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Tick-borne diseases widespread in horses: Many infections go undetected, study finds

In a study led by the University of Agder (UiA), published in Microorganisms, researchers examined 331 horses from four counties. The results show that a full 66% of the horses have been exposed to one or more tick-borne viruses or bacteria.

"That's insanely high. Well over half of the horses tested positive for one or more tick-borne infectious agents," says Ph.D. Research Fellow Hanne Kloster, who led the study.

Highest in Scandinavia

Researchers took blood samples from horses in four counties: Agder, Vestfold and Telemark, Vestland and the former Viken county. They tested the blood samples for antibodies against three types of tick-borne diseases: Lyme disease, anaplasmosis and tick-borne encephalitis (which is caused by TBE).

If horses have antibodies against a bacterium or virus in their blood, it means that they have been infected by it at some point.

Antibodies against Borrelia were most prevalent, with findings in 47% of the horses. Antibodies against Anaplasma were found in 34%, while antibodies against TBE virus were detected in 10%.

"This is higher than similar findings in both Sweden and Denmark," says Kloster.

Many get sick without it being detected

The horse owners also completed a questionnaire. Of the 331 horses in the study, only one had been treated with antibiotics for tick-borne diseases. This may indicate that many horses have been sick without their owners noticing.

"If you have a horse with non-specific issues, you should consider testing for tick-borne diseases. This is particularly important in areas with a lot of ticks," says Kloster.

She recommends horse owners be more vigilant. "I would advise horse owners to use tick repellent on their horses and be diligent in checking them regularly for ticks. This applies to all types of horses. We found infection in competition horses, trotters and Icelandic horses, so none are exempt from the risk."

Differences between coast and inland

The study also revealed differences between the regions. Anaplasma was found in about half of the horses along the coast of Agder and Vestland, but in less than two out of ten horses in Vestfold and Telemark and the former Viken county.

TBE antibodies were equally common in Vestland as in the other counties.

"In Western Norway, no cases of TBE have been detected in humans. So the fact that we found quite a lot of antibodies in horses is quite surprising, and interesting for further research," says Kloster.

Could be a measure of infection

The findings are important for both horse owners and the general public. Ticks are spreading further north in Norway than before and can easily move from animals to humans.

"We can use horses as indicators of the infection levels in different areas," says Kloster.

The discovery of antibodies against the TBE virus, which can lead to meningitis, are particularly interesting, she finds.

"TBE can, at worst, be fatal for horses if untreated. This disease has also increased significantly among humans in Norway in recent years," says Kloster.