Brucillosis of sheep in China

• Chinese Academy of Agricultural Sciences, China

Three ACIAR projects are linked with China's goal of increased animal production, and this one addresses the need for disease-free stud rams. Recent estimates suggest that brucellosis affects about 5% of China's sheep and goats. Much of it appears to be due to Brucella melitensis, an important cause of abortions, infertility and reduced yields in small ruminants and also of serious human disease. The Harbin Veterinary Research Institute has developed an effective live vaccine (M5) of B. melitensis, which is now widely used in the northern pastoral regions.

A related organism, B. ovis, which is commonly found in sheep-producing countries, including Australia, causes a contagious epididymitis of rams. Although it has been isolated from rams in Xinjiang Province in recent years, its prevalence in China remains unknown.

This project seeks to adapt the Elisa (Enzyme-linked Immuno Absorbent Assay) tests developed in Australia for diagnosis of Brucella abortus infection in cattle and B. ovis infection in sheep, and use them in China to diagnose B. melitensis infection in sheep and goats, and B. ovis infection in sheep. The first objective will be to determine the prevalence of these infections in the major sheep-producing areas of the northern provinces. The second will be to develop tests capable of distinguishing between vaccinated and naturally infected animals, using highly specific reagents obtained by monoclonal antibody techniques and cloning of the bacterial DNA.

The Elisa method offers several advantages over conventional diagnostic tests. It requires only very small amounts of antigen which facilitates the use. Highly purified or costly antigens can be used for large-scale testing programs covering the whole country. Moreover, a single laboratory can provide the necessary antigens, simplifying the problems of standardisation.

The test allows selective measurement of antibody isotypes, and once set up in a laboratory it is readily adapted to the diagnosis of any infection for which suitable antigens are available. In the case of B. melitensis, researchers expect to identify and purify a lipopolysaccharide antigen, which will allow easy detection of the infection in sheep and goats.

Discrimination between responses to virulent wild-type and attenuated vaccine strains of the same species is more difficult to achieve. Most of the antigenic differences are probably restricted to small regions ('epitopes') of individual proteins, and a reliable test, capable of detecting infection in the majority of animals at any one time, will probably require the identification of several such epitopes. Monoclonal antibodies are extremely valuable reagents for such identification and analysis. However their extreme monospecificity may limit their usefulness when, for example, experimental techniques result in conformational changes in the proteins.

The development of vectors that allow expression of cloned DNA sequences provides a new approach to the problem of selecting strain-specific epitopes of bacterial antigens which are recognised by the immune system of the natural host. The method is very sensitive, and can screen large numbers of colonies in a single operation. As a powerful tool for the selection of relevant peptide sequences, it would have application in many fields.

Once they have identified appropriate antigens and reagents for tests against these infections, the scientists can determine the prevalence and significance of B. melitensis and B. ovis infection in northern China. Using the Elisa test, they will be able to monitor the effectiveness of the M5 vaccine developed at Harbin. They will also investigate the extent to which vaccination with M5 confers cross-protection against infection with virulent B. ovis. Use of this live vaccine would not be acceptable in areas from which B. melitensis has been eradicated - or in Australia, where the organism does not occur - and these experiments will be done at the Harbin Veterinary Research Institute.

If testing reveals B. ovis infection in areas free of B. melitensis, the project may also include selection of attenuated strains of B. ovis for testing as potential vaccines.