Climate change and increased urbanisation are driving an increase in dengue and other mosquito-borne diseases and the Pacific is responding with a suite of new interventions and tactics.
The World Health Organization (WHO) says dengue is endemic in about 125 countries this year, and dengue cases and associated deaths increased by 30% in 2023 compared to 2022.
The first few months of 2024 have seen French Polynesia and Samoa report an increase in dengue cases, with Samoa on the brink of declaring a dengue fever outbreak as this issue went to print.
The Pacific Community (SPC) says from 2012 to April 2023, there were 98 reported mosquito-borne viral outbreaks in the region, 72% of them dengue fever.
SPC Senior Epidemiologist, Dr Salanieta Saketa says while the organisation has capturing outbreak reports through its Epidemic Intelligence system for 40 years, “we struggle with [getting] adequate or accurate data from countries.”
She says sharing and consolidation of data across the region, plus event-based surveillance (where information is gathered from communities, social media and other sources in those countries without lab testing facilities), would improve planning and implementation of dengue responses, and importantly, early warning systems.
“Dengue and malaria are considered climate sensitive diseases. So climate change is a real issue in the region, and it is affecting how vectors are, their behavior in terms of procreating and being able to transmit disease.”
WHO also notes the role of climate change—and in particular higher average temperatures, extended rainy seasons and increased humidity— in influencing the incidence of dengue.
Changing climate and increased international travel has seen dengue present in parts of Europe and the US it has previously not been reported.
Mosquito mobility
Mosquitos are mobile. Their dried eggs can survive journeys between nations, for example on the inside of a tyre, and hatch much later when they come into contact with water.
Professor Tom Burkot of Australia’s James Cook University is Co-Lead of the Pacific Mosquito Surveillance Strengthening for Impact (PacMOSSI) project. He says many Pacific Islands have not conducted a mosquito survey for some time.
“Distributions of mosquitoes change all the time. It’s really an important thing for countries to understand what species they have present and the distribution of those species on the different islands. So when a dengue outbreak occurs, you … know what mosquitoes are on this island, where the outbreak is, and we know their behaviours.”
The situation in the Pacific is particularly complex because there are 12 different species that are capable of transmitting dengue.
“The thing about these 12 different mosquitoes … is they all have slightly different behaviours. So the sorts of interventions that will work for one won’t work for the other,” Dr Burkot says.
PacMOSSI aims to assist Pacific Island countries with the most effective mosquito surveillance and control plans possible, within the resource constraints faced by the region’s health sectors. The project also provides research grants so countries can test strategies for themselves.
“What happens right now is in a lot of countries, they do things to respond to a dengue outbreak, but oftentimes they don’t have the resources to be more proactive to try to reduce the risk of an outbreak occurring. And so we’re trying to communicate what we know from other parts of the world, where the evidence is strong, that if you do certain activities, you can reduce the risk of dengue transmission or you can reduce the scale of the epidemic,” Dr Burkot adds.
Expanding the toolkit
One of the most common, and visible methods of mosquito control in many Pacific countries is outdoor fogging or spraying of insecticides.
But Dr Burkot says generally this is not recommended, as it can kill a lot of other insects, can lead to resistance in target mosquito populations, and has only a transient effect.
“You spray it, you kill the mosquitoes that are flying at that moment in time. And then an hour later, or probably 15 minutes later, it’s not killing any more mosquitoes. So it’s a very expensive intervention.
“Local governments like to do fogging because it’s something that’s very visible to the population, ‘that your government is concerned about the health of the people, and so we’re spraying insecticides to protect you from dengue.’ But all the evidence suggests that it is not a very effective approach.”
This month a study detailing a newer style of intervention, the introduction of Wolbachia bacteria in Aedes aegypti mosquitoes in Fiji, Vanuatu and Kiribati, showed positive results. The bacteria stops the mosquitos from being able to carry the dengue virus, so they cannot transmit it between humans.
