Norwegian Red Foxes Have More Trichina, But Less Scabies Than Previously

The most frequently-found Trichinella species in the red fox, T. nativa, is strongly resistant to freezing and thawing, making it ideally suited to survival in the Norwegian climate. Trichinella nativa is only slightly infectious in pigs, which means that the parasite represents little danger for people through the consumption of pork. The doctorate also shows that fox scabies is less common now than it was in the 1990's and that the percentage of healthy foxes with antibodies to the parasite has increased. This finding indicates that the scabies mite and its fox host have to a certain extent adapted to each other.

For her doctorate, Rebecca Davidson investigated the incidence and distribution of scabies mites (Sarcoptes scabiei) and different species of trichina worms (Trichinella spp.) in red foxes in Norway by looking at blood antibodies levels combined with direct examination for the parasites. Trichina live for a short period in the intestine of both animals and man. Here they produce larvae that establish a chronic infection of the body musculature and are called muscle trichina. Infection is spread by eating meat from affected animals.

Once in people, the parasite can cause serious illness. People are usually infected by eating pork from trichina-containing pigs. In Norway, however, all swine carcases are checked and to find trichina in Norwegian pork is extremely rare, so the risk to humans is minimal. Inspection for trichina is, however, very costly, and authorities are considering whether other and equally good ways of monitoring the incidence of trichina exist.

EU-regulations allow for the establishment of trichina-free regions for pork production. However, trichina may still exist in local wildlife, and there is a need to obtain information on the incidence of this parasite in Norway. The level of trichina in red foxes is a good indicator of the general level of trichina in nature and this is the basis for the study of trichina occurrence in red foxes that were shot during the mid-1990s and during the period 2002 - 2005.

Trichina found throughout Norway

Davidson found trichina in red foxes from all over Norway. The incidence was higher in south-eastern Norway compared to the rest of the country, and fell as one went northward. These differences may be related to geographical differences in red fox density. Two species of trichina were found in the red fox - T. nativa and T. britovi.

The species of trichina that most readily infect pigs, T. spiralis and T. pseudospiralis, were not found. The species that were found, T. nativa and T. britovi, have only a limited to moderate ability to infect pigs and are therefore of limited relevance in regard to Norwegian swine herds. Additionally, modern pork production has innate, effective barriers against infection from wild animals. Nonetheless, the countrywide distribution of trichina in red foxes will make it difficult to declare trichina-free regions in this country.

Resistant to freezing and thawing

Studies have shown that T. nativa is the dominant trichina species of red foxes in Norway. It was already known that this species of trichina tolerates prolonged freezing. The ability of muscle trichina to survive in an animal after its death will also influence its likelihood of transmission to scavenging animals. Davidson showed, for the first time, that T. nativa readily survives repeated freezing and thawing, which is a condition for survival in carcases during the spring. This, together with the known ability to survive prolonged freezing, shows that T. nativa is well-adapted to our cold climate. The Norwegian red fox population appears to be increasing, and it is logical therefore to expect that the incidence of trichina will increase with the increasing concentration of the fox population.

Hard times for scabies

At the end of the 1970's and during the 1980's the red fox population was severely reduced from scabies. The scabies mite destroys the skin of the fox, causing severe itching and hair loss, and severely affected animals lose weight and die. Scabies is still an important disease of red fox, however, the red fox population appears to be increasing. It was therefore desirable to see how the parasite affects the red fox population today, compared with a decade ago. The number of animals found with overt scabies was compared to the number of animals with antibodies to the parasite. During the 1990's, few foxes had antibodies without also showing illness, while in the 2000's, the number is much higher. These findings indicate that infection with scabies mite is now less likely to cause overt disease in foxes, and/or that the fox has developed resistance to the parasite.

Rebecca K. Davidson defended her Ph. D. thesis, entitled "Trichinella and Sarcoptes infections in red foxes (Vulpes vulpes) in Norway", at the Norwegian School of Veterinary Science on April 2, 2009.