Vaccination of pigs with the S48 strain of Toxoplasma gondii--safer meat for human consumption

Abstract

As clinical toxoplasmosis is not considered a problem in pigs, the main reason to implement a control strategy against Toxoplasma gondii (T. gondii) in this species is to reduce the establishment of T. gondii tissue cysts in pork, consequently reducing the risk of the parasite entering the human food chain. Consumption of T. gondii tissue cysts from raw or undercooked meat is one of the main sources of human infection, with infected pork being considered a high risk. This study incorporates a mouse bioassay with molecular detection of T. gondii DNA to study the effectiveness of vaccination (incomplete S48 strain) in its ability to reduce tissue cyst burden in pigs, following oocyst (M4 strain) challenge. Results from the mouse bioassay show that 100% of mice which had received porcine tissues from vaccinated and challenged pigs survived compared with 51.1% of mice which received tissues from non-vaccinated and challenged pigs. The presence (or absence) of T. gondii DNA from individual mouse brains also confirmed these results. This indicates a reduction in viable T. gondii tissue cysts within tissues from pigs which have been previously vaccinated with the S48 strain. In addition, the study demonstrated that the main predilection sites for the parasite were found to be brain and highly vascular muscles (such as tongue, diaphragm, heart and masseter) of pigs, while meat cuts used as human food such as chop, loin, left tricep and left semitendinosus, had a lower burden of T. gondii tissue cysts. These promising results highlight the potential of S48 strain tachyzoites for reducing the number of T. gondii tissues cysts in pork and thus improving food safety.

Figure 1

Average porcine temperature per experimental group following vaccination with S48 tachyzoites. Rectal temperatures of each pig was recorded daily prior to challenge. Between days 0 – 14 post vaccination, treatment groups G1 and G4, and G2 and G3 are identical. The red cut off line indicates the normal rectal temperature for pigs. G1 = Oocyst (M4) challenged pigs, G2 = Vaccinated (S48) and oocyst (M4) challenged pigs, G3 = Vaccinated (S48) pigs, G4 = Negative control pigs.

Figure 2

T. gondii IgG ELISA. T. gondii IgG ELISA results for all pigs from week 0 to 70 post vaccination. Plots show observed ELISA value of each pig (small dots), estimated mean ELISA value of each group (large solid dots joined by solid line) and corresponding 95% confidence intervals (shaded region). The vertical dashed line indicates the point at which pigs were challenged with 1000 M4 T. gondii oocysts. The horizontal dashed line indicates when animals were classed as seropositive for T. gondii.G1 = Oocyst (M4) challenged pigs, G2 = Vaccinated (S48) and oocyst (M4) challenged pigs, G3 = Vaccinated (S48) pigs, G4 = Negative control pigs.

Figure 3

Kaplan-Meir survival curves. Kaplan-Meir survival curve in relation to vaccination highlighting proportion of mice that survived when fed tissues from G1 and G2 pigs (A). All mice which received tissues from vaccinated and challenged pigs (G2) survived (blue line) until the end of the bioassay (day 42). Only 51.1% of mice which received tissues from unvaccinated and challenged pigs (G1) survived until the end of the bioassay (day 42). G1 = Mice inoculated with porcine tissues from pigs which were vaccinated (S48) and oocyst (M4) challenged. G2 = Mice which were inoculated with porcine tissues from pigs which were oocyst (M4) challenge. Kaplan-Meir survival curve from mice fed different tissues from pigs in G1 highlighting proportion of mice that survived when fed different tissue types (brain, food and other) (B). Brain = brain tissue, Food = pooled sample of chop, loin, left tricep and left semitendinosus, Other = pooled sample of diaphragm, heart, tongue and masseter.

Figure 4

Mouse survival rate related to specific porcine tissues (experimental groups G1 and G2). All mice which received homogenised tissue from G2 pigs survived. Not all mice which received porcine tissues from G1 pigs survived, with only 40% of mice which received brain tissue surviving, followed by 26.7% which received food and 86.7% receiving other.