The organizers of this symposium felt that it could be helpful if the first paper provided a general introduction to the concepts and techniques associated with this rapidly developing area of biology. It is widely but wrongly believed to be a complex and difficult subject. The details of some of the manipulations are sometimes extremely complex, but the general principles are not. Of those responsible for this paper, Andrew Simpson and Tina Walker are active workers in the field, but Roland Terry is not. Following the principle embodied in an old Arabic proverb (Fig. 1), roughly translated as ‘What one fool can understand, another can’, Roland Terry presented the paper.

Three species of filarial worms, Wuchereria bancrofti, Brugia malayi and Brugia timori, are the causative agents of lymphatic filariasis in man, defined by the characteristic tropism of adult worms of each species for the afferent lymphatics. Reproductive activity leads to the release of large numbers of microfilariae, which circulate in the vascular system, and upon ingestion by an appropriate mosquito vector, develop through to infective third-stage larvae (L3) within 10–14 days. After a subsequent bloodmeal, the infective larvae enter the definitive host via the wound and mature to the adult stage over several months, involving two moults, during which the entire nematode exoskeleton (cuticle) is replaced.

Schistosomiasis is a chronic debilitating disease which accounts for considerable human mortality and morbidity and is second only to malaria in its prevalence and severity. Despite several years of concentrated effort to combat the disease with molluscicides and chemotherapy, schistosomiasis is still spreading, and it is thought that 500 million people are currently exposed to infection (larotski & Davis, 1981).

During the past decade, enormous technological developments have occurred in biology that have led to significant and revolutionary advances. New techniques of DNA cloning, restriction-enzyme analyses and nucleotide sequencing are providing a mass of data concerning the genomes of a wide variety of organisms. Such insights are having a great impact upon many areas of biological investigation, and would seem to be of considerable potential value for studies of taxonomy and population biology. The application of these new approaches to the characterization and identification of parasitic helminths has only recently begun, but they promise to become powerful additional tools for this purpose. Better methods of characterization are required for more precise definition of the parasites of man and domestic animals and for determining vectors and intermediate hosts as well as possible animal reservoir hosts. Moreover, a greater understanding of the genetic diversity of parasitic organisms is required since many helminths, which are morphologically similar, show marked differences in epidemiologically significant factors such as infectivity, pathogenicity, immunogenicity and drug sensitivity.

As part of an integrated programme investigating human schistosomiasis, work which involves epidemiological surveys and detailed immunological studies as well as biochemical investigations, we have, over the last three years, been cloning schistosome genes in a variety of plasmid and lambda vector systems. In this lecture we present a review of some selected aspects of work primarily aimed at production of experimental vaccines against the disease but which, on a broader front, is also concerned with developmental regulation of gene expression around the parasite's life-cycle. Specifically, we are interested in cloning three groups of genes. First, those encoding surface antigens; second, those associated with sexual maturity and egg production; and third, antigens which may provide a basis for a specific immunodiagnostic test.

Sporozoites of P. falciparum and other Plasmodia appear to be fairly simple antigenically, in that there is a dominant antigen, the circumsporozoite (CS) protein that forms the sporozoite surface coat (Potocnjak, Yoshida, Nussenzweig & Nussensweig, 1980; Santoro et al. 1983). Consequently, the CS protein and the gene encoding it have now been studied in considerable detail (Ellis et al. 1983; Godson et al. 1983; Ozaki et al. 1983; Dame et al. 1984; Enea et al. 1984). In contrast to sporozoites, the asexual blood stages of P. falciparum are antigenically complex. Two-dimensional gel analyses of immunoprecipitated, biosynthetically labelled antigens indicate that repeated infection with P. falciparum results in the synthesis of antibodies against a large number of distinct antigens (Perrin & Dayal, 1982; Brown et al. 1981, 1983). In further contrast to the sporozoite, the asexual blood stages of different P. falciparum isolates exhibit a high degree of antigenic heterogeneity (Brown et al. 1983; Hall et al. 1983; McBride, Walliker & Morgan, 1982). Much of this antigenic diversity is no doubt due to allelic differences but clonal populations of parasites may also have the capacity to undergo antigenic variation (Hommel, David & Oligino, 1983).

Smallpox has been eradicated by immunization with live vaccinia virus. Now, using genetic engineering, infectious vaccinia virus recombinants are being constructed which express genes from other pathogens. These viruses can stimulate specific immunological responses against the foreign antigen in vaccinated animals and can protect against disease caused by the corresponding pathogen. In this paper we describe how recombinant vaccinia viruses are constructed and illustrate the potential of this vector system for expression of genes from parasitic pathogens.

The human malaria parasite, Plasmodium falciparum, is currently being actively studied by molecular biologists. It is hoped that the use of recombinant DNA techniques in this area will give new insights into the biology of the organism and, at the same time, provide new approaches to diagnosis and vaccine development.

Our own studies employ the blood stages of the parasite and cover three main areas: enzymes of importance in parasite metabolism; antigens of potential use in a subunit vaccine; and repetitive DNA as a probe able to distinguish genetically different isolates of P. falciparum and as a species-specific diagnostic tool in human and mosquito infections.

Human leishmaniasis is a world-wide public health problem with more than 400000 new reported cases/year (Anon, 1984) in the tropics and subtropics. Were (1985) has estimated that Leishmaniases affect about 20 million people in the Third World. In hospitals where treatment is available, initially all positive cases are treated equally; it is the prognosis and follow-up that varies according to the causative organism. A wide variety of species or subspecies of Leishmania may be present in a small verrucose lesion developing after an infected sandfly bite or appear in lesions as the disease progresses. Depending on which Leishmania is present a clinial decision must be made whether to treat with drugs of varying toxicity or not.