Correlation between number of human cases of myiasis caused by the reindeer warble fly (Hypoderma tarandi) and weather conditions during summer in northern Scandinavia

Abstract

The reindeer warble fly (<i>Hypoderma tarandi</i>) causes myiasis in reindeer and caribou (<i>Rangifer tarandus and subspecies</i>) and aberrant hosts such as humans. Of 22 human cases reported 1982–2016, 16 were children and 18 were residents in or visited northern parts of Norway or Sweden. Of a series of 39 new human cases in Norway 2011–2016 (reported 2017), 32 were children, 32 were resident in Finnmark (northernmost county of Norway), one was a visitor to Finnmark (most likely infested there), 17 were infested in 2012 and 10 in 2013. There are to our knowledge no human cases reported from Finland, although the <i>H. tarandi </i>infestation level in reindeer is high in the reindeer husbandry area of the country and many people live there. Consequently, the differences in geographical distribution and in distribution between children and adults, and between years, relative to where <i>Rangifer</i> and <i>H. tarandi</i> live, strongly indicate the presence of important identifiable drivers affecting the number and distribution. Meteorological data for June–August 2011–2016 from five meteorological stations in Finnmark applied in statistical analyses demonstrated that low proportion of days suitable for<i> H. tarandi </i>flying in July–August, combined with low proportion of days with rain in August, resulted in high number of cases in the particularly cold summer 2012. In contrast, in the particularly warm summer 2013, high proportion of days suitable for <i>H. tarandi</i> flying during July–August gave a high number of cases, and particularly high mean temperature in June tended in the same direction.