Over 300 million people worldwide are at risk of contracting leishmaniasis according to the World Health Organization. The protozoan responsible for this disease, Leishmania, is spread through infected sand flies, where it colonises the gut lumen and is exposed to the local microbiota. Understanding the relationship between the sand fly gut microbiota and Leishmania may provide clues to improved prevention of the disease, which is endemic in several regions across the globe including Brazil. In a recent study in Parasites & Vectors, Rod Dillon from Lancaster University, UK, Mauricio Sant’Anna from the Federal University of Minas Gerais, Brazil, and colleagues investigate how colonisation of the gut of Lutzomyia sand flies with yeast and bacteria impacts the establishment of a Leishmania population, and conversely how colonisation with Leishmania impacts subsequent infection with the insect bacterial pathogen Serratia marcescens. Here Dillon and Sant’Anna discuss the implications of these findings for the reducing the incidence of leishmaniasis, as well as the importance of open access and international collaborative efforts in this field.
How much of a public health burden is leishmaniasis in Brazil, and more broadly in the Americas?
MS: According to the World Health Organization, there were over one million cases of cutaneous leishmaniasis in the world during the last five years. Visceral leishmaniasis, the far more dangerous type of leishmaniasis if not treated, was responsible for 300,000 cases, with over 20,000 deaths annually. In the Americas, more than 90 percent of visceral leishmaniasis cases occur in Brazil. From 1980 to 2005 Brazil registered over 50,000 new cases of visceral leishmaniasis, most of which were concentrated in the northeast region. It is not just fatalities but loss of income and cost of treatment that creates a huge burden on the population, especially for those with a lower income.
Interestingly, the disease has spread throughout the country in recent years and become more urbanised since the early 1980s. The main reason for this is the high adaptability of the insect vector to urban and human-modified environments. Lutzomyia longipalpis, the main vector species transmitting Leishmania infantum in the country, has been captured more in urban and semi-urban areas in comparison to forested environments, showing a clear urbanisation trait.
My hometown, Belo Horizonte is endemic for visceral leishmaniasis. I have relatives whose friends or family have become infected with L. infantum. An extra complication comes with the fact that the symptoms (fever, weight loss, loss of appetite, etc) can be mistaken for other illnesses, so complicating early diagnosis, which is crucial for effective treatment.
What led to your interest in gut bacteria and parasite transmission in insect vectors, and more specifically your work on Leishmania and sand flies?
RD: My first interests were in the co-evolution of insect and plant interactions but I soon discovered that there was a hidden layer of potential mediators in the form of microbes found in insects and plants. This led to a career as an insect microbiologist where I started working on microbiological agents for the control of locusts. During that time (the 1980s) we had the idea that gut microbes may have a protective role in preventing insects from succumbing to disease. We eventually published a review on this in 2004 and I like to think that this helped to encourage research in this area. The idea of a protective gut microbiota and ‘probiotics’ has gained acceptance, and within the insect research world it has become the subject of intense research, for example in mosquitoes. These were new ideas at the time and there wasn’t a great deal of interest in my work!
After studying plant feeding insects I wanted a change and the idea of working with medically important insects was appealing. The main challenge initially was to adjust from working on a locust 600 mm in length to the tiny 2 mm sand fly. Sand flies actually feed on plants, it is only the female that needs blood for egg production and so I like to think of sand flies as plant feeding insects with an occasional blood feeding habit. When I started working on sand flies in the 1990s I always wanted to study gut microbes in sand flies. At that time there was a belief that the gut of sand flies was sterile – this now seems such a naive view. I studied gut microbes in sand flies with a group in Cairo, Egypt, and we found that these insects do contain a diverse community of microorganisms. My hunch then was that these gut microbiota may be important in influencing Leishmania transmission and this current study is the fruition of some of this work. Funding for this was a long time coming and support from the Leverhulme Trust was vital in starting this up.
What were the key findings of your recent study in Parasites & Vectors?
RD: There were two key findings. The first was that yeasts and bacteria that inhabit the gut of this tiny fly may prevent colonisation of the gut by the medically important Leishmania. We also found that two microbe species were potentially more effective than one in preventing colonisation. So a diverse fly microbiota is likely to resist colonisation by Leishmania. The question here is whether many of the wild sand flies possess a diverse microbiota and if this providing a naturally occurring ‘probiotic’ effect against Leishmania. Perhaps we would see more cases of leishmaniasis if these microbes were not present. This is a complex area because the number of cases depends on other factors such as the presence of animal reservoirs of the parasite.
