Review
Toxoplasmosis in sheep—The last 20 years

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Abstract

Sheep are important to the economy of many countries because they are a source of food for humans Sheep are commonly infected with the protozoan parasite, Toxoplasma gondii. Infection with the parasite may cause early embryonic death and resorption, fetal death and mummification, abortion, stillbirth, and neonatal death. Severity of infection is associated with the stage of pregnancy at which the ewe becomes infected, the earlier in gestation, the more severe the consequences. Infected sheep meat is a source of T. gondii infection for humans and carnivorous animals. Most sheep acquire T. gondii infection after birth, and less than 4% of persistently infected sheep transmit the parasite vertically to the next generation. Recent studies by a group of researchers in England reported that repeat ovine transmission of T. gondii may be more common than previously believed but these findings are soley based on PCR data and require additional data using other techniques to verify the findings. Following infection with T. gondii, sheep develop humoral and cell-mediated immune responses against the parasite that provides effective protection against disease in subsequent pregnancies. A commercial vaccine is available, comprising a live, incomplete strain of T. gondii. The vaccine is administered to sheep prior to mating to protect against lamb losses due to toxoplasmosis. In the present paper, information on the prevalence, transmission, and control of ovine toxoplasmosis in the last 20 years is reviewed.

Introduction

Toxoplasma gondii infection is widely prevalent in humans and animals worldwide (Dubey and Beattie, 1988). Humans become infected post-natally by ingesting tissue cysts from undercooked meat, consuming food or drink contaminated with oocysts, or by accidentally ingesting oocysts from the environment. The ingestion of undercooked infected lamb is considered an important source of infection for humans. Cook et al. (2000) identified eating uncooked lamb as a risk factor for T. gondii infection in pregnant women in Europe. In a retrospective study of 131 mothers in the USA who had given birth to children infected with T. gondii, 50% recalled having eaten uncooked meat (Boyer et al., 2005). Toxoplasmosis also causes heavy economic losses to sheep industry worldwide (Dubey and Beattie, 1988, Buxton et al., 2007). We previously reviewed worldwide information on ovine toxoplasmosis up to 1988 (Dubey and Towle, 1986, Dubey and Beattie, 1988). In the present paper, information on prevalence, transmission, epidemiology, immunology and control of ovine toxoplasmosis since 1988 is reviewed.

Section snippets

Serologic prevalence

Antibodies to T. gondii have been found in sheep worldwide (Table 1). The prevalence of antibodies in ewes was more than twice that in lambs, but results were dependent on the age of lambs sampled (Lundén et al., 1994, Gorman et al., 1999, Figliuolo et al., 2004, Rozette et al., 2005, Dumètre et al., 2006, Ragozo et al., 2008). Sero-prevalence was shown to increase with age, reaching 95% in 6-year-old ewes in some flocks (Dubey and Kirkbride, 1989a), suggesting that most animals acquire

Experimental toxoplasmosis in sheep

In the last 20 years, several authors reported on experimental studies of ovine toxoplasmosis using different strains of T. gondii. Results from these studies are summarized below.

Vaccine

A live vaccine (Toxovax®) is commercially marketed in the UK, France and New Zealand for reducing losses to the sheep industry from congenital toxoplasmosis (Buxton and Innes, 1995). This vaccine was initially developed in New Zealand (O’Connell et al., 1988, Wilkins et al., 1988, Wilkins and O’Connell, 1992), and further efficacy studies were conducted by Buxton and his colleagues in Scotland. The vaccine consists of a modified strain (S48) of T. gondii, originally isolated from an aborted

Concluding remarks

Despite the advances in our understanding of ovine toxoplasmosis, several aspects of the disease in sheep require further research effort. These include understanding whether there may be breed differences or genetic differences relating to disease susceptibility in sheep In addition, the extent to which T. gondii-associated repeat abortion may occur in flocks, and the need for a non-viable vaccine to prevent infection and abortion.

Acknowledgement

I would like to thank Drs. H.R. Gamble. Lee Innes, and Dolores Hill for their help.

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