Dynamics and epidemiology of Toxoplasma gondii oocyst shedding in domestic and wild felids

Abstract

Oocyst shedding in domestic and wild felids is a critical yet understudied topic in Toxoplasma gondii ecology and epidemiology that shapes human and animal disease burden. We synthesized published literature dating from the discovery of felids as the definitive hosts of T. gondii in the 1960s through March 2021 to examine shedding prevalence, oocyst genotypes, and risk factors for shedding. Oocyst shedding prevalence in many geographic regions exceeded the commonly accepted 1% reported for domestic cats; crude prevalence from cross‐sectional field studies of domestic cat shedding ranged from 0% in Australia to 18.8% in Africa, with greater variation in reports of oocyst shedding in free‐ranging, wild felids. Shedding in wild felid species has primarily been described in captive animals, with attempted detection of oocyst shedding reported in at least 31 species. Differences in lifestyle and diet play an important role in explaining shedding variation between free‐ranging unowned domestic cats, owned domestic cats and wild felids. Additional risk factors for shedding include the route of infection, diet, age and immune status of the host. It is widely reported that cats only shed oocysts after initial infection with T. gondii, but experimental studies have shown that repeat oocyst shedding can occur. Factors associated with repeat shedding are common amongst free‐ranging felids (domestic and wild), which are more likely to eat infected prey, be exposed to diverse T. gondii genotypes, and have coinfections with other parasites. Repeat shedding events could play a significant yet currently ignored role in shaping environmental oocyst loading with implications for human and animal exposure. Oocyst presence in the environment is closely linked to climate variables such as temperature and precipitation, so in quantifying risk of exposure, it is important to consider the burden of T. gondii oocysts that can accumulate over time in diverse environmental matrices and sites, as well as the spatial heterogeneity of free‐ranging cat populations. Key directions for future research include investigating oocyst shedding in under‐sampled regions, genotyping of oocysts detected in faeces and longitudinal studies of oocyst shedding in free‐ranging felids.

Factors influencing tissue cyst yield in a murine model of chronic toxoplasmosis

“…While there is no argument with regard to lethality of the mouse hypervirulent Type I strains (RH, GT1), there is considerable variation in the observed LD50 values dependent on the passage history of the parasite and the mouse background used in infection (25). Indeed, in vitro passage of ME49 in tissue culture selects for mouse hypervirulence resulting in a 2 order of magnitude or greater reduction in the LD50 value (from 10e 4 to 100 or fewer organisms) (26,43,44). This is observed to be the case with the lab adapted ME49, and ME49∆HXGPRT used here.…”

Molecular detection and dense granule antigen 6 genotyping of feline Toxoplasma gondii in Phayao, Thailand

“…In general, T. gondii oocysts are not frequently found during routine stool examinations with only 1% of infected cats reportedly excreting oocysts in a given period [14,28]. Based on our findings, the high rate of T. gondii-infected cats in this area could contribute to oocyst burden in the environment, although infected cats only excrete oocysts early on after infection [27,29].…”

“…For example, a wide range of warm-blooded animals serves as intermediate hosts, whereas cats and other felids serve as the main definitive host of this parasite. Infected domestic cats significantly contribute to the spread of Toxoplasma oocysts throughout the environment [13,14]. Thus, these felines play an important role in maintaining the sexual reproduction phase of T. gondii life cycle by producing a high number of oocysts in their feces.…”

New Avenues to Design Toxoplasma Vaccines Based on Oocysts and Cysts

“…Although it is commonly assumed that cats only shed oocysts the first time they get infected and only for a short period of time, a growing body of evidence suggests that cats might shed oocysts more than once if they become immunosuppressed ( 27 , 28 ), infected with other coccidian parasites such as Cystoisospora ( 29 ) or with heterologous Toxoplasma strains ( 30 , 31 ). This, together with the extraordinary infectivity and durability of oocysts, supports the idea that cats play a critical role in the epidemiology of toxoplasmosis.…”