The disease is caused by filariae (a type of nematode transmitted by blood-sucking arthropods) in the lymphatic system, subcutaneous tissue, abdominal cavity, and thoracic cavity of vertebrate definitive hosts. The clinical manifestations of filariasis caused by the two types of filariae are very similar. The acute phase involves recurrent lymphangitis, lymphadenitis, and fever, while the chronic phase leads to lymphedema and elephantiasis, severely harming the health and economic development of residents in endemic areas.

bubble_chart Epidemiology

Filariasis is one of the five major parasitic diseases in China. Both men and women, young and old, can be infected. The peak infection rate of microfilariae in endemic areas is mostly between 21 and 30 years of age. Bancroftian filariasis is distributed worldwide, mainly prevalent in tropical and subtropical regions; Malayan filariasis is limited to Asia, primarily endemic in Southeast Asia. In China, filariasis is prevalent in 16 provinces, municipalities, and autonomous regions, including Shandong, Henan, Anhui, Jiangsu, Shanghai, Zhejiang, Jiangxi, Fujian, Guangdong, Guangxi, Hainan, Hunan, Hubei, Guizhou, Sichuan, and Taiwan, located in the central and southern parts of the country.

bubble_chart Pathological Changes

1. Acute phase allergic and inflammatory reactions: The secretions, metabolites, and decomposition products of larvae and adult worms, as well as the uterine discharge of female worms, can stimulate local and systemic reactions in the body. In the early stages, the lymphatic vessels may exhibit intimal swelling and endothelial cell proliferation, followed by inflammatory cell infiltration in the vessel walls and surrounding tissues, leading to thickening of the lymphatic vessel walls, impaired valve function, and the formation of lymphatic thrombi. The infiltrating cells include a large number of eosinophils.
2. Chronic phase obstructive pathological changes: Lymphatic system obstruction is a critical factor in the chronic symptoms of filariasis. Due to stimulation by adult worms, lymphatic vessels dilate, valves become incompetent, and lymph stasis occurs, resulting in pitting lymphedema. Subsequently, inflammatory cell infiltration, endothelial cell proliferation, and luminal narrowing lead to lymphatic vessel occlusion. Granulomas form around dead adult worms and microfilariae, consisting of inflammatory cells, macrophages, plasma cells, and eosinophils, ultimately causing lymphatic embolism. Increased pressure in the distal lymphatic vessels leads to varicosities or rupture, with lymph fluid leaking into surrounding tissues. Clinical manifestations vary depending on the site of obstruction.
   • Elephantiasis: This is the most common symptom in advanced-stage filariasis. The initial stage of elephantiasis is lymphedema. In limbs, it often presents as pitting edema, which subsides when the limb is elevated. As fibrosis progresses, non-pitting edema develops, which does not subside with elevation, and skin elasticity is lost. Eventually, elephantiasis develops, characterized by limb enlargement, extensive fibrosis, fat deposition, dilated lymphatic vessels, and retained lymph fluid, along with epidermal hyperkeratosis or verrucous thickening.
   • Hydrocele testis: Obstruction of lymphatic vessels in the spermatic cord and testis causes lymph fluid to accumulate in the tunica vaginalis, leading to hydrocele. However, in some cases, acute inflammatory reactions may be the cause, and the condition can resolve after anti-inflammatory treatment.
   • Chyluria: This condition results from obstruction of the urinary and abdominal lymphatic vessels in patients with Bancroftian filariasis.
   • In addition to the above conditions, filarial nodules in the female breast are not uncommon in endemic areas. Rarely, filariasis can also affect the eyes, spleen, chest, back, neck, arms, and other sites, causing granulomas, pericarditis, chylous pleural effusion, chylous hemoptysis, and microfilariae in the bone marrow.
3. Occult filariasis: Also known as tropical pulmonary eosinophilia, it presents clinically with nocturnal asthma or cough, accompanied by fatigue and low-grade fever. Blood tests show marked eosinophilia and significantly elevated IgE levels. Chest X-rays reveal diffuse millet-sized shadows in the mid-to-lower lung fields.

