High levels of congenital transmission of Toxoplasma gondii in longitudinal and cross-sectional studies on sheep farms provides evidence of vertical transmission in ovine hosts

Abstract

Recent research suggests that vertical transmission may play an important role in sustaining Toxoplasma gondii infection in some species. We report here that congenital transmission occurs at consistently high levels in pedigree Charollais and outbred sheep flocks sampled over a 3-year period. Overall rates of transmission per pregnancy determined by PCR based diagnosis, were consistent over time in a commercial sheep flock (69 %) and in sympatric (60 %) and allopatric (41 %) populations of Charollais sheep. The result of this was that 53. 7% of lambs were acquiring an infection prior to birth: 46. 4% of live lambs and 90. 0 % of dead lambs (in agreement with the association made between T. gondii and abortion). No significant differences were observed between lamb sexes. Although we cannot distinguish between congenital transmission occurring due to primary infection at pregnancy or reactivation of chronic infection during pregnancy, our observations of consistently high levels of congenital transmission over successive lambings favour the latter.

Toxoplasma gondii Seroprevalence in Domestic Animals and Humans in Mymensingh District, Bangladesh

“…Chronic T. gondii infections in ruminant dams may cause abortion by reactivation of the parasite during pregnancy [5]. Although most infections in humans are asymptomatic, abortion may also occur after congenital T. gondii infection [3].…”

High frequency detection ofToxoplasma gondiiDNA in human neonatal tissue from Libya

“…Further recent studies on sheep have revealed that congenital transmission may be sufficient to explain the maintenance of T. gondii in natural populations of sheep without requiring new infections by oocysts secreted by cats. [22][23][24][25] In the case of these sheep studies, it was demonstrated that PCR detection of T. gondii DNA in the umbilical cord was a good indicator of infection when aborted lamb brain tissue was compared in the same animal. Nevertheless, a challenge to be encountered in humans (as may be the case in sheep and other animals) is to understand whether these individuals are truly asymptomatic carriers of vertical transmission.…”

“…22 Appropriate protocols were used at each stage of the DNA purification and any subsequent PCR amplifications to prevent cross-contamination as described previously. 21,23,25,35 PCR detection of T. gondii Detection of T. gondii was carried out using a nested PCR amplification of both surface antigen gene 1 (sag 1) as described previously 24,36 and surface antigen gene 3 (sag 3) 37 with subsequent modifications. 38 Pure T. gondii DNA from RH, Martin, 17695 and COR strains were used as positive controls in all PCR experiments.…”

“…[16][17][18] However, the frequency of this mode of transmission in natural populations of animals, or humans, is less clear. PCR based studies on the transmission of T. gondii in several mammalian species, e.g., mice, woodmice 17,[19][20][21] and sheep [22][23][24][25] have suggested that congenital transmission may be occurring at a higher frequency than previously thought. Although higher frequencies of congenital transmission are reported in these studies, in many cases there seems to be no measurable disease effect on the neonates in these species.…”

“…1 However, some studies have suggested that more sensitive techniques, such as PCR or parasite detection by bioassay, are capable of detecting parasites in seronegative individuals (e.g., in rodents, 29,30 pigs 31 and humans 32 ). In the PCR based studies on sheep, 22,23 the measurement of congenital transmission was conducted utilising umbilical cord tissue as a proxy for neonatal tissue. In those studies, comparison of PCR detection of Toxoplasma DNA in umbilical cord tissue with internal tissues (brain) from infected and uninfected aborted lambs showed very strong agreement.…”