The world poultry industry is under great stress due to parasitic diseases; coccidiosis being one which is responsible for substantial economic losses worldwide. In 1948 the first research paper was published about the treatment of coccidiosis with sulphaquinoxaline. After six decades, researcher's attention has focused on DNA vaccination, especially as certain anticoccidials have failed due to drug resistance and residues. Thus far vaccination is partially successful but is accompanied by disadvantages: e.g. instability, inferiority control, cost-effectiveness, and inefficiency in opposition to a large number of coccidian strains which are prevalent in different geographical areas. Due to developments whereby genetically engineered DNA can be administered in vaccine form to provoke cellular and humoral immune responses; there has been huge development in the practical application of this field. In the last decade a number of DNA vaccines employing different strategies have been tested to produce appropriate immune responses against coccidiosis. The DNA fragments taken from all the four important species, E. tenella, E. necatrix, E. maxima and E. acervulina were able to provoke appropriate immune responses against challenging infections with homologous species; however, most of them were not able to provoke a response with heterologous infection. The shared DNA antigen in two different species of Eimeria; E. tenella and E. acervulina was able to produce sufficient immune responses, not only against these species but also against E. necatrix, but not against E. maxima. E. maximum is the biggest and most complex of all the seven species and it has come ahead as a challenge for DNA vaccine researchers.

A DNA-construct coding for the elastase of the parasite Schistosoma mansoni was prepared from adult S. mansoni worm RNA which was reverse transcribed into cDNA. The gene coding for the elastase was amplified using primers specific for the sequence of cercarial elastase and was cloned into a mammalian expression vector. Expression of the elastase gene at the transcriptional level was achieved for the first time in transfected mammalian cells (COS-7) and was also successful in muscle tissue of mice injected with the DNA-construct. These mice developed antibodies recognizing in Western blots the elastase from cercarial secretions. Also, these antibodies reacted in immunofluorescence tests with the preacetabular glands of cercariae, i.e. the site of origin for elastase. Thus, the DNA-construct induced the expression of elastase in mice and formation of antibodies that recognized the native antigen.