Biomedical Engineering Reference
In-Depth Information
vector, by injecting larvae into the primary host when the insect has a blood
meal. The larvae grow into adult threadworms within the primary host, a
process that takes place over a year or more, and the cycle continues. The
symptoms and severity of the disease are largely determined by the anatomic
destinations of the migrating worms, and on the total load of worms, as mul-
tiple infections in the same person lead to a continuously increasing filarial
burden.
Wuchereria bancrofti, Brugia malayi, and Brugia timori tend to cause
lymph system obstruction, which can lead to elephantiasis. Elephantiasis is a
condition wherein one or more extremities, usually the legs, are chronically
swollen.
Other filarial worms preferentially inhabit the fatty tissue within the subcutis
of skin: loa loa (the African eye worm), Mansonella streptocerca,and
Onchocerca volvulus. The subcutaneous tissue is also the cause of infection
by a nematode in Class Camallanida: Dracunculus medinensis (see below).
Mansonella perstans and Mansonella ozzardi live in the abdominal peritoneum.
Onchocerca volvulus is the cause of river blindness, the second leading
cause of infection-produced blindness (behind trachoma, caused by Chlamydia
trachomatis, Class Chlamydiae, Chapter 13). The ocular pathogenicity of
Onchocerca volvulus is caused by an endosymbiont, Wolbachia pipientis (see
discussion in Class Alpha Proteobacteria, Chapter 5).
In addition to the filarial infections for which humans are the natural,
primary host, there are reported cases of so-called zoonotic filariasis, in
which humans are a dead-end host. The zoonotic infections, as in the filarial
infections for which humans are the natural hosts, are all transmitted by
blood-feeding insects. The filaria live for a time in human tissues, where
they eventually die, producing a localized inflammatory reaction. Although
various species of filaria are found in a wide assortment of animal hosts,
including birds and reptiles, only mammalian hosts have been associated
with zoonotic filariasis in humans [92].
Nematoda
Secernentea
Spirurida
Camallanida
Dracunculoidea
Dracunculidae
*Dracunculus
Dracunculus medinensis is the single pathogenic species in Class
Dracunculidae. Dracunculiasis, also known as guinea worm, has a dramatic
clinical presentation and treatment. The adult female worm bursts forth from
the skin, usually just above or below the knee. The astute physician coaxes
