Toxoplasma gondii in animal and human hosts
Burrells, Alison Clair
The protozoan parasite Toxoplasma gondii (T. gondii) is an important zoonotic pathogen, which has the ability to infect all warm blooded mammals including humans, with approximately one third of the human population predicted to be infected. Transmission of the parasite to the foetus during pregnancy can result in miscarriage, however, a child infected during pregnancy may go on to develop clinical symptoms such as retinochoroiditis (ocular toxoplasmosis), hydrocephalus or learning difficulties in later life. Post-natally acquired infection in humans is generally asymptomatic, however, individuals who are immunocompromised may develop ocular toxoplasmosis or toxoplasmic encephalitis. T. gondii type II is reported to be the predominant genotype in Europe and the United States, but currently very little information exists about the prevalence and genotypes present within Great Britain. Consumption of T. gondii tissue cysts from raw or undercooked meat is a main source of infection for humans, with infected pork being considered a high risk. Currently the “gold standard” for assessing the viability of infective T. gondii tissue cysts is by an in vivo mouse bioassay. However, more recent ethical requirements to reduce, refine or replace experimental animals raises the question as to whether molecular technologies could be incorporated into these studies to reduce mouse numbers. The main aims of this PhD were to: (i) determine the prevalence and genotypes of T. gondii within different wildlife populations and humans in Great Britain; (ii) determine whether vaccination of pigs with a live attenuated strain of T. gondii would reduce the load of viable T. gondii tissue cysts within this species; (iii) study the viability and dissemination of tissue cysts from oocyst and bradyzoite infected pigs and (iv) to compare mouse bioassay with molecular detection of T. gondii DNA from experimentally infected pigs. The main findings of this work show that the prevalence of T. gondii within carnivorous wildlife varied from 6.0% to 44.4% depending on the host species with type II being the predominant lineage identified, however, type III and two alleles for type I were also present. In humans, serological detection of the parasite from a group of Scottish blood donors from Glasgow and Dundee (n=1403) was determined at 13.0%, molecular detection of T. gondii in human brains (n=151) from the Sudden Death Brain Bank show a prevalence of 17.9%. A correlation between increasing age and an increase in the detection of parasite was identified from both study groups. T. gondii strain genotyping using DNA extracted from human brains identified alleles for type I and III, however, no direct link between cause of death and detection of parasite DNA could be made. Live vaccination and subsequent oocyst challenge of pigs showed a significant reduction in the establishment of viable T. gondii tissue cysts. Mouse bioassay clearly demonstrates this result, where 100% of mice that were inoculated with homogenised tissues from vaccinated/challenged pigs survived, compared to the survival of only 51% of mice, which received homogenised tissues from non-vaccinated/oocyst challenged animals. In addition, porcine tissues from pigs challenged with either oocysts or bradyzoites did not show a significant difference in mouse survival following bioassay of these tissues. Challenge with either stage of the parasite (oocysts or bradyzoites) showed a preference to form tissue cysts in brains and highly vascular muscles (tongue, diaphragm, heart or masseter) of pigs. The findings, comparing mouse bioassay with molecular detection of parasite DNA from homogenised porcine tissue (prior to inoculation into mice), showed similar levels of detection. However, mouse bioassay was more sensitive and also provides evidence of parasite viability. In conclusion, this research not only provides current figures for prevalence and genotypes of T. gondii in both wildlife and humans in Great Britain, it also successfully answers the question as to whether live vaccination of pigs with the S48 strain can reduce the tissue cyst burden. These promising results show the potential of a vaccine against T. gondii in producing safer pork for human consumption. Although the mouse bioassay still remains the most sensitive method for the detection and viability assessment of tissue cysts, further research should be carried out in this area, perhaps incorporating a technique such as magnetic capture qPCR, to enable an effective in vitro technique to be developed.