Virulence in Mice of a Toxoplasma gondii Type II Isolate Does Not Correlate With the Outcome of Experimental Infection in Pregnant Sheep

Abstract

Toxoplasma gondii is an apicomplexan parasite that infects almost all warm-blooded animals. Little is known about how the parasite virulence in mice extrapolates to other relevant hosts. In the current study, in vitro phenotype and in vivo behavior in mice and sheep of a type II T. gondii isolate (TgShSp1) were compared with the reference type II T. gondii isolate (TgME49). The results of in vitro assays and the intraperitoneal inoculation of tachyzoites in mice indicated an enhanced virulence for the laboratory isolate, TgME49, compared to the recently obtained TgShSp1 isolate. TgShSp1 proliferated at a slower rate and had delayed lysis plaque formation compared to TgME49, but it formed more cyst-like structures in vitro. No mortality was observed in adult mice after infection with 1–105 tachyzoites intraperitoneally or with 25–2,000 oocysts orally of TgShSp1. In sheep orally challenged with oocysts, TgME49 infection resulted in sporadically higher rectal temperatures and higher parasite load in cotyledons from ewes that gave birth and brain tissues of the respective lambs, but no differences between these two isolates were found on fetal/lamb mortality or lesions and number of T. gondii-positive lambs. The congenital infection after challenge at mid-pregnancy with TgShSp1, measured as offspring mortality and vertical transmission, was different depending on the challenged host. In mice, mortality in 50% of the pups was observed when a dam was challenged with a high oocyst dose (500 TgShSp1 oocysts), whereas in sheep infected with the same dose of oocysts, mortality occurred in all fetuses. Likewise, mortality of 9 and 27% of the pups was observed in mice after infection with 100 and 25 TgShSp1 oocysts, respectively, while in sheep, infection with 50 and 10 TgShSp1 oocysts triggered mortality in 68 and 66% of the fetuses/lambs. Differences in vertical transmission in the surviving offspring were only found with the lower oocyst doses (100% after infection with 10 TgShSp1 oocysts in sheep and only 37% in mice after infection with 25 TgShSp1 oocysts). In conclusion, virulence in mice of T. gondii type II isolates may not be a good indicator to predict the outcome of infection in pregnant sheep.

Treatment with Bumped Kinase Inhibitor 1294 Is Safe and Leads to Significant Protection against Abortion and Vertical Transmission in Sheep Experimentally Infected with Toxoplasma gondii during Pregnancy

“…These results are in line with those from a pregnant mouse model of toxoplasmosis, in which 100% protection against pup mortality and 93% protection against vertical transmission in the surviving offspring was accomplished with BKI-1294 (28). The increased protection of BKI-1294 in pregnant mice compared to pregnant sheep could be explained by the lower offspring mortality and vertical transmission of T. gondii in mice compared to that in sheep (36).…”

“…At this time point, the infection has already been established and the parasites have been widely distributed in the animals. In this experiment, pregnant ewes that were infected but not treated showed 100% fetal mortality as previously described (36). Clinical observations in the BKI-1294-treated group revealed high protection against fetal/lamb mortality, as 5 out of 7 (71%) of the ewes gave birth to healthy lambs (13 out of 17 fetuses/lambs were born healthy), with statistically improved fetal/lamb survival rates compared to those of the infected, untreated group.…”

“…In this study, lambs born from twin pregnancies in the infected and treated group exhibited slightly lower birth weights than the uninfected group receiving vehicle alone, but since no significant differences were found between lambs that were PCR positive and those that were PCR negative, it is likely that this lower growth of the twin fetuses is a consequence of decreased food intake in the dams from the infected, treated group for 6 days (from 96 to 101 days of pregnancy, associated with marked fever due to T. gondii infection), as previously described (37). Likewise, all lambs born in the infected and treated group were healthy, while in a previous study using the TgShSp1 isolate in pregnant sheep, weak lambs were often observed (36). The rectal temperatures from infected and treated pregnant ewes were lower than those of infected, untreated pregnant ewes on days 5 and 7 p.i.…”

“…However, in abortions during the acute phase of the disease caused by the infection of pregnant ewes with TgShSp1 oocysts (days 7 to 10 p.i. ), T. gondii was scarcely detected in fetal tissues and placental thrombosis was often found (36). In lungs, which are commonly infected by T. gondii (44), no lesions or parasites were detected in lambs born in the BKI-1294-treated group.…”

“…However, using the ELISA based on T. gondii soluble antigens, only one ewe that gave birth to a stillborn lamb and two dams that gave healthy lambs seroconverted, while the remaining ewes were seronegative at the end of the sampling period. The findings that soluble antigens are exposed to the immune system during replication (42) and that much lower humoral immune responses to soluble antigens were found compared to those of ewes infected with lower oocyst doses (36) suggest that there was only low replication of the parasite throughout the experiment. In contrast, SAG1 and BAG1 antibodies in the infected and treated group may be derived from their enhanced expression in vivo, as predicted by the multinucleated complexes found in vitro after BKI-1294 treatment of T. gondii-infected cultures (25).…”