Genotyping Toxoplasma gondii from the first national survey of feral swine revealed nearly 40% seroprevalence in the adult animals, and the presence of highly virulent parasite genotypes

Authors: Dubey, Jitender; CERQUEIRA-CÉZAR, CAMILA - Non ARS Employee; MURATA, FERNANDO - Non ARS Employee; VERMA, SHIV - Non ARS Employee; Kwok, Oliver; PEDERSEN, KERRY - Animal And Plant Health Inspection Service (APHIS); Rosenthal, Benjamin; SU, CHUNLEI - University Of Tennessee

Submitted to: Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2019
Publication Date: 11/19/2019
Citation: Dubey, J.P., Cerqueira-Cézar, C.K., Murata, F.H., Verma, S.K., Kwok, O.C., Pedersen, K., Rosenthal, B.M., Su, C. 2019. Genotyping Toxoplasma gondii from the first national survey of feral swine revealed nearly 40% seroprevalence in the adult animals, and the presence of highly virulent parasite genotypes. Parasitology. 1-8. https://doi.org/10.1017/S0031182019001586.
DOI: https://doi.org/10.1017/S0031182019001586

Interpretive Summary: Among these zoonotic pathogens, the protozoan parasite T. gondii is perhaps the most ubiquitous, having been identified in the tissues of a variety of animal hosts, including both mammalian and avian species. Toxoplasma gondii is estimated to chronically infect one third of the world’s human population, causing ocular toxoplasmosis in immunocompetent individuals and often-fatal encephalitis in the immunocompromised, as well as birth defects and mortality following vertical transmission to developing fetuses. Humans become infected postnatally by eating undercooked meat infected with T. gondii tissue cysts or by ingesting oocysts from the environment. Cats (domestic and wild) are the main reservoir of infection because they are the only hosts that can excrete the environmentally resistant stage, the oocyst. Oocysts are highly infectious for people and animals. The presence of T. gondii in feral swine is considered a good indicator of contamination in the environment because they are omnivores with a generalist diet, and become infected both by ingesting oocysts while rooting and eating tissues of infected animals. Transmission of T. gondii has been documented in free-ranging domestic pigs through cannibalism. In the present study, antibodies to T. gondii were found in 27.7% % of 1517 feral swine across the USA and viable T. gondii was isolated from tissues of 77 pigs. Genetic typing of T. gondii isolates revealed moderate genetic diversity. The results will be of interest to biologists, parasitologists, public health workers, and hunters.

Technical Abstract: Case studies have raised the concern that feral swine may constitute an important and growing reservoir of toxoplasmosis, but no national survey of infection in the United States has been conducted, limiting our understanding of the magnitude of the problem or the zoonotic risk they may pose to consumers of hunted game or of domestic pork. We therefore endeavored to pair serological survey with parasite isolation and bioassay in order to understand the prevalence and composition of such parasites. From 2012-2017, tissues of 1517 feral swine accross the USA were collected for the isolation of viable Toxoplasma gondii. Antibodies to T. gondii were first determined in serum samples and the tissues of seropositive pigs were bioassayed in mice. Antibodies were detected in 421 (27.7%) of 1517 pigs tested by the modified agglutination test (MAT, 1:25 or higher). Antibody positive rates increased significantly with age, with 10.1% of juveniles, 16.0% of sub-adults, and 38.4% of adults testing positive. Myocardium (50 g) from each of the 232 seropositive were digested in pepsin and bioassayed in mice. Viable T. gondii was isolated from 77 pigs from 21 states. Twelve of the 77 isolates were pathogenic to outbred Swiss Webster mice. Seventy-five of the 77 stains could be propagated further in cell culture and were genotyped. For genotyping, DNA extracted from cell culture-derived tachyzoites was characterized by PCR restriction fragment length polymorphism (RFLP) using the genetic markers SAG1, SAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1 and Apico. Genotyping revealed 15 ToxoDB genotypes, including genotype #5 (haplogroup 12) - 43 isolates, #24 -11 isolates, #2 (haplogroup 3) - 4 isolates, #3 (haplogroup 2) - 2 isolates, #4 (haplogroup 12) - 2 isolates, #216 – 2 isolates, #221 - 2 isolates, #289 – 2 isolates, and 1 isolate for each of genotypes #1 (haplogroup 2), #39, #66, #260, #261, #297 and #299. Genotype #5 is the most frequently isolated, acounted for 57% (43/75) of the isolates, followed by #24, acounted for 15% (11/75). Genotypes #260, #289, #297 and #299 are new types. Genotype #289 was highly virulent to mice and originated from feral swine collected in Louisiana on the same day at the same location. Genotype #216 was previously demonstrated to be highly virulent to mice. Our results document moderate genetic diversity of T. gondii in feral swine in the USA, with the genotye #5 (haplogroup 12) dominant in continental USA, whereas genotype #24 (10/14) dominant in Hawaii islands, suggesting different population structure of the parasites among the two distinct genographical locations. Feral pigs appear to play an important role in transmission of T. gondii in the environment, and they often carry different T. gondii genotypes than do farm animals, including highly virulent genotypes which may pose increased risk to consumers of wild game.