Implications of Ascaris Co-infection - Ascaris: The Neglected Parasite - page 91

whether the co-infected host is resistant or susceptible to the pathogenesis
of RSV infection. A diseased individual may prevent larval migration
through the lung because prevailing anti-viral Th2 responses may poten-
tiate quicker formation of white spots and overproduction of mucous
( Figure 4.4 B) may trapmigrating larvae. On the other hand, the lungs of an
individual who has been challenged with RSV infection but mounts
a strong Th1 response to the virus may facilitate passage of migrating
Ascaris larvae through the lungs by antagonizing the Th2 responses
triggered by the migrating larvae, and dampening the infiltration of
eosinophils and neutrophils leading to white spot formation.
Pre-existing Th1 responses affecting Ascaris larval migration in
a similar way may also be encountered in individuals infected with
hepatitis B virus (HBV), a common infection in African populations. This
virus can be passed frommother to child and chronic infection can lead to
cirrhosis of the liver with infiltration of immune cells ( Figure 4.4 C) in
response to Th1 and Th17 immune responses. 75 In the lungs, Mycobacte-
rium tuberculosis infection leads to the formation of granulomas
( Figure 4.4 D) that contain, but do not completely eliminate, the
M. tuberculosis bacilli. 76 Lung granulomas that form in response to
M. tuberculosis do so in the context of ongoing Th1 responses maintained
by chronic low-level antigen stimulation and the cellular composition is
quite different to that of granulomas surrounding Schistosome eggs. 77
Pre-existing Th1 responses present in the lungs of a person with tuber-
culosis may reduce Th2-mediated white spot formation in response to
migrating Ascaris larvae potentiating Ascaris infection.
Gastrointestinal Tract
Co-infection with Other Geohelminths
The GI tract is likely to be inhabited by nematode infections in addition
to Ascaris and these include hookworm (Necator americanus or Ancylostoma
duodenale), Trichuris trichiura or Strongyloides stercoralis. Several studies have
shown that co-infection with two or more geohelminth infections is
a common finding in populations living in endemic areas 78 demonstrating
that GI nematode infections such as Ascaris do not require exclusive use of
the GI tract to survive. The immune responses generated by other
GI-dwelling nematodes are of a similar skewas that generally described for
Ascaris infections in that Th2 and immunoregulatory immune responses
are dominant. 79 The extent to which antigen-specific immune responses
generated by one GI nematode infection are able to influence responses to
a second GI nematode infection is currently unknown. However, it can be
hypothesized that in multi-geohelminth co-infections mutually beneficial
immune responses are generated via the bystander effects of immuno-
regulatory cytokines, particularly to cross-reactive antigens.