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First Epizootics of Blue Tongue Disease in Bulgaria. Dynamics of Immunity, Virus Carriage and Reservoirs of Infection

ABSTRACT The general data from the literature about the duration of the viremic period in different kinds of ruminants are contradictory. It was affirmed that it is the shortest in sheep and goats—28 days in average and the longest in large ruminants (8). The differences in the investigations depend mainly on the sensitivity of the methods of isolation of the virus. In the Blue tongue outbreaks on the island of Lesbos in 1999 it was found that the viremic period in sheep and goats varies from 27 to 54 days. The duration of the viremia is influenced by reinfection with other viral serotypes circulating in the field, as well as other unknown factors facilitating the reaction of the virus in the affected animals. Thus Sergeev (1983) (1) and Leudke (3) assume that the viremia in large ruminants can continue up to three years. Similar data have been presented by some American authors, which explain the appearance of Blue tongue disease after a long and heavy winter on the North American continent. Syurin and Fomina, 1979 (2) consider that the viremic period in large ruminants can be protracted to 700 days. However, more recent investigators share the opinion that the viremic period in large ruminants has a duration of 35–49 days, with a variance of 100 days, which is the maximum survival time of the erythrocytes. Australian and American authors, using polymerase chain reaction (PCR) detected the nucleotide sequences of Blue tongue disease in ruminants in reconvalescent cattle up to the 180th day after their experimental contamination with a few serotypes of the Blue tongue virus, circulating in both countries. Reliable differences in the duration of the viremic period in the American and Australian virus isolates were not found, therefore it was definitely accepted that cattle can e considered completely virus free after the 180th day of the beginning of the infection. Investigations reflected in this publication aim to trace the dynamics of the immunity and the duration of the viremia of Blue tongue disease in ruminants in the course of the epizootics in Bulgaria in 1999. The objective was to investigate the dynamics of the antibody formation as one of the most important factors of immunity of sheep vaccinated with an attenuated vaccine against the disease. Another task was to investigate the duration of the viremic period in small and large ruminants in the regions affected with the disease and to estimate the risk of the naturally created reservoirs of Bluetongue infection in Bulgaria.