Significant familial differences in the frequency of abortion and Toxoplasma gondii infection within a flock of Charollais sheep

Abstract

A study was carried out to investigate the frequencies of abortion and congenital Toxoplasma gondii infection within 27 families (765 individuals) of a pedigree Charollais sheep flock maintained on a working farm in Worcestershire, UK, since 1992. Pedigree lambing records were analysed to establish the frequency of abortion for each family. The frequency of congenital infection was determined for each family by PCR analysis of tissue samples taken from newborn lambs. A total of 155 lambs were tested for congenital T. gondii infection, which were all born during the study period 2000–2003. Significant differences in the frequency of abortion between sheep families within this flock were observed with frequencies ranging between 0% and 48% (P<0·01). Significantly different infection frequencies with T. gondii were also observed for different families and ranged between 0% and 100% (P<0·01). Although the actual cause of each abortion was not verified, a highly significant positive correlation was found to exist between the frequency of abortion and the frequency of T. gondii infection in the same families (P<0·01). The data presented here raise further questions regarding the significance of congenital transmission of T. gondii within sheep populations, the possible successive vertical transmission of T. gondii within families of sheep, and the potential role of inherited genetic susceptibility to abortion with respect to T. gondii infection. This work invites further study into the epidemiology of ovine toxoplasmosis and may have implications for sheep husbandry methods in the future.

Outbreak of ovine congenital toxoplasmosis in Iran, confirmed by different diagnostic methods

“…There are a lot of unclear aspects of its congenital importance. According to the results of the recent researches in the UK, Toxoplasma vertical transmission can occur more frequently than what was previously thought (Duncanson et al 2001;Morely et al 2005Morely et al , 2007Hide et al 2009;Innes et al 2009;Dubey 2009), but Edwards and Dubey (2013) reported that this problem is resolved in the involved flock after one year or two years. If this kind of abortion takes place, it needs to be dealt with either by taking prophylactic measures or culling infected sheep from the flock which is more economical.…”

“…It was first described by Hartley et al (1954) which even today is known as one of the major causes of sheep abortion world wide (Buxton 1991). Another characteristic of vertical transmission of T. gondii is that this kind of transmission can occur more frequently than what was previously thought (Duncanson et al 2001;Morely et al 2005Morely et al , 2007Hide et al 2009;Innes et al 2009;Dubey 2009). …”

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.…”