The molecular epidemiology of Toxoplasma gondii with specific reference to transmission in sheep
Much research has been conducted on the life cycle of T. gondii by means of investigation of experimentally infected animals. However, much less has been achieved in studies of naturally infected populations. In particular, controversy surrounds the relative importance of congenital transmission, which is generally believed to be very low. I report a detailed analysis of T. gondii transmission, with specific reference to congenital transmission, in a naturally infected sheep population. Nucleic acid amplification methods (PCR) were developed for use in the detection of T. gondii from artificially and naturally infected tissue samples. PCR amplification of both the SAG1 gene and TOX element were optimised to enable definitive detection of T. gondii in tissue samples from a range of animals including experimentally infected mice and naturally infected sheep. A study of the relationship between lamb mortality and T. gondii was carried out at a farm in Worcestershire (Average lamb mortality for Suffolk Cross was 10.5%). A single study of a lambing period in spring 1999, using PCR amplification techniques, demonstrated that congenital transmission was occurring in 61% of lambings (n=88) (Duncanson et al, 2001). To further confirm these results, a three-year longitudinal study was conducted using a sample of 255 ewe pregnancies. This extended study also revealed high levels of congenital transmission (59%). Separate studies on two independent flocks (Suffolk Cross and Charollais) showed levels of congenital transmission at 61% and 53% respectively. One implication of these high levels of congenital transmission is that a significant number of lambs may be born infected with T. gondii but show no significant detrimental effects to health. Due to the lack of clinical disease demonstrated in live born lambs congenitally infected with T. gondii, these animals enter the food chain for consumption and potential transmission of the parasite to humans.