2012 – another bank vole and hantavirus year?

Bank vole (Photo: Ulrike Rosenfeld)

Scientists and forestry agencies have registered signs of a possible increase in hantavirus infections in 2012. As a rule, this infectious disease which is mainly transmitted by bank voles has a mild course with flu-like symptoms; in isolated cases however severe courses of disease with kidney failure requiring dialysis may occur. According to the forestry agencies, beeches and oaks particularly in Baden-Wuerttemberg, Hesse, Bavaria, Northrhine-Westphalia, Lower Saxony and Thuringia produced a particularly large number of fruit („full mast“) last year, which might lead to an increase in the number of bank voles. In the frame of a study in Baden-Wuerttemberg carried out in autumn 2011, the Friedrich-Loeffler-Institut and the Julius Kühn-Institut detected a high population density of bank voles which might lead to an explosion of the population this year. Over the past few months, the Robert Koch-Institut also has registered an increase in human hantavirus infections which are notifiable to the local health authorities pursuant to the Infection Protection Act. In most years, the number of infections decreases after a peak in summer; during the past autumn and winter season however this was not the case. A similar phenomenon was observed at the turn of the year 2009/2010 prior to the hantavirus outbreak year 2010. Again, regions with known hantavirus endemic areas (e.g. the Swabian Alb, the Bavarian Forest, the area around Osnabrück, the Münsterland) are particularly affected.

In Germany, hantavirus infections have been known since the mid-1980s. Since mandatory notification was introduced in 2001, an annual average of 500 cases has been reported. Data collected Europe-wide reveal that a significant increase in the number of infections must be expected at regular intervals of 2 to 3 years. Such increases were observed in Germany in 2007 and 2010 with 1688 and 2017 cases, respectively. These outbreaks were mainly caused by a hantavirus called Puumalavirus which is transmitted to humans by bank voles. The reasons for the strong fluctuations in the number of notified cases are unclear. An association with the population density of bank voles and their hantavirus infection rate is likely. A strong fruit production (mast) of beeches and other relevant tree species plays a crucial role for the population dynamics of rodents. An increased availability of food caused by the beech mast increases the survival probability of bank voles in the winter half-year. In extreme cases, rodents even might reproduce in winter, which might lead to an increase in the population density at the beginning of the subsequent year. These populations will then begin to reproduce further in spring and may be the source of human infections, if their infection rate is sufficiently high.

Hantavirus disease in humans begins with increasing fever, headaches as well as back and abdominal pain. Often, vision disorders occur. In severe cases, a drop in blood pressure and dysfunction of internal organs, especially kidney failure, are observed over the next days. Blood tests show a reduction of the thrombocyte count and an increase in serum creatinine; in urine samples protein and red blood cells are detected frequently. As a rule, the patient will recover completely, once the clinically critical phase has been overcome.

Detection of hantavirus antibodies in the blood is used for specific diagnostics. Analysis of the genetic material of the virus, which is only detectable in the patient’s blood during the first 1 to 3 weeks of disease, is of particular importance. Physicians and diagnostic laboratories are kindly asked to contact the national consiliary laboratory for hantaviruses at the Charité.

Further information and tips for prevention can be found in the information sheet „Informationen zur Vermeidung von Hantavirus-Infektionen“, which is available on the homepages of the Charité, FLI, JKI and RKI.