Abstract
Toxoplasma gondii is an important zoonotic agent with high genetic diversity, complex epidemiology, and variable clinical outcomes in animals and humans. In veterinary medicine, this apicomplexan parasite is considered one of the main infectious agents responsible for reproductive failure in small ruminants worldwide. The aim of this study was to phenotypically characterize 10 Spanish T. gondii isolates recently obtained from sheep in a normalized mouse model and in an ovine trophoblast cell line (AH-1) as infection target cells. The panel of isolates met selection criteria regarding such parameters as genetic diversity [types II (ToxoDB #1 and #3) and III (#2)], geographical location, and sample of origin (aborted foetal brain tissues or adult sheep myocardium). Evaluations of in vivo mortality, morbidity, parasite burden and histopathology were performed. Important variations between isolates were observed, although all isolates were classified as “nonvirulent” (< 30% cumulative mortality). The isolates TgShSp16 (#3) and TgShSp24 (#2) presented higher degrees of virulence. Significant differences were found in terms of in vitro invasion rates and tachyzoite yield at 72 h post-inoculation (hpi) between TgShSp1 and TgShSp24 isolates, which exhibited the lowest and highest rates, respectively. The study of the CS3, ROP18 and ROP5 loci allelic profiles revealed only type III alleles in ToxoDB #2 isolates and type II alleles in the #1 and #3 isolates included. We concluded that there are relevant intra- and inter-genotype virulence differences in Spanish T. gondii isolates, which could not be inferred by genetic characterization using currently described molecular markers.
Highlights
The cosmopolitan apicomplexan parasite Toxoplasma gondii can infect almost all homoeothermic species [1]
The virulence degree of Toxoplasma gondii strains has been conventionally determined according to the cumulative mortality rate in outbred laboratory mice
T. gondii clonal lineages I, II and III have been traditionally classified as highly virulent (100% lethality, LD100 = 1; type I), intermediate virulent (99–30%, LD50 ≥ 1000; type II) and nonvirulent (< 30%, LD50 > 105; type III) [4, 41]
Summary
The cosmopolitan apicomplexan parasite Toxoplasma gondii can infect almost all homoeothermic species [1]. It is estimated that approximately one third of the global human population is infected by this obligate intracellular protist, and its high prevalence values in primary livestock species support its consideration as an important risk to food safety [2]. The T. gondii global population was assumed to be structured in three clonal lineages associated to virulence. Two decades of scientific effort in isolation and both molecular and phenotypic characterization of the parasite led to the discovery of a much more complex reality involving a population structure with at least 16 haplogroups worldwide [9,10,11] and a virulence degree classification under debate [12]. Regarding strains that circulate in European and Spanish sheep livestock, there are several genotyping studies that demonstrate the total predominance of type II strains, which coexist with small percentages of type III and recombinant strains [13,14,15]
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