Combining spatial analysis and host population genetics to gain insights into the mode of transmission of a pathogen: The example of Toxoplasma gondii in mice.

Toxoplasma gondii is a ubiquitous highly prevalent zoonotic protozoan. Cats are the definitive hosts, while all other warm-blooded animals are intermediate hosts for this parasite. Commensal rodents, being the main prey of cats, are probably the major reservoir for T. gondii in the domestic environment. Rodents can acquire infection after ingestion of oocysts that have sporulated in the environment. However, experimental evidence shows that vertical transmission can be sufficient for the perpetuation of transmission between generations of mice. In natural settings, the relative epidemiological importance of vertical transmission over oral transmission is a matter of debate and raises the question of the possibility of a T. gondii cycle in the absence of cats. In the present study, we took advantage of an extensive survey of commensal rodents in Dakar, Senegal, where the house mouse is the predominant putative reservoir of T. gondii. Mice genotypes and spatial location through GPS referencing of all trapping localizations were investigated in relation to T. gondii infection in eight sites of the city of Dakar and on Goree Island. In each sampling site, the occurrence of over-prevalence zones of T. gondii infection was investigated through Kulldorf's statistic using SaTScan software. Genetic structure and relatedness between mice were investigated within each over-prevalence zone, in order to find clues of transmission between related mice. Within each of the four over-prevalence zones identified across nine sites, infected mice belonged to more than one genetic group. No association between the degree of relatedness and the occurrence of T. gondii infection could be detected. These findings suggest an environmental source of infection for mice associated with localized putative foci of environmental contamination and support an oral route of infection for mice from Dakar rather than a cycle based on vertical transmission. However, further investigations based on a denser sampling in different epidemiological contexts are recommended.