NAME: Wuchereria bancrofti
SYNONYM OR CROSS REFERENCE: Bancroftian filariasis, Lymphatic filariasis, elephantiasis
CHARACTERISTICS: Wuchereria bancrofti is a filarial nematode that, as an adult, is a thread-like worm(1,2,3). The female nematodes are 10 cm long and 0.2 mm wide, while the males are only about 4 cm long(1,3). The adults reside and mate in the lymphatic system where they can produce up to 50 000 microfilaria per day(1). The microfilaria are 250-300 µm long, 8 µm wide and circulate in the peripheral blood. They can live in the host as microfilaria for up to 12 months(1,4). Adult worms take 6 to 12 months to develop from the larval stage and can live between 4 and 6 years(1,4).
PATHOGENICITY/TOXICITY: In filarial endemic areas, there are three groups of patients recognized(5). The first group, considered endemic normals, are exposed to the nematode but have not been infected(5). The second group have been exposed, infected, and have microfilaria in their peripheral circulatory system, but remain asymptomatic.(5). Asymptomatic infections can go undetected for years, and with lymphatic filariasis (LF) it may eventually result in internal damage which is not easily diagnosed(5). The third group are those who are chronically infected and present with lymphoedema (which affects 16 million people), hydrocoele and elephantiasis(5,6). Acute (bacterial) dermato-lymphangio-adenitis (ADLA), another condition that can result from infection, presents with fever, chills, swelling and lymphoedema. ADLA usually occurs when an adult worm dies and the lymph vessels surrounding it are inflamed due to the host's immunological response(2,5). ADLA normally occurs in older children and youth and remains with the infected individual throughout life(5). Chronic ADLA attacks can cause renal disease, haematuria, proteinuria, chyluria, nephritic syndrome and glomerulanephritis(2,5). Patients with LF can also have rheumatic problems, cystitis with urethral obstruction, fibrosing mediastinitis, tropical vaginal hydroceles and bladder pseudotumors(5). Another indication of LF is pulmonary eosinophila which is characterized by paroxysmal cough and wheezing and, even though the patient harbours adult worms, there are no microfilarias in the blood(5). The most disabling of health problems caused by LF is elephantiasis, a permanent swelling of a limb (usually lower limbs although it can effect arms, breasts and genitalia). Streptococci bacteria can infect the affected limb, worsening the condition(5,6). Certain markers predispose patients to chronic filarial disease, including a high dose of the infectious agent, a pre-existing bacterial infection, or a specific host response(7).
EPIDEMIOLOGY: Wuchereria bancrofti is endemic in 78 countries and affects 128 million people worldwide(1). This nematode is widespread throughout humid and tropical zones of Asia, Africa, the Americas and the Pacific islands, and is common in areas with poor socioeconomic levels(1,2,7). Wuchereria bancrofti is the infectious agent in 91 % of LF cases. LF is also recognized as the second most disabling mosquito-borne disease next to malaria(1,4,5,6). To date, 44 million people experience clinical disease; however, 76 million suffer pre-clinical damage to their renal and lymphatic systems(5). An estimated 1.3 billion people in endemic regions are at risk of developing LF each year and, even though not fatal, it still remains the leading cause of infirmity, permanent disability, and chronic morbidity(1). Vector control and the mass distribution of pharmaceuticals have proven the most effective measures in containing this epidemic(1,4).
INFECTIOUS DOSE: Infection usually involves numerous exposures to this organism. It is not uncommon that an individual receive 2700 to 1,000,000 bites from infected mosquitoes (approximately equivalent to 10 to 20 years of exposure) before becoming infected(1).
MODE OF TRANSMISSION: The disease is usually transmitted through the bite of an infectious mosquito. Overall there are 6 genera and 70 species of mosquitoes responsible for the spread of Wuchereria bancrofti(1).
INCUBATION PERIOD: The incubation period is variable and often difficult to determine. Both microfilaria and adult worms have been observed in patients as early as 6 months and as late as 12 months after infection(4).
COMMUNICABILITY: This disease is not transmitted from person-to-person. Mosquitoes, however, can be infected by humans if they ingest microfilaria during a blood meal of an infected individual(1). The mosquito remains infectious for only 10-14 days after consuming an infected blood meal(1).
RESERVOIR: Humans are the only known reservoir for Wuchereria bancrofti(1).
ZOONOSIS: Humans are infected by mosquitoes who act as vectors, as well as a developmental reservoir(1).
