Wuchereria bancrofti and Brugia species

Causative agents of lymphatic filariosis

Parasites and occurrence. About 120 million people in 80 countries suffer from lymphatic filariosis caused by Wuchereria bancrofti or Brugia species (one-third each in India and Africa, the rest in southern Asia, the Pacific region, and South America), and 1.1 billion people are at infection risk (WHO, 2000). (Table 10.4). Humans are the only natural final hosts of W. bancrofti and the most widely disseminated Brugia strains. There are, however, other Brugia strains using also animals as final hosts (cats, dogs, and monkeys).

Life cycle and epidemiology. The intermediate hosts of W. bancrofti and B. malayi are various diurnal or nocturnal mosquito genera (Table 10.4). The development of infective larvae in the insects is only possible at high environmental temperatures and humidity levels; in Wuchereria bancrofti the process takes about 12 days at 28 °C. Following a primary human infection, the filariae migrate into lymphatic vessels where they develop to sexual maturity. Microfilariae (Mf) do not appear in the blood until after three months at the earliest (B. malayi, B. timori) or after seven to eight months (W. bancrofti). Tables 10.4 and Fig. 10.17 show their specific characteristics. The adult parasites survive for several years.

Pathogenesis and clinical manifestations. The pathologies caused by W. bancrofti and Brugia species are very similar. The initial symptoms can appear as early as one month p.i. although in most cases the incubation period is five to 12 months or much longer. The different courses taken by such infections can be summarized as follows:

■ Asymptomatic infection, but with microfilaremia that can persist for years.

■ Acute symptomatic infection: inflammatory and allergic reactions in the lymphatic system caused by filariae ! swelling of lymph nodes, lymphangitis, intermittent recurrent febrile episodes, general malaise, swellings on legs, arms, scrotum and mammae, funiculitis, orchitis.

Table 10.4 Filarial Species Commonly Infecting Humans

Species and

Distribution

Vector

Localization

Microfilariae:

Pathology

length (cm)

of adults

characteristics

and periodicity

Wuchereria

Southeast

Mosqui

Lymphatic

244-296 im,

Lymphangitis

bancrofti

Asia, Pacific,

toes:

system

sheathed,

and lymph

FF: 2.4-4.0

trop. Africa,

Culex,

in blood,

adenitis, ele-

99: 5.0-10.0

Caribbean,

Anopheles,

nocturnal,

phantiasis

trop. South

Aedes

diurnal or

America

subperiodic2

Brugia

South and

Mosqui

Lymphatic

177-230 im,

Lymphangitis

malayi

East Asia

toes:

system

sheathed, in

and lymph-

FF: 2.2-2.5

Anopheles,

blood, nocturnal adenitis, ele-

99: 4.3-6.0

Aedes,

or subperiodic

phantiasis

Mansonia

Brugia timori

Indonesia

Mosqui-

Nocturnal

toes:

periodic

Anopheles

Loa loa

Tropical

Flies:

Subcutaneous 250-300 im,

Skin swel

FF: 3.3-3.4

Africa

Chrysops

connective

sheathed, in

lings, infec

99: 5.0-7.0

tissue

blood, diurnal

tion of con

periodic

junctiva

Onchocerca

Africa, Central

Black flies: Subcutaneous 221-358 im,

Skin no-

volvulus

and South

Simulium

connective

unsheathed,

dules, der

FF: 2.0-4.5

America

tissue

in skin, not

matitis, eye

99: 23-50

periodic

lesions

Mansonella

Africa, South

Midges:

Peritoneal

190-200 im,

Normally

perstans

America

Culicoides

and pleural

unsheathed, in

apathogenic

FF: 4.5

cavities

blood, nocturnal

99: 7.0-8.0

subperiodic

Mansonella

Tropical

Midges:

Subcutaneous 180-240 im,

Skin edema,

streptocerca

Africa

Culicoides

connective

unsheathed,

dermatitis

FF:3

tissue

in skin,

93

not periodic

Mansonella

Central and

Midges:

Peritoneal

173-240 im,

Normally

ozzardi

South America Culicoides

cavity

unsheathed,

apathogenic

FF:3

in blood (not

99: 6.5-8.1

periodic)

1 See Fig. 10.1 for details on differentiation of microfilariae.

2 Subperiodic: periodicity is not pronounced.

3 No exact data are available.

■ Chronic symptomatic infection: chronic obstructive changes in the lymphatic system ! hindrance or blockage of the flow of lymph and dilatation of the lymphatic vessels ("lymphatic varices")! indurated swellings caused by connective tissue proliferation in lymph nodes, extremities (especially the legs, "elephantiasis"), the scrotum, etc., thickened skin (Fig. 10.16). Lymph-uria, chyluria, chylocele etc. when lymph vessels rupture. This clinical picture develops gradually in indigenous inhabitants over a period of 10-15 years after the acute phase, in immigrants usually faster.

