Tropical diseases are a group of infectious diseases caused by parasites, mosquitoes, worms, bacteria, or viruses. In order to establish and spread, they need warm and humid conditions and are mainly spread in tropical and subtropical parts of the world. But global warming means that today it is on average 1.5 degrees warmer and 10 per cent wetter during the summer months in Sweden, compared to the 1990s. This means that some tropical diseases have taken hold in Europe and are coming closer and closer to us in Sweden.
Invasive mosquitoes are spreading northwards
Flaviviridae is a family of RNA viruses spread by mosquitoes and ticks and includes several different diseases. The invasive mosquitoes that are spreading further into northern and western Europe are bringing diseases that were previously only found in tropical areas. For example, the Asian tiger mosquito, which can carry dengue, Zika, and chikungunya viruses, has now established itself in 337 regions in 13 European countries. In 2022, 71 people were infected with dengue in Europe and globally, around 400 million people are infected with the disease every year. Although the symptoms of the disease are generally mild the first time you get sick, they put a heavy burden on the healthcare system.
"In Skåne, we have rapid tests for dengue fever for people who have been infected abroad. We have not had any domestic cases of these diseases and the Asian tiger mosquito is not yet present in Sweden, but we diagnosed 13 people last year as a result of travelling in Asia, South America, and Africa," says Gülsen Özkaya Sahin, senior consultant at Clinical Microbiology, Infection Prevention and Control at Laboratory Medicine in Region Skåne and associate professor of virology at Lund University.
The mosquito, Culex modestus, which causes West Nile fever is also spreading rapidly in Europe. In 2023, European countries reported 965 cases, including 73 deaths, to the European Centre for Disease Prevention and Control. The number of reported cases is lower than in 2022, but the number of affected regions is the highest since the peak in 2018, indicating a wide geographical circulation of the virus. According to mosquito researcher Anders Lindström, the mosquito has been present in Sweden since 2016. We have not yet had a case of the disease, but Gülsen Özkaya Sahin believes that, with regard to the ongoing global warming, it is only a matter of time before we have our first case in Sweden.
"Unlike our common mosquitoes, which thrive in the shade and don't like the sun and wind, the Nile fever mosquito flies all day and night and is quite cheeky."
The virus has its reservoir in birds and is spread by blood-sucking female mosquitoes. Humans and horses can also be infected, but due to the very short duration and low level of the virus, it cannot spread further. This is why humans and horses are called 'dead-end hosts'. But there are exceptions. The virus can spread between humans via blood transfusion, organ transplantation, and during pregnancy and breastfeeding.
"In 80 percent of cases it is a silent disease, but one-fifth develop flu-like symptoms with or without a skin rash. In less than one per cent, however, the disease is severe with signs of meningitis. Patients with neuroinvasive diseases also have a higher mortality rate."
During the period July-November each year, there are regional outbreaks of West Nile fever in different countries in Europe. Blood centres in Sweden are currently managing this with a four-week travel cure. As there are cases of West Nile fever as high in Europe as in Germany, Region Skåne has now prepared a contingency plan with screening of blood donors.
"It's not a question of if, but when we get our first case of West Nile fever. Suspected samples of the disease are currently being sent from all over Sweden to the Public Health Agency for testing. If it turns out to be a domestic case, it will be the signal that we should start screening all blood donors in Skåne. Everything is prepared and it will only take a few days before we can start. On average, this will mean testing 250 blood samples per day in Region Skåne."
At present, there is no vaccine and no specific treatment. Gülsen Özkaya Sahin, who is also the chair of the Scientific Affairs Subcommittee and a board member of the European Society of Clinical Microbiology and Infectious Diseases (ESCMID), is therefore planning to work with other stakeholders to launch a research project on West Nile fever to develop screening tools:
"The first step is to develop a diagnostic test to detect those already infected. The next step for the future is to try to develop a vaccine."
NTM Infections
One group of infections—so-called NTM (non-tuberculous mycobacteria) infections—is caused by mycobacteria that occur naturally in soil and water, among other things. There is evidence that higher temperatures and increased humidity make the bacteria thrive and multiply more easily. Globally, NTM infections are a major and growing problem, including in Sweden, but it is unclear how many people are infected.
