Biology Reference
In-Depth Information
Co-infection with Diarrhea-causing Pathogens
Diarrheal diseases are one of the most common categories of life-
threatening diseases for children in developing countries. In some
settings the attack rate can be as high as 12 episodes per year
80
and
diarrhea can be caused by several different pathogenic organisms
(
Figure 4.3
). Despite the high prevalence of Ascaris infection in areas
where diarrhea is common, there are surprisingly few reports on the
effects of co-infection with Ascaris on diarrheal diseases. Diarrheal path-
ogens could potentially alter the impact of Ascaris infection through
modifications of the luminal environment of the GI tract, and through the
generation of strong pro-inflammatory immune responses.
In the GI tract, there are a number of factors that could influence Ascaris
infection in a host co-infected with a diarrheal-causing pathogen. In
general mucous production is increased in diarrheal disease
81
and this
may hinder the penetration of the gut wall by newly-hatched L3 larvae.
Ascaris adult nematodes are large organisms requiring significant levels of
nutrients to maintain their body size. Enteric pathogens causing diarrhea
may reduce the availability of nutritional sources such as microflora for
consumption by adult nematodes, a consequence of mechanisms acti-
vated to flush out the invading diarrheal pathogen.
Ascaris nematodes do not attach to the gut, and instead swim against the
peristaltic flow to remain in position. Anything that disrupts smooth
muscle contractability controlling gut motility (peristalsis) may affect the
ability of Ascaris nematodes tomaintain their position in the gut. Tcells exert
some control over peristalsis in Trichinella spiralis-infectedmice and smooth
muscle contractability is positively correlatedwithTrichinella expulsion.
82
If
a similar mechanism applies to Ascaris infection, any immunosuppressive
immune response that reduces T cell responses in Ascaris (for example, in
HIV infection) may help Ascaris remain in the GI tract.
Immune Responses and Nematode Development
As described for migrating larvae, co-infection with Th1-inducing
diarrheal pathogens may benefit an established Ascaris co-infection by
helping to reduce detrimental Th2-associated effector mechanisms.
However, it is possible that an established immune response generated by
a co-infecting pathogen may benefit Ascaris beyond dampening the
immune clearance mechanisms. Scattered evidence from mouse models
of other medically important helminth infections suggests that immune
responsiveness may provide cues for helminth development.
83
e
86
In Schistosome infections, a lack of immune responsiveness, particu-
larly frommonocytes/macrophages and CD4
Tcells,
83,84
was associated
with poor development of adult Schistosome parasites. There is also
evidence in mouse experiments with the filarial nematode Brugia malayi
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