■ Tropical, pulmonary eosinophilia: syndrome with coughing, asthmatic pulmonary symptoms, high-level blood eosinophilia, lymph node swelling and high concentrations of serum antibodies (including IgE) to filarial antigens. No microfilariae are detectable in blood, but sometimes in the lymph nodes and lungs. This is an allergic reaction to filarial antigens.

Diagnosis. A diagnosis can be based on clinical symptoms (frequent eosinophilia!) and finding of microfilariae in blood (blood sampling at night for nocturnal periodic species!). Microfilariae of the various species can be differentiated morphologically in stained blood smears (Table 10.4, Fig. 10.17) and by DNA analysis. Conglomerations of adult worms are detectable by ultrasonography, particularly in the male scrotal area. Detection of serum antibodies (group-specific antibodies, specific IgE and IgG subclasses) and circulating antigens are further diagnostic tools (Table 11.5, p. 625). The recent development of a specific ELISA and a simple quick test (the ICT filariosis card test) represents a genuine diagnostic progress due to the high levels of sensitivity and specificity with which circulating filarial antigens can now be detected, even in "occult" infections in which microfilariae are not found in the blood.

Therapy. Both albendazole and diethylcarbamazine have been shown to be at least partially effective against adult filarial stages. However, optimal treatment regimens still need to be defined. Adjunctive measures against bacterial and fungal superinfection can significantly reduce pathology and suffering.

Control and prevention. In 1997, the WHO initiated a program to eradicate lymphatic filariosis. The mainstay control measure is mass treatment of populations in endemic areas with microfilaricides. Concurrent single doses of two active substances (albendazole with either diethylcarbamazine or iver-

Fig. 10.16 a Infection with Wuchereria bancrofti: elephantiasis; b infection with Loa loa: eyelid swelling; c onchocercosis: cutaneous nodules caused by Onchocerca volvulus; d blindness caused by O. volvulus; e Trichinella spiralis; larvae in rat musculature; f larva migrans externa. (Images a, b, d: Tropeninstitut Tübingen, c: Tropeninstitut Amsterdam; f: Dermatologische Klinik der Universitüt Zürich.) ►

— Nematode Infections

Onchocerca Volvulus

— Microfilariae of Various Filarial Species

— Microfilariae of Various Filarial Species

Wuchereria Bancrofti Life Stages

Fig.10.17 Differential diagnosis of microfilariae in human blood: sheathed, large: 1 Loa loa: tip of tail (1a) with several nuclei; 2 Wuchereria bancrofti: tip of tail (2a) without nuclei; 3 Brugia malayi: tip of tail (3a) with single nucleus. Unsheathed, smaller: 4 Mansonella perstans: tip of tail (4a) rounded with densely packed nuclei, often in several rows reaching nearly to the tip of the tail; 5 Mansonella ozzardi: tip of tail (5a) pointed, tip free of nuclei.

Fig.10.17 Differential diagnosis of microfilariae in human blood: sheathed, large: 1 Loa loa: tip of tail (1a) with several nuclei; 2 Wuchereria bancrofti: tip of tail (2a) without nuclei; 3 Brugia malayi: tip of tail (3a) with single nucleus. Unsheathed, smaller: 4 Mansonella perstans: tip of tail (4a) rounded with densely packed nuclei, often in several rows reaching nearly to the tip of the tail; 5 Mansonella ozzardi: tip of tail (5a) pointed, tip free of nuclei.

mectin) are 99% effective in removing microfilariae from the blood for one year after treatment. Mass-treatment with albendazole or ivermectin is also expected to have a controlling effect on intestinal nematodes (Ascaris, hookworms, Strongyloides, Trichuris). Measures to avoid mosquito bites are the same as for malaria.

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Responses

  • paul
    How to differentiate loa loa from wucheria?
    7 years ago
  • Fatima
    How to distinguish sheathed and unsheathed microfilariae?
    7 years ago
  • ANNE
    How to differential microfilaria species?
    6 years ago

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