bubble_chart Clinical Manifestations

The clinical symptoms during the acute phase manifest as lymphangitis, lymphadenitis, and erysipelas-like dermatitis. Lymphangitis is characterized by a retrograde course, where a red subcutaneous line can be seen developing centrifugally during an attack, colloquially referred to as "fire flow (erysipelas of shank)" or "red line." It can occur in both the upper and lower limbs but is more common in the lower limbs. When the inflammation involves the superficial fine lymphatic vessels of the skin, diffuse redness and swelling appear in the local skin, with a shiny surface, tenderness, and a burning sensation, known as erysipelas-like dermatitis. The affected areas are mostly seen in the middle and lower parts of the calf. In the case of *Wuchereria bancrofti*, if adult worms reside in the lymphatic vessels of the scrotum, it can lead to funiculitis, epididymitis, or orchitis. Along with local symptoms, patients often experience fear of cold, fever, headache, and joint soreness, collectively termed filarial fever. Some patients may only present with chills and fever without local symptoms, which may indicate deep lymphangitis and lymphadenitis.

During the chronic phase, obstructive sexually transmitted disease varies depending on the site of obstruction, leading to diverse clinical manifestations, including elephantiasis, hydrocele testis, chylous stranguria, filarial nodules in the breast, and occasionally ocular filariasis. Filariasis can also cause granulomas in the spleen, chest, back, neck, and arms, as well as filarial pericarditis, chylothorax, chylous hemoptysis, and microfilariae in the bone marrow.

The clinical presentation of occult filariasis includes nocturnal paroxysmal asthma or cough, accompanied by fatigue and low-grade fever. Blood tests reveal extreme eosinophilia and significantly elevated IgE levels. Chest X-rays may show diffuse millet-sized shadows in the middle and lower lung fields.

It is divided into etiological diagnosis and immunological diagnosis. The former includes detecting microfilariae and adult worms in peripheral blood, chylous stranguria, and aspirated fluids; the latter involves detecting filarial antibodies and antigens in serum.

1. Etiological Diagnosis

(1) Blood examination for microfilariae: Due to the nocturnal periodicity of microfilariae, the optimal time for blood collection is between 9 PM and 2 AM.

1) Thick blood membrane method: Take 60µl (3 large drops) of peripheral blood and smear it into a thick film. After drying, perform hemolysis and microscopic examination. Staining can reduce missed diagnoses and help identify the species.
2) Fresh blood drop method: Place one large drop of peripheral blood in physiological saline on a slide, cover it immediately, and examine under a microscope to observe the activity of microfilariae. This method is suitable for teaching and health promotion activities.
3) Concentration method: Take 1–2 ml of venous blood, perform hemolysis, centrifuge the sediment, and examine the residue under a microscope. This method improves detection rates but requires venous blood and is more complex.
4) Diethylcarbamazine daytime provocation method: Administer 2–6 mg/kg of diethylcarbamazine orally during the day, and collect blood for examination 30–60 minutes later. This method is useful when nighttime blood collection is inconvenient but may lead to missed diagnoses in low-level infections.

(2) Examination of body fluids and urine for microfilariae: Microfilariae can also be found in various body fluids and urine, such as hydrocele fluid, lymph, ascites, chylous stranguria, and urine. Direct smears of these fluids can be stained and examined microscopically, or methods like centrifugal concentration or thin membrane filtration can be used. For chylous fluids, add ether to dissolve the fat, remove the fat layer, dilute with water 10-fold, centrifuge at 1500–2000 rpm for 3–5 minutes, and examine the sediment.

