Date: Wed 28 Aug 2019
Source: Outbreak News Today [edited]
As of [Mon 26 Aug 2019], about 560 human cases have been reported, much more this time of year than usual and even more than 2015 when 859 people across the country suffered from the illness. Most cases of illness are reported from central Sweden (the Dalarna region, Gavleborg and Orebro), but an increasing number of reports are also starting to come in from other regions, especially in northern Sweden.
Since the number of illness cases is usually highest in September in Sweden, the outbreak is expected to grow further in the coming weeks.
Infections in Sweden are mainly seen in forest and field hares and rodents, but the disease has been reported in several other species, including other mammals, birds, amphibians, insects, and ticks.
Tularemia, or harpest as it is known as in Sweden, is one of the most common native zoonoses in people in Sweden. People are infected mainly through mosquitoes, but also through direct contact with sick or dead animals and by inhalation of, for example, infectious dust.
The number of reported cases of harpest in humans varies from year to year, in recent years there have been reported between about 100 to over 800 cases per year. More than 90 percent of reported human cases have fallen ill in Sweden.
[This outbreak, primarily in central Sweden, continues to increase in size. _Francisella tularensis_ subsp. _tularensis_ (type A) is associated with lagomorphs (rabbits and hares). Wild rodents are also frequently infected, and occurrence of human cases is usually linked to these host species. _F. tularensis_ is transmitted primarily by ticks and biting flies, and is highly virulent for humans and domestic rabbits.
Tularemia is largely confined to the Northern Hemisphere and is not normally found in the tropics or the Southern Hemisphere. _F. tularensis_ subspecies _holarctica_ naturally infects several mammalian wildlife species in northern Europe, in particular, mice, rabbits, hares, beavers, voles, lemmings, and muskrats. The ticks _Dermacentor reticularis_ and _Ixodes ricinus_ are vectors for the bacterium, although previous research has suggested that mosquito bites are the most frequent route of transmission to humans in Sweden.
In addition to vector transmission, tularemia may be spread via contact with infected animals or environmental fomites by inhalation, or by ingestion of the poorly cooked flesh of infected animals or contaminated water.
Tularemia can be transmitted by aerosol, direct contact, ingestion, or arthropods. Inhalation of aerosolized organisms (in the laboratory or as an airborne agent in an act of bioterrorism) can produce a pneumonic form. Direct contact with, or ingestion of, infected carcasses of wild animals (such as cottontail rabbit) can produce the ulceroglandular, oculoglandular, oropharyngeal (local lesion with regional lymphadenitis), or typhoidal form. Immersion in or ingestion of contaminated water can result in infection in aquatic animals. Ticks can maintain infection transstadially [pathogen remains with the vector from one life stage ("stadium") to the next] and transovarially [transmission of a pathogen from an organism (as a tick) to its offspring by infection of eggs in its ovary], making them efficient reservoirs and vectors.
The manifestations of infection in this growing outbreak in parts of Sweden are not stated. If vector borne, the most common form would be ulceroglandular with an ulcer at the introduction site and enlarged lymph nodes in that area. If waterborne, oropharyngeal would be most common. Among flying vectors, deer flies are often brought up but mosquitoes are also relevant as shown in this Swedish publication:
Thelaus J, Andersson A, Broman T, et al: _Francisella tularensis_ subspecies _holarctica_ occurs in Swedish mosquitoes, persists through the developmental stages of laboratory-infected mosquitoes and is transmissible during blood feeding. Microb Ecol. 2014; 67(1): 96-107; <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3907667/
In Sweden, mosquitoes are considered the major vectors of the bacterium _Francisella tularensis_ subsp. _holarctica_, which causes tularemia. The aim of this study was to investigate whether mosquitoes acquire the bacterium as aquatic larvae and transmit the disease as adults. Mosquitoes sampled in a Swedish area where tularemia is endemic (Orebro) were positive for the presence of _F. tularensis_ deoxyribonucleic acid throughout the summer. Presence of the clinically relevant _F. tularensis_ subsp. _holarctica_ was confirmed in 11 out of the 14 mosquito species sampled. Experiments performed using laboratory-reared _Aedes aegypti_ confirmed that _F. tularensis_ subsp. _holarctica_ was transstadially maintained from orally infected larvae to adult mosquitoes and that 25% of the adults exposed as larvae were positive for the presence of _F. tularensis_-specific sequences for at least 2 weeks. In addition, we found that _F. tularensis_ subsp. _holarctica_ was transmitted to 58% of the adult mosquitoes feeding on diseased mice. In a small-scale in vivo transmission experiment with _F. tularensis_ subsp. _holarctica_-positive adult mosquitoes and susceptible mice, none of the animals developed tularemia. However, we confirmed that there was transmission of the bacterium to blood vials by mosquitoes that had been exposed to the bacterium in the larval stage. Taken together, these results provide evidence that mosquitoes play a role in disease transmission in part of Sweden where tularemia recurs." - ProMED Mod.LL]
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