Professor Dave Chadee is an internationally recognised expert in vector-borne diseases and their control. His often groundbreaking research ranges across the disciplines of entomology, parasitology, environmental science, climate change and epidemiology. With 300 publications, he has added significantly to the literature in these fields and impacted public health management around the world. He is an authority on mosquitoes, Aedes aegypti in particular, and the development of surveillance systems and methodologies for controlling the spread of diseases such as dengue fever, the fastest growing mosquito-borne disease, affecting an estimated 390 million people each year.

NIHERST interviews Dave Chadee

Q: You grew up on your family’s cocoa estate in Tableland. Did that rural environment influence what you went on to study?
A: It certainly shaped me from a very early age. I have often told the story of being badly stung by caterpillars at about age four. My father put some in a jar, explaining their development from chrysalis to pupa and then to the butterflies that finally emerged as I looked on with great fascination. That’s when my eyes were opened to science. The experience captured my imagination, as did all the other wildlife around me. We caught fish and songbirds too, with fishing lines and traps we had to make from scratch. And today I am in a profession where I have had to invent traps!

Q: Was school as much fun?
A: I never liked school but now I love it! I thought it was too restrictive. I think the education system needs to be a little more flexible so students can learn at their own pace. My parents were primary school teachers and we were one of very few families who had encyclopaedia. So I had some advantages academically. I was certainly not an ace student though. But I exceeded my parents’ expectations so they were quite happy.

Q: One surprising and unusual aspect of your academic background is that you didn’t study science subjects at A levels. Why not? And how did you manage the leap into advanced sciences at university?
A: At Naparima College I did English, Geography, History and GP because they did not offer A levels in biology or zoology. When I applied to Dalhousie in Canada to do a BSc in Biology, they accepted my O levels in chemistry and biology. I worked hard and ended up with a first class
honours degree.

Q: When you did your honours project on “The reasons for the high incidence of Aedes aegypti in Trinidad”, had you already decided to specialise in entomology?
A: Yes. I returned home in 1976 to do the research project at the Ministry of Health’s Insect Vector Control Division. Having completed my first degree, I went on to do my MPhil at UWI, St Augustine under the distinguished parasitologist and entomologist, Dr Elisha Tikasingh, working on the Culex caudelli mosquitoes in the Aripo Wallerfield Forest and Savannah.

Q: What was the reason for the high incidence in Trinidad? Was it not the same in Tobago?
A: The high incidence of Aedes aegypti in Trinidad was due largely to the inadequacy of the water supply system which resulted in the need to store water thus providing breeding sites for the mosquito. At that time, Tobago was still free of Aedes aegypti.

Q: Your research, from very early in your career, has been mostly operational (in the field) rather that laboratory based. Did your area of interest require that or did you not like lab research as much?
A: Laboratory experiments are much easier to conduct because you control everything: temperature, age and sex of mosquitoes and so on. Field experiments can be affected by the vagaries of the weather and human and animal interference, but reveal more realistic behavioural patterns which can be used to design control programmes or develop behavioural models.

For example, when I started at Insect Vector Control Division, we had problems with the insecticides being used to control Aedes aegypti immatures. So I developed operational studies because the literature showed the insecticide was effective for up to 13 weeks and the advisory was to plan treatment cycles every 10 weeks. The duration of the programme at Vector Control was every three months but the mosquitoes were still surviving in these treated containers. Our studies revealed that the insecticides were being diluted by householders using and refilling the treated containers or by rain falling from the eves. For the experiments cited in the literature, the drums were covered but here, the householders’ drums were open and exposed to the elements. We found that if water in the drums was used and refilled in five to six days, there was no mosquito mortality. And if rain fell into the containers, the insecticide would only last five weeks. So as a result of this operational research, the abatement programme changed immediately. This led to training programmes for vector control workers and to community health education programmes.

Q: Other research you did around that time led to the introduction of the international regulation of spraying commercial aircraft. Can you tell us about that?
A: In 1984, Le Maitre and Chadee published a 20-year report of all the insects collected on board aircraft landing at Piarco International Airport in Trinidad, including mosquitoes, many of which were vectors of diseases, thereby demonstrating that mosquitoes were able to be  transported on aircraft. This provided the international community with the evidence required for the “blocks away spray program”, which is used on board aircraft
prior to landing in tropical countries. This and other work are good examples of the quality of our research and its relevance to international public health.

Q: During the 1978 yellow fever outbreak, did the Ministry enlist your expertise?
A: Yes. We started doing work in seven forested areas. We embarked on an intensive collection of mosquitoes as well as sick or dead monkeys, to test for the yellow fever virus. Then I proposed we study the biology, ecology and behaviour of the four species of Haemagogus mosquito, i.e. the known forest vectors of yellow fever. It was the lack of understanding of the ecology and behaviour of these mosquitoes which resulted in our inadequate mosquito surveillance programme. We identified the peak biting times of these mosquitoes to determine the time of day yellow fever was transmitted and whether they took blood meals at canopy or ground level, etc.

