Biology Reference
In-Depth Information
by tissue-invasive Ascaris larvae, which can be inactivated by antibody. 13
The other idea that is often raised is that the larvae are keeping ahead of
the immune response by ceasing to expose antigens to which the immune
response has already begun to respond at an earlier stage of infection. But
this would require no overlap in antigenicity of the different develop-
mental stages, which may be unlikely. Also, if the “keeping ahead”
hypothesis were valid, and immune responses to early larval stages were
effective, how then could Ascaris infections be so long-lived, and involve
continuous recruitment for decades 14 ? Some ideas on this are presented
later in this chapter.
ALLERGENS
One reason that Ascaris has consistently attracted attention is the
allergic-like reactions that it causes. These appear to be of two broad
types. The first is severe and rapid-onset reactions (allergic rhinitis, tears,
etc.) that are occasionally reported by laboratory workers exposed to the
adult worms or extracts thereof. These cases are apocryphal and the
nature of the inciting agent is unknown. Given that a highly volatile
material appears to be involved, there is no particular reason to believe
that it could be due to a protein allergen, which would not be expected to
diffuse through air so readily unless volatilized or droplet-dispersed.
Also, there is no evidence yet that this reaction is strictly immunological.
The other type of allergy-related reaction is those associated with infec-
tions with Ascaris, the classic example being Loeffler's syndrome. 15 This is
considered to be a pulmonary type I immune hypersensitivity reaction
that involves pulmonary eosinophilia and probably also IgE antibody-
mediated events. 16 Loeffler's syndrome has long been associated with
ascariasis, but has not been consistently researched, despite the morbidity
known in arid areas with periodic, high-level seasonal transmission
resulting in large numbers of larvae passing through the lungs. 17,18 The
syndrome has possibly become such a routine feature of ascariasis in
human populations that it is seldom reported, but, when asked, physi-
cians working in endemic areas say it is common and harmful (L. Savioli,
personal communication). Whether mortality associated with Loeffler's
syndrome is significant is simply not known, though an indication of the
potential is indicted by “Mass treatment for ascariasis of aboriginal chil-
dren in Queensland, Australia, has reduced the mortality (mostly attrib-
uted to 'pneumonia') from 29
54% of the total deaths to 8% in the last few
years” (quoted in 19 ). Ascaris-associated pulmonary hypersensitivity
reactions, and liver white spot, are also known in animals such as cattle
and sheep that are exposed to A. suum eggs. 20 So, Ascaris has long been
infamous for the pulmonary immune hypersensitivities it can cause, yet
e
Search WWH ::




Custom Search