Toxoplasmosis risk from horse meat underlined in study

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Stained example of Toxoplasma gondii tachyzoite. Tachyzoites are one of the three infectious stages of the parasite. They are typically crescent shaped with a prominent, centrally placed nucleus. Photo: DPDx Image Library (Public domain), via Wikimedia Commons
A stained example of Toxoplasma gondii tachyzoites. Tachyzoites are one of the three infectious stages of the parasite. They are typically crescent shaped with a prominent, centrally placed nucleus. Photo: DPDx Image Library (Public domain), via Wikimedia Commons

The risk of horse meat as a potential source of toxoplasmosis in humans has been underlined in research in Serbia.

Toxoplasmosis is a parasitic disease caused by Toxoplasma gondii. It is estimated that a third of the world’s population is infected. Most have no symptoms, although occasionally some may suffer a mild flu-like illness for a few weeks or months.

A small number may develop eye problems. Severe symptoms such as seizures and poor coordination may occur in those with a weak immune system. Infections can also cause problems for unborn babies.

T. gondii is considered one of the most successful parasites due to its omnipresence and wide array of hosts, including all mammals.

Toxoplasmosis is usually spread by eating poorly cooked food that contains cysts, or exposure to infected cat feces. The parasite is known to reproduce sexually only in cats.

Olgica Djurković-Djaković and her colleagues, writing in the journal Parasites & Vectors, said although horse meat was typically eaten rare or under-cooked, information on the risk to humans of T. gondii from infected horse meat was scarce.

The study team set out to determine the presence of T. gondii infection in slaughter horses in Serbia, and to attempt to isolate viable parasites from heart tissue samples and identify strains.

Their study included horses from all regions of Serbia slaughtered at two abattoirs between June 2013 and June 2015.

Blood samples underwent molecular testing for the presence of specific antibodies to T. gondii.

A total of 105 slaughter horses were sampled. In all, 48.6% of the examined horses were seropositive when assessed at a cut-off value of 1:6. Positive results indicated exposure to the parasite.

Viable parasites were isolated from two mares; both being T. gondii type III. The strains were isolated from one weakly seropositive animal and from another which was completely seronegative.

This, they said, agreed with the results of a recent review which showed very low agreement between blood test results and the presence of T. gondii cysts, especially in large animals including cattle and horses.

These were the first cases of isolation of T. gondii from horses in Serbia, the researchers reported.

“The isolation of viable T. gondii parasites from slaughter horses points to horse-meat as a potential source of human infection,” they concluded.

However, the fact that viable parasites were isolated from horses with only a serological trace of T. gondii infection suggested that blood testing may not be adequate to assess the risk of toxoplasmosis from horse-meat consumption.

The life cycle of Toxoplasma gondii. Graphic: Centers for Disease Control / Alexander J. da Silva, PhD / Melanie Moser (Public domain), via Wikimedia Commons
The life cycle of Toxoplasma gondii. Graphic: Centers for Disease Control / Alexander J. da Silva, PhD / Melanie Moser (Public domain), via Wikimedia Commons

The study team noted that viable cysts have been shown to persist for many months in various organs in experimentally infected horses.

Discussing their findings, the researchers said the reported seroprevalence of T. gondii in horses worldwide ranged from 0 to 100%, and from 0 to 53% in Europe. Recent studies in Europe have shown seropositivity rates of 1.7% in Greece, 10.8% in Spain, 3% and 17.6% in Italy, 23% in the Czech Republic, 37.8% in Romania, and from 13 to 90% (depending on the cut-off) in France.

They said it was difficult to assess whether the reported result represented real differences in the seroprevalence of T. gondii infection or possibly reflected the use of different tests, different cut-offs for the test, or differences in the study samples regarding age and/or origin of the animals.

Reports of T. gondii isolations from naturally infected horses were quite rare, they noted, and information regarding the risk of its transmission from infected horse meat to humans was scarce and only inferred.

In France, one study described three cases of human toxoplasmosis, in which the probable source of infection was eating raw horse meat (in two cases imported from Canada and Brazil).

The risk was further emphasized by the isolation of a type III-like strain from horse meat (originating from Canada) obtained from the first patient’s butcher.

In a 2015 study, T. gondii DNA was detected in 43% of 231 horse meat samples from French supermarkets. Strain isolation was attempted from 118 samples, but no parasites were isolated, which the authors considered to reflect low distribution of cysts in skeletal muscles, therefore indicating a low risk of human T. gondii infection from consuming infected horse meat.

“However, quite the opposite was concluded in a study of the relative contribution of sheep, beef and pork products to human T. gondii infection in the Netherlands, which showed a high public health risk even in the case of low prevalence of parasite cysts in animal tissues, if the meat is inadequately processed.

“Therefore, as horse meat is generally consumed rare or under-cooked, the risk for human infection may be high even in the case of low cyst burden in horse tissues.”

About 400 horses a year are slaughtered in Serbia. The country used to be an important horse producer and exporter, but the production has been significantly decreasing over the last decades. Local consumption of horse meat currently constitutes about 0.01 kg per person per year.

The research team comprised Djurković-Djaković, Ivana Klun, Aleksandra Uzelac, Isabelle Villena, Aurélien Mercier, Branko Bobić, Aleksandra Nikolić, Irena Rajnpreht, Marieke Opsteegh, Dominique Aubert, Radu Blaga, and Joke van der Giessen. They are variously affiliated with Serbia’s University of Belgrade, the University of Reims Champagne-Ardenne in France, the University of Limoges in France, Serbia’s National Institute for Public Health and the Environment, and the University of Paris-Est in France.

The first isolation and molecular characterization of Toxoplasma gondii from horses in Serbia
Ivana Klun, Aleksandra Uzelac, Isabelle Villena, Aurélien Mercier, Branko Bobić, Aleksandra Nikolić, Irena Rajnpreht, Marieke Opsteegh, Dominique Aubert, Radu Blaga, Joke van der Giessen and Olgica Djurković-Djaković
Parasites & Vectors 2017 10:167
DOI: 10.1186/s13071-017-2104-x

The study, published under a Creative Commons License, can be read here.

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