Filariasis: An Endemic Infectious Disease in India

In India, patients present with diseases on a daily basis that we only read about in the United States: scrub typhus, typhoid fever, leprosy and malaria to name a few. Therefore, the physicians training and treating this patient population learn to build many of their differential diagnoses around disease presentations, which would be considered very obscure by American physicians. I learned this lesson very quickly in the outpatient clinic during my week on CHAD (the community health rotation). One day, when I was working with a young Indian physician, a man came in for interpretation of blood work. He reported having a fever for the past couple weeks along with fatigue. He had also noted painful swellings in his groin region (painful, swollen lymph nodes). Consequently, he had come to the clinic for further workup. The patient’s blood work showed evidence of anemia as well as a significant increase in eosinophils. The doctor informed me that this patient’s increase in blood eosinophils most likely indicated a parasitic infection, specifically filariasis. Although I have often read that eosinophilia may indicate a parasitic infection, parasites are not usually the first consideration in such circumstances and it certainly wasn’t on my differential diagnosis list. In the end, we treated the patient for filariasis with a combination of Albendazole and Ivermectin. The intern informed me that liberally treating suspected filariasis without using diagnostic test to confirm the etiology is an extremely common practice in India, since filarial diseases are so prevalent and are very commonly seen.

Filariasis is a disease with very few acute symptoms that develops and festers over many years and can ultimately leads to elephantitis (enlargement of the limbs and genitalia secondary to lymphatic obstruction). There are actually three manifestations of filariasis that can develop in humans, depending on what area of the body the worm occupies: lymphatic filariasis, the most commonly seen manifestation that causes elephantitis, subcutaneous filariasis and serous cavity filariasis. It is lymphatic filariasis that presents the main global challenge, as it has currently infected an estimated 120 million people worldwide, with over 1/3 of those cases resulting in serious disability and disfigurement. Furthermore, India, with its dense populations and booming cities, harbors a huge percentage of the global burden of the disease (more than 40%). One study done in India estimated that a minimum of $850 million dollars is lost every year resulting from a combination of treatment cost and missed working days.

Filariasis is caused by a thread-like filarial nematode worm. There are nine different nematode worms capable of causing the disease. The most common disease-causing nematode seen in India is known as Wuchereria bancrofti. These nematodes infect humans via mosquito bites. When an infected mosquito bites its host, it deposits microfilariae into the skin. These larvae travel through the dermis and ultimately gain access to the lymphatic system. They then take up residence in the lymphatics and the larvae mature into adults over the course of almost a year. These adult nematodes can then survive for up to five years in the lymphatic system, during which time they mate and produce many thousands of offspring per day.

Filariasis is a silent disease that is often acquired during childhood but takes many years for disease manifestations to surface. And in fact, those infected later in life may never develop outward manifestations of the disease. However, newer studies have shown that many outwardly healthy adults infected with filarial nematodes may actually have hidden lymphatic pathology as well as kidney damage. Some patients, however, will suffer acute disease manifestations including acute adenolymphangitis, filarial fever and tropical pulmonary eosinophilia. Acute adenolymphangitis characteristically presents with painful lymphadenopathy and fever (most likely what my clinic patient was suffering from). The symptoms most likely result from immune-mediated responses to dying worms within the lymphatic system. The inflammation resolves within one to two weeks without intervention, but tends to recur frequently. Patients infected with filarial nematodes can also present with filarial fever, characterized by limited episodes of fever without obvious lymph node enlargement. This isolated febrile illness is often confused with other tropical diseases, such as malaria. Rarely, acute filariasis can also cause tropical pulmonary eosinophilia, manifested as nocturnal wheezing. The wheezing results from immune hyperresponsiveness to microfilaria in the lungs.

Chronically infected patients will show disease symptoms. Symptoms are more often seen in men than women. Long term infection will lead to chronic edema, harding of the tissues and hyperpigmentation of the skin. When the lymphodema is severe, it is referred to as elephantitis, which can be disabling as well as very disfiguring. In the rural villages around Vellore, I saw multiple advanced cases of elephantitis in people who obviously suffered from longstanding infection. Men with filariasis can also develop hydroceles (fluid filled enlargements around the testicles), which can lead to permanent genital damage. In addition to the irreversible lymphodema that develops with chronic infection, there are acute episodes of local inflammation involving skin, lymph nodes and lymphatic vessels. These acute inflammatory episodes are not a direct result of the filarial infection, but rather a result of superimposed bacterial infections of the skin secondary to damaged lymphatic channels and suppressed immune responses.

Until recently, lymphatic filariasis was difficult to diagnose in the communities where it is most prevalent. Filarial infections generally had to be detected microscopically in the blood, which was very difficult without advanced laboratory technology, not to mention very costly. A new test allows finger prick drops of blood to be used to detect circulating parasitic antigens without the need to isolate actual parasites. With this new card test, advanced laboratory technology is unnecessary and the cost is much lower. However, as my opening story illustrated, doctors working in endemic areas will often treat suspected filariasis without using a confirmatory test.

The recommended treatment for killing adult filarial worms is a combination of albendazole and ivermectin. A combination of diethylcarbamazine (DEC) and albendazole has also been shown to be effective. The primary goal of treatment in endemic areas is to interrupt the mosquito transmission cycle by eliminating microfilariae from the blood of infected patients. If patients are not treated until the disease is in the advanced stages of elephantitis, they can still benefit from anti-parasitic treatment. However, the most significant intervention in advanced stage disease is most likely prevention of superimposed bacterial and fungal superinfections, which advances the pathology and causes more suffering than the infection itself. The WHO has proposed a treatment strategy to eliminate lymphatic filariasis, which involves treating the whole population in an endemic area with once yearly albendazole plus ivermectin or DEC over the course of 4-6 years. Alternatively, the WHO has suggested fortifying common table salt used in cooking with DEC in endemic regions.

While lymphatic filariasis is usually not a life threatening disease, it exerts incredible social impact, especially on impoverished people in society. The grotesque complications resulting from longstanding infection result in social stigmatization for men and women as well as physical limitation that inhibits livelihood. Therefore, it is an important disease which is easily treatable in early stages and can be eliminated with proper education and treatment campaigns.

Sources:
Leder, Karin PhD and Peter Weller MD, FACP. "Epidemiology, pathogenesis, and clinical features of lymphatic filariasis." Ed Peter Weller. UpToDate. 1 Oct 2008. 5 Feb 2009. www.uptodate.com/online/content/topic.do?topicKey=parasite

World Health Organization. "Lymphatic Filariasis." Revised September 2000. 5 Feb 2009. www.who.int/mediacentre/factsheets/fs102.en.print.html