50 Aust Vet J Vol 76, No 1, January 1998 Evaluation of the gamma-interferon assay for eradication of tuberculosis in a goat herd E LIÉBANA a , A ARANAZ a , JJ URQUÍA b , A MATEOS a and L DOMÍNGUEZ a Objective To evaluate the usefulness of the gamma- interferon assay in the diagnosis of caprine tuberculosis in comparison with a single intradermal tuberculin test, and to obtain a group of animals free from this infection in a herd with a high prevalence. Design An immunological study involving four serial comparative gamma-interferon and single intradermal tuber- culin tests. Animals A herd of 87 goats of Guadarrama breed. Procedure Serial testing and segregation of animals. Results We found that the number of infections detected by the gamma-interferon test was considerably greater than the number detected by the single intradermal tuberculin test. A group of 10 animals was negative to both tests in two consecutive rounds and three kids were negative in the last round of testing. Conclusions Gamma-interferon assay is appropriate for diagnosis and eradication of tuberculosis in goats. This test is able to detect early Mycobacterium bovis infection. Avian reactors with simultaneous increased reaction to bovine PPD in the gamma-interferon assay (designated as avian B reac- tors) should be considered test positive for M bovis. By serial testing with the gamma-interferon and the single intradermal tuberculin tests, and a policy of segregation of kids at birth, it is possible to achieve a group of animals test negative for tuberculosis from a herd of goats with high immunoreactivity to this infection. Aust Vet J 1998;76:50-53 Key words: Tuberculosis, goat, Mycobacterium bovis, gamma-interferon, eradication. B ovine and caprine TB is an important endemic infectious disease in Spain. The eradication policy for this disease started in 1965 with the implementation of the SIDT test as the official method for detection of tuberculous animals. The prevalence of bovine TB is still one of the highest within the European Union with 5.2% herds infected in 1995. Until now the eradication efforts have only been directed to the cattle population. There is no official data on the prevalence of caprine TB in Spain, although it is generally known that goat populations are heavily affected by the disease. 1 Tuberculosis in goats has also been reported in France, Italy, Great Britain, Germany, USA, Africa, India, Australia and New Zealand, 2 although there is little recent data on the prevalence of this infection. In the past TB has been considered rare in goats and it was thought that this species is naturally immune to tuberculosis. However, several studies show that goats possess no particular resistance to the disease. 2,3 Over the last 5 years in our laboratory we have cultured M bovis from samples of up to 100 goats orig- inating from different regions of Spain. These findings could suggest that the disease is widespread in this country and may be a more serious problem than previously recognised. Clinical diagnosis of this disease is not easy 4 as not all goats with TB show signs of disease. Some will gradually lose condi- tion, develop enlarged lymph nodes and die. Macroscopic lesions in goats are similar to those described in cattle, however in most animals they are widespread and acid-fast bacilli are generally difficult to find and few in number. 5,6 The single intradermal tuberculin test applied to goats has a reported sensitivity of 82 to 93.8%, 2,6 although there is a considerable controversy on the performance of this test in goats. IFN-γ is a potent modulator of the cellular immune response to M bovis infections. The inherent ability of the circulating leucocytes to produce IFN-γ in response to mycobacterial anti- gens rises sharply once the cell-mediated immune response to the latter is established. T-cells rather than antibodies play the dominant role in the immune response to tuberculosis, 8 and it has been found that the production of IFN-γ by in-vitro stimula- tion of sensitied lymphocytes is a very good indicator of T-cell response and correlates with disease. Wood et al 9 described an in-vitro cellular assay for diagnosis of bovine TB based on detection of the production of IFN-γ in a whole blood culture system. The assay is based on in-vitro stimulation of lymphocytes with bovine or avian tuberculin PPD. Lymphocytes from uninfected cattle do not respond to produce IFN-γ, conversely those from animals infected with M bovis secrete IFN-γ. After production and characterisation of monoclonal antibodies specific for bovine IFN-γ, 10 an EIA for detection of IFN-γ was developed for the diagnosis of bovine TB. 11 Extensive studies have been conducted for comparison of the sensitivity and specificity of the SIDT test and the EIA for IFN- γ in cattle. In a field trial in Australia with 6754 animals the sensi- tivity obtained for the IFN-γ test was 77 to 94% compared to a sensitivity of 65.6% for the SIDT test in the same population. 12 In a further study with 1362 cattle the IFN-γ was found to have a Departmento de Patología Animal I (Sanidad Animal), Facultad de Veterinaria, Universidad Complutense de Madrid, Avda Puerta de Hierro s/n 28040, Madrid, Spain b Consejería de la Comunidad Autónoma de Madrid, Instituto Técnico de Desarrollo Agrario, Ronda de Atocha, Madrid, Spain EIA Enzyme immunosorbent assay IFN-γ Gamma-interferon HPC Cetyl pyridinium chloride OD Optical density PBS Phosphate-buffered saline PPD Purified protein derivative SIDT Single intradermal tuberculin TB Tuberculosis