Evidence that primary infection of Charollais sheep withToxoplasma gondiimay not prevent foetal infection and abortion in subsequent lambings

Abstract

A study carried out on a sheep farm examined whether Toxoplasma gondii foetal infection and associated abortion occur in successive lambings. We identified 29 ewes that gave birth to lambs on at least 2 successive years over our study period, 2000-2003. Tissue samples from the progeny of these ewes were analysed by PCR to determine infection status with T. gondii. T. gondii-infected lambs were born in 31% of successive pregnancies. T. gondii-positive lambs were aborted in successive pregnancies in 21% of lambings during study period, 2000-2003. The frequency of successive abortions within this flock over the period 1992-2003 was 18%. If a lamb was congenitally infected there was a high risk (69%) that the successive lamb from that ewe would also be congenitally infected. Similarly, if a lamb was aborted there was a high risk (55%) of abortion in the next lamb produced. These data suggest that life-long immunity to T. gondii infections may not always be acquired following an initial infection and raises the question as to whether the mechanisms of T. gondii transmission prior to and during ovine pregnancies are fully understood.

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.…”