A protected shrew in Central Europe carries a virus known to cause fatal brain inflammation in horses, Austrian researchers have discovered.
Scientists at the University of Veterinary Medicine Vienna, in research published in the open-access journal, PLoS ONE, say the bicoloured shrew, Crocidura leucodon, carries the Borna virus. Infection with the virus causes fatal encephalitis in horses. Cases are incurable.
The mechanisms of transmission have until now been unclear, but the research work has confirmed the virus can be passed on from the insect-eating shrews to horses.
Borna disease is named after the German city of same name, which saw a cluster of cases more than 100 years ago. It mainly affects horses and sheep, and in rare cases cattle and rabbits. A single case of an infected dog has been reported.
Infected horses seclude themselves from the herd and suffer from depression and general disorientation. Ultimately, the infection is fatal.
Researchers have long been in the dark concerning the transmission mechanism of the virus. The bicoloured shrew was one suspect, but definitive proof was missing.
Norbert Nowotny and Jolanta Kolodziejek, from the university’s Institute of Virology, and Herbert Weissenböck, from the Institute of Pathology and Forensic Veterinary Medicine, worked with a colleague in Germany to carry out a study on 107 shrews from the German region of Saxony-Anhalt.
All the shrews were found dead, and 58 of them were bicoloured shrews, 14 of which carried the Borna virus. No Borna viruses were detected in the other shrew species.
By examining tissue samples, it was discovered that the shrews carried significant amounts of virus in almost all their organs, including the mucus membranes of the oral cavity, the respiratory tract and the skin. This meant that dead skin scales from these animals might be infectious.
“We were surprised to discover significant amounts of viruses in the shrews’ skin,” said Weissenböck, a pathologist and co-author of the study.
“Usually, viruses are found deeper inside a transmitting organism and are excreted in urine and faeces.
“In horses, the virus first affects the olfactory brain area, so we assume that infection occurs via the respiratory tract rather than the digestive tract.”
The bicoloured shrew lives exclusively in Central Europe, the region where Borna disease occurs. This area is mostly within Germany, but also includes the eastern part of Switzerland and Vorarlberg in western Austria.
Virologist Nowotny explains: “The distribution of bicoloured shrews remains fairly constant because the animals stay within their habitats. They do not like to move around.”
In fact, the number of incidents of Borna disease has actually declined in the last few years. There are about 100 cases a year.
A genetic analysis of viruses taken from the shrews and infected horses provided further proof that the bicoloured shrew acts as a “pathogen reservoir”.
The viral strains found in the shrews corresponded exactly with the strains from sick horses in the same region.
This supported the assumption that the virus sub-types developed in bicoloured shrews in a particular region over the course of centuries and did not generally spread beyond that area. These sub-types can then be transmitted to horses.
The transmission mechanism of the Borna disease is remarkable in that the virus is not transmitted from one infected animal to another, so an intermediate host is needed – the bicoloured shrew. The disease is therefore not directly contagious. That is why it never affects whole herds, but individuals.
Lead author Nowotny said: “The best way to avoid transmission is to keep shrews away from the stables. Normal hygiene measures should be sufficient.”
The article, The bicolored white-toothed shrew Crocidura leucodon (HERMANN 1780) is an indigenous host of mammalian Borna disease virus, by Ralf Dürrwald, Jolanta Kolodziejek, Herbert Weissenböck and Norbert Nowotny was published in the April 3 edition of the journal PLoS ONE.