1) Direct worm detection: For patients with active lymphatic inflammation or those who develop lymph nodules after treatment, use a syringe to aspirate adult worms from suspicious nodules or excise the nodules and dissect the tissue under a dissecting microscope or naked eye. The obtained worms are killed, fixed, and cleared in nematode clearing solution for microscopic examination and species identification.
2) Pathological section examination: Prepare pathological sections from suspicious nodules using standard methods. If the nodules are filarial, adult worms will be visible at the center, surrounded by typical filarial sexually transmitted disease changes.

2. Immunological Diagnosis can be used as an auxiliary method.

(1) Intradermal test: Cannot be used for definitive diagnosis but is suitable for epidemiological surveys.
(2) Antibody detection: Many methods exist, but currently, the indirect fluorescent antibody test (IFAT) using frozen sections of adult filarial worms, the immunoenzyme staining test (IEST), and the ELISA using soluble antigens from adult worms or microfilariae of Brugia malayi show high sensitivity and specificity.
(3) Antigen detection: Recent domestic studies using monoclonal antibodies against filarial antigens have made preliminary progress in detecting circulating antigens of Wuchereria bancrofti and Brugia malayi through ELISA double-antibody and dot-ELISA methods.

bubble_chart Diagnosis

The diagnosis is confirmed by detecting microfilariae and adult worms in peripheral blood, chylous urine, or effusion fluid through laboratory tests.

bubble_chart Treatment Measures

The main therapeutic drug is hetrazan (also known as diethylcarbamazine, DEC). Hetrazan has a killing effect on both types of filariae, with better efficacy against Malayan filariasis than Bancroftian filariasis, and a stronger effect on microfilariae than adult worms. The commonly used regimen of hetrazan in China is a 4.2g 7-day course for Bancroftian filariasis, and a 1.5–2.0g 3–4-day course for Malayan filariasis. After taking the medication, patients may experience allergic reactions due to the massive death of microfilariae, presenting symptoms such as fever, shivering, and headache, which should be promptly managed. To reduce the side effects of hetrazan, medicated salt containing hetrazan is now widely used in prevention and control efforts. The salt is prepared at a concentration of 0.3%, calculated as an average daily intake of 50mg of hetrazan per person. Consuming this for six months can reduce the microfilariae positivity rate to below 1% in moderate and low endemic areas, with mild side effects. In recent years, China has successfully developed a new antifilarial drug, furapyrimidone, which kills both microfilariae and adult worms and is effective against both types of filariae. A 7-day course with a total dose of 140mg/kg body weight shows better efficacy than hetrazan for Bancroftian filariasis.

For patients with elephantiasis, in addition to hetrazan for killing worms, traditional Chinese medicine and mulberry leaf injections combined with bandaging therapy or heat and bandaging therapy can be used. For patients with scrotal elephantiasis or hydrocele, surgical treatment such as tunica vaginalis inversion can be performed. For patients with chylous stranguria, mild cases may recover with rest; alternatively, 1% silver nitrate can be used for renal pelvis lavage. Severe cases can be treated with microsurgical lymphangio-venous anastomosis, yielding good results.

bubble_chart Cure Criteria

After thorough treatment, the absence of clinical symptoms or signs, and no microfilariae detected in laboratory tests indicate a cure.

bubble_chart Prevention

1. Mass Screening and Treatment: Early detection of patients and carriers, along with timely treatment, not only ensures public health but also reduces and eliminates sources of pestilence. The screening should target all residents aged one year and above, with the requirement that over 95% of the population undergoes blood collection.
2. Mosquito Prevention and Elimination
3. Strengthen epidemiological surveillance in areas that have met the basic criteria for filariasis elimination. During surveillance, attention should be paid to:
   1. Re-examining and re-treating previously positive sexually transmitted disease cases; conducting supplementary screenings and treatments for those who were not tested before; meanwhile, enhancing the management of the floating population, identifying patients, and providing timely treatment until they test negative.
   2. Intensify mosquito vector surveillance in households with positive blood test results. Upon discovering infected mosquitoes, expand blood testing and mosquito elimination efforts to surrounding populations centered on the infected households, to clear infection hotspots and prevent further transmission.