Filariasis overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Ahmed Elsaiey, MBBCH [2]
Overview
Filariasis is a parasitic and infectious tropical disease caused by thread-like parasitic filarial worms called nematodes. These include Wuchereria bancrofti, Brugia malayi, and Brugia timori. They are all transmitted by mosquitoes. The infection occurs when an infected mosquito bites an individual introducing the larvae into the skin. These larvae then spread to various sites such as lymphatic vessels, subcutaneous tissues or serous cavities. The larvae then mature to be adult filariae, then adult worms which produce microfilariae which are carried by another mosquito and the cycle continues. Filariasis is endemic in some Asian, African and South American countries where it affects 120 million individual annually. Screening is important for early detection and the two commonly used tests are dipstick colloidal dye immunoassay and ICT filariasis test kit. If left untreated, patients with filariasis may progress to develop lymphoedema, hydrocele, skin pigmentation, and chyluria. Filariasis has a good prognosis in early cases but the chronic cases may progress to disability. The most common symptom of filariasis is elephantiasis and scrotal swelling. Patients with filariasis present with filarial fever which is self-limited fever. Diagnosis of filariasis is made by identifying microfilariae on a giemsa stained thick blood film. Blood must be drawn at night, since the microfilaria circulate at night. There are also PCR assays available for making the diagnosis. Ultrasound also can help in the diagnosis by detecting the moving living worm which shows filarial dance sign. The recommended treatment for patients with filariasis is albendazole combined with ivermectin. A combination of diethylcarbamazine (DEC) and albendazole is also effective. Prevention of the disease involves wearing appropriate clothing, avoiding outbreaks, use of insecticides, and spatial repellents.
Historical Perspective
Filariasis is believed to be found since the 16th century when Jan Huygen Linschoten put an overall idea about the disease after his trip to Goa. Moving forward through the 19th century there were many discoveries regarding filariasis, the infective worms, and the arthropod vectors. In 1866, microfilariae were detected in urine and blood. Ten years later in 1876, Joseph Bancroft discovered the adult worm which is responsible for the infection and named it as "Wuchereria bancrofti". Through the next years till 1900s, more discoveries and description of the life cycles of the worms were made.
Classification
Filariasis disease can be classified based on the site of infection. It is caused by different types of roundworms that infect a particular site in the body. A group of these worms infect the lymphatic vessels causing lymphatic filariasis. Others infect serous cavities and subcutaneous tissues.
Filariasis can be also classified into acute and chronic filariasis based on the duration of symptoms.
Pathophysiology
Filariasis infection occurs when a larva carrying mosquito bites individual skin introducing these larvae into the skin. The larvae then enter the patient's blood through the skin wound and spread to the different sites such as lymphatic vessels, subcutaneous tissues or the serous cavities. At these sites, the larvae matures in a six to twelve months period into the adult filariae which can live up to fifteen years. Reproduction takes place between the male and female adult worms producing microfilariae which are premature organisms with sheath that circulate the blood in case they are settled in the lymphatic vessels. During another blood meal, the mosquito takes up the microfilariae, then these microfilariae lose their sheath within two weeks to be larvae that enter the human body. When a human is bitten by a mosquito, the cycle restarts again. Pathogenesis of the disease depends on number of factors including immune response of the patient, the number of secondary bacterial infections, the accumulation of the worm antigens, release of Wolbachia bacteria from the worm and the genetic predisposition.
Causes
Filariasis is caused by the parasitic organisms nematodes which are round worms or thread worms that infects mainly lymphatic vessels causing lymphatic filariasis. The three main nematodes that cause lymphatic filariasis are Wuchereria bancrofti, Brugia malayi and Brugia timori. Other nematodes include Loa loa (the eye worm), Mansonella streptocerca, and Onchocerca volvulus that cause subcutaneous filariasis. Mansonella perstans and Mansonella ozzardi cause serous cavity filariasis.
Differentiating filariasis from other diseases
Lymphatic filariasis must be differentiated from other causes of lower limb edema, such as chronic venous insufficiency, acute deep venous thrombosis, lipedema, myxedema, cellulitis and causes of generalized edema.
Epidemiology and Demographics
Lymphatic filariasis is widely distributed all over the world and affects as many as 120 million individual worldwide. It is also responsible for disability in about 40 million patient. It affects children below 5 years and the probability of infection increases with age. The causative worms are more found in the tropical areas, Asia and Africa.
Risk factors
Common risk factors in the development of filariasis are exposure to mosquitoes for long periods of time, getting bitten by them multiple times and people living in tropical areas for long time.
Screening
Screening is important especially among the people of Asian countries like Malaysia, China and India. It is important among people who work in agriculture fields. Agriculture fields may be inhabited by the infected mosquitoes and vectors, making these people more vulnerable to getting infected. Two known tests for the screening of filariasis are dipstick colloidal dye immunoassay and Immunochromatographic technique (ICT) filariasis test kit.
Natural history, Complications, and Prognosis
If left untreated, patients with lymphatic filariasis may progress to develop lymphatic dilation and impaired lymphatic drainage. Common complications of filariasis include chronic lymphedema, hydrocele, skin pigmentation, and renal impairment like chyluria. Prognosis is generally good in early cases, but in chronic cases the disease can leave an individual severely disabled with genital damage.
Diagnosis
History and Symptoms
Filariasis can have varied clinical presentations depending on their cause. The most common symptom is elephantiasis. Other symptoms include fever, headache, pruritus, pulmonary symptoms, scrotal and leg swelling.
Physical examination
Filariasis patients appear toxic on presentation due to pain. They present with fever called filarial fever. It is an acute self-limited fever present in the beginning of the disease. Edematous plaques may be observed and it is a sign of acute dermatolymphangioadenitis. In onchocerciasis, blindness occurs and subcutaneous nodules may be found. Genitourinary manifestations also observed in filariasis include hydrocele, chyluria, hematuria and scrotal elephantiasis.
Laboratory and Findings
Labarotary findings consistent with the diagnosis of filariasis include identifying microfilariae on thick blood film stained with Giemsa stain. The blood sample is drawn at night as the microfilaria circulate at night. There are also PCR assays available for making the diagnosis.
X ray
There are no x ray findings associated with filariasis.
CT scan
There are no CT scan findings associated with filariasis.
MRI
There are no MRI findings associated with filariasis.
Ultrasound
Ultrasound can be used to detect the presence of the adult worms in the lymphatics. It is also used in estimating the effectiveness of the medical therapy. Dilated lymphatic channels and living worm movement known as the filarial dance sign are noticed on the ultrasound.
Other imaging findings
There are no other specific imaging findings for filariasis.
Other diagnostic findings
There are no additional diagnostic findings for filariasis.
Treatment
Medical Therapy
The mainstay treatment for patients with filariasis is albendazole (a broad spectrum anthelmintic) combined with ivermectin. A combination of diethylcarbamazine (DEC) and albendazole is also effective. All of these treatments are microfilaricides; they have no effect on the adult worms.
Surgery
Medical therapy is the first-line treatment option for patients with filariasis. Surgery is usually reserved for patients with chronic lymphedema with failure of medical treatment and worsened presentation such as lymphatic venous anastomoses. Hydrocelectomy can also be performed for intractable cases of hydrocele.
Primary prevention
There are many primary preventive measures available for filariasis. Wearing appropriate clothing, Avoid outbreaks, insecticides and spatial repellents, bed nets and mass drug treatment programs are effective ways to prevent filariasis.