Equine Science Update
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Tick-borne infections are common in Danish horses according to a recent report.  Results of a survey published in Acta Veterinaria Scandinavica showed that many Danish horses had antibodies to Borrelia burgdorferi sensu lato  (the group of Borrelia species known to cause Lyme disease) and Anaplasma phagocytophilium (the cause of equine granulocytic anaplasmosis).

In northern Europe these micro-organisms are transmitted by the hard-bodied tick Ixodes ricinus (known as the sheep tick or castor bean tick).
Written by Mark Andrews. Published online 24.02.10
© Copyright Equine Science Update  2010
Tick-borne disease in Danish horses
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Not all horses that come into contact with the infection will develop signs. In endemic areas, it has been estimated that although 30-40% horses may be seropositive for Borrelia burgdorferi sensu lato, only 5-10% will show signs of disease. Similarly, up to 50% of animals may be seropositive for Anaplasma phagocytophilium in endemic areas. However, clinical signs are rarely recognised and most horses recover without treatment - so the condition infection usually goes unnoticed.

The study, overseen by Dr Anders M Bojesen of the Department of Veterinary Disease Biology, Faculty of Life Sciences, University of Copenhagen, Denmark aimed to evaluate the seroprevalence of B. burgdorferi s. l. and A. phagocytophilium in Danish horses.

In total, 390 horses of various breeds were sampled, from all regions of Denmark. Only healthy horses were included in the survey. Horses with signs or recent history of untreated infectious disease were excluded.

Lead researcher, Marie Hansen collected and tested the samples. She used a commercially available test (SNAP ® 4DX ®), which, although intended for testing canine blood samples, has been shown to be useful for detecting B. burgdorferi s. l. and A. phagocytophilium antibodies in horse serum.

Previous studies with the test suggest that it is likely to detect antibodies for up to five months after infection with A. phagocytophilium and for up to nine months after infection with B. burgdorferi s. l.  

The researchers found that overall, 29.0% horses were seropositive for B. burgdorferi s. l. and 22.3% were seropositive for A. phagocytophilium. This was a higher proportion than had been found in previous studies in neighbouring countries.  They explain: “Whether the differences in Denmark versus Sweden and Germany are due to an increase in the number of infected ticks since the latter investigations were performed, or whether the prevalence in Denmark in fact is higher will however remain unknown until data from prevalence studies with a comparable design and method from all countries in the region are performed.“

There was no significant correlation between risk factors investigated, including breed, gender, age, use and housing and the occurrence of antibodies against B. burgdorferi s. l.

However they did find that older horses were more likely to have been infected with A. phagocytophilium. Compared with horses aged between 1-4 years old, those aged 11-20 years were 2.3 times more likely to be seropositive for A. phagocytophilium, and horses over 21 years old were 3.3 times more likely to be seropositive.

Horses seropositive for B. burgdorferi s. l. were likely to be seropositive for A. phagocytophilium and vice versa.

The researchers conclude “these findings warrant further attention to these infections in horses particularly with regard to improved means for detection of active infections, which may contribute to a better understanding of these diseases and their impact on horse behaviour and welfare.”


The full report is available

Seroprevalence of Borrelia burgdorferi sensu lato and Anaplasma phagocytophilium in Danish horses.
Hansen MGB, Christoffersen M, Thuesen LR, Petersen MR, Bojesen AM.
Acta Vet Scandinavica (2010) 52, 3
doi: 10.1186/1751-0147-52-3

Photo courtesy Dr Mitsuo Ishii