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Serological Survey Of Antibodies To Toxoplasma

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Ciamek Ghazaei D.V.M
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Citation: C. Ghazaei : Serological Survey Of Antibodies To Toxoplasma . The Internet Journal of Veterinary Medicine. 2005 Volume 2 Number 1

Keywords: Toxoplasma gondii | Food animals | Seroprevalence | ELISA

 

Abstract

Toxoplasmosis is one of the most prevalent parasitic infections of man and livestock and its transmission has usually been attributed to ingestion of undercooked or raw meat from infected livestock with the infection rate in those animals being an important risk predictor of human disease, high in Iran and Ardabil State. Looking for this public health problem we tested serum samples from cattle, goat, sheep and chicken from the State of Ardabil, Iran for IgG antibodies to Toxoplasma gondii by enzyme-linked immunosorbent assay (ELISA). Antibodies toToxoplasma gondii were found in 30.00% (60/200) of sheep, 15.00% (30/200) of goat and 9.00% (18/200) of cattle, without positive samples in chicken. Despite differences in feeding habits of each species the rate of infection of tested animals could be better attributed to livestock management methods, which improvement could reduce infection.


Introduction

Infection by the protozoan parasite Toxoplasma gondii, is widespread in humans and many other species of warm-blooded animals. Although the course of disease is generally benign, it can cause significant morbidity and mortality in the developing fetus and in immunocompromised individuals, including humans with Acquired Immunodeficiency Syndrome - AIDS or submitted to cancer chemotherapy.

Among livestock, sheep and goat are more widely infected with Toxoplasma gondii than cattle and chicken. This parasite is a major cause of abortion, with significant economic losses to sheep and goat breeders. The infection does not usually cause clinical symptoms in cattle and in chicken.

Recent studies showed that a small percentage of affected individuals acquire infection in the uterus, but the majority becomes exposed to Toxoplasma gondii by ingestion of undercooked or raw meat containing tissue cysts, ingestion of oocysts shed by infected cats or consumption of contaminated drinking water or fresh vegetables. Toxoplasma gondii have been demonstrated in mutton including in Iran, goat meat, beef and chicken.

Although Toxoplasma gondii is found in most parts of the world, there have been relatively few recent reports on small ruminants, cattle and chicken in Iran. Epidemiological surveys still remain the most useful way of assessing the relative importance of different sources of Toxoplasma gondii infection in humans. Since contaminated meat is a significant infection source to man, it is particularly beneficial to ensure continuous surveillance of T. gondii prevalence in animal species destined for human consumption.

Material and Method

Serum was collected from a total of 750 food animals from Ardabil State, Iranl: 200 being from extensive breed cattle slaughtered at abattoirs in the city of Ardabil, 200 from semi-intensive breed goat from farms of the Ardabil region, 200 from extensive breed sheep slaughtered at abattoirs in the city of Ardabil and 150 from intensive breed chicken slaughtered at abattoirs in the city of Ardabil. Cattle, sheep and chicken blood was collected during slaughter, immediately after killing, and goat blood was collected by venipuncture. The serum was separated from the clot by centrifugation at 1000g for 10 min, mixed (1:1, v/v) with phosphate buffered glycerol, pH 7.2, and stored at -20°C until use. RH strain Toxoplasma gondii tachyzoites salt soluble antigen was prepared from infected mouse peritoneal fluid as described elsewhere, except for one step of mammalian cell exclusion by adhesion to sterile pre-packed Sephadex G50 columns. The antigen was adjusted to 1mg protein/ml and stored at -70°C until use. ELISA was performed as described elsewhere, using high protein binding- certified microplates (Sigma) coated with 100 ml/well of Toxoplasma gondii antigen 10 mg/ml. Serum sample, diluted 1:100 in PBS-T, was added to each well and bound IgG detected with species-specific anti IgG peroxidase conjugate, with optical density (OD) was measured in a microplate reader. In each plate, we included positive, usually obtained from an experimentally infected animal from the same species, negative and threshold controls, all previously determined by Indirect Immunofluorescence Assay (IFA). The threshold control, obtained from the dilution of the positive serum of known IFA titer in standard negative serum, was used to clearly distinguish reactive from non-reactive serum samples in multiple plate assays; the absorbance of the threshold serum was taken as the lowest level of identifiable positive reaction. The reactivity index (RI) of the samples was defined as the ratio of the average absorbance of the samples by the average absorbance of the threshold serum, being positive when RI 3 1.0. All serum was tested in duplicate, with reproducibility inter and intra tests higher than 99.00%.

Results and Discussion

The ELISA results were shown both as frequency of infection in the Table1,and also as their reactivity distribution in the Figure1.We found 31.00% of seropositivity in sheep, 17.00% in goat, 11.00% in cattle, without positive sample in chicken with a 95.00% confidence interval for each measure, which was significant higher in sheep than in goats and cattle, and absent in chicken. Their reactivity distribution shows clearly the expected bimodal distribution of infected and non infected animals.

Thumbnail: Table 1: Frequency of seropositivity for T. gondii among livestock from Ardabil state, Iran. Comparison between frequencies by ...
Table 1: Frequency of seropositivity for T. gondii among livestock from Ardabil state, Iran. Comparison between frequencies by c2 test showed that the highest frequency was found in sheep, with cattle and goat with similar intermediate frequencies and chicken with lowest seropositivity.

Thumbnail: Figure 1: Distribution of anti-Toxoplasma gondii in livestock, showing typical bimodal distribution in infected species
Figure 1: Distribution of anti-Toxoplasma gondii in livestock, showing typical bimodal distribution in infected species

The results of this survey would suggest that small ruminants play a more important role as source of toxoplasmosis than cattle as discussed elsewhere. Nonetheless, despite the lower seroprevalence detected in cattle, consumption of mutton is much greater than that of beef or goat meat and this thus increases the importance of sheep as source of local infection. Our current understanding of the epidemiology of toxoplasmosis leads us to think that herbivores acquire infection by ingestion of pasture and water contaminated with Toxoplasma gondii oocysts shed by cats. The differences in rate of infection could be attributed both to differences in susceptibility to T. gondii or to differences in management methods. Since sheep is bred under extensive management, it is more likely to be exposed to Toxoplasma gondii oocysts in pasture and water than goats, which are supplied with better water and food quality under semi-intensive management. Although, lower seropositivities in cattle samples compared to those in sheep may be attributed to differences in susceptibility, since both species are bred under extensive management. On the other hand, the absence of infection in the chickens can also be attributed to the management method, as these animals are bred in highly intensified management. Our data showed that, out of the three infected species, the lowest seroprevalence occurred in cattle, but in view of the typical preference for beef, bovine protein cannot be ruled out as a significant source of human infection, including as processed products. The high infection rate in sheep might have local implications because, in the State of Ardabil and Iran, mutton is more popular as source of animal protein, turning into a potential source of human toxoplasmosis of increasing importance.

These data suggest that it is possible to significantly reduce the risk of Toxoplasma gondii infection in using intensive farm management with adequate measures of hygiene, confinement, and prevention.

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