Immunity to bovine ephemeral fever

• Harbin Veterinary Research Institute, China

Bovine ephemeral fever (BEF) is an important disease in Australia, China, Africa and Southeast Asia. ACIAR Project No. 8455 -- which involved collaborative studies between CSIRO and the Harbin Veterinary Research Institute (HVRI) -- demonstrated, through epidemiological studies, that BEF in China has many similarities to the disease in Australia and has been a continuing problem there since 1956. Very major epidemics are separated by years of minor epidemics, perhaps due to antigenic drift of BEF viruses, factors affecting insect vectors, or a combination of factors.

Major epidemics of BEF in China occurred in 1976 and 1983. The disease extended to fifteen provinces, severely influencing the milk output of dairy cows and the work capacity of draught cattle and buffaloes. In the most recent epidemic, 1988-89, infection spread to two more provinces. In a typical outbreak, 1-2% of cattle die, 30-40% are sick, 1-2% abort, and milk production falls dramatically. Preliminary analyses suggest there are a number of vector species. As the insects cannot be eliminated, vaccination is the only way of controlling the disease, but currently used live-virus vaccines have been unsatisfactory in China.

Serological tests used in earlier studies are not regarded by the experienced CSIRO virologists as being sufficiently specific for continued disease surveillance or for experimental work to produce a vaccine. And when a suitable vaccine is developed, continual surveillance of the disease will be essential in order to determine where and when to vaccinate and also to estimate vaccine production requirements in advance of need.

In the present project, virologists from CSIRO and HVRI will use a newly developed ELISA, and other tests, to help refine the present BEF antibody and antigen detection systems -- both in order to gather epidemiological information and to facilitate assays used in experimental work to produce a new vaccine. The major objectives of the project will be to locate the critical antigenic areas of BEF virus relevant to the induction of immunity, determine whether these antigens are present on a range of strains of BEF viruses from various epidemics in Australia, cross-compare suitable virus strains from Australia and China, and produce a suitable killed whole-virus or split-product vaccine by an optimised growth method.

Currently China and Australia are using live attenuated vaccines that have been propagated in tissue culture. These are expensive to produce, require continuous cold storage, and either do not induce immunity in vaccinated cattle or induce it for a short time only. A non-living vaccine prepared from specific BEF virus-derived immunogenic antigens would be reliable, would require only cool temperature storage, and would eliminate the risk that vaccine is blamed for BEF outbreaks. There is a chance that an immunogenic protein derived from both the Australian and Chinese strains of BEF would form the basis of a vaccine suitable for use in both countries. Costs associated with the manufacture of such a vaccine on a commercial scale are not available at this stage. However, these would be well offset by savings resulting from the prevention of mortality in draught and dairy animals, loss of calves and loss of milk production.

The project will be conducted in three streams of investigation: the development of diagnostic tests; immunochemistry and molecular biology of the virus and its antigens; and research on the host, its reaction to infection, and the mechanism of protective immunity. As the work progresses, candidate vaccine antigens will be tested for their capacity to induce long-lasting immunity in experimental animals and in the field. Most of the laboratory work will be done at CSIRO Long Pocket Laboratories and at HVRI, with exchange of scientists taking place between the two countries. In the course of the project, a number of Chinese scientists will be trained in virological and computing techniques.