Abstract
Tularemia is an emerging zoonosis caused by the Gram-negative bacterium Francisella tularensis, which is able to infect a range of animal species and humans. Human infections occur through contact with animals, ingestion of food, insect bites or exposure to aerosols or water, and may lead to serious disease. F. tularensis may persist in aquatic reservoirs. In the Netherland, no human tularemia cases were notified for over 60 years until in 2011 an endemic patient was diagnosed, followed by 17 cases in the 6 years since. The re-emergence of tularemia could be caused by changes in reservoirs or transmission routes. We performed environmental surveillance of F. tularensis in surface waters in the Netherlands by using two approaches. Firstly, 339 samples were obtained from routine monitoring -not related to tularemia- at 127 locations that were visited between 1 and 8 times in 2015 and 2016. Secondly, sampling efforts were performed after reported tularemia cases (n = 8) among hares or humans in the period 2013–2017. F. tularensis DNA was detected at 17% of randomly selected surface water locations from different parts of the country. At most of these positive locations, DNA was not detected at each time point and levels were very low, but at two locations contamination was clearly higher. From 7 out of the 8 investigated tularemia cases, F. tularensis DNA was detected in at least one surface water sample collected after the case. By using a protocol tailored for amplification of low amounts of environmental DNA, 10 gene targets were sequenced. Presence of F. tularensis subspecies holarctica was confirmed in 4 samples, and in 2 of these, clades B.12 and B.6 were identified. This study shows that for tularemia, information regarding the spatial and temporal distribution of its causative agent could be derived from environmental surveillance of surface waters. Tracking a particular strain in the environment as source of infection is feasible and could be substantiated by genotyping, which was achieved in water samples with only low levels of F. tularemia present. These techniques allow the establishment of a link between tularemia cases and environmental samples without the need for cultivation.
Highlights
Tularemia is a zoonosis with a human, animal and environmental component
Two different approaches were used for environmental surveillance for F. tularensis in the Netherlands
For the first approach, sampling locations were selected independent of recent tularemia cases
Summary
Tularemia is a zoonosis with a human, animal and environmental component. The disease is caused by the Gram-negative bacterium Francisella tularensis, which is able to infect different species of animals besides humans. Disease in humans and animals is mostly caused by subspecies tularensis (type A) and holarctica (type B) (Hestvik et al, 2015). The infectious dose is very low and infection can cause many different symptoms, ranging from fever and skin ulcers to life-threatening pneumonia. Human infections occur through direct contact with infected animals, ingestion of food or water, insect bites or exposure to aerosols or water (WHO, 2007). Disease incidence due to infections with subspecies holarctica has been shown to be higher near lakes and rivers (Desvars et al, 2015). F. tularensis has been detected in various types of surface waters and sediments (Petersen et al, 2009; Broman et al, 2011; Janse et al, 2017) where the bacterium can be hosted by free-living protozoa and may reside in biofilms (Abd et al, 2003; Sinclair et al, 2008; van Hoek, 2013)
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