VACCINEStion of living bacteria, the host’s immune system may not be able to contain them and
they could then spread round the body. That is instead of remaining localised they become
systemic.
n The usual route of administration is by injecting intra-muscularly. The major site
or response is then localised but the subsequent reactive cells and antibodies need
to circulate round the body to be protective. An oral route of infection may be more
effective, but only after extensive testing can the correct route be determined.
n Once the bacteria in the vaccine have stimulated an immune response they should
then be destroyed by the reaction that they provoked. Those bacteria that remain live
could become virulent and potentially harmful.
n The host must be healthy and able to respond to the vaccine. If a host has not de-
veloped an immune system or its immune system is damaged or compromised then
the vaccine will not be effective.n 9. 3CUTANEOUS LEISHMANIASIS
A strain of mice known as BALB/c mice which are normally highly susceptible to
Leishmaniainfection were treated with BCG and the result was that the severity of the
infection was reduced. Under experimental conditions, naive hosts infected with L. major
and viable BCG were found to have smaller lesions. BALB/c mice treated in a similar
manner were protected against systemic infection. Complete protection against L. major
cutaneous disease was achieved when BCG immune mice were challenged with viable
BCG and parasites.
BCG has also been used in attempts to protect humans. A vaccine containing 6.4 ×
188 heat-inactivated Leishmania mexicanaamastigotes plus BCG was prepared and admin-
istered to a group of volunteers. After three intra-dermal injections of the vaccine, 49
out of 52 sufferers of localised cutaneous leishmaniasis caused by the parasites of
L. brasiliensiswere cured within 32 weeks. A CMI response to Leishmaniawas instrumental
in the recovery but whether or not the cure was due to the reacting antigen (the epi-
topes) of the Mycobacteriaand Leishmaniacross-reacting, or due to the cytokines pro-
duced modulating the immune response is matter for debate.
A vaccine that expresses Leishmaniaprotective antigen in BCG is a possible goal.
However if that target is to be achieved then there will have to be identification and cloning
of the parasite genes responsible for the production of immunogenic antigens. Following
that, there must be insertion of these genes into a bacterium so they can produce the anti-
gens in vitro. The synthetic antigens have then to stimulate the host’s response.
The current thinking with regard to the function of an anti-protozoan parasite vaccine
is to sensitise the host so as to be able to prevent or control growth and multiplication
of the parasite and reduce the pathological effect upon the host.
n SUMMARY
Vaccination against the cattle lung worm (Dictyocaulus viviparus) is the only truly anti-
parasite vaccine available. A vaccine should be able to produce memory cells in the host
so that once contact is made with the pathogen, a rapid response to destroy the infection
is set in operation.
The target antigen has to be clearly identified and produced either in culture or by recom-
bination techniques so that, once admistered, it can stage an immune response without