The protection of humans and domestic animals against parasitic infections remains a major goal, especially in light of developing of drug resistant strains in many parasite species. "Classic" vaccines are based on attenuated infective stages of protozoan and helminth parasites. Although such vaccines are effective in confering host immunity against several Protozoan (coccidiosis, giardiosis, toxoplasmosis) diseases and one helminth (dictyocaulosis) they are very unstable and expensive. Recombinant techniques enable to obtain protective antigens quickly and in considerable quantities, cultivating of the bacteria and purification of the recombinant protein is less expensive than the maintenance of host animals and isolation of the protective antigens from harvested parasites. Moreover, the cloned protective antigens may be deprived of epitopes responsible for immunopathology. However, at present only one anti-parasite recombinant protein vaccine is commercially available (TickGARD). Such a situation may result from that many protective parasitic antigens cannot be expressed in bacteria or yeast in anative from. DNA vaccines present many advantages over protein ones. Firstly, the antigenic proteins synthesised within the host cell possess an appropriate molecular structure and undergo a posttranslational modifications specific for a native protein. The next advantage of DNA vaccines is that DNA is easier to handle and more resistant than proteins to temperature changes. DNA vaccines are likely to induce novel mechanisms of i mm une response, which may be beneficial in case of parasitic invasions. Costs of DNA vaccines are comparable, and may be even lower, in comparison to recombinant protein vaccines. The main obstacle preventing the use of DNA vaccines is still Jack of the complete knowledge conceming mechanisms of their action. Vaccines based on transgenic plants (=edible vaccines), expressing the protective parasitic antigens, present another promising approach in research on anti-parasitic vaccines. Such vaccines may be of special importance in prevention of infections with gastrointestinal parasites.
The aim of the study was to evaluate an influence of vaccination of the final host on F. hepatica development in intermediate hosts. Fluke eggs were isolated from the biliary tracts of calves vaccinated orally with recombinant cysteine proteinase of F. hepatica after the challenge infection and from control calves which received the infection only. To asses the ef fect of the vaccine on egg "hatch rate" the eggs were transferred to the Petri dishes with distilled water and incubated at 25°C for 16-19 days. They were subsequently exposed to light for about 2 h, at a temperature of 27 ± 1°C, to stimulate sprouting of the miracidia and asses the egg hatchability. In order to evaluate infectivity and pathogenicity of the miracidia, single miracidium infections of Lymnea truncatula by F. hepatica were carried out under laboratory conditions using 4-mm-high snails. The prevalence of snail infections with F. hepatica was calculated using the ratio between the number of cercariae-shedding snails in each group and that of surviving snails. It appeared that the eggs isolated from immunized calves demonstrated significantly lower hatchability than the eggs isolated from non-vaccinated control hosts. Also, the proportion of infected snails as well as their mortality were lower after exposition to miracidia originating from vaccinated calves. It is suggested that effectors of the immune response in vaccinated calves inhibited in part biological activity of cysteine proteinases of the fluke which are known to be involved in egg shell formation, penetration of host's tissues and worm feeding.