Genetically modified live attenuated parasites as vaccines for leishmaniasis

Leishmaniasis causes significant morbidity and mortality worldwide and is an important public health problem. Even though it is endemic in developing countries in tropical regions of the world,in recent years economic globalization and increased travel has extended its reach to people in developed countries. Leishmania is usually spread by the bite of the female sandfly. In addition, naïve populations can be exposed to Leishmania infection through transfusion of blood and blood products from infected asymptomatic individuals. There are several clinical forms of leishmaniasis caused by different species of the parasite. In some cases, the only possible cure for this disease is drug treatment. However, prolonged use of such drugs has led to parasite drug resistance. At present there are no effective vaccines against Leishmania. Many vaccine strategies have been pursued, including the use of whole cell lysate, killed, avirulent or irradiated parasites. Additionally, DNA vaccines and purified or recombinant parasite antigens have also been tested. Most of these strategies have shown some degree of effectiveness in animal models but little or no protection in humans. There is now a general consensus among Leishmania vaccine researchers that parasite persistence may be important for effective protective response and could be achieved by live attenuated parasite immunization. In this article we reviewed the efforts in developing genetically defined live attenuated Leishmania parasites as vaccine candidates with the goal of achieving a low level of parasite persistence without being virulent in the host and inducing protective immunity.