Cross-Sectional Survey of the Seroprevalence of Puumala Hantavirus in Austria

Introduction

H antaviruses belong to the family Bunyaviridae and can cause two significant human diseases: hemorrhagic fever with renal syndrome (HFRS) in Asia and Europe, and hantavirus cardiopulmonary syndrome in the Americas (Klempa 2009). The main mode of transmission is via inhalation of aerosols from excreta of persistently infected rodents and insectivores.

In Europe the predominant serotype is Puumala (PUU) hantavirus, which causes nephropathia epidemica, the mildest form of HFRS, although severe and lethal infections have also been reported (Hoier et al. 2006). The incidence of human hantavirus infections correlates with the population size of the bank vole (Myodes glareolus), the main reservoir host for PUU hantavirus (Klempa 2009).

In Austria approximately 10–30 cases are reported annually, although >70 cases were reported in 2002 and 2007 (Heinz 2011). The increased hantavirus activity seen in the two epidemic years has been related to a periodically high abundance of bank voles following mast years with high production of oak and beech seeds (Klempa 2009; Heinz 2011). Based on the mode of transmission and circulation of hantavirus in nature, increases in average temperatures in Europe resulting from global climate change may lead to significant increases in PUU hantavirus-associated nephropathia epidemica outbreaks via its impact on reservoir host populations, at least in western and central Europe (Klempa 2009).

Further, environmental changes such as changing agricultural practices, human population movements, deforestation, and irrigation projects may affect the geographic distribution and abundance of the rodent reservoir, and thus hantavirus epidemiology, and might create conditions that increase the risk of human exposure to hantaviruses (Klempa 2009).

Likewise, a risk assessment of vector-borne and zoonotic diseases in Europe by the European Centre of Disease Prevention and Control lists hantavirus infections among 10 vector-borne and zoonotic diseases that have the greatest potential to affect European citizens (Senior 2008). In line with this, hantavirus infection is suspected to be the prevailing cause of renal failure associated with infectious diseases in central Europe (Faulde, et al. 2000).

Farmers, forestry workers, and particularly members of the armed forces, are occupationally exposed to hantaviruses, and are traditionally considered as high-risk groups for hantavirus infection (Hukic et al. 2010). Moreover, foreign military assignments in areas with high endemicity of these viruses might increase the risk of exposure for military personnel.

Surveillance is a key element in controlling emerging and re-emerging vector-borne and zoonotic diseases, but asymptomatic and nonspecific mild infections may escape the surveillance system and result in underestimation of the true incidence.

The aim of our study was to document the seroprevalence of PUU hantavirus in Austria, and to assess the risk of occupational exposure to this virus in military personnel.

Materials and Methods

Based on the assumption that hantavirus antibodies are detectable even decades after infection (Settergren et al. 1991), in 2009 we conducted an exploratory national cross-sectional seroprevalence survey on 526 healthy Austrians volunteering for international military missions, including 222 professional soldiers and 304 civilians. The study was approved by the Research Ethics Committee of the Austrian Armed Forces.

Before taking part in a mission all volunteers, military personnel as well as civilians, undergo a medical check-up, including routine laboratory investigations, at the Military Hospital Vienna. A commercial ELISA (Hantavirus Puumala IgG; Institute Virion/Serion GmbH, Wurzburg, Germany) was used to screen serum samples for hantavirus-specific antibodies in accordance with the manufacturer's protocol. The ELISA detects antibodies directed against the nucleocapsid protein of hantavirus serotype Puumala with >99% sensitivity and specificity, according to the manufacturer.

All participants answered an epidemiologic questionnaire on demographic characteristics, occupational contact with animals, outdoor behavior, and previous international military operations.

Results

Between April and June 2009, a total of 526 individuals from all nine federal states of Austria were enrolled in the study. The group comprised 222 (42.2%) professional soldiers and 304 (57.8%) civilians 18–60 years old (median age 26.0 years); 514 (97.7%) were male and 12 (2.3%) were female. Information on previous military deployments abroad and demographic characteristics of the seven subjects testing seropositive are outlined in Table 1.

