The molecular epidemiology of Toxoplasma gondii with specific reference to transmissions in sheep

Much research has been conducted on the life cycle of T. gondii by means ofinvestigation of experimentally infected animals. However, much less has beenachieved in studies of naturally infected populations. In particular, controversysurrounds the relative importance of congenital transmission, which is generallybelieved to be very low. I report a detailed analysis of T. gondii transmission,with specific reference to congenital transmission, in a naturally infected sheeppopulation.Nucleic acid amplification methods (PCR) were developed for use in thedetection of T. gondii from artificially and naturally infected tissue samples.PCR amplification of both the SAG1 gene and TOX element were optimised toenable definitive detection of T. gondii in tissue samples from a range of animalsincluding experimentally infected mice and naturally infected sheep.A study of the relationship between lamb mortality and T. gondii was carried outat a farm in Worcestershire (Average lamb mortality for Suffolk Cross was10.5%). A single study of a lambing period in spring 1999, using PCRamplification techniques, demonstrated that congenital transmission wasoccurring in 61% of lambings (n=88) (Duncanson et al, 2001). To furtherconfirm these results, a three-year longitudinal study was conducted using asample of 255 ewe pregnancies. This extended study also revealed high levels ofcongenital 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 asignificant number of lambs may be born infected with T. gondii but show nosignificant detrimental effects to health. Due to the lack of clinical diseasedemonstrated in live born lambs congenitally infected with T. gondii, theseanimals enter the food chain for consumption and potential transmission of theparasite to humans