Mosquitoes are not just an annoying summer problem. They can be carriers of serious infections, some of which are already appearing in Europe under conditions that were previously less common. West Nile fever deserves special attention, since a large part of those infected have no symptoms and may not even realize that they have had the infection. Against the backdrop of higher temperatures, longer warm seasons and the spread of the Asian tiger mosquito in Bulgaria, prevention is becoming increasingly important. What are the real risks, which symptoms should not be underestimated and how can we protect ourselves from diseases transmitted by mosquitoes… Dr. Anelia Zasheva, an infectious diseases specialist at the Military Medical Academy, spoke to FACTI.
- Dr. Zasheva, what are the most dangerous infections that can be transmitted through a mosquito bite, and which of them pose a real risk to people in Bulgaria?
- When we talk about the “most dangerous” infections, it is necessary to distinguish the global burden of diseases from the real risk to Bulgaria. Worldwide, malaria remains the leading cause of mortality among mosquito-borne infections. Yellow fever, severe dengue, which usually develops with repeated illness, Japanese encephalitis and the neuroinvasive form of West Nile infection can be particularly serious and life-threatening. Chikungunya is rarely fatal, but it can cause prolonged and severe joint pain, and Zika infection is particularly dangerous during pregnancy due to the risks to the fetus.
The immediate risk for people in Bulgaria is West Nile infection. Its main vectors are mosquitoes of the genus Culex, and the virus circulates between mosquitoes and birds. Malaria, dengue, chikungunya and Zika are currently mainly associated with travel. The establishment of the Asian tiger mosquito creates the possibility of an imported virus being transmitted locally, but this is a potential risk.
- West Nile fever is among the diseases that are already occurring in our country. Why do about 80% of infected people have no symptoms and how does this make it difficult to detect the infection?
- There is no single reason why one person remains asymptomatic and another becomes ill. The clinical outcome depends on the interaction between the virus and the body – age, immune response, concomitant diseases, individual genetic characteristics, viral strain and probably the amount of virus at the time of infection. In most people, the immune system limits the multiplication of the virus before a clinically apparent disease develops.
According to various epidemiological and clinical studies, about 20% of people develop symptoms such as fever, headache, fatigue, muscle pain, sometimes rash, nausea or enlarged lymph nodes. These symptoms are non-specific and can easily be attributed to another viral infection, especially in the summer season. Therefore, the registered cases do not represent the total spread of the infection. The real circulation is estimated not only by diagnosed patients, but also by examining blood donors, mosquitoes, birds and horses.
- What are the first symptoms that should make us seek medical help after a mosquito bite – especially if we have been abroad or in an area with established circulation of the infection?
- It is not necessary to seek medical attention after every bite. The usual redness, itching and slight swelling are a local reaction to the mosquito's saliva and most often pass without complications.
A medical consultation is necessary if in the following days or weeks a fever, severe headache, severe fatigue, muscle or joint pain, pain behind the eyes, rash, nausea or vomiting occur. Immediate help should be sought in case of confusion, drowsiness, neck stiffness, seizures, sudden muscle weakness or paralysis, difficulty breathing, persistent vomiting, severe abdominal pain, bleeding or fainting.
A particularly important rule is that any fever during or after traveling to an area with malaria should be evaluated urgently - even if the person has taken prophylaxis. The doctor should be informed exactly where, when and for how long the patient has traveled, as well as what prophylaxis he has taken. If dengue is suspected, aspirin and nonsteroidal anti-inflammatory drugs should not be taken arbitrarily until the diagnosis has been ruled out, as they can increase the risk of bleeding.
- How serious can the complications of West Nile infection be and which people are most at risk of severe nervous system damage?
- Approximately one in 150 infected people develops neuroinvasive disease - meningitis, encephalitis or involvement of the anterior horns of the spinal cord, which can resemble poliomyelitis and cause acute flaccid paralysis. In the most severe cases, the respiratory muscles may also be affected.
It is important to note that the severe form is rare compared to all diagnosed cases, but when it develops, it can be life-threatening. Among patients with neuroinvasive disease, the mortality rate in some studies and seasons reaches 20%, and some of those who have recovered may have prolonged weakness, memory and concentration disorders, tremors or other neurological consequences.
The risk increases with age, especially in people over 60-65 years of age. People with a suppressed immune system, transplant patients and patients with oncological diseases, diabetes, chronic kidney disease or arterial hypertension are particularly at risk. However, severe cases have also been recorded in young people. There is still no approved vaccine or specific antiviral treatment.
- The Asian tiger mosquito already has established populations in Bulgaria. How can we recognize it and does its presence mean that there is already a real risk of dengue, chikungunya and Zika in our country?
- The Asian tiger mosquito – Aedes albopictus – is relatively small and dark. It has clearly defined white or silvery rings on its legs and a single light longitudinal line on the upper side of the thorax. Unlike many common mosquitoes, it bites aggressively mainly during the day, most often outdoors. However, a photo alone cannot always reliably distinguish it from other similar species. Observations can also be submitted through the NCCSPB Mosquito Alert project. Its presence is a necessary, but not sufficient condition for the local spread of dengue, chikungunya or Zika. It takes an infected traveler to arrive in Bulgaria while the virus is circulating in their bloodstream; a local mosquito to bite them; the temperature to allow the virus to multiply in the mosquito; and then the same mosquito to bite another person.
