Biology Reference
In-Depth Information
pulmonary inflammation in humans
137
e
139
and asthma severity markers,
are one of the most important mechanisms of helminth elimination in
experimental animal models.
9,140
It can be speculated that the highly
Ascaris
-resistant population (whether atopic or not) may also overreact
with this cytokine-mediated defense mechanism, causing tissue damage
and increasing the severity of pulmonary symptoms. In this regard, it has
been reported that during an experimental helminth infection there was
first a strong IL-17 response that caused lung damage, which was
resolved by a parasite-induced Th2 responses that included IL-10 and M2
macrophages.
141
However, the actual role of these cytokines on the
severity of asthma deserves more investigation. The current possibility of
detecting very low levels of proteins in the circulation makes it possible to
simultaneously evaluate the immunomodulator molecules and these
cytokines during human ascariasis.
PARASITE INFECTIONS, ALLERGY,
A
ND THE HYGIENE HYPOTHESI
S
The hygiene hypothesis, formulated after the epidemiologic finding of
lower prevalence of hay fever associatedwith poor hygiene conditions and
other protecting factors,
142
predicts that allergic diseases aremore frequent
where the improvement of hygiene conditions has been really successful,
making microbial infections infrequent in early childhood. Several mech-
anisms explain how the hygiene hypothesis works. Immune deviation to
a predominant Th1 response due to bacterial and viral infectionswas, at the
beginning, the most obvious explanation; but there is recent evidence that
one of the most potent Th1 inducers, tuberculosis, instead of protecting is
positively associated with allergy.
143
An important point is that, according
to the hypothesis, it is expected that allergic diseases have a lowprevalence
where hygiene conditions are poor, and this actually occurs at least in some
geographical locations, but this is likely to be because of chronic helminth
infections.
65,82,144
More interestingly, in several moderate- to low-income
countries of the tropical zone, the prevalence of asthma and other
allergic diseases is high and concurs with early exposure to bacterial and
viral infections.
38
e
41,46,145
e
150
Therefore, helminthiases may explain not
only why poor hygiene conditions are associated with low allergy
frequency,
151,152
but also why the increasing trend in the prevalence of
allergies is more general and not restricted to affluent countries with good
hygiene conditions.
153
Typically, soil-transmitted helminth infections are susceptible to
change in frequency when hygiene conditions improve. During the last
decade, the immunosuppressive effects of chronic, heavy helminth
infections have been described in both humans and other animals, being
