Page 945 - Veterinary Immunology, 10th Edition

immunoglobulins. These proteases generate Fab fragments that
VetBooks.ir bind and mask parasite antigens. In addition, F. hepatica tegumental

protein suppresses production of IFN-γ and IL-12 by acting directly
on dendritic cells and possibly suppressing signaling by NF-κB.

Many helminths suppress host immunity by promoting the
production of Treg cells and IL-10. Because F. hepatica infestation is
such a strong inducer of Th2 responses, it can adversely affect an
animal's ability to mount Th1 responses and interfere with

diagnostic tests such as the whole blood IFN-γ assay used to
diagnose bovine tuberculosis (Chapter 33). Sheep infected with H.
contortus may become specifically suppressed so that they are
unreactive to H. contortus, even though they remain responsive to

unrelated parasites. Ostertagia ostertagi, Oesophagostomum radiatum,
and Trichostrongylus axei infestations depress calf lymphocyte
responses to mitogens. In other helminth infections, such as
trichinosis, infected animals are nonspecifically

immunosuppressed. This immunosuppression is reflected in a
lowered resistance to other infections, a poor response to vaccines,
and prolongation of skin graft survival.
Parasitic helminths produce a family of immunomodulatory

peptides called helminth defense molecules that resemble
mammalian cathelicidins. One of these molecules produced by F.
hepatica can be endocytosed by host macrophages in which it
prevents endosomal maturation, impedes antigen presentation, and

inhibits antigen carriage to the cell surface in conjunction with
MHC class II molecules

Vaccination

It is not surprising, considering the nature of the host response to

parasitic worms and the availability of cheap and effective
anthelmintics, that vaccines against helminth parasites are not
widely available. Nevertheless, the emergence of anthelmintic

resistance and environmental concerns raised by excessive chemical
use have resulted in an increased interest in anti-parasite vaccines.
Vaccine use is predicated on the assumption that a host's immune
response can control or prevent an infestation. This is not always
obvious in helminth infestations, and traditional vaccines may be of

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