Abstract

Waterborne outbreaks of protozoal parasites are increasingly causing fatal disease in marine wildlife. Population expansions of felids near marine estuarine environments have led to increased exposure of marine wildlife to highly infectious Toxoplasma gondii oocysts shed in the feces of cats that are dispersed by storm events. 53 Toxoplasma isolates were obtained from mustelids that stranded between 1998–2003 with toxoplasmosis (ranging in disease from chronic asymptomatic infection to fatal encephalitis). Using population genetic and multi-locus PCR-DNA sequencing to study the evolution, emergence, and transmission of the Toxoplasma strains isolated established that over 72% of the sea otters were infected with new genetic variants of Toxoplasma gondii, referred collectively as Type X. Depending on the locus investigated, Type X strains possessed only one of two allelic types that had independently assorted across the strains examined; either a genetically distinct allele or a Type II allele. Genome-wide CGH array and WGS analyses confirmed that Type X is a sexual clade of natural isolates that have undergone relatively limited genetic admixture and are the result of a cross between Type II and a mosaic of two distinct ancestries referred to as “gamma” and “delta.” When assayed through mice, a subset of Type X strains (19/53; 36%), which had expanded in sea otters as chronic asymptomatic infections, were highly pathogenic (LD₁₀₀=1 parasite). To identify murine virulence genes within this naturally occurring population, we performed a genome scan and identified four QTLs with LOD scores greater than 3.8. Targeted disruption of ROP33, the strongest candidate from among 16 genes within the highest QTL on Chromosome VIIa established that ROP33 is a murine virulence locus. The ability of this highly pathogenic, Type X mouse-virulent clone to expand asymptomatically and cause the majority of sea otter infections supports a virulence shift model whereby generalist parasites like Toxoplasma utilize their sexual cycles to produce new strains with expanded biological potential that can be naturally selected among their vast intermediate host range, such as the southern sea otter, to maximize their transmission. Our work establishes a rationale for how virulent strains can be maintained cryptically in nature across the parasite’s expanded host range, and act as reservoirs for epidemic disease.

* Presenting author