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Disease incidence and rodent density may suggest infectious reservoir

Rodents are often carriers of Ljungan virus. It has been determined that the numbers of animal present in nature covaries with the number of patients presenting with certain diseases such as intrauterine fetal deaths. It should be noted that Ljungan virus causes this same medical condition in infected animals.

Based on these facts, several scientists investigating the Ljungan virus are working with the following hypothesis:

When the number of rodents in nature increases, humans exposure to the Ljungan virus is increased. The Ljungan virus is transmitted from animals to humans, causing several medical conditions in humans as well.

Rodent density varies significantly between different years in the north of Sweden. Below is an example of an association between the density of small rodents in the north of Sweden and the number of cases of intrauterine fetal death in humans in the same region.

Unique data base of rodent abundance in Sweden
Rodent populations in the north fluctuate on a three- to four-year cycle of abundance, whereas small rodent populations in southernmost Sweden are non-cyclic. During peak abundance, voles may be 300 times more abundant than immediately after population declines in northern Sweden.

In Sweden long-term records on small rodent abundance are available as part of the ongoing National Environmental Monitoring Programme run by the Swedish Environmental Protection Agency. The data is part of a unique data base on rodent abundance in Sweden – the longest continuous time series of rodent population fluctuations in the world.

For additional details, please click on the link below.
National patient registry of the Swedish National Board of Health and Welfare – a gold mine of epidemiological information
Disease incidence can be estimated based on the National patient registry of the Swedish National Board of Health and Welfare covering all hospitals in Sweden. Information on age, sex, previous hospitalizations, all diagnosis (causative diagnoses and underlying diagnosis), geographic area of residence, and dates for hospitalization is available for computer analysis. The diagnosis is based on the International Classification of Diseases (ICD). Patient identity is protected in the use of these data bases.

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