Therefore, the risk is possible, but it depends on many factors. The presence of tiger mosquitoes in our country does not mean that they are infected or that these diseases are already endemic. Local outbreaks of dengue and chikungunya in other European countries show that such a chain of transmission is possible. For Zika, Aedes albopictus is considered a less efficient vector compared to Aedes aegypti.
- How climate change – higher temperatures, longer summers and milder winters – change the spread of mosquitoes and the infections they carry?
- Higher temperatures accelerate the development of mosquitoes and, under the right conditions, shorten the time it takes for the virus to become infectious in the mosquito itself. Longer summers extend the transmission season, and milder winters facilitate the survival of eggs and the establishment of invasive species.
Precipitation also matters. Rain creates water habitats, but periods of drought can also increase the risk when people start storing water in open containers. The relationship is not completely linear - extreme heat and prolonged drought can also reduce some populations.
- Could Bulgaria become a more suitable environment for these diseases?
- Bulgaria can become, and to some extent has already become, more suitable for certain mosquitoes and infections. This is evidenced by the establishment of the tiger mosquito and the periodic circulation of the West Nile virus. But climate is not the only factor. International travel, urbanization, water management, vector surveillance and timely control also determine whether an outbreak will occur. Therefore, a more favorable environment does not mean that Bulgaria will inevitably become a tropical endemic zone.
- What should people who travel to countries with widespread malaria, dengue, yellow fever or other infections know? When is a vaccine needed, and when is drug prophylaxis needed?
- It is best to consult with an infectious disease specialist four to eight weeks before departure. Not only the country is evaluated, but also the specific region, season, altitude, duration and mode of travel, as well as the age, presence of pregnancy, concomitant diseases and immune status of the traveler. Even when traveling at the last minute, consultation makes sense.
A vaccine is used for yellow fever, which is recommended for some countries, and for others the certificate is a condition for entry. One dose provides long-lasting, usually lifelong protection, but the vaccine has contraindications and requires individual assessment, especially in immunosuppressed, pregnant women, young children and people over 60 years of age. For travelers to malarial areas, the main specific method is drug prophylaxis. The choice between the different drugs depends on local resistance, contraindications and the route. They are started before the trip and continued during the stay and a certain period after return, which varies for the individual drugs.
There is no drug prophylaxis for dengue. There is a vaccine, but it is not a universal recommendation for every tourist and its use depends on age, data on previous infection, length of stay and current national recommendations.
Neither the vaccine nor drug prophylaxis replaces protection from bites: repellent, covering clothing, nets, air-conditioned rooms and appropriately treated nets or clothing.
- Can we distinguish a simple mosquito bite from a situation in which we are likely to be infected with a virus or parasite? And does it make sense for a person to test their blood prophylactically after a bite?
- No. An infected mosquito does not look different, and the bite itself does not indicate whether the pathogen has been transmitted. Itching, redness and swelling are a reaction to the mosquito's saliva, not to the virus or parasite.
Prophylactic testing after a single bite in a person without symptoms usually has no medical meaning. If done too early, the test may be negative despite the infection having occurred. On the other hand, some antibody tests may cross-react with other similar viruses or indicate an old, rather than a current infection.
Tests are ordered when there is a combination of symptoms, an appropriate incubation period, and actual epidemiological exposure. Exceptions may include special recommendations for pregnant women after possible Zika exposure, measures for blood donation, or specific instructions from health authorities.
- What are the most effective measures to limit mosquitoes around our homes? How important is it to remove stagnant water and why can even small containers become mosquito breeding grounds?
- Removing stagnant water is one of the most effective measures, especially against the tiger mosquito. Once a week, we should empty, wash and scrub or turn over all containers that can hold water: pot bases, buckets, watering cans, children's toys, vases, tires, gutters, covers, birdbaths and pet bowls.
It is important not only to pour out the water, but also to scrub the walls. The tiger mosquito lays desiccation-resistant eggs just above the waterline, and these can remain attached to the container until it is flooded again. The larvae do not need a swamp for development - a small amount of water in a forgotten container is enough.
Rainwater tanks should be tightly covered, pools - properly maintained, and window and door screens - strong. When water cannot be removed, only approved larvicides can be used and exactly as directed. Spraying against adult mosquitoes has a temporary effect and cannot replace the removal of breeding sites.
- What is the most important message to people during the summer season – what should we really be afraid of and what can we do to reduce the risk of infections without becoming unnecessarily anxious?
- We should not be afraid of every bite. For the vast majority of people, it will only cause short-term itching and redness. But we should not underestimate the fact that conditions in Europe are changing and some mosquito-borne infections are now a real, albeit relatively rare, possibility.
The most sensible approach is to be informed without panicking: to use repellent and appropriate clothing, to limit areas with stagnant water and to put nets on windows. If you have a fever or unusual symptoms after traveling, you should seek medical attention and always report where you were and for how long.
It is not the mosquito itself that should scare us, but the ignored fever after a risky trip, the missed warning symptoms, and the lack of basic prevention. Most risks can be minimized with a few consistent and completely doable habits.