Bears as reservoirs of zoonotic parasites: a 50 year retrospective review based on serological and genetic evidence

Authors: Dubey, Jitender; MURATA, FERNANDO - Non ARS Employee; CERQUEIRA-CEZAR, CAMILA - Non ARS Employee; Kwok, Oliver; SU, CHUNLEI - University Of Tennessee

Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2021
Publication Date: 6/3/2021
Citation: Dubey, J.P., Murata, F., Cerqueira-Cezar, C., Kwok, O.C., Su, C. 2021. Bears as reservoirs of zoonotic parasites: a 50 year retrospective review based on serological and genetic evidence. Journal of Parasitology. 107(3):519-528. https://doi.org/10.1645/21-16.
DOI: https://doi.org/10.1645/21-16

Interpretive Summary: Food safety research is of paramount importance for agriculture and the public. Foodborne protozoon infections are a leading cause of death from foodborne illness in the United States, especially for individuals with weak immune systems such as children and HIV patients. USDA research in this area has borne undeniable results – including helping to cut the prevalence of Toxoplasma gondii by as much as 50 percent in the United States. The USDA provided the veterinary, clinical, and public health communities an indispensable resource by disseminating up to date scientific information on toxoplasmosis and its prevention. Humans become infected mostly by ingesting food and water contaminated with oocysts or by eating infected under cooked meat. Bears, especially black bears, are excellent sentinels of environmental contamination with T. gondii because they eat almost anything vegetation, fruits, and animals. Hundreds of bears are hunted yearly in USA and humans can not only acquire toxoplasmosis but also another deadly parasite, Trichinella; authors review a case in a human who acquired both infections simultaneously after eating bear meat. Here, authors reviewed 50- year data on prevalence, epidemiology, clinical disease, diagnosis and genotypes of T. gondii. T. gondii prevalence in bears in Pennsylvania is around 80% and has been stable for 40 years. This information will support veterinarians, physicians, and federal agencies seeking to advance additional research needed in this area regarding human health. This reviews literature from the past five decades, including but not limited to scientific discoveries made by USDA scientists. No experiments or surveys were performed since the redirection of USDA’s program on toxoplasmosis.

Technical Abstract: Toxoplasma gondii infections are common in humans and animals worldwide. The present review summarizes worldwide information on the prevalence of clinical and subclinical infections, epidemiology, and genetic diversity of T. gondii infections in bears. Seroprevalence estimates of T. gondii in black bears (Ursus americanus) are one of the highest of all animals. In Pennsylvania, USA, seroprevalence is around 80% and has remained stable for the past 4 decades. Approximately 3500 bears are hunted yearly in Pennsylvania alone. Validity of different serological tests is discussed based on bioassay and serological comparisons. Seroprevalence in grizzly bears (Ursus arctos) is lower than in black bears. Even polar bears (Ursus maritimus) are infected; infections in these animals are ecologically interesting because of the absence of felids in the Arctic. Clinical toxoplasmosis in bears is rare and not documented in adult animals. Few reports of fatal toxoplasmosis in young bears need confirmation. Viable T. gondii has been isolated from black bears and a grizzly bear. Genetic diversity of isolates based on DNA from viable T. gondii isolates is discussed. Genetic typing of a total of 26 T. gondii samples from bears by 10 PCR-RFLP markers revealed 8 PCR-RFLP ToxoDB genotypes: genotype #1 (clonal Type II) in 3, #2 (clonal Type III) in 8, #4 (haplogroup 12) in 3, #5 (haplogroup 12) in 3, #74 in 5, #90 in 1, #147 in 1, and #216 in 2. This result suggests relatively high genetic diversity of T. gondii in bears. Overall, T. gondii isolates in bears range from those circulating in domestic cycle (genotypes #1 and #2), to those mainly associated with wildlife (such as #4 and #5 together known as haplogroup 12). A patient who acquired clinical Trichinella spiralis infection after eating undercooked bear meat also acquired T. gondii infection. Freezing of infected meat kills T. gondii, including the strains isolated from bears.