VACCINES

n 9.1 THE IDEAL VACCINE

Until quite recently the only commercially available anti-parasite vaccine was Dictal

(developed in 1959). Dictal is a vaccine against Dictyocaulus viviparus, the cattle lung

worm and the cause of cattle bronchitis. A similar type of vaccine was developed in

1978 as protection against the dog hookworm Ancylostoma caninumbut was only used in

the USA.

The above two vaccines contain live, attenuated, irradiated larvae in capsules. More

recently a vaccine against the cysticercus of the sheep tapeworm Taenia ovishas been manu-

factured using recombinant technology.

Serious attempts to produce an anti-parasite vaccine against the various human and do-

mestic animal parasites have been ongoing since before the second world war. The great

target for all would-be producers is to manufacture an anti-malaria followed by an anti-

Leishmania, anti-Trypanosomaand an anti-Schistosoma(both human and cattle) vaccine. In

addition there has been and still is research into protecting domestic animals against a

variety of protozoan and helminth parasites, such as Theileria, Trypanosoma, Eimeria,

Fasciola hepatica,Schistosoma bovis etc. Protection of cattle against Schistosoma bovis,

Theileria parvaandT. annulatahas involved the use of live, attenuated infectious stages.

Killed Leishmania mexicanatogether with BCG (Bacillus Calmette–Guérin) has been used

in human trials and gave enough protection at the time to warrant further development.

An ideal vaccine will have to have the following properties:

n Generate enough antibodies to be able to recognise the presence of the parasite antigen.

n Prevent the parasite becoming established in its preferred site within the host’s body.

n Allow for the boosting of antibodies during subsequent infections.

n Activate antibody-dependent cell-mediated cytotoxicity (ADCC).

n Induce a T cell cytotoxic-dependent response capable of killing the parasites directly

or indirectly.

n Prevent the parasite from reaching any form of maturity and/or multiplying asexually.

n 9.1.2 THE TARGET ANTIGEN

A vaccine has to identify that part of the parasite which constitutes the reactive antigen,

the immunogenic antigen or the epitope, provoking an immune response. The epitope

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