I also wanted to determine for the first time in science, the diel oviposition periodicity of these mosquitoes, which is the exact time of day that these mosquitoes lay their eggs, so these could be harvested to determine whether female mosquitoes passed the virus onto their eggs – “transovarial transmission”.

Q: Was that the impetus for pursuing your PhD at University of Dundee in Scotland?
A: In 1984, I left for Dundee to do my PhD, looking at the oviposition periodicity of Aedes aegypti. I studied two strains of Aedes aegypti mosquitoes, one from Africa, one from Trinidad, and did field experiments in Trinidad. We looked at the impact of mosquito population density as well as physical interference on those egg-laying patterns to establish control systems including the development of an effective assay method to identify the “skip oviposition” behaviour because females do not lay all their eggs in one container but rather spread their genetic material around in many containers. Today, there are probably about 25 different mosquito traps which are sold internationally, all of whose developers use this data to enhance the collection mechanism and to provide the evidence for the efficiency of these traps. The icing on the cake for me was that through these local field trials, we established, for the first time in science, the oviposition periodicity of Aedes aegypti in nature. And our methodology has now become the gold standard for similar field studies. So it was really pioneering work when we started. Apart from dawn and dusk egg-laying peaks, we also saw the influence of the rising and setting sun, with more females laying on the eastern side than on the western side of houses. Those are the kinds of findings that have helped in the placement of these traps. Other research helped us improve outdoor and indoor spraying in communities and homes.

From the work I had done, I realised the mosquitoes’ biting times were different from the times we were spraying. To control mosquitoes by truck spraying, we must make contact with the mosquitoes while they are flying in order to kill them. So we established peak biting times. We also investigated the indoor resting behaviour (90 per cent of Aedes aegypti hide in bedrooms on dark surfaces), oviposition, and evaluated the efficacy of the truck spraying programme. All of these findings were the building blocks for vector surveillance and control strategies. So our work had very practical implications for the control of that mosquito worldwide, helping to reduce manpower, costs, logistics, transportation, insecticide use, by using an evidence-based approach. During the latter stage of my career, we developed strategies to break disease transmission within a week because we knew exactly where the mosquitoes were, exactly where to create barriers, etc. Unfortunately, the protocols for these control strategies are very often not followed.

Q: In T&T, are the scale and frequency of outbreaks significant for us? It doesn’t seem like a lot of people, looking at the case numbers.
A: Cases from Trinidad are generally under-reported and perhaps represent one third of the actual number of cases. This is not unique to Trinidad and Tobago but happens internationally as well.

Q: You left the public health service in 2004 to take up a full-time lecturer post at UWI. What are some areas you have focused on research wise?
A: Yes, I welcomed the new challenge. Since 2000, I have been working on a sterile insect technique (SIT) project first implemented in South East Asia. We developed control strategies for the chikungunya virus transmitted by the Aedes aegypti mosquitoes in the Caribbean region. SIT is an alternative approach where the male mosquitoes are sterilised through radiation, so the females inseminated by these males are unable to produce fertilised eggs.

In Trinidad, we’ve also done some trials which have confirmed about 90 to 95 per cent of Aedes aegypti females inseminated by sterile males produced nonviable eggs. We are now developing SIT programmes with Paraguay, Brazil, Costa Rica, Guatemala and Mexico, with funding from the International Atomic Energy Agency.

Q: Can you tell us about the traps you have been working on over the years?
A: A group from Tulane University approached me in 2003 to work on a joint project funded by the Bill and Melinda Gates Foundation to manufacture “Alot traps”. But the traps didn’t conform to what I thought should be done and proved not to be very effective. So I asked to be separated from them. Instead, with a colleague from the University of Queensland, Australia, I developed the sticky and double sticky traps, which are far more effective in collecting adults and immatures. We ran trials in Trinidad of all three traps and have now proven that the double sticky is a far superior method to all others. We have also just come up with another version of Gravid Adult Trap – the “GAT trap”- which removes both adult Aedes aegypti mosquitoes as well as the eggs from the community in which it is placed. Within six weeks we removed something like 25,000 eggs in Curepe and over 2,000 adult mosquitoes. And the methodology also allows us to test the dead mosquitoes for viruses using molecular tools.

Q: Many of our scientists leave Trinidad to pursue careers abroad. Why did you remain here and what would you advise young people about this?
A: I wanted to come back, partly because my research is on vector borne diseases in the tropics but mainly because of my family. My parents and I were very close.

But if you are working in science in the Caribbean region, you have to be prepared to live in a society where not much of what you do may be understood or appreciated. To young people, I would say that doing research and getting it published in international journals is very exciting and also possible here in the Caribbean. That’s why these awards are important in that they let young people know what can be achieved here at home. So I say more power to your organisation and please continue rewarding people who try to make a difference.