In 2018, Outbreak Thursday reported on a tularemia outbreak connected to mice-contaminated wine at a German vineyard. This week, we are covering an outbreak in Sweden that is causing increasing concern among public health officials due to high incidence compared to previous years. The outbreak has reached nearly 800 cases so far in 2019 and is poised to surpass 2015 as the highest yearly total in more than 50 years. We will look at the epidemiology for the current outbreak as well as longer-term trends, including the possibility of elevated incidence in future years.

The Disease

Tularemia, commonly known as rabbit fever, is caused by the Francisella tularensis bacterium and occurs predominantly in the Northern Hemisphere, particularly temperate and cooler regions, including Scandinavia. Symptoms can vary, depending on how the bacteria are introduced into the body, but usually include fever and swollen lymph nodes. Diagnosis is difficult due to the relative rarity of cases and nonspecificity of the disease’s presentation, but blood tests and culture can be used for confirmation. Most patients treated with antibiotics recover fully, but without treatment, the disease can be life-threatening. Outbreaks in humans typically result from infections via skin contact with an infected animal (eg, rabbits, cats, rodents) or tick bites; however, infections also result from deer fly bites, inhalation of contaminated aerosols such as dust, consumption or use of contaminated water (or wine), and mosquito bites. Tularemia is not known to exhibit human-to-human transmission.

Sweden’s Outbreak

Sweden began reporting elevated tularemia incidence in late July and August of this year, primarily in the northern and central areas of the country. The Swedish National Veterinary Institute reported that the infection had also been detected in dead hares in several counties (or länen). By early August, it became clear to authorities that July had an unusually high number of cases compared to what would be expected based on historical trends. Sweden reported 591 domestically acquired human cases in August, more than 21 times the incidence compared to the same period in 2018. The highest yearly total in recent years was in 2015 (859 total cases), and the current outbreak has far exceeded the 2015 figures at this point in the year (344 cases through August 2015). As of September 9, incidence had slowed to 100 new cases per week; however, this is still unusually high compared to what is expected for this time of year. According to Swedish authorities, there have been nearly 800 tularemia cases in 2019, as of September 9. 

The majority of cases in the current outbreak have been reported in Dalarna (252 cases) and Gävleborg (147 cases) counties (länen). Dead hares infected with the disease have been reported in Dalarna, Norrbotten, and Västra Götaland, indicating that there may be additional areas at risk for zoonotic transmission. The seasonal peak for tularemia in Sweden is typically around August and September, so continued elevated incidence can be expected over at least the next month. It is unknown if this outbreak will surpass the 859 cases reported in 2015, Sweden’s highest incidence since its record in 1967 (2,700 cases), but it is certainly well within reach. Authorities have not indicated the primary mode or vector of transmission for the current outbreak. 

The European Centre for Disease Prevention and Control (ECDC) recommends that people in affected areas avoid drinking untreated water, prevent mosquito and tick bites (eg, mosquito repellant, protective clothing), and avoid contact with sick or dead animals. Individuals who handle wild animals should wear protective gloves, and all game meat should be thoroughly cooked to reduce the risk of infection. Additionally, ECDC notes the risk of transmission via organ transplants and cautions clinicians to consider tularemia when evaluating potential organ donors.


Potential Link to Climate Change

A review of nearly 3 decades of tularemia surveillance in Sweden revealed an overall increase in tularemia incidence and the number of local outbreaks, particularly since the mid-1990s. Tularemia in Sweden also shows signs of seasonality, with cases increasing in summer and early fall as people have greater exposure to contaminated water and mosquito activity. Interestingly, among the myriad of potential exposure and transmission routes, mosquitoes are the primary vectors for F. tularensis in Sweden. Sweden, like many countries, is also feeling the effects of climate change, exhibited by an increase in annual temperature, particularly in northern Sweden, which overlaps with tularemia-endemic areas. Previous Outbreak Thursday posts have remarked upon the connection between rising temperatures due to climate change and increases in the incidence of mosquito-borne diseases. Research also indicates that increasing temperatures could contribute to the spread of mosquitoes to new geographic areas or could lengthen the number of days per year during which such diseases could be transmitted in Europe. Models of tularemia transmission in Sweden are being assessed to better understand the connections between these factors, but additional research is needed to elucidate any potential trends or patterns.

Conclusion

Because tularemia infection can result from multiple sources beyond just arthropod vectors, climate change and mosquito population dynamics are only a fraction of the many factors influencing its incidence. That being said, Sweden looks poised to record the 2 highest yearly tularemia incidence in the past half century within a span of only 4 years. It is certainly possible that these events are signaling an emerging trend.

It is unclear how much of a role climate change is playing in the increased incidence of tularemia in Sweden, but it is clear that annual incidence is increasing. As we continue to monitor the current outbreak and future tularemia seasons, public health authorities must remain vigilant and continue adapting and implementing effective communication and intervention efforts to mitigate these risks.

Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly to allow others to learn from these experiences in order to improve global outbreak response capabilities.    

Photo courtesy of CDC/Dr. J.M. Clinton (https://phil.cdc.gov/Details.aspx?pid=6467)

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