Page 946 - Veterinary Immunology, 10th Edition

little use. Despite this, a recombinant T. ovis vaccine has been
VetBooks.ir produced that can induce protective immunity in sheep. This

vaccine contains a cloned oncosphere antigen (To45W) with a
saponin-based adjuvant. It stimulates a response that prevents

parasite penetration of the intestinal wall. Similar single-antigen
recombinant vaccines are highly effective against E. granulosus in
sheep and cattle.
Protection against some helminths has been obtained by the use

of live irradiated organisms. The most important of these is the
vaccine used to protect calves against pneumonia caused by the
lungworm Dictyocaulus viviparus. In this vaccine, second-stage
larvae hatched from ova in culture are exposed to 40,000 R X-

irradiation, and two doses of these larvae are then fed to calves. The
larvae can penetrate the calf's intestine, but since they are unable to
develop to the third stage, they never reach the lung and are thus
nonpathogenic. During their exsheathing process, the larvae

stimulate the production of antibodies that can block reinfection.
The efficiency of this vaccine depends very much on timing and on
the size of the challenge dose since even vaccinated calves may
show mild pneumonic signs if placed on grossly infected pastures.

The major helminth antigens are of two types, soluble excretory-
secretory products and antigens bound to the parasite surface
(somatic antigens). Important secretory products include
periredoxin, annexins, galectins, heat-shock proteins, HMGB

homologs, and S100 homologs. The immunodominant antigens of
nematodes are polyprotein allergens and antigens that act as lipid-
binding proteins. Another important somatic antigen is the enzyme
γ-glutamyl transpeptidase. Some somatic antigens, such as those in

the parasite gut, are hidden since they are not normally exposed to
the host's immune response and may therefore be potential
candidates for vaccines. For example, vaccination of lambs and kids
against the intestinal aminopeptidase of H. contortus (called H11)

has resulted in the development of antibodies against Hemonchus
intestinal proteases. These presumably interfere with worm
digestion and reduce both parasite numbers and fecundity. Because
these molecules do not enter the tissues they are not normally
recognized by the sheep immune system. As a result, up to five

doses of this vaccine are required for optimal protection. This

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