Overall 196 (37.3%) persons declared at least one previous assignment abroad: 125 (63.8%) in Kosovo, 34 (17.3%) in Bosnia-Herzegovina, 55 (28.1%) in the Golan Heights, and 20 (10.2%) in Cyprus.

Among the 526 individuals, 7 (1.3%) tested positive on serologic screening. All were male. Seroprevalence did not increase with age: seropositive individuals were 19, 22, 24, 24, 26, 38, and 60 years old. All 7 (100%) had regular outdoor activities, in contrast to 421 (80%) individuals in the group overall (p=0.354). None of the seropositive individuals declared occupational contact with animals.

There were also no significant differences between professional soldiers and civilians in seroprevalence of PUU hantavirus (p=1.0), or between individuals with and without previous military employment abroad (p=0.715). Of the seven seropositive individuals, 3 (42.9%) were professional soldiers, and 3 (42.9%) had been on a mission abroad previously. One of the professional soldiers had previous employment in Bosnia-Herzegovina and the Golan Heights; another had two previous deployments in Cyprus. Among the four seropositive civilians, one had been on a mission abroad in the Golan Heights.

All seven seropositive individuals had permanent residency in Austrian federal states where nephropathia epidemica had previously been reported (Heinz 2011): four in Styria, two in Upper Austria, and one in Burgenland. None of the seven recalled a feverish illness with apparent involvement of the kidneys.

Discussion

The 1.3% seroprevalence rate found in the present study is comparable to that seen in other central European countries (Sanfeliu et al. 2011). In addition, a serosurvey of internal medicine patients in Austria in 1999 revealed closely similar results with an overall hantavirus seroprevalence of 1.2% (Aberle et al. 1999). Thus our study found no change in PUU hantavirus seroprevalence in Austria during the past decade.

Bosnia-Herzegovina has been identified as a European hotspot for hantavirus seroprevalence, with seropositive rates up to 16.1% in former soldiers from endemic regions (Hukic et al. 2010), thus the finding that previous international military operations were not associated with a higher risk of exposure to PUU virus was somewhat unexpected. However, only 55 out of the 526 volunteers examined, and only 1 out of 7 seropositive individuals, had been in Bosnia previously. These very small numbers do not allow one to draw any conclusions and there were no statistically significant differences.

Further, the finding that military personnel and civilians were similarly affected is in contrast to previous studies (Clement et al. 1996; Wells et al. 1997), but is in line with the results of a recent study in neighboring Switzerland, where rates in military personnel and civilians were equally low (Schultze et al. 2007). One possible explanation may be the fact that both the civilians and the military personal examined in the present screening study had participated in the same missions previously.

In the present study, none of the seropositive individuals recalled an illness compatible to nephropathia epidemica. This may reflect the fact that in contrast to infections with other serotypes, PUU hantavirus infections in Europe are often asymptomatic or nonspecific, with a presumed ratio of 10:1 between subclinical and clinical infections (Settergren et al. 1988).

The limitations of the present study should be noted. First, the overall percentage of seropositivity of 1.3% is too low to obtain definitive conclusions for the variables surveyed. Second, screening was performed solely for PUU hantavirus, which is the only serotype detected in Austria so far, and did not include other serotypes like Dobrava-Belgrade virus (DOBV). Taking into account that DOBV is present in some of the countries where volunteers were employed, we cannot exclude possible cross-reactions from antigenically similar hantavirus serotypes, as demonstrated by Sanfeliu and associates (Sanfeliu et al. 2011). However, Hukic and colleagues have shown that in endemic regions in Bosnia-Herzegovina the prevalence of DOBV (1.1%) is much lower than the prevalence of PUU hantavirus (5.1%) (Hukic et al. 2010). The aim of the present study was to investigate its seroprevalence in Austria, and to identify possible risk factors for hantavirus exposure, independently of possible cross-reactions with other hantavirus serotypes. Therefore we do not consider testing solely for PUU hantavirus to be a major limitation.

In conclusion, our serosurvey did not reveal any change in PUU hantavirus seroprevalence in Austria during the years 1999–2009. With the limitation that the overall number of seropositive subjects is too low to obtain definitive conclusions, no difference in the risk of exposure to PUU hantavirus could be detected after comparing military personnel with the general population.