Wolbachia was introduced to select communities through a pilot in 2019. In the communities tested, “there was sufficient evidence that Wolbachia is well established in the mosquito population” says Cameron Simmons, who is the Director of Global Implementation at the World Mosquito Program, a not for profit operated by Monash University in Melbourne. Simmons says while Wolbachia has taken hold in the areas it was introduced, the public health outcomes in Fiji have been less clear. He attributes this to the (global) unreliability of dengue data due to the pandemic, and a lack of traction from local health authorities.
“We would love the Ministry of Health and Medical Services to be more interested in the outcomes of this project, because really the data on dengue case burden and incidence, that’s the Ministry of Health’s data. But we’ve struggled a little bit to have interest from the ministry in the outcomes of this program. And part of that is that continuity, some key people left the ministry over time. Part of it is reflection [of the fact] we’re a small NGO, we do not have people placed in Suva and Nadi, we don’t have a presence in Fiji to keep knocking on doors.”
Islands Business asked the ministry for comment, but received no response before we went to print.
New Caledonia has had a national Wolbachia program for five years. Simmons says they are now “waiting with bated breath” for a recommendation from WHO in response to the Wolbachia pilots. While it has not stopped the roll out of the intervention in other countries, it could ease the way.
“I think there is no other intervention at the moment at this scale that compares to this,” he says, adding that a WHO recommendation would enable some lower income countries to sit down with international donors and development agencies and discuss investment in such interventions.
WHO’s Western Pacific Region office in Fiji told Islands Business that the ‘guideline development process’ has begun and that “there is a prospect of refining WHO recommendations when additional high-certainty evidence becomes available.”
Simmon says there are other obvious places where Wolbachia [has potential] such as Honiara (Solomon Islands), Samoa, Tonga and Cook Islands. “But no intervention is free. And we, both the World Mosquito Program and the country pilots, need funds to be able to enable the activities to happen,” he stresses.
There are other pilots underway. Last year the Marshall Islands government selected Oxitec to trial the release of genetically engineered Aedes aegypti mosquitos on densely populated Ebeye, where a severe dengue outbreak sparked a state of health emergency in 2019.
Oxitec’s technology involves production of non-biting ‘Friendly’ male mosquitoes over time, which mate with invasive, biting Aedes aegypti females. Their female offspring cannot survive, which means fewer biting female mosquitoes in the following generations.
Meanwhile 20 countries have licensed a dengue vaccine (Dengvaxia) with other vaccines in development. As to whether this might be an option for the Pacific, WHO told Islands Business: “introduction of a new vaccine is a national decision. WHO advises countries to implement certain measures and put in place certain steps before countries introduce new vaccines. Vaccination should be considered a part of an integrated dengue prevention and control strategy. This includes adherence to other disease preventive measures such as well-executed and sustained vector control.”
Dr Saketa says SPC produced a position paper on dengue vaccines in 2016, and at that time there were some reservations about the introduction of a vaccine, but that a lot has been done since then, and they are following its progress.
“We’re really interested, we’re looking at it, watching, and hope to get more evidence around the use of this new vaccine. And we know that countries have to prepare because one of the criteria for introduction of a vaccine is that it’s recommended for those [countries] that have endemic dengue.”
Dr Saketa says countries can only prove that by having serious prevalence studies. Again, it comes back to the need for better and more consistent data.
She notes that while SPC does not oppose fogging/insecticide use, “we know that insecticide resistance is one of the major things that’s affecting the use of those [methods]. “ We’re working to see if we can have a standardised, more informed way of advising countries in terms of the use of insecticide and also to be able to address the issue around insecticide resistance.”
But she says SPC is very supportive of the trial of new technologies in dengue control; Wolbachia, Oxitec in Marshall Islands and a new sterilization/irradiation technique in French Polynesia and Cook Islands .
“We’re looking forward to results and evaluation of these to see how they can also be implemented in other countries if they wish to have these tools as well.”
