CHAPTER 36 Lymphatic Filariasis

Epidemiology

About 120 million people worldwide are infected with organisms causing lymphatic filariasis, 90% of them with W. bancrofti. B. timori occurs only on some small islands in Indonesia, and B. malayi is limited to Asia and several Pacific island groups including Indonesia and the Philippines. Infections due to W. bancrofti occur in sub-Saharan Africa, the Nile delta in Egypt and Sudan, south Asia, the western Pacific, and Central and South America, including islands in the Caribbean. The largest number of cases occurs in south Asia and tropical sub-Saharan Africa. Large-scale, aggressive control programs recently have decreased the prevalence of disease in many areas. Distribution of disease may be highly focal within affected areas (Fig. 36.1).

Fig. 36.1 Distribution of organisms causing lymphatic filariasis.

(Courtesy Nutman TB, Kazura JB. Filariasis. In: Guerrant RL, Walker DH, Weller PF, eds. Tropical Infectious Diseases, 2nd edn. Philadelphia: Churchill Livingstone; 2006:1154)

Mosquitoes transmit the parasites associated with LF, and several genera are capable of transmitting the infection. In Africa and the Pacific, the Anopheles species predominates, whereas in urban areas, including India, Culex species are more common. Aedes and Mansonia (transmitting only B. malayi) species are present in some Pacific islands. The variety of mosquito vectors is a factor in the widespread and patchy distribution of LF. There is no animal reservoir for Wuchereria bancrofti, although cats in some parts of Southeast Asia are naturally infected with Brugia malayi.

Etiology

Larvae injected into the skin by the bite of the mosquito migrate to the lymphatics and develop into adult worms. Adult worms reside in the lymphatics and begin to produce microfilariae that are injected into the blood after 8 months in the case of W. bancrofti and 3 months in B. malayi.¹ Adult worms may produce microfilariae for approximately 5–10 years, and microfilariae may live up to 1 year (Fig. 36.2). Microfilariae show a predominantly nocturnal periodicity, corresponding to the biting habits of the vector mosquitoes, though subperiodic strains are found in some geographical regions. Periodicity may also be influenced by the circadian rhythms of the human host.²

Fig. 36.2 Life cycle of Wuchereria bancrofti. During a blood meal, an infected mosquito introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound (1). They develop into adults that commonly reside in the lymphatics (2). The female worms measure 80–100 mm in length and 0.24–0.30 mm in diameter, while the males measure about 40 mm by 0.1 mm. Adults produce microfilariae measuring 244–296 μm by 7.5–10 μm, which are sheathed and have nocturnal periodicity, except the South Pacific microfilariae which have the absence of marked periodicity. The microfilariae migrate into lymph and blood channels moving actively through lymph and blood (3). A mosquito ingests the microfilariae during a blood meal (4). After ingestion, the microfilariae lose their sheaths and some of them work their way through the wall of the proventriculus and cardiac portion of the mosquito’s midgut and reach the thoracic muscles (5). There the microfilariae develop into first-stage larvae (6) and subsequently into third-stage infective larvae (7). The third-stage infective larvae migrate through the hemocoel to the mosquito’s proboscis (8) and can infect another human when the mosquito takes a blood meal (1).

(Adapted from Centers for Disease Control and Prevention DPDx.)

The prevalence of disease in a given area depends on a complex interaction of factors including intensity of transmission, genetic susceptibility, immunologic tolerance, prior treatment, and coinfections.³ Infection rates increase with age, representing the accumulation over a lifetime of adult-stage worms through continued exposure to infected vector mosquitoes. Frequency of disease manifestations also increases with age, though likelihood of developing clinical disease varies geographically. Overall, approximately one-third of those infected have some overt manifestations of disease.⁴