Leishmania spp and Leishmaniasis: Part 2 Lecture Notes
Key Words and Terms
Leishmania species, Leishmaniasis, protozoa, macrophage, procyclic promastigote, amastigote, metacyclic promastigote, blood born disease, Lipophosphoglycan (LPG), sand fly, insect vector
Leishmania spp (species) are protozoans that cause Leishmaniasis in humans. They belong to the same family as the Trypanosoma species and are closely related. They are intracellular parasites. The incidence of disease is more wide-spread then that of T. cruzi and is present in Africa, the Middle East, regions around the Mediterranean Sea, and Central and South America. This is also a disease of poverty and is present in underdeveloped regions of the world, but in contrast to T. cruzi, development of these countries is not really stopping the transmission of Leishmania species. Recently the disease in moving from rural areas to more urban areas, and as it moves it comes into contact with susceptible individuals (there are ~300 million susceptible people in the world) and the incidence may increase. In 2007 there were approximately 12 million people in the world that were infected.
There are many different species of Leishmania and the disease that they cause is directly linked to the species of Leishmania with which a person is infected. The cutaneous form of Leishmaniasis is caused by L. major, L. tropica, or L. mexicana. The cutaneous form is characterized by a skin lesion that is normally self-healing. The mucocutaneous form is caused by L. braziliensis and this is characterized by degradation of the mucosal membranes and can cause severe disfigurement. The most severe form of Leishmaniasis is the visceral form that is caused by L. infantum, L. chagasi, and L. donovani. This form is characterized by severe enlargement of the liver and spleen, fever, weight loss, and anemia. The mortality rate is highest with the visceral from of Leishmaniasis.
Even though the disease has been around for a long time, it wasn’t until 1900 – 1903 when the Leishmania organisms were first characterized. William Leishman, a British medical doctor that was analyzing samples from patients in India, and Charles Donovan, an Irish investigator, noticed that the macrophages in the spleen of infected people are heavily infected with organisms that looked similar to trypanosomes; they were initially called Leishman-donovan bodies. In 1904, Leonard Rogers and in 1908, Charles Nicolle took spleen tissue from patients and put it in culture and they were able to show that a second extracellular form of the organism appeared in these cultures. This indirectly indicated that the organisms had a life cycle similar to Trypanosoma and possibly an insect vector. It wasn’t until 1924 that John Sinton discovered the first clues about the vector. He noticed that the incidence of visceral Leishmaniasis was associated with the distribution of a specific species of sand fly. In 1928, Knowles was able to detect Leishmania in sand flies yet it wasn’t until 1942 that Swaminath formally demonstrated that sand flies could transmit Leishmania to humans. Around this time (1940s) the life cycle of Leishmania was beginning to be understood; the parasite replicates prolifically in the sand fly gut and is then transmitted to humans/dogs/rodents after a blood meal via regurgitation. The parasite then invades and replicates within the macrophages of the host.
In 1984-1985 David Sacks at the National Institute of Health (NIH) identified the infective stage of Leishmania. He showed how the different forms of the parasite are resistant or susceptible to the complement system. He showed that the procyclic promastigote form differentiates to the complement resistant metacyclic promastigote form in the sand fly mid gut. In 1990, Malcolm McConville and David Sacks showed that differentiation into metacyclic promastigote forms involves the surface molecule Lipophosphoglycan (LPG), which is a major component of the surface coat of the parasite. In the procyclic promastigote form, LPG is a shorter molecule with sugar side chains terminating in galactose. In the metacyclic promastigote form, LPG is an elongated molecule with sugar side chains that terminate in arabinose. This modification of LPG promotes detachment of the infective form of the parasite from the mid-gut of the sand fly. He later went on to show in 1994, that vectoral competence for different species of Leishmania is associated with LPG polymorphisms. This means that the ability of specific species of sand flies to transmit a specific species of Leishmania is based on different polymorphisms of the LPG molecule. This showed the molecular properties of the parasites that caused specific associations with vectors.
Since the portion of the life cycle that occurs in the insect has been studied extensively, the next frontier is to understand replication in the human host, especially because the parasite replicates inside macrophages, which is normally the cell that would kill the parasite. We do not completely understand how the parasite is able to survive within macrophages. It is now known that the entry of the parasite into the macrophage occurs via receptor mediated phagocytosis using complement receptors CR1 and CR3, which bind to the complement proteins, which have opsonized the parasite. Jean Claude Antoine at the Pasteur Institute has shown that Leishmania survives in intracellular compartments that share properties with lysosomes – they are acidic, contain a full set of lysosomal enzymes (hydrolases), and the membranes contain lysosomal membrane proteins (i.e. MHC II). These parasites can replicate quite well, despite the harsh conditions.
When the sand fly bites a human, dog, or rodent, it regurgitates the organism into the bite wound. The organism then infects macrophages and replicates intracellularly. In the sand fly the parasite is in the promastigote form that changes from procyclic to metacyclic, and the metacyclic promastigote form is the infectious form that is regurgitated into the bite. This form invades macrophages and then transforms into the amastigote form, which replicates with in the macrophage. The doubling time is ~12 hours and the life cycle takes several days. Eventually the macrophage is lysed and the amastigote form in released into the blood.
Control of Human Transmission
The control of transmission is complicated because it is difficult to limit exposure of humans with sand flies because they are very small insects. The simple measures that have proved very efficacious in control of the spread of malaria (i.e. bed nets), have not worked in this specific case because the sand flies are so small they can pass through the bed nets. There is a strong need for new, less toxic drugs to treat the different forms of Leishmaniasis. The drugs that are currently being using throughout the world to treat Leishmaniasis are very toxic, but there are some new drugs that could prove useful to treat the disease.