Many horses are infected by ticks without it being detected

Researchers found surprisingly many traces of tick-borne diseases in horses. The findings may also say something about the risk of infection for humans.

“I would advise horse owners to use tick repellent on their horses and be diligent in checking them regularly,” says Hanne Kloster.

(Photo: Shutterstock / NTB)

Walter Norman Wehus COMMUNICATIONS ADVISER

Presented by: University of Agder

Published 14 May 2025 - 00:01

In a study led by the University of Agder, researchers examined 331 horses from four counties in Norway.

The results show that a full 66 per cent of the horses had 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 researcher Hanne Kloster. She led the study.

Facts about tick-borne diseases:

Lyme disease can cause rashes, fever, and joint pain. In some cases, it can lead to neurological issues.
Anaplasma can cause flu-like symptoms and is particularly serious for sheep.
The TBE virus can cause meningitis in both animals and humans.
There is a vaccine against the TBE virus for humans, but not for animals.

Highest in Scandinavia

Researchers took blood samples from horses in four counties: Agder, Vestfold and Telemark, Vestland, and the former Viken county.

Then they tested the blood samples for antibodies against three types of tick-borne diseases: Lyme disease, anaplasmosis, and tick-borne encephalitis (caused by TBE).

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

Antibodies against Borrelia were most prevalent. The researchers found this in 47 per cent of the horses. They found antibodies against Anaplasma in 34 per cent, while antibodies against the TBE virus were detected in 10 per cent.

Hanne Kloster has studied tick-borne diseases in horses. She is now working on a similar study in dogs.

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

Many get sick without it being detected

The owners of the horses also filled out a questionnaire. Of the 331 horses in the study, only one had been treated with antibiotics for a tick-borne disease.

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 that 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 areas

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 fewer than two out of ten horses in Vestfold and Telemark and the former Viken county.

TBE antibodies were just as 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 farther north in Norway than they have before. They also move easily 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 believes.

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

Reference:

Kloster et al. Seroprevalence of IgG Antibodies Against Borrelia burgdorferi Sensu Lato, Anaplasma phagocytophilum, and Tick-Borne Encephalitis (TBE) Virus in Horses in Southern Norway, Microorganisms, vol. 13, 2025. DOI: 10.3390/microorganisms13040771

———

Read the Norwegian version of this article on forskning.no