Filariasis refers to parasitic infection caused by filarial parasites. The clinical manifestation and treatment of filariasis depends on the type of filarial parasitic worm (nematode) involved. The following table lists the parasite and the filarial disease caused.
Filarial parasite |
Filarial disease |
|---|---|
Onchocerca volvulus |
Onchocerciasis |
Wuchereria bancrofti |
Bancroftian filariasis (lymphatic filariasis) |
Brugia malayi and Brugia timori |
Malayan filariasis (lymphatic filariasis) |
Loa loa |
Loiasis |
Mansonella species |
Mansonelliasis |
Dirofilaria species |
Dirofilariasis |
All filarial infections cause some type of skin problems in addition to systemic manifestations.
Filariasis is predominantly a disease of tropical countries and affects the lives of millions of people. Filariasis is spread from host-to-host via the bites of certain flies and mosquitoes that act as vectors for the parasites. For example the filarial nematode Onchocerca volvulus enters the skin from the bite of an infected blackfly (Simulium species). The nematode is in its third larval stage before it moults to its fourth larval stage in 4-6 weeks. Then after some months it moults again to become a juvenile adult that matures and mates after about a year to produce microfilariae. These adult worms live and mate encased in fibrous nodules just below the skin surface and can live there for up to 10-15 years.
It has been discovered that a type of Rickettsia (Wolbachia spp.) has a symbiotic relationship with filarial nematodes. The bacteria initiates the filarial disease process with a severe adverse inflammatory reaction.
The cutaneous signs and symptoms vary depending on the type of filarial infection.
Onchocercal dermatitis is the most common sign/symptom of onchoceriasis. The skin becomes very itchy and a red spotty rash is often present. Scratching often leads to bleeding, ulcers and secondary infections. The lower trunk, pelvis, buttocks, thighs and legs appear to be the most affected. Microfilariae are the main cause of this dermatitis.
In addition to onchodermatitis, nodules containing adult worms called onchoceromas develop just below the skin surface. These nodules can be up to several centimetres in diameter and are usually firm, flattened or bean-shaped, movable and non-tender. They most often occur over bony parts of the body such as the skull, ribs, scapulae, elbows, and knees.
If left untreated, onchoceriasis can lead to chronic skin changes. Skin loses its elasticity and layers of the skin start to die and form scar tissue. This very thin wrinkled skin results in an elephant-skin appearance that later turns into something like lizard skin.
The most obvious symptom of lymphatic filariasis is elephantiasis. This is a condition where the limbs swell to the point that they resemble an elephant’s foreleg in size, texture and colour. This thickening of the skin and swelling of underlying tissues is caused by parasites lodged in the lymphatic system, a network of channels, nodes and organs that maintain proper fluid levels in the body. Elephantiasis appears to be due an immune response to the dead and dying worms in the lymphatic system. Mainly limbs, breasts and genitalia are affected. Without treatment continued lymphatic damage will lead to irreversible elephantiasis.
The initial bite from a redfly infected with the Loa loa parasite causes redness, swelling and itching. As the parasite moults to larval stage 4 it causes red urticarial papules (itchy red spots) in the skin of the affected area. Then over the next 3 months or more as it develops into the adult form, patients may experience pain and numbness of the affected area, temporary swelling of a limb, itchiness and occasionally hives.
Most patients infected with M perstans do not experience any symptoms. Newcomers to an endemic area who become infected appear to experience more symptoms than local inhabitants. Symptoms include swelling of the arms, shoulders, and face; abdominal pain; itchiness; fatigue; and joint pain.
The most common symptom of a patient infected with M streptocerca is chronic itching dermatitis that is often confined to one part of the torso. Constant scratching may thicken the skin.
Humans are poor hosts for this filarial parasite as the worm usually dies before reaching sexual maturity. However in recent years there has been a reported increase in the number of human dirofilariasis cases. The head, neck, breasts, arms, legs and scrotum are most commonly affected. A flesh-coloured to red inflamed tender nodule develops. This can be painful and cause burning and itching.
There are four key treatment methods available:
Treatment depends on the infecting parasite.
Filarial disease |
Treatment options |
|---|---|
Onchocerciasis |
|
Lymphatic filariasis |
|
Loiasis |
|
Mansonelliasis |
|
Dirofilariasis |
|
The best way to prevent filariasis is by preventing spread of the parasites by vector control. This means eliminating or controlling infected flies and mosquitoes. Research is underway to find a biological agent that will eliminate the vector and is inexpensive, non-toxic to humans and deployable on a large scale.
For some filarial infections long-term or intermittent treatment with antihelminthic therapy have been suggested.
