Direct microscopy is widely used for the diagnosis of parasitic infections although
it often requires an experienced microscopist for accurate diagnosis, is labour intensive
and not very sensitive. In order to overcome some of these shortcomings, molecular
or nucleic acid-based diagnostic methods for parasitic infections have been developed
over the past 12 years. The parasites which have been studied with these techniques
include the human Plasmodia, Leishmania, the trypanosomes, Toxoplasma gondii, Entamoeba
histolytica, Giardia, Trichomonas vaginalis, Cryptosporidium parvum, Taenia, Echinococcus,
Brugia malayi, Wuchereria bancrofti, Loa loa and Onchocerca volvulus. Early methods,
which involved hybridisation of specific probes (radiolabelled and non-radiolabelled)
to target deoxyribonucleic acid (DNA), have been replaced by more sensitive polymerase
chain reaction (PCR)-based assays. Other methods, such as PCR-hybridisation assays,
PCR-restriction fragment length polymorphism (PCR-RFLP) assays and random amplified
polymorphic DNA (RAPD) analysis have also proved valuable for epidemiological studies
of parasites. The general principles and development of DNA-based methods for diagnosis
and epidemiological studies will be described, with particular reference to malaria.
These methods will probably not replace current methods for routine diagnosis of parasitic
infections in developing countries where parasitic diseases are endemic, due to high
costs. However, they will be extremely useful for genotyping parasite strains and
vectors, and for accurate parasite detection in both humans and vectors during epidemiological
studies.