Biology Reference
In-Depth Information
animal experimental models showing the capacity of
Ascaris
antigens to
induce parasite-specific IgE response and allergic symptoms.
61
e
64
However, since the relationship with allergy is not as evident as in ani-
sakiasis, and some reports have demonstrated the immunosuppressive
effect of chronic, heavy load infections in rural populations,
65
there is the
erroneous belief that ascariasis only induces immune downregulation.
ASCARIASIS INFLUENCES THE PREVALENCE
OF ASTHMA
Ascariasis has been associated with significantly enhanced likelihood
of asthma
66
but this topic is controversial because studies have shown
conflicting results. In some population surveys, the infection is a predis-
posing factor for IgE sensitization and asthma,
66
e
74
while in others it is
protective.
75
e
78
There are several reasons for these apparently contra-
dictory findings; the most important is that, in fact, ascariasis induces
both suppression and enhancement of allergy symptoms according to
the intensity of the infection.
79
e
82
Most of the studies concluding that
this parasitic infection is associated with a low prevalence of allergy
(or negative skin tests to common allergens) have been undertaken in
heavily infected populations
79
and vice versa; those associated with
higher prevalence of asthma have been performed in urban, lightly
infected communities. Why the infection results in these two contrasting
effects is unknown. Whether it reflects just a higher exposure to the
parasite or is the result of more complex gene
e
environment interactions
remains to be solved.
When analyzing the effect of ascariasis on the prevalence of asthma, it
is necessary to define how the diagnosis was made. Some surveys use the
specific IgE to
Ascaris
extract as a marker for ascariasis, a sensitive way to
evaluate exposure to this parasite,
83
but disregard the potential bias from
cross-reactivity between mites and
Ascaris
on the specificity of the test.
84
There is evidence suggesting that mite
eAscaris
cross-reactivity influences
the results of epidemiological surveys.
49
Not all authors adjust for this
confounding variable, but in a study showing that specific IgE against
A. lumbricoides
extract was a risk factor for the number of positive skin
tests and bronchial hyper-reactivity, the logistic regression analysis
showed that the significance disappeared when correcting for specific IgE
to mites and cockroaches.
69
We will further analyze the mite
eAscaris
cross-reactivity in this
chapter. Here it is important to say that in spite of these and other limi-
tations related to the study design and the heterogeneity of the pop-
ulations surveyed, the number of publications supporting the association
between ascariasis and a high prevalence of asthma or asthma-associated
