Parasitology
Control of gastro-intestinal nematode parasites of ruminants with vaccines
Dr David Smith
Recent Moredun-driven vaccine studies based on parasite gut membrane
proteins have helped define the potential benefits and limitations
of this approach.
An 11 month long trial in South Africa with native worm antigens in
sheep naturally infected with Haemonchus demonstrated that a recombinant
vaccine based on the gut membrane principle should confer substantial
benefits. This experiment was conducted in a climatic zone fairly
typical of that experienced by commercial grazers in the tropics and
sub-tropics. However, similar collaborative studies in Louisiana,
USA where climatic conditions are highly favourable to Haemonchus
development, showed that such a vaccine was unlikely to be of value
in young grazing lambs subjected to a very heavy challenge.
Cross-protection trials to determine whether the same Haemonchus antigens could be effective against either Ostertagia ostertagi or Teladorsagia circumcincta were not encouraging, however. Likewise, no protection was conferred against T. circumcincta when its own gut membrane proteins were used as antigen. With O. ostertagi the effect of vaccination with homologous antigens was more positive in that worm egg output was partially suppressed. This is the most encouraging result published to date with regard to immunisation against this species. Interestingly these proteins cross-protected quite efficiently against Haemonchus, indicating that the basis for the relative homologous antigen vaccine failure with T. circumcincta or O. ostertagi was likely to be because these species do not ingest sufficient immunoglobulin for the approach to be effective.
Considerable progress has been achieved regarding the definition of the protective components in H-gal-GP, a Moredun patented Haemonchus gut membrane protein complex which is highly effective when used as a vaccine antigen. A series of fractionation and other experiments has identified a metalloprotease and an aspartyl protease as the best protective candidates within the complex. The cDNAs encoding these have been cloned and expressed in bacteria, but the resultant recombinant proteins were insoluble and not protective. Attempts to express them as soluble proteins in a Pichia yeast system have recently met with success in the case of the metalloprotease and the intention is to evaluate this product in a sheep protection trial as soon as possible.
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