"In particular, human-to-human lung infections are on the rise and are difficult to treat because the bacteria are highly resistant. As a result, mortality rates have also increased, but more research is needed to understand the complex relationship between climate change and NTMs," says Gabriela Godaly, Professor of Clinical Microbiology at Lund University.
Vibrio infections
As water gets warmer, bacteria multiply faster. With more rainfall and heavy downpours, there is also a risk of bacteria spreading when sewage systems overflow and end up in our waterways. Vibrio bacteria, which are mainly found in brackish water on the east coast of Sweden, grow when the water is warmer than 20 degrees. They enter the body through wounds or other skin lesions and can cause everything from mild skin infections to severe cases of swim fever.
“Vibrio infections, "swimmer's fever”, is on the rise, especially in Blekinge, and occasional cases have also been seen along the coasts of Skåne during the hot summers when the water temperature rises," says Kristian Riesbeck, Professor of Clinical Bacteriology at Lund University and Chief Physician at Labmedicin Skåne.
E. coli and Klebsiella are also two common groups of bacteria that are naturally found in lakes and oceans and also in the human and animal intestinal tract. Klebsiella bacteria, which can cause diarrhea and urinary tract infections, are often resistant to several types of antibiotics, making treatment of the infections they cause more difficult.
"Wastewater containing residues from people treated with antibiotics finds its way into our waterways via sewage treatment plants. These antibiotic residues, mainly from synthetic antimicrobials, end up in the water and accumulate in the bottom sludge. It can take up to five years for the environment to break down these antibiotics. As a result, the bacteria naturally present in our waters may acquire resistance to antibiotics."
...and tick-borne diseases
During the 18th and 19th centuries, malaria was present in Sweden, but it was eradicated thanks to improved living conditions, together with the introduction of quinine as a medical treatment. The last case of indigenous malaria in Sweden was reported in 1933.
"Malaria mosquitoes still exist in Sweden, even though the disease has been eradicated. However, it has long been discussed that the risk of malaria spreading further north in Europe is imminent now that the climate is getting warmer," says Kristina Persson, a researcher at Lund University and senior physician at the Department of Clinical Chemistry and Pharmacology in Region Skåne.
More acute are the tick-borne diseases that are increasing throughout Sweden. Milder winters and longer summers are increasing the survival of ticks, and they are both increasing in number and spreading further up the country, including in the mountains.
"In the past, researchers had to go into the forest to find and study ticks, but now it is enough to go to an ordinary city park."
The tick-borne parasite Babesia, which causes the disease babesiosis, is something Kristina Persson has been studying. Like the malaria parasite, it causes anaemia in its host by eating the red blood cells, leading to similar symptoms to malaria with fever and fatigue. The disease has been relatively unknown until now, but new studies show that more people in Skåne than previously known have antibodies to the disease, so they must have been exposed to the infection at some point. The disease is probably underdiagnosed because the symptoms are so vague.
A healthy person will usually experience flu-like symptoms at most, but as with all tick-borne diseases, a person with a weakened immune system or someone without a spleen can become very seriously ill.
The aggressive taiga tick and migratory bird tick species
The taiga tick, which has now established itself in Sweden, can spread a more serious variant of TBE that has not been found in the country before. The aggressive migratory bird ticks, belonging to the hyalomma tick species, have also been found in a few isolated cases in Sweden. It is twice the size of the common tick, fast and moves like a spider. It is known to spread Crimean-Congo haemorrhagic fever, a serious disease with a mortality rate of up to 40 percent.
"This variant is not normally found in Sweden, but the occasional specimens that have been found have probably arrived with migratory birds. So far, it is difficult for it to survive in winter here, but in southern Europe, cases of haemorrhagic fever are increasing."
It is not easy for healthcare professionals to correctly diagnose the diseases caused by ticks, as many of the symptoms are similar. The disease panorama has changed with climate change and over the last 20 years new diseases have emerged as more tick species have become established in Sweden.
"There is a vaccine against TBE that also bites against the more serious variant, but for other diseases it is important to avoid being bitten. Wear full-coverage clothing in the woods and fields, tuck your trousers into your socks and wear boots. If you do find a tick, remove it as quickly as you can. TBE is spread immediately at the bite, but for the other tick-borne diseases it usually takes a couple of hours before the infection is transmitted," concludes Kristina Persson.