Filariasis

Introduction


It is estimated that 1 billion people (20% of the world's population) are at risk of acquiring Filarial infection. The disease is endemic in at least 80 countries of the world.

Filariasis is a chronic disease that causes a number of long term problems like, hydrocoele (accumulation of fluid), elephantiasis of the leg/scrotum, lymphoedema (swelling of subcutaneous tissues) and chyluria (passing of chyle -a turbid white/yellow fluid- in urine). The clinical manifestations of filariasis develop very slowly. The nature and severity of the manifestations differ in persons who belong to endemic areas from those in persons who have recently travelled to such an area.





Cause and Pathogenesis


Filariasis is caused by filarial worms which are called Wucheria bancrofti ,Brugia malayi, or Brugia timori. Humans are the definitive host. No definite animal reservoir hosts are known. The adult worms live in the lymph nodes and lymphatics. The adult worms produce large numbers of microfilariae about 6-12 months after infection. Mosquitoes become infected by ingesting microfilariae when they suck blood from infected human beings. The infected mosquitoes can introduce the microfilariae into new hosts. Repeated and prolonged exposure to the microfilariae is a pre requisite for getting the infection. The microfilariae take roughly about 1-2 weeks to develop into infective forms. The adult worms cause dilatation of the lymphatics and vessel wall thickening. Inflammatory changes that occur cause proliferation of connective tissue, twisting of the lymphatics, and damage to the valves. Lymphoedema with hard and brawny changes in the overlying skin can develop. The effects of filariasis are due to both the direct effects of the worms and the result of the defence mechanisms of the body. Death of the adult worm leads to fibrosis and obstruction of the lymphatics.

Symptoms and Signs


The symptoms that occur in persons affected by filariasis are:


Fever with chills,
Swelling in the area where the adult worms are present
Enlargement of the lymph nodes
Recurrent episodes of fever with chills can occur
Inflammatory changes develop in the lymphatics and in the lymph nodes. (This is usually associated with fever and local swelling.)
The inflammation in the lymphatics spreads to the neighbouring areas and also spreads in a reverse direction. The lymphatics can become thickened and inflamed. The veins in the region can get involved and this results in inflammatory changes in them.
The changes in the lymphatics can involve the upper and the lower extremities. If lymphatic damage leads onto lymphatic obstruction it causes pitting oedema. Thickening of the subcutaneous tissues can progress to hyperplasia and ultimately to elephantiasis of the involved limb. Infected males develop disease in the scrotum which may cause hydrocoele which in advanced stages may leads to elephantiasis of the scrotum.





Investigations and Diagnosis

Diagnosis can be made only by detection of the parasites. Microfilariae can be found in blood and body fluids. Timing of the blood collection is important and should be done in the night. This is because the microfilariae circulate in the blood only in the night time (based on the nocturnal periodicity of the microfilariae). Elevated eosinophil count in the peripheral blood supports the diagnosis. Other tests not commonly done are, assays for antigens of W. bancrofti and polymerase chain reaction based assays for DNA. Ultrasonogram of the scrotum may help to detect extent of involvement.

Treatment and Prognosis


Filariasis can be effectively treated by oral administration of drugs like, Diethyl carbamazine (DEC) 6 mg/ kg /day once a day or in divided doses for 2-3 weeks. This drug kills the microfilariae very effectively while the effect on the adult worms is slow. The drug is started in small doses and gradually increased over a course of few days. Side effects like swelling of the lymph nodes, abscess, ulceration, fever, headache, body ache and allergic reactions can occur due to the killing of the microfilariae and the adult worms. Several courses of treatment may be required. Supportive management includes, bed rest, elastic bandages for reducing the oedema and antibiotic drugs to combat the secondary infection. Ivermectin has also been used in single or two divided doses (starting dose of 20mic per kg followed by 400 mic per kg). For chronic lymphatic obstruction elevation of affected limb, elastic stockings can be tried. Injecting sclerosants can treat small hydrocoeles. Surgical management may also be necessary. For elephantiasis, surgical procedures like removal of the excess of subcutaneous tissue or lymphovenous shunt may be necessary. Early cases have a good prognosis if properly treated. Cases identified in the late stages have a poor prognosis.

Prevention


Avoidance of mosquito bites by use of mosquito nets or repellents.