Discuss the influence of environmental factors and animal reservoirs on the transmission of diseases caused by African trypanosomes and Leishmania parasites.


To answer this question first must understand what they are. These are both trypanomastid, protozoan parasites with kinetoplasts. This is an extracellular DNA that consists of maxi and mini circles of DNA which contain enzymes for mitochondria. They both have indirect life cycles where they require 2 hosts. Draw below:
Trypanasome:
Metacyclic -> long slender -> short stumpy -> procyclic -> epimastigote -> metacyclic
Leishmania:
Promastigote (infectious) -> Amastigote (back to sand fly)
Here it is clear to see that vectors play a part in their transmission.
Nature of sandfly vs Tsetse
Tsetse is definitive host, takes up parasite on tenerial feed. Within the fly it matures and becomes infective. The actual tsetse fly is not that common as it lays pupae, not eggs. Also it prefers feeding on livestock or game rather than humans.
Sandfly is much more common as each fly lays around 100 eggs. They lay them into organic matter such as soil.
What is the relevance of this? It means that we can control the transmission of both of these diseases by controlling the vector. For example use of repellents or insecticides are crucial. This could also involve prophylactic treatment of individuals going to areas infested by this organism.
Consider the turnover of each of these flies is fairly rapid so another species must be involved –animal reservoir.
The animal reservoir
The role of the animal reservoir varies depending on the nature of the disease. Generally acute diseases depend a lot more on the animal reservoir than chronic. This is seen in the different subspecies of T. brucei.
T. Brucei Rhodesiense – found in east Africa
Acute – sleeping sickness
Heavily dependent upon animal reservoir
T. Brucei Gambiense – Found in west Africa
Chronic – Asymptomatic carriers for many years
Human – Tsetse – Human transmission commonly seen.

Both of these diseases has a characteristic 2 phase disease progression:
·         Phase 1 pre CNS – Chancre and swollen lymph nodes (characteristic in gambiense – winterbottom sign). Maybe some Anaemia
·         Phase 2 CNS – meningo encephalitis and sleeping sickness. Various neurological problems.
With Rhodesiense the progression to phase 2 is much faster and hence the animal reservoir is important. With all species of brucei they do not produce much virulence in livestock or game.
How does this relate? It means that for trypanosomes particularly for rhodesiense treatment of the livestock is essential -> pentamide is given prophylactically.
Also the epidemiology shows increased incidence in cattle markets. Also for the other form it is important to catch the disease in humans, so being able to spot winter bottoms sign and also card agglutination tests are important.
Consider with Leishmania in spain, and brazil the role of dogs.
 Environmental factors
With both the nature of environment can influence spread. Consider chenopod -> rodent leishmania in Tunisia. Also movement of animals towards game animal in Africa. (consider Masai farmers intentionally avoid this).
Human behaviour

This is another factor and is particularly leishmania is the role of human behaviour, spread particularly by mechanical IVDUs.
Originally written by Jennifer Albus

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