In villages across the Malaysian state of Sabah, locals are being asked to do something unusual: expose themselves to—potentially malaria carrying—mosquitoes. The person behind this request is third year PhD student Rebecca Brown (@REBrown552), who is studying the distribution of malaria vectors in the region. Here, Rebecca walks us through the steps of conducting her tropical field research, explains why a parasite that typically infects monkeys has become a cause of concern for human health, and gives us a glimpse into life in northern Borneo.
Kampung life and mosquito vectors of disease in Malaysian Borneo
In seizing the opportunity to show you what Institute entomologists do when we are not desk-bound or on the hunt for coffee in Glasgow, I’d like to introduce you to my field of research: vector borne disease. My research focuses on the mosquito vectors of monkey malaria in Malaysian Borneo; I’m going to take you through the stages of conducting my field experiments, whilst hopefully giving an insight into village (or kampung in Malay) life.
The location of my field site is northern Sabah, an area where Plasmodium knowlesi malaria is endemic. Plasmodium knowlesi parasites are naturally hosted by long-tailed and pig-tailed macaques and are transmitted to people by female anopheline mosquitoes when they blood-feed. Due to morphological similarities with other malaria species, only recently, with the advent of molecular diagnostics, has the extent of the infection in the human population been realised.
You may have heard about the forest clearance in Borneo to accommodate palm oil plantations, which despite benefiting the economy has caused significant ecological disruptions. Scientists speculate that an increase in human P. knowlesi infections has been a repercussion, because habitat removal has brought monkeys and people closer together. Also, Malayan anophelines enjoy the cool temperatures of the dense forest, but environmental change may have disrupted their usual ‘forest-dwelling’ habits.
As the problem of P. knowlesi in Sabah is relatively new, little is known about the behaviour of its mosquito vectors or the environmental conditions that lead to people becoming infected. Last year one mosquito species, Anopheles balabacensis, was incriminated as the primary transmitter of P. knowlesi malaria, but only from one district in Sabah… so, it was my job to expand on this geographically to find out about the distribution of this vector in other districts. But how?
Step 1: Site selection and meeting the kampung chiefs
We selected 11 villages for study, spanning a range of altitudes and districts, for which information on community malaria prevalence is already available. The first port of call if you want to work in a village or hire local people is to ask the Ketua Kampung (KK), or village leader, for permission. The ‘wild-goose chase protocol’ is used for finding the KK, which involves a lot of asking around—thankfully Malaysian people are very friendly and helpful. On meeting, we explain that we would like to catch mosquitoes in their village and are sometimes met with puzzled expressions, but they are always interested and accommodating.
Step 2: Hiring people and explaining the job
The technique we use to collect mosquitoes is called ‘human-landing catch’. In this method, we require people to expose their legs so we can collect any mosquitoes that land on them to feed; currently there is no reliable alternative to examine mosquito behaviour. The malaria prevalence in Sabah is very low, only 1.84 cases per 1000 persons per year. However, the risks are clearly explained to participants and a course of prophylaxis (antimalarial drugs) is offered for the period of work. From each village, we employ six individuals to help us.
Step 3: Experiment setup
At each village, mosquito collections are performed in peri-domestic areas (the garden area, within 5m of houses), farmland and forest patches to find out what habitat poses the greatest malaria risk/has the highest vector abundance. Families often own a small area of land near the home for subsistence farming, common crops are: palm oil, rubber trees, cabbages, lettuces, rice and corn. From 6pm-12am, three teams of two people do human-landing catches (the peak biting time of An. balabacensis is 6pm-8pm).
Step 4: Identification and recording of species
Mosquitos are immediately killed by storing them at -20°C overnight. The following morning, the species of Anopheles are identified morphologically and the time of collection is recorded. Normally we continue collections in one village for four nights.
Step 5: Follow-up screening
One month later we return to the village to check-up on the people we employed by performing a finger-prick blood-smear. This is a standard procedure to check for malaria parasites, and due to the prophylaxis and low infection prevalence, the result is always negative.
Step 6: Adapt to surroundings
Finally, throughout the whole process we must keep in mind the unpredictability of fieldwork; things don’t always go to plan and sometimes events are just out of our control. In the event of heavy rainstorms or surprise insecticide fogging at our study sites, we need to be prepared to roll with it and kick back with a coconut!
It is a privilege to visit and work with remote communities in Sabah. I’ve learned that a strong local team is essential, such as mine with Nemran Bin Rubit (driver) and Mohn Fazreen Abdullah (field assistant). Three months of communicating largely through hand gestures, nondescript noises/songs and one word sentences paid off: we found An. balabacensis in all districts sampled, as well as other vectors of malaria, and there was high variability in Anopheles species and abundance between villages. Importantly, one An. balabacensis was infected with P. knowlesi, highlighting that this species of mosquito is a vector of this malaria parasite in further districts of Sabah. We also found that malaria vectors are common in agricultural land as well as in forest patches, suggesting that the palm/rubber plantation habitat is just as suitable for these previously ‘forest-dwelling’ anophelines. These key aspects about anopheline ecology are crucial for designing a vector control program to effectively reduce human malaria in Sabah.
With special thanks to:
Dr Heather Ferguson (University of Glasgow)
Professor Chua Tock Hing (Universiti Malaysia Sabah)
Dr Kimberly Fornace and the Monkeybar project (MRC)
The local communities and our mosquito catching teams