challenge exposures to monitor BHV-1 multiplication. At both 91 and 121 days post vaccination (PV), calves were also stressed for 5 days with dexamethasone (DEX) to induce reactivation of BHV-1 and nasal swabs were obtained. tk- BHV-1 multiplied in the nasal mucosa of IN vaccinated calves and was also recovered after DEX treatment. Likewise, tk- BHV-1 was isolated from the buffy coat fraction of IV vaccinated calves, but not from nasal swabs. tk- BHV-1 vaccination reduced the multiplication of tk+ BHV-1 in the nasal mucosa, but did not completely prevent development of a persistent infection by the challenge virus. The phenotypes of viruses isolated from the nasal swabs and buffy coats were analyzed by enzyme assays of extracts from virus-infected cells and by plaque autoradiography. These assays showed that tk- BHV-1 can persist for at least 3 months in vaccinated calves and may also be transmitted from vaccinated to control calves without reverting in vivo to tk+. The results demonstrate that the tk- BHV mutant is attenuated and efficacious as a vaccine.
INTRODUCTION of concurrent research projects or study facilities of the Division of Veterinary Surveys for selected zoonoses were Medicine, Walter Reed Army Institute of conducted on small wild mammals, ex-Research (WRAIR). These included cluding rodents, located in an undevelop-leptospirosis, listeriosis, rickettsiosis, viral ed forest and swamp area of Aberdeen infections, toxoplasmosis, and filariasis. Proving Grounds, Maryland. Diseases Findings of this extensive study are selected for study were within the scope summarized in this report.
Two seronegative sheep were infected intravenously with 109 PFU each of bluetongue virus (BTV) serotype 10 and BTV serotype 17. One animal experienced a mild bluetongue-like disease, and both experienced a short-duration viremia and developed neutralizing immune responses to both virus serotypes. Progeny virus was isolated from venous blood from each animal by using conditions in which reassortment could not have occurred during isolation. Electropherotypes were determined for the progeny viruses from the infected sheep, yielding strikingly similar results for the two animals. In both sheep, serotype 10 dominated among the progeny, accounting for 92% of the progeny. Serotype 17 was rarely isolated and accounted for 3% of the progeny analyzed. The remaining 5% of the progeny clones were reassortant and derived genome segments from both serotypes 10 and 17. Analysis of the parental origin of genome segments in the small number of reassortant progeny analyzed suggested that selection of specific genome segments may have occurred in the infected sheep. These data indicate that reassortment of genome segments occurs, at low frequency, in sheep mixedly infected with BTV. Bluetongue virus (BTV) is an orbivirus, a member of the family Reoviridae, with 23 serotypes recognized worldwide * Corresponding author.
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