Lentiviruses of goat and sheep - Genetic characterization and molecular epidemiology

Viruses isolated from Norwegian goats have a genetic sequence so different from the same type of virus in other countries that that they may be considered a separate genetic group of SMLV.

Both goats and sheep may become infected by this type of virus, and transmission is predominantly from goat to sheep. Mapping of virus variants in goats and sheep will provide knowledge of the distribution of the virus and the degree of genetic variation, which will be important for the establishment of reliable techniques of diagnosis and control.

Caprine Arthritis Encephalitis (CAE) is widespread among Norwegian goats, while mædi-visna (MV) outbreaks in sheep have only appeared sporadically since the disease was first demonstrated here in Norway in the 1970's. Both CAE and MV are characterised by persistent inflammation, particularly in the lungs, udder, joints and central nervous system.

The title of small ruminant lentivirus (SRLV) has been coined to comprise these two diseases. It was originally believed that CAE- and MV-virus were two different viruses that were specific for either goats or sheep.

By sequencing the entire genome of a virus from a goat with CAE, researchers discovered clear dissimilarities in sequences from Norwegian animals and from known virus isolates from small ruminants in other countries.

It is important to establish an overview of the virus variants in sheep and goats in Norway, and to document the differences between these variants. Therefore, viruses from both pure sheep and goat herds, and in combined herds, were cultured and sequenced.

The same virus variant in sheep and goats

A higher than expected number of sheep test positive for SRLV. Databases of genetic relationships indicate that both goats and sheep in combined herds may be infected by the same virus variant. Transmission probably occurs in the direction of goat to sheep, because there are more SRLV-seropositive goats than sheep.

No features of the virus were found that would indicate specificity for either goats or sheep. Infection with the virus produces characteristic pathological changes in both sheep and goats.

Genetic comparisons show that Norwegian SRLV constitute a separate genogroup and this should be taken into consideration when evaluating procedures for detection of virus. Sheep can to a lesser degree also be infected of the well-known MV virus variant. This virus type was not found in Norwegian goats, in contrast to the situation outside Norway.

Small ruminant lentivirus is a more variable group of viruses than was previously thought. A control program for SRLV should take into consideration that goat and sheep function as reserves of infection for each other, and should therefore include both species.

This doctoral project was carried out at the Department of Food Safety & Infection Biology, Section of Microbiology, Immunology and Parasitology, of the Norwegian School of Veterinary Science.

Britt Gjerset defended her Ph. D. thesis, entitled "Lentiviruses of goat and sheep. Genetic characterization and molecular epidemiology.", at the Norwegian School of Veterinary Science, on May 14, 2009.

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