Phlebotomus papatasi sand fly feeding. Source: Frank Collins, CDC – Nat. Center for Infectious Diseases, NCID, Div. of Parasitic Diseases, DPDx; Leishmaniasis

Leishmaniais a protozoan parasite responsible for leishmaniasis, a vector-borne disease that causes painful and often disfiguring tissue destruction.Leishmania majoris known for causing cutaneous leishmaniasis and ismainly foundin the Middle East and North Africa.Leishmania donovanicauses visceral leishmaniasis, where the tissue destruction occurs in the organs. This manifestation is primarily known to occur in India and East Asia.

In order to complete its lifecycle,Leishmaniaundergoes two vital development stages. One stage is in a mammalian (e.g. human) host and the other in the sand fly digestive system.Leishmania在沙蝇主机生存是一个重要的鹿e to investigate as different sand fly species vary in their ability to transmit the disease (approximately 30 species of sand fly can spread at least 20 species ofLeishmania). This measure of transmission ability is called vector competence and understanding the factors that affect it can aid attempts to eradicate this often-deadly disease.

Lifecycle in the sand fly

The sand fly feeds on blood from an infected mammal and takes up the amastigote form of the parasite into the abdominal midgut. Here the parasite transforms to the procyclic promastigote form. The parasite is then thought to attach to the midgut wall and finally migrate back towards the mouthparts. Just before it reaches the mouthparts it converts to its mammalian-infectious form, the metacyclic promastigote, ready for when the sand fly takes its next blood meal. The parasite is passed on because the sand fly regurgitates the mixture that lingers in its mouth parts into the bite before feeding.

Pruzinova and colleagueslooked into parasite developmental success in two competent and two non-competent (refractory) sand fly species and found some surprising results. The first part of their study consisted ofin vitroexperiments and can be broken down into 3 main findings (although more aspects can be found in the paper itself):

1. death in the midgut is not sand fly-species-specific
2. time since blood meal affects midgut toxicity
3. midgut proteases are likely not the direct cause of death.

The second part of the study looks at how thein vitrofindings relate to theirin vivofindings.

Death by midgut is not sand fly species-specific

This experiment investigates whether the midgut contents of different sand fly species have varying levels of toxicity toL. donovaniandL. major。The authors dissected out midguts from four sand fly species and mixed them with the parasites. Surprisingly, the midguts of the two refractory sand fly hosts did not cause higher mortality than the two competent sand fly species.

Time since blood meal affects midgut toxicity

Again, midguts were dissected out from four sand fly species, but at different time intervals since feeding, and incubated with parasites. All sand fly species midguts were most toxic towards the end of the process of digestion of their blood meal. After analysing these data, the authors concluded that parasite death in thesein vitroexperiments is not so much dependent on sand fly species, but rather, dependent on time since the sand fly last fed.

Midgut proteases are likely not the direct cause of death

Previous explanations for parasite death in refractory flies have included the hypothesis that sand fly proteases are responsible. This study incubatedL. donovaniparasites with rabbit blood or human haemoglobin and/or with a commercially bought proteinase. Interestingly the parasites were killed by blood/haemoglobin and the proteinase together, but not by the proteinase on its own. The authors argue that, instead of previous opinion, it could be toxins produced during blood digestion (e.g haem) that are harmful to the parasites rather than the digestion agents (e.g. proteases) themselves. This suggestion fits with the above finding where midgut contents were more toxic the longer the sand fly had been digesting.

How do these findings relate to thein vivoenvironment?

Excitingly, this group also conducted anin vivostudy where the two susceptible and two refractory sand fly species were infected withL. donovani。寄生虫发展研究by dissecting out each section of the gut at different times post-blood meal and identifying the parasite life stage. An additional explanation that has been suggested for parasite mortality in the sand flyPhlebotomus papatasiis parasite inability to attach to the gut wall. The data here show that the parasites do survive and develop inP. papatasibut disappear after sand fly defecation. Therefore, it seems in thisin vivoscenario thatL. donovanilose grip in the gut rather than being killed by toxins.

A grip mechanism has been described as the strategy that enablesL. majorto be successful inP. papatasi,wherebyparasite lipophosphoglycan (LPG)binds tosand fly galectins。The results from this study by Pruzinova and colleagues therefore support the idea thatL. donovanido not possess the appropriate LPG for binding.

Conclusion

This study looked at several aspects ofLeishmaniadevelopment within different sand fly species and found that any death caused by the contents of the midgut, specifically proteases, was not significantly species-specific but dependent on the stage of blood digestion. In addition, as seen by thein vivostudy,L. donovanican initially survive in a refractory sand fly species but are eventually lost through defecation.