A Serological Survey of Toxoplasmosis in Pigs in Ghana

Abstract

Detection of Toxoplasma gondii infection by the direct demonstration of parasites is difficult due to the intracellular development of the organism, whilst the tissue culture technique is cumbersome and slow. Several workers have, therefore, relied on the demonstration of specific anti-f. gondii antibodies in serum and body fluids of both human and animal populations to obtain epidemiological data on toxoplasmosis. The applicability of this approach is based on the ability to differentiate acute infection, which is of major concern to the clinician from chronic or asymptomatic infection using the class of specific anti -Toxoplasma antibodies. The work reported in this thesis was carried out, firstly, to obtain reliable information on the prevalence of anti-71 gondii antibodies in pigs in Ghana. Secondly, the study sought to find out the distribution of T. gondii infection among pig breeds in the three defined ecological zones of the country, namely, the Coastal Savannah, the Forest Belt and the Guinea Savannah. Thirdly, the present investigation sought to determine whether there is a relationship between antibody prevalence and reported abortions/stillbirths in pigs on particular farms in Ghana. To achieve these objectives the IFAT, which is a highly sensitive and specific test with high correlation with the standard Dye test was used to validate a microplate-ELISA as an immunoassay for mass screening of sera for anti-7. gondii antibodies. 100 sera from pigs randomly selected from the three defined ecological zones were examined for anti-r. gondii antibodies using IFAT, followed by the microplate- ELISA. Fifty-five (55) sera tested positive in both IFAT and microplate-ELISA whiles thirty-six (36) sera were negative. Six (6) sera, which were positive in IFAT, tested negative by microplate-ELISA whereas three (3) sera, negative with IFAT, were positive by microplate-ELISA. Compared to the IFAT, the microplate-ELISA had a sensitivity and specificity of 90.2% and 92.3% respectively. There was also a high correlation (r = 0.82) between IFAT and microplate-ELISA. The specificity of anti-7! gondii antibodies in pig sera detected by the IFAT and microplate-ELISA was confirmed by their reactivity with T. gondii antigens in the Western immunoblot assay. This was achieved by comparing antigen bands recognized by specific antibodies in a WHO T. gondii positive human reference serum to that of positive pig sera. Anti-T. gondii antibodies in both the human reference serum and pig sera recognized antigens located in similar bands with molecular weights ranging from 12 to 74 kDa. Anti- T. gondii antibodies in pig sera were dominated by antibodies that recognized antigens located in the 26, 30, 32 and 60 kDa band regions, including the major surface antigen of the tachyzoite, the p30. In all, 614 pigs were sampled and examined for anti-'T. gondii antibodies using the microplate-ELISA. A nationwide 39% T. gondii seroprevalence in pigs was obtained in the present study with prevalence rates of 43.9%, 30.5% and 42.5% in the three main ecological zones, namely, the Coastal Savannah, the Forest belt and the Guinea Savannah. Based on the result of the microplate-ELISA, it was found that pigs in the Forest belt had a significantly lower (P<0.05) antibody prevalence than those in both the Guinea and Coastal Savannah zones. Also, anti-7! gondii antibody prevalence was found to increase with the age of pigs. It was also found that pigs reared under the intensive management system generally had lower (P<0.05) T. gondii antibody prevalence than those under semi-intensive management. Again, female pigs were found to have a significantly higher (P<0.01) antibody prevalence than male pigs. The result of this work showed a difference (P<0.05) in T. gondii seroprevalence between Large White (38.8%) and crossbreed pigs (46.8%). However, the differences in antibody prevalence between the exotic Large White (38.8%) and the indigenous Ashanti Black (42.5%) and also Ashanti Black (42.5%) and crossbreed (46.8%) pigs were not significant. Market pigs had a significantly lower (P<0.05) antibody prevalence than breeder pigs. Also, this study showed a high positive relationship between T. gondii antibody prevalence as determined by microplate-ELISA and abortions/stillbirths in 20 sows on a farm where abortions and stillbirths were reported to occur frequently. Of the 20 sows, 14 (70%) that had aborted before had specific antibodies against T. gondii. 2 (10%) of the pigs were pregnant and both had anti-2”, gondii antibodies. Three other sows (15%) were seropositive and had farrowed one piglet each. The pig that had the highest antibody titre in this study was a sow that had lost all its seven piglets through stillbirth. The high specificity and sensitivity (92.3% and 90.2%) obtained for the microplate- ELISA compared to IF AT in the present study suggest that the microplate-ELISA is an efficient serological assay for the mass screening of sera for anti-7! gondii antibodies in pig sera. The results obtained in this thesis also suggest a possible relationship between antibody prevalence and abortions/stillbirths in sows in Ghana. Thus, toxoplasmosis might probably be a possible cause of piglet losses through unexplained abortions/stillbirths reported by some pig farmers during the present survey. The 39% T. gondii antibody prevalence in pigs obtained nationwide also suggests that significant proportions of the pig population in Ghana have been exposed to T. gondii infection. Pork containing viable T. gondii tissue cysts could thus be a potential source of T. gondii infection in humans.