Filariasis: Description
The term filariasis refers to a group of diseases caused by small, parasitic nematodes (filariae) that are transmitted to humans through the bite of infected mosquitoes or horseflies. From the blood, the worms migrate to different target tissues, depending on the worm species, where they multiply. Filarioses are divided into three groups:
- Lymphatic filariasis: The worms live especially in the lymphatic vessels.
- Serous filariasis: the worms colonize the abdomen or chest.
Filariasis occurs predominantly in tropical countries – mainly in tropical Africa, Southeast Asia, South America, Central America and the Caribbean. In other countries such as Germany, the infections can be introduced by travelers. It is estimated that about 200 million people worldwide are infected with filariae.
Life cycle of filariae
If an infected human is stung by a blood-sucking insect, the insect can ingest microfilariae while drinking. In the insect, the microfilariae develop into infective larvae, which can then re-enter a human body during the next blood meal.
Since the parasites reproduce in humans, they are the primary host. Mosquitoes and horseflies, on the other hand, are secondary hosts because they are only necessary for the transmission of the parasites to humans.
Lymphatic filariasis is the most common form of filariasis, with about 120 million people infected worldwide. It can be caused by three different filarial species:
- Wuchereria bancrofti (responsible for about 90 percent of cases, found in Africa and Asia)
- Brugia malayi (mainly in South and Southeast Asia)
- Brugia timori (mainly in southeastern Indonesia)
The worms clog the vessels and constantly trigger new local inflammatory reactions. This disrupts the lymphatic drainage, causing an increasing swelling of the affected part of the body to develop over time.
It takes one to two years after infection for the worms to become fully grown and sexually mature and produce microfilariae. Therefore, the infection is often discovered very late or not at all. As elephantiasis, the disease does not become apparent for months to years without adequate medical treatment.
Subcutaneous filariasis
Subcutaneous filariasis is divided into two major syndromes:
- Loa loa filariasis
- Onchocerciasis (river blindness)
Loa loa filariasis
The disease is transmitted by horseflies of the genus Chrysops. These live particularly in forested areas (preferably on rubber tree plantations), are diurnal and are attracted by human movements and wood fires. Especially during the rainy season, one should protect oneself from this type of horsefly.
The parasites live and move under the skin (at a speed of about one centimeter per minute). Sometimes you can even see the worms through the thin skin on your fingers or breasts. Or they migrate into the conjunctiva of the eyes, where they are then also clearly visible. Colloquially, they are therefore also called “African eye worm”.
Onchocerciasis (river blindness)
After the bite of an infected blackfly, the larvae of the onchocerciasis pathogen enter the subcutaneous tissue. There they develop into the adult worms, which mate and produce microfilariae. These remain in the tissue under the skin, as in Loa Loa, where they cause inflammatory reactions. Also possible is an infestation of the cornea in the eyes, which leads to blindness if left untreated.
Serous filariasis
The parasite can be transmitted by various mosquito species. The hatching worms settle in the pleural cavity (between the lung and pleura), in the pericardium or in the abdominal cavity. There they mate and produce microfilariae, which are absorbed into the insect from the blood of the infected person when the mosquito bites again.
Filariasis: symptoms
As a rule, Europeans are only at risk of infection during longer trips to the tropics. If corresponding symptoms occur, the patient should always inform the doctor about past travel activities.
Lymphatic filariasis: symptoms
In lymphatic filariasis, symptoms appear no earlier than three months after infection. Some people show few symptoms at the beginning, while others complain of acute symptoms. Possible early signs of lymphatic filariasis include:
- inflammation and swelling of the lymph nodes
- increased number of certain immune cells in the blood (eosinophilic granulocytes)
The adult worms obstruct the lymphatic passages and cause recurrent inflammation of the lymphatic vessels and nodes (lymphangitis, lymphadenitis). The resulting lymphatic congestion causes swelling. After many years of progression, elephantiasis may result:
In addition to the changes in the extremities, elephantiasis also damages the lungs. If this is impaired in its function, long-term damage also occurs in many other organs. The chronic lung disease manifests itself particularly in the form of nocturnal asthma attacks, recurrent fever attacks and an increase in pressure in the pulmonary arteries (pulmonary hypertension).
Full-blown elephantiasis is rare in Europe and is generally only observed in emerging and developing countries. Worldwide, lymphatic filariasis is the second leading cause of long-term disability, according to the World Health Organization (WHO).
Subcutaneous filariasis: symptoms
In subcutaneous filariasis, the worms colonize the skin and underlying tissues. Itching is often the main symptom, and swelling and bumps are common accompanying symptoms.