The second finding was that Leishmania may potentially benefit the sand fly host under certain circumstances; its presence reduces fly mortality due to an insect pathogen. The main question that was asked in the past was whether Leishmania had a harmful effect on the fly. No one really asked whether there may be benefits for the fly. The results for these experiments were surprising because the insect pathogen is also capable of killing Leishmania. So for the fly and Leishmania it is a case of ‘better together’ regarding the presence of this bacterial pathogen. In practice Leishmania may have positive or negative effects on the fly depending on the circumstances.
Biological control of vectors is one approach to eliminating Leishmania and other vector-borne diseases. What advice would you give researchers working in this area, as a result of the findings from your study?
RD: A pioneering North American, early 20th century, insect microbiologist Ed Steinhaus said “comprehensive understanding of the biology of insects requires that they be studied in ecological context with microorganisms as an important component of the system”. I think the advice is that all aspects of the ecology of the vector needs to be considered and that the ecology at the microbial level is just as important as the macro ecology. I am an avid supporter of local biocontrol approaches. The development of fungal or bacterial pathogens as biocontrol agents needs to consider all aspects of the microbial ecology of the insect-Leishmania-mammalian interaction.
What impact do you think your work into Leishmania will have on reducing the burden of leishmaniasis?
RD and SM: The treatment of visceral leishmaniasis has greatly changed over recent years due to emerging drug resistance. In the absence of a vaccine with a protective effect, leishmaniasis control relies on insecticide spraying, with associated pesticide resistance. What we propose with our work is an alternative way to interfere with Leishmania development within the sand fly gut by harnessing their own microbes. We also study how bacteria and Leishmania itself activate the flies’ innate immune system to try and understand how the vector modulates their gut inhabitants populations. The aim is to find tools we can use to disrupt Leishmania development inside their insect vectors and reduce disease transmission. These tools might be quite subtle, for example some manipulation of the local environment around people’s houses or more of an intervention through novel techniques to switch on the sand flies immune system to kill the Leishmania.
Climate change is predicted to make ‘tropical’ disease more of a problem in non-tropical regions. How do you see climate change influencing leishmaniasis?
SM: This is a possibility. The presence of sand flies in new areas is a potential risk factor for leishmaniasis transmission. Recent work has shown evidence of an increasing risk of sand fly establishment in new parts of European countries such as Germany and Switzerland. If global temperatures increase over the next years, sand flies could be able to colonise areas that they were not able to colonise before.
What do you think has been the biggest benefit of collaborating with international researchers in your recent study, and in your field of research in general?
RD: It is difficult to be successful in bioscience research without collaborating these days. We always collaborate with international researchers; these collaborations are built on personal relationships in which there is a large element of trust and openness. Of course it brings new perspectives and is also personally enriching.
MS: Collaboration is the key word in science. Scientists do not survive in their research fields on their own! As a student I was involved in ‘sandwich’ post-graduation programmes, having the opportunity to start my PhD in my home country and go abroad for work experience, returning to defend my viva. That time abroad was of great importance for me, not only to give me new scientific knowledge but also to bring me invaluable personal experience. Later on as a junior researcher, I had the opportunity to work as a postdoc in three UK universities for over ten years, definitely paving the way to my scientific career. All these years working with top scientists from the sandfly-Leishmania field of work taught me that collaborative work is the key to success.
How important is open access in your field of research? How is open access viewed by the scientific community in Brazil?
MS: Researchers, not only in Brazil but worldwide are publishing their work more and more in open access journals, simply because it hits a broader audience, increasing their chance to reach a larger number of readers. For researchers in developing countries like Brazil, a clear disadvantage are the publication fees, which can be expensive. Most grant applications do not cover publication fees and certain university departments only cover publication of papers above a certain impact factor.
RD: Open access is increasingly important, particularly for neglected tropical disease work, some grant awarding bodies are now demanding that results of their funded work are published in this way. We are in a period of transition with traditional publishing models breaking down, and the addition of social media outlets and personal blogs is further blurring the boundaries. Obviously for scientists in economically deprived communities open access is a fantastic asset. The publication costs for the scientist can be an issue but these may be included in grant costings or sometimes can be funded via the university, as is happening at Lancaster University.