Toxoplasma gondii tissue cyst formation and quantitative density of tissue cysts in shoulders of pigs 7 and 14 days after feeding infected mice tissues

Authors: RANI, S - University Of Maryland; CERQUEIRA-CÉZAR, CAMILA - Non ARS Employee; MURATA, FERNANDO - Non ARS Employee; SADLER, M - Non ARS Employee; Kwok, Oliver; PRADHAN, K - University Of Maryland; Urban, Joseph; Hill, Dolores; Dubey, Jitender

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2019
Publication Date: 4/10/2019
Citation: Rani, S., Cerqueira-Cézar, C.K., Murata, F.H., Sadler, M., Kwok, O.C., Pradhan, K., Urban Jr, J.F., Hill, D.E., Dubey, J.P. 2019. Toxoplasma gondii tissue cyst formation and quantitative density of tissue cysts in shoulders of pigs 7 and 14 days after feeding infected mice tissues. Veterinary Parasitology. 269:13-15. https://doi.org/10.1016/j.vetpar.2019.04.004.
DOI: https://doi.org/10.1016/j.vetpar.2019.04.004

Interpretive Summary: Toxoplasma gondii infection is widely prevalent in humans and animals, and toxoplasmosis continues to be a public health concern worldwide, including the USA. Humans become infected postnatally by ingesting infected uncooked/undercooked meat or food and water contaminated with oocysts excreted by cats. Among the meat sources of T. gondii, pork is considered important in the epidemiology of toxoplasmosis in the USA because beef and commercially raised poultry are rarely infected with T. gondii. Pigs are highly susceptible to T. gondii infection with a minimum infective dose of 1 oocyst. Toxoplasma can persist in tissues of pigs for life. However, there are no quantitative data about density of parasites in pork for risk assessment. How soon after infection, T. gondii forms tissue cysts in pork is also unknown. In the present study authors found that the parasite encysted in pork as early as 7 days after infection of pigs and even small helpings (50g) contained tissue cysts. Therefore, precautions should be used while handing pork and all pork should be cooked thoroughly before human consumption, and uncooked pork should never be fed to cats. Results will be of interest to veterinarians, public health workers, and parasitologists.

Technical Abstract: Toxoplasma gondii infection is widely prevalent in humans and animals, and toxoplasmosis continues to be a public health concern worldwide, including the USA. Humans become infected with T. gondii postnatally by ingesting infected uncooked/undercooked meat or food and water contaminated with oocysts excreted by cats. Among the meat sources of T. gondii, pork is considered important in the epidemiology of toxoplasmosis in the USA because beef and commercially raised poultry are rarely infected with T. gondii. Previous studies have shown that pigs can be easily infected with T. gondii, and the parasite can persist in pork for more than 2 years. However, there are no quantitative data on distribution of T. gondii in pork for risk assessment. How soon after infection, do T. gondii forms tissue cysts in pork is also unknown. In the present study eight serologically negative ~ 3 months old pigs were fed mouse tissues infected with VEG (Type III) strain of T. gondii and euthanized 7 (4 pigs) and 14 days (4 pigs) post-inoculation (p.i.). Meat from the right shoulder of each pig was bioassayed for T. gondii tissue cysts by peptic digestion and by feeding it to T. gondii free cats. Feces of cats fed pork were tested for oocyst excretion. From each pig, the shoulder muscle part was cut at 6 random spots into small 5, 10 and 50 g portions. Extreme care was taken to use different scalpels and forceps to minimize cross contamination among 17 samples (6 replicates of 5 and 10 g portions and 5 replicates of 50 g). From the 4 pigs euthanized at 7 days p.i., a composite of ~200g of leftover meat from each shoulder was collected for bioassay in cats. All 8 pigs developed T. gondii antibodies (modified agglutination test, MAT, 1: 80 or higher) and viable T. gondii was isolated from shoulder meat of each pig. The 4 cats fed pork from the pigs euthanized 7 days p.i., excreted millions of oocysts. The density of T. gondii, based on mouse infectivity, varied within 5-50 g samples from each pig, and between pigs within the same group, day 7 versus day 14 p.i.. Overall, the rate of isolation of T. gondii increased with sample size of meat bioassayed. There were no appreciable differences in mouse bioassay results obtained with day 7 versus day 14 infected pigs. Results demonstrate that tissue cysts are formed early in infection and they are unevenly distributed. Therefore, precautions should be used while handing pork and all pork should be cooked thoroughly before human consumption.