Parasitology - Blood and Tissue Protozoa (Leishmania) Flashcards
Outline Leishmania.
- Leishmania species are intracellular parasites of the reticulo-endothelial (phagocyte) system. They are related to trypanosomes, but exist in only two morphological forms: amastigotes (non-flagellate forms), which occur in the infected lesion, and promastigotes (flagellate forms that lack an undulating membrane), which occur in the insect vector or in laboratory culture
- Causes leishmaniasis, a vector-borne disease that is transmitted by female sand flies.
- Human infection is caused by various species that infect mammals and cause a variety of different diseases. These species include: L. donovani, L. mexicana, L. tropica, and L. viannia (including L. braziliensis).
- The different species are morphologically indistinguishable, but they can be differentiated by isoenzyme analysis, molecular methods, or monoclonal antibodies.
Outline the epidemiology of Leishmania spp.
- Leishmaniasis is found in about 88 countries exposing 350 million people, 12 million people infected, ~1.3 million new cases every year
- 20,000-30,000 deaths per year
- Mostly prevalent in the tropics and subtropics ranging from rain forests in Central and South America to deserts in West Asia.
- Associated with malnutrition, population displacement, poor housing, a weak immune system and lack of resources.
- Leishmaniasis is a zoonosis. The natural reservoirs include foxes, dogs, jackals, cats rodents.
What is the vector for Leishmania spp.?
Female sand fly
What are the different types of leishmaniasis?
- Several distinct types of disease are recognized, although they are caused by morphologically identical parasites.
- Cutaneous leishmaniasis (oriental sore) is the least troublesome, causing a boil-like swelling on the face or other exposed part of the body. The central part of the lesion may become secondarily infected with bacteria, but the leishmania organisms reside in the raised, indurated edge of the lesion.
- The sore usually heals spontaneously, leaving a scar, but with some species a more severe disseminated cutaneous leishmaniasis may occur. Parasites of the Leishmania mexicana complex may cause a destructive lesion of the outer ear (Chiclero’s ulcer).
- In mucocutaneous leishmaniasis (espundia), which is associated with the L. braziliensis complex, disfiguring lesions of the mouth and nose may be caused.
- However, the most serious form of leishmaniasis is visceral leishmaniasis (kala azar), which is a life-threatening disease involving the whole of the reticulo-endothelial (phagocytic) system. There are estimated to be around 500 000 cases a year in the world, with the greatest burden in north east India and Bangladesh. A late complication of kala azar, post-kala azar dermal leishmaniasis, may be confused with leprosy or other skin conditions.
Outline which species of Leishmania are associated with which forms of leishmaniasis.
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L. donovani: visceral leishmaniasis (kala-azar, dumdum fever)
- Over 90% of new cases occur in six countries: Bangladesh, Brazil, Ethiopia, India, South Sudan, and Sudan
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L. tropica: cutaneous leishmaniasis (Oriental sore, Delhi boil)
- About 95% of CL cases occur in the Americas, the Mediterranean basin, and the Middle East and Central Asia
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L. braziliensis (Subgenus L. Viannia): mucocutaneous leishmaniasis (American leishmaniasis, espundia, chiclero ulcer)
- Almost 90% of mucocutaneous leishmaniasis cases occurs in the Plurinational State of Bolivia, Brazil and Peru
Outline the lifecycle of Leishmania spp.
- Leishmania procyclic promastigotes differentiate in sandflies into infective, non-dividing metacyclic promastigotes, which are located ready for transmission at the stomodeal valve (an invagination of the foregut into the midgut).
- During blood meal, the sandfly regurgitates metacyclic promastigotes, together with immunomodulatory parasite-derived proteophosphoglycans and various salivary components.
- The metacyclic promastigotes are then phagocytosed by one of several possible cell types that are found in the local environment.
- After establishing an intracellular residence, metacyclic promastigotes transform into aflagellate amastigotes.
- Amastigotes undergo replication within host cells, which rupture when too many amastigotes are present, allowing reinfection of local phagocytes.
- The transmission cycle is complete when infected phagocytes are taken up by another sandfly with the blood meal, and amastigotes then convert into promastigotes in the sandfly midgut.
What can bee seen in this image?
Leishmania tropica amastigotes in a biopsy specimen from a skin lesion. An intact macrophage is filled with amastigotes
What are the clinical symptoms/syndromes of leishmaniasis?
- The same microorganism can cause a range of diseases depending upon the host’s immune response
- incubation period <1 yr
- gradual onset of fever, diarrhoea, chills, anaemia and sweating
- marked enlargement of liver and spleen, weight loss; kidney damage if involved
- persistence of disease causes deeply pigmented granulomatous areas of skin (post-kala-azar dermal leishmaniasis);
- if left untreated, death within 1-2 years.
What can be seen in this image?
Cutaneous leishmaniasis sore.
What can be seen in this image?
Mucocutaneous leishmaniasis.
- Lesions spread to mucoid tissues (oral, pharyngeal and nasal) and lead to their destruction and hence sever deformity.
- Partial or total destruction of mucous membranes of the nose, mouth and throat
- Caused by L. braziliensis, L. mexicana and L. peruviana.
What can be seen in this image?
Visceral leishmaniasis.
- L. donovani in visceral leishmaniasis are rapidly eliminated from the site of infection, hence there is rarely a local lesion.
- Parasites localise and multiply in the mononuclear phagocytic cells of spleen, liver, lymph nodes, bone marrow, intestinal mucosa and other organs.
- 1-4 months after infection, there is occurrence of fever, with a daily rise to 102-104º F, accompanied by chills and sweating.
How is leishmaniasis diagnosed?
In the cutaneous or mucocutaneous form of the disease, typical intracellular amastigotes may be recognized in Giemsa-stained smears (pictured) of material obtained from tissues at the margin of the lesion. Free amastigotes are commonly seen because of rupture of the macrophage host cell.
Material should also be cultured in Novy, MacNeal and Nicolle’s (NNN) medium or a modification thereof. This is a rabbit blood agar containing antibiotic to prevent bacterial contamination and a buffered salt overlay solution in which the parasites grow as promastigotes. Incubation is maintained for up to 3 weeks at room temperature (not 37°C).
In kala azar, spleen puncture is the most reliable method of diagnosis, but sternal marrow aspirate is safer and is usually preferred. Smears and cultures are made and examined as for cutaneous leishmaniasis. Various serological tests have been designed, but demonstration of the parasite by microscopy or culture is preferable whenever possible.
How is leishmaniasis treated and controlled?
The pentavalent antimony compounds, sodium stibogluconate and meglumine antimoniate, have been used traditionally in all forms of leishmaniasis, but they are toxic and therapy often fails.
Amphotericin B is effective, but poorly tolerated, and the less toxic lipid-based formulations are preferred.
A phosphocholine derivative, miltefosine, offers considerable promise in kala azar.
Protection from sand flies and insect repellants.
What can be seen in this image?
Kala-azar survivor.
With progression of visceral leishmaniasis, skin develops hyperpigmented granulomatous areas (kala-azar means black disease).
What are the differences in morphology of the different forms of trypomastigotes?
