Overview 

Filariasis is an infectious disease that has affected millions of people worldwide. The disease which is caused by a roundworm spreads through mosquito bites. The classic symptoms of filariasis include swelling of the lymph nodes, tissues and scrotum and thickening of the skin. The infection is usually acquired during childhood but often remains undetected at this stage.  Recognizing filariasis as early as possible helps in more practical management and prevention of the disease.

Introduction

Filariasis is a tropical disease (disease that is indigenous to the tropical regions of the world ) carried by mosquitos. The major causative organism of filariasis is a parasitic roundworm. The most common species are Wuchereria bancrofti, Brugia malayi, and Brugia timori. The infective larva spreads to the human bloodstream during a mosquito bite. The most common species of mosquito that act as carriers for filariasis are Aedes, Anopheles, Culex, Mansonia, and Ochlerotatus.¹

Filariasis is considered an endemic in 72 countries in the world. According to WHO “over 882 million people in 44 countries worldwide remain threatened by lymphatic filariasis and require preventive chemotherapy to stop the spread of this parasitic infection”.The disease is more common in the tropics and subtropical climates of Asia, Africa, the Western Pacific, South America, and the Caribbean.² It is the second most common cause of permanent deformity and disability worldwide.¹

Novel highly sensitive diagnostic tools confirm that filariasis is first acquired during childhood, with often one-third being infected at less than 5 years old. Initially, it remains undetected but after puberty, the patient starts developing symptoms. Diagnosing filariasis at an early stage plays an important role not only in the management but also in the eradication of the disease.³

Understanding filariasis

Causes and transmission

90% of filariasis infection is caused by the parasite Wuchereria bancrofti. When the infected blood reaches the mosquito, the worms develop into highly active infective larvae in 10 -12 days.⁴ When a mosquito bites, these larvae enter the human bloodstream. It reaches the lymph nodes and reproduces within the lymph nodes. Thus it obstructs the lymphatic drainage. The worms can live up to 6- 8 years and can produce millions of microfilariae - immature larvae. Occluded lymph nodes result in an increased risk of bacterial and fungal infection in the patients. Humans are called the primary reservoir of these larvae and mosquitos are the vectors.¹

Symptoms of filariasis

At least half of the infected individuals do not show any symptoms until they get tested. The disease gradually damages the lymphatic system. The symptoms can be acute or chronic. Acute symptoms are those that are seen in the early stages. The chronic symptoms appear in later stages. The common acute symptoms include:

• Sudden swelling of lymph nodes
• Fever- filariatic fever 
• Chills
• Body ache 
• Sudden infections
• general weakness

The chronic symptoms include:

• chronic lymphedema - swelling of the tissues
• Elephantiasis - skin thickening 
• Hydrocele - scrotal/breast swelling
• Kidney damage
• Increased risk of infections
• Tropical pulmonary eosinophilia with excessive cough, breathlessness, wheezing⁵
• Filarial abscess - causing localised swelling and pain until rupture with discharge of adult dead worms¹

Diagnostic Criteria for Filariasis

Diagnostic Methods

Clinical Diagnosis

Recognition of symptoms and physical examination are crucial in the diagnosis of filariasis. The disease is usually subclinical—without symptoms in the early stages but progresses into the lymphatic system over time. Usually, the affected children do not show any symptoms. Sometimes the symptoms will be visible only in the late 20s or 30s. The most common symptom seen is a hydrocele.¹ However clinical diagnosis alone cannot give an accurate diagnosis in the early stages of the disease since the early symptoms are not specific.

Laboratory Diagnosis

Blood Smear Test

• According to the Centers for Disease Control (CDC), blood smear is the gold standard for diagnosis
• Blood samples are taken from the veins or finger or heel stick
• Samples are taken preferably after 8 pm due to the nocturnal periodicity - worms move in blood only at night
• The samples are viewed under a microscope to detect the filarial worms¹

Antigen Detection Tests

• Rapid diagnostic tests like the Immunochromatographic Test(ICT card) and filariasis Test Strip (FTS) are used worldwide
• Commonly used in endemic areas
• Detects the presence of filarial antigen in the blood 
• The Filariasis Test Strip is considered the standard indicator for monitoring and surveillance in the WHO’s Global Program to Eliminate Lymphatic Filariasis.⁷

Antibody Tests

• Filariasis patients will have high levels of antibody IgG4 in the blood
• Antibody tests have certain limitations as it cannot distinguish a past infection from a current infection⁶
• Brugia Rapid point-of-care cassette test (BRT) can detect the presence of IgG4 antibody against Brugia species in blood.

PCR (Polymerase Chain Reaction)

• An advanced diagnostic tool for antigen detection 
• It can also detect latent infections¹

Imaging Techniques

• Scrotal ultrasounds can be used to assess the movement of filarial worms
• A pattern of worm movement - "filarial dance sign" can be detected in the lymphatics of men¹

WHO recommendations

WHO initiated a program called the Global Programme to Eliminate Lymphatic Filariasis (GPELF)  in the year 2000. They aim to eliminate the transmission (spread) and prevent new cases through mass drug administration (MDA), according to WHO all but 3 of 72 endemic countries have successfully established this program. They recommend conducting surveys to assess the spread twice in 2 to 3-year intervals after stopping MDA.

WHO recommends using thick blood smears (20–60 μl) of finger-prick blood to diagnose filariasis. According to WHO Filariasis Test Strip (FTS) should be used in endemic for W. bancrofti and Brugia Rapid Test for  Brugia species.

The recommended treatment protocol through mass drug administration involves a single dose of two medicines given together, albendazole (400 mg) plus either ivermectin (150–200 mcg/kg or diethylcarbamazine citrate (DEC) (6 mg/kg). Efficient mosquito control is necessary to prevent the spread of the disease. WHO supports strategies like insecticide-treated nets, indoor spraying, and personal protection for mosquito control.

Summary

Filariasis is a tropical infectious disease caused by a roundworm. It is an endemic in many parts of the world and is spread by mosquitos. Humans act as the reservoir of the worms. Most often filariasis infection is acquired during childhood but goes undiagnosed in the early stages due to the lack of symptoms. Filariasis is characterised by symptoms like hydrocele, elephantiasis and swelling of lymph nodes. The major diagnostic criteria involve the clinical evaluation of symptoms and laboratory tests like blood smear test, antigen detection, antibody detection, PCR test and ultrasonography. Filariasis is a disease that can cause severe disability if not treated as early as possible. Hence accurate diagnosis and mosquito control are important for the prevention and elimination of the disease.

FAQs

What is the best diagnostic test for filariasis?

The Filariasis Test Strip (FTS)  which diagnostic test based on antibody detection is one of the best tests recommended by the WHO for monitoring filariasis.

How is filariasis diagnosed?

Filariasis can be discovered by various tests like blood smear test, the antigen detection tests like  Immunochromatographic Test(ICT card), Filariasis Test Strip (FTS), antibody detection tests like Brugia Rapid point-of-care cassette test (BRT), PCR tests and ultrasonography.

What is the rapid test for lymphatic filariasis?

Some examples of rapid tests are the Filariasis Test Strip (FTS), and the Brugia Rapid point-of-care cassette test (BRT).

What is the prognosis for filariasis? 

Filariasis has a good prognosis if detected early