Often, those infected with this form of filariasis have no symptoms except for occasional itching. The typical “calabar bump” may develop in various parts of the body – as a reaction of the immune system to the worm and its excretions.
It is a local, sudden swelling that persists for one to three days. It is usually not particularly painful, but is very itchy. In addition, the area may be slightly red.
Symptoms of onchocerciasis (river blindness).
The adult (adult) worms form tangles under the skin that are palpable from the outside as painless nodules. Such a worm-filled skin nodule is called an onchocercoma.
Patients complain of severe itching, the skin becomes inflamed and may thicken like leather (lichenification). The skin color (pigmentation) may disappear in some areas, resulting in a kind of “leopard skin pattern”. In the long term, the entire skin of the body changes – one speaks of so-called “paper or old man’s skin”.
Recent studies suggest a possible link between the worm infection and a disease that has only been studied in more detail for a few years – the so-called “head nodding syndrome”. This is a particular form of epilepsy observed in some children in Uganda and South Sudan. In those affected, food or cold can trigger an epileptic seizure. The exact background to the development of the disease is not yet known.
Most people with serous filariasis have no symptoms. When symptoms do occur, they are usually not dangerous and do not result in disability. Therefore, serous filariasis has been studied less intensively than the other filarioses.
Filariasis: causes and risk factors
The various filarioses are transmitted by different mosquitoes or horseflies. These insects are therefore also called disease vectors. In principle, travelers to tropical countries should familiarize themselves with the typical diseases and infections in the respective destination country before the trip.
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Disease vector |
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Lymphatic filariasis |
Mosquitoes of the species Aedes (partly diurnal), Anopheles, Culex, Mansonia (all mainly nocturnal) |
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Subcutaneous filariasis |
Brakes of the genus Chrysops, black flies (exclusively diurnal) |
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Serous filariasis |
Culicoides mosquitoes (active mainly in the morning and evening hours) |
Filariasis: examinations and diagnosis
Microscopic detection of microfilariae in the patient’s blood ensures the diagnosis of filariasis. Depending on which mosquitoes are believed to have transmitted the pathogen, the blood sample should be taken at different times: This is because the microfilariae have adapted to the biting habits of the vector insects:
In onchocerciasis, the microfilariae do not enter the blood at all – the parasites can only be detected directly under the skin.
If the search for microfilariae is unsuccessful, certain tests can be used to look for specific antibodies in the blood.
If internal organs are already affected, imaging techniques (e.g. computer tomography, magnetic resonance imaging) can be used to determine more precisely the damage that has already occurred.
Filariasis: Treatment
- Diethylcarbamazine (DEC)
- Ivermectin
- Suramin
- Mebendazole
In principle, these drugs are very effective in killing the filariae. It is more problematic to recognize the disease at all, so that the appropriate treatment measures can be initiated.
In some filarioses, the death of the worms triggers a strong immune reaction in the body, so that glucocorticoids (“cortisone”) must also be given. They have an anti-inflammatory and depressant effect on the immune system (immunosuppressive), which can prevent a possible excessive immune reaction.
Filariasis: surgery
In onchocerciasis, surgery can be used to remove the worms from under the skin. In Loa loa disease, worms can be cut out of the conjunctiva of the eye if they are discovered there.
Filariasis: course of the disease and prognosis
The adult worms can survive in the host for several years. It can take several months to years for microfilariae to appear in the blood, so that an infection is only noticed late or not at all. However, the sooner it is properly treated, the better the prognosis.
In lymphatic filariasis, the development of the disfiguring lymphedema (elephantiasis) can be avoided by consistent therapy.
Onchocerciasis is the most threatening filariasis for the native population because of the often severe damage to eyes and skin. However, with timely treatment, the prognosis is considerably better.
Serous filariasis is considered comparatively harmless in terms of disease severity and possible complications.
Filariasis: Prevention
- Wear long, light-colored clothing.
- Use mosquito repellents (as spray, gel, lotion, etc.). Make sure that the products are tropicalized and recommended by organizations such as the WHO.
- Keep in mind that repellents are only ever locally effective on the skin area to which they are applied.
- Use a mosquito net when sleeping. Mosquito nets impregnated with repellents are recommended.
- Avoid riverbeds and wetlands, where insects are most likely to be present.
- Consult a tropical medicine doctor/travel medicine specialist a few weeks before departure about possible medications to protect against infections and about necessary travel vaccinations.
- If you take malaria prophylaxis with doxycycline during the trip, it is very likely to be effective against lymphatic filariasis and onchocerciasis as well.
