Page 930 - Veterinary Immunology, 10th Edition

VetBooks.ir Immunity to Helminths

Worm burdens tend to increase slowly over time and reach a peak
before slowly declining. This decline may reflect protective

immunity. The major defense against gastrointestinal helminths is a
type 2 adaptive immune response. The success of this response
depends upon the animal's age, its nutritional status, and its
genetics and the site of infection, as well as the species of worm
involved and whether the infection is sudden and large or a much

slower trickle infection. Thus lambs infected with Nematodirus
battus rapidly develop resistance, while immunity to Teladorsagia
circumcincta is slow to develop. It takes up to a year for lambs to

fully develop their resistance to nematodes. Adult sheep generally
harbor only a few adult nematodes but periodic re-exposure is
required to maintain their immunity. Resistance to abomasal
worms develops more slowly than resistance to intestinal worms. A
reduction in resistance to nematodes is seen in ewes from 2 weeks

prior to 6 weeks after lambing as a result of periparturient
immunosuppression. One consistent feature of intestinal nematode
infestations is the very wide variation in parasite load within a

population. Most animals harbor a few worms, but a few animals
harbor a lot of worms.

Innate Immunity

Innate factors of host origin that influence helminth burdens

include the age, sex, and most important, the genetic background of
the host.
The influence of sex on helminth burdens appears to be largely
hormonal. In animals whose sexual cycle is seasonal, parasites tend

to synchronize their reproductive cycle with that of their hosts. For
instance, ewes show a “spring rise” in fecal nematode ova, which
coincides with lambing and the onset of lactation. Similarly, the
development of helminth larvae in cattle in early winter tends to be

inhibited until spring in a phenomenon called hypobiosis. The
larvae of Toxocara canis may migrate from an infected bitch to the
liver of the fetal puppy, resulting in a congenital infection. Once

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