Leishmania tropica belong to a large group of flagellated protozoa that live intracellularly in macrophages in skin tissue and require host switching between sand flies or butterfly mosquitoes and vertebrates for their dissemination. They are causative agents of cutaneous leishmaniasis, also known as oriental bubonic disease, which is mainly prevalent in southern Europe and Asian countries. The protozoa are able to survive phagocytosis when they enter the bloodstream and multiply intracellularly in macrophages in the blood.
What are Leishmania tropica?
The flagellated protozoa Leishmania tropica form a subspecies of the genus Leishmania and are also known as hemoflagellates because of their characteristic lifestyle. They require host switching between humans or other vertebrates and the sand fly (Phlebotomus) or the butterfly mosquito (Nematocera) to spread. Host switching is associated in each case with a switch between the flagellated (promastigote) and unflagellated (amastigote) forms of the pathogen. In the infected mosquito, promastigote pathogens mature and actively move to the mosquito’s biting apparatus by means of their flagella. When the mosquito bites into a blood vessel of a human or other host animal, the flagellated pathogens enter the surrounding tissue. They are recognized by the immune system as hostile and are therefore phagocytosed by polymorphonuclear neutrophil granulocytes (PMN). Leishmania tropica survive phagocytosis and are initially protected intracellularly. They reach their actual host cells, macrophages, after apoptosis of the PMN and renewed phagocytosis – in this case by macrophages. They transform intracellularly within the macrophages into the amastigote form and can proliferate by division. After re-release of the pathogen in the blood, an uninfected mosquito or an already infected mosquito can ingest pathogens via its proboscis, which convert back to the amastigote form in the mosquito, completing the cycle.
Occurrence, distribution, and characteristics
Leishmania tropica is distributed mainly in countries of western and central Asia. Endemic occurrence is shown in a strip from Turkey to Pakistan, in parts of India, in Greece and in some areas of North Africa. The parasite is infectious only when it is introduced directly into the bloodstream in the flagellated form. Naturally, infection occurs through the bite of infected sandflies or butterflies. The pathogens are located in the mosquito in the immediate vicinity of the mosquito’s sucking apparatus. They are flushed out with the anticoagulant secretion that the mosquito ejects into the bite wound to prevent blood clotting and are immediately transported into the surrounding tissue. In the tissue, they are captured and phagocytosed by the first wave of immune defense against pathogens, the polymorphic neutrophil granulocytes, but for the most part they manage to survive phagocytosis by producing chemokines that prevent PMNs from releasing their proteolytic substances. In addition, the flagellated form of the pathogen is able to secrete chemokines that suppress certain chemokines in neutrophil granulocytes that normally attract other leukocytes such as monocytes and NK cells. By secreting an enzyme that increases the mean survival time of neutrophil granulocytes from normally a few hours to two to three days, the pathogens can “wait” for macrophages, their definitive host cells, to appear. They actively assist their host granulocyte in secreting chemokines that attract macrophages. Apoptosis, the programmed and ordered cell death of PMNs, stimulates macrophages to phagocytose the apoptotic cells without releasing their proteolytic substances. The amastigote Leishmania tropica can thus be taken up unrecognized and unharmed by the macrophages together with the fragments of the granulocytes and are now intracellularly safe, so to speak. In the macrophages, the pathogens transform from the promastigote to the amastigote form and multiply by cell division.
Diseases and disorders
Leishmania tropica is the causative agent of a cutaneous form of leishmaniasis. The bite of an infected sandfly transmits the pathogen into the skin tissue, causing symptoms of the disease to appear after an incubation period averaging two to eight months.In exceptional cases, the incubation period can be much longer, up to several years. Leishmaniasis tropica leads to dry, highly keratinized skin bumps that are painless and do not itch. If left untreated, the skin bumps usually heal on their own after 6 to 15 months, but sometimes leave disfiguring scars. After the disease has healed, there is usually lifelong immunity. In rare cases, recurrent (recurring) cutaneous leishmaniasis may occur after one to 15 years. Usually, the recurrent form of the disease manifests itself in multiple papules that slowly enlarge at the irregular margins and slowly keratinize and heal from the center. There are relatively few pathogens in the papules. Unlike visceral forms of the disease (which affected the viscera), cutaneous leishmaniasis tropica is usually more benign, but usually leaves unsightly scars. Few systemically acting antibiotics and also a locally applied antibiotic are available for treatment. Neither vaccination nor other directly preventive measures to prevent infection are non-existent. The best protection is to use a mosquito net at night in areas at risk and apply mosquito repellent during the day.