VECTORS: Mosquitoes are the vector for this nematode and certain genera and species appear to transmit the infectious agent in particular geographical locations(1). At least 43 species of Anopheles mosquitoes are responsible for the infection of people in West Africa, rural Southeast Asia and parts of the Southern Pacific(1). Anopheles mosquitoes, in particular, transmit the nocturnal periodic form of the worm(1). Other genera like Aedes, Ochleratus, and Downsyomia, which have 20 different species among them, spread both the nocturnal and diurnal supperiodic forms of the nematode, particularly in the Pacific islands and parts of Southeast Asia(1). Like the Anopheles genus, 6 species of the Culex genus infect humans with the nocturnal periodic form of Wuchereria Bancrofti, but in East Africa, the Middle East, urban Southeast Asia and Latin America(1).
DRUG SUSCEPTIBILITY: A combination of both albendazole and ivermectin, or albendazole and diethylcarbamazine are effective in eliminating of microfilaria, but only disrupt the adult female's reproductive ability(1,4,5). Drug treatment regimes may not get rid of the adult worms so long term therapeutic courses are often employed(2). These nematodes have also shown susceptibility to avermectin, piperazines and suramin (Bayer 205)(8).
SUSCEPTIBILITY TO DISINFECTANTS: Most microorganisms are susceptible to 1 % sodium hypochlorite and 2 % glutaraldehyde, including Wuchereria bancrofti(9).
PHYSICAL INACTIVATION: The microfilaria of Wuchereria bancrofti has been observed to be susceptible to freezing(10).
SURVIVAL OUTSIDE HOST: Not known.
SURVEILLANCE: To monitor suspected cases of LF, laboratories use Giemsa-stained peripheral blood films, phosphatise detection of microfilaria, Knott concentration procedure and membrane filtration techniques(1). More recently, ELISA, PCR, Lymphoscintigraphy, and ICT tests have been used to detect the organism(1,11). The new technology has enabled epidemiologist to monitor the presence of this agent in the mosquito vector. Vector analysis in combination with patient diagnosis has allowed a more comprehensive picture of Wuchereria bancrofti and its relevance to human health.
Note: All diagnostic methods are not necessarily available in all countries.
FIRST AID/TREATMENT: Appropriate drug therapy, such as ivermectin and albendazole or diethylcarbamazine (DEC) and albendazole, to treat LF(1,2). A medical follow-up should be done after the treatment to ensure the elimination of both microfilaria and adult worms in the patient(12).
PROPHYLAXIS: While a combination of either albendazole and ivermectin or albendazole and diethylcarbamazine (DEC) is given once a year in endemic areas, it has no effect on transmission by mosquito(1,12). It is recommended to stay away from mosquito breeding sites, particularly at key feeding periods, and to protect oneself with sprays or lotions that contain active ingredients like deet that repel mosquitoes, or by using bed nets impregnated with insecticides such as permethrin(12).
LABORATORY-ACQUIRED INFECTIONS: No laboratory-acquired infections have been reported to date.
PRIMARY HAZARDS: Accidental ingestion of infected eggs or parenteral inoculation with infective larvae are the primary hazards(13). Arthropods used routinely in laboratory research that are contaminated with filariasis also pose a risk(13,14).
SPECIAL HAZARDS: Certain sensitized individuals can have allergic reactions to some antigenic components of nematodes(13).
RISK GROUP CLASSIFICATION: Risk group 2.
CONTAINMENT REQUIREMENTS: Containment Level 2 facilities, equipment, and operational practices for work involving infectious or potentially infectious materials, animals, cultures.
PROTECTIVE CLOTHING: Lab coat. Gloves when direct skin contact with infected materials is unavoidable. Eyes protection must be used where there is a known or potential risk of exposure to splashes(15).
OTHER PRECAUTIONS: All procedures that may produce aerosols, or involve high concentrations or large volumes should be conducted in a biological safety cabinet (BSC). The use of needles, syringes and other sharp objects should be strictly limited. Additional precautions should be considered with work involving large scale activities(15).
SPILLS: Allow aerosols to settle and, wearing protective clothing, gently cover spill with paper towels and apply appropriate disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time before clean up (30 min)(15).
DISPOSAL: All wastes should be decontaminated before disposal either by steam sterilization, incineration or chemical disinfection(15).
STORAGE: The infectious agent should be stored in a sealed and identified container(15).
REGULATORY INFORMATION: The import, transport, and use of pathogens in Canada is regulated under many regulatory bodies, including the Public Health Agency of Canada, Health Canada, Canadian Food Inspection Agency, Environment Canada, and Transport Canada. Users are responsible for ensuring they are compliant with all relevant acts, regulations, guidelines, and standards.
UPDATED: August 2010
PREPARED BY: Pathogen Regulation Directorate, Public Health Agency of Canada.
Although the information, opinions and recommendations contained in this Pathogen Safety Data Sheet are compiled from sources believed to be reliable, we accept no responsibility for the accuracy, sufficiency, or reliability or for any loss or injury resulting from the use of the information. Newly discovered hazards are frequent and this information may not be completely up to date.
Public Health Agency of Canada, 2010