Filarial Worms I : Structure, Infection, & Disease
Andrew G. Campbell, Ph.D.
Parasitic Filarial Worms - Background
• Thread-like worms transmitted by insect vectors (flies and mosquitoes) • collectively infect AT LEAST 100 million worldwide • considered major obstacles to development in endemic countries • "major players" – Wuchereria bancrofti / Brugia malayi lymphatic filariasis – Onchocerca volvulus - river blindness – Loa loa - loaiasis
•Taxonomy •Lymphatic Filariae •Nonlymphatic Filariae •Disease •Diagnosis, Treatment, & Control
KINGDOM PHYLUM CLASS ORDER FAMILY GENUS/SPECIES Wuchereria bancrofti Brugia malayi
ANIMALIA NEMATODA ...view middle of the document...
• mature within 1 year from infectious larvae • mature sizes
– females: 8 cm (3 inches) by 0.3 mm – males: 4 cm (1.5 inches) by 0.1 mm
• live within the lumen of lymph vessels, where they pair and produce microfilariae • continue shedding microfilariae for at least 4 years (possibly as long as 40 years!)
W. bancrofti adult male, x4.
• extremely small in comparison to adults (approx. 0.3 mm vs. 4-8 cm) • produced by actively mating adult pair and migrate to bloodstream • can survive in the host for as long as 1.5 years • must be ingested by mosquito in order to continue development • develop, but do not multiply, in the insect vector • in most cases nocturnally periodic
W. bancrofti microfilaria
Microfilarial (MF) periodicity
• MF numbers in the blood of humans are typically high at night and low/absent during the day • possible basis for periodicity – oxygen tension of the peripheral blood? – parasite circadian rhythm? experiments that reverse sleeping patterns or manipulate O2 levels alter MF periodicity • optimal parasite acquisition occurs when microfilariae peak numbers coincide with female mosquito feeding patterns • when not in peripheral blood, microfilariae aggregate in lung capillaries
Disease caused by lymphatic filariae
• presence of adult worms in lymphatics causes inflammation, dilation of the vessel, and some obstruction of lymph flow • immune responses are ellicited by dead and dying worms, leading to fibrosis of the vessel wall with subsequent blockage of lymph flow (lymphedema) • lymphedema can lead to disfiguring abnormalities such as elephantiasis (enlargement of affected organs) • it is generally thought that development of elephantiasis requires repeated and protracted infection
Cross section of lymph vessel occupied by adult W. bancrofti ,x54
Examples of severe Elephantiasis
Several cases of megascrotum
Disease caused by microfilariae
• microfilariae of lymphatic filariae do not appear to be very immunostimulatory and thus are not believed to be responsible for pathology in most infected persons • some persons develop a hypersensitivity syndrome known...