In a first-of-its kind study on the Galapagos Islands, EPI researcher Sadie Ryan investigated social and ecological factors associated with dengue, as well as the presence of the mosquito that spreads it.
With its turquoise waters and abundant wildlife, Ecuador’s Galapagos Islands have long been a vacation dream of many. Tourist guides highlight playful sea lions and giant tortoises, but they do not talk about the mosquitoes. For the more than 30,000 people who live there, emerging epidemics of mosquito-borne disease such as dengue, Zika, and chikungunya, which causes fever and joint pain, are an ongoing threat.
A new study led by UF Emerging Pathogens Institute faculty member Sadie J. Ryan, examined the social and ecological factors associated with dengue fever in the Galapagos Islands and the presence of Aedes aegypti, an invasive mosquito and vector for dengue between people.
“People think of the Galapagos in terms of protecting the ecological systems and wildlife, but we rarely think about the fact there are human diseases there,” says Ryan, who is also an associate professor of medical geography in the College of Liberal Arts and Sciences.
This is the first study of its kind in the Galapagos, and the first time household-level risks of dengue have been rigorously explored off the mainland of Ecuador. The study was published in the International Journal of Environmental and Public Health in a special issue on mosquito-borne diseases.
Dengue is considered a newly emerging disease in the Galapagos Islands since cases were first detected in the early 2000’s. The disease, and A. aegypti, were eradicated 600 miles away on Ecuador’s mainland in the 1970s due in large part to a well-funded political push for mosquito-borne disease eradication throughout South America, and widespread applications of the controversial insecticide DDT. But dengue re-emerged in the 1980s because of relaxed vigilance in vector control measures associated with reduced international funding. Eventually all four serotypes of the virus were co-circulating within Ecuador’s coastal lowlands.
For her research, Ryan collaborated with the Biosecurity Agency of the Galapagos and colleagues from the Universidad San Francisco de Quito, the Escuela Superior Politecnica del Litoral, and SUNY Upstate Medical University. Ryan’s graduate student, Catherine Lippi, and post-doctoral fellow Gabriella Hamerlinck, were involved in the research and analysis and coauthored the study. The team conducted household questionnaires and entomological surveys on the two most populated islands — Santa Cruz and San Cristobal. (Red stars on the map below mark study sites.) Statistical models were developed to identify risk factors for the presence of self-reported dengue infection and A. aegypti presence.
Ryan emphasized, “The Galapagos Islands, and other island systems, present a unique challenge for vector control, given the difficulty in surveying and controlling mosquitoes across diverse landscapes separated by the ocean.”
Dengue fever is a serious disease throughout Ecuador and Latin America, with an estimated 16 million cases occurring in the Americas annually. Globally, about 20,000 people die from dengue each year. But the greater problem from a public health perspective is the more than 290 million cases where people are infected with the dengue virus but are asymptomatic. They constitute a vast but hidden and under-surveyed infection reservoir which allows the pathogen to remain in circulation in certain regions. Typically, Ecuadorian public health agents implement vector control programs that include fumigation, larvicide applications and mosquito surveys.
A. aegypti has been dubbed a “town mosquito” because it likes puddles of clean rainwater, and will even make do with water pooled on tarps, in discarded tires or plant pots. Prior studies show that females of this species, which take blood meals from people, tend to stay within about 1,300 feet of the spot where they emerge from their larval stage to that of an adult. They do not naturally carry viruses but pick them up when they probe or bite an infected human or animal. It is the movement of people, not the A. aegypti mosquito, that propels dengue rapidly around or between neighborhoods, communities and cities.
Ryan’s team found that the knowledge and attitudes of study participants were significantly associated with disease risk. They found that homes were more likely to have A. aegypti present if they had no screens on windows or doors, if they did not cover water containers, and if the residents perceived dengue as difficult to prevent. The most important household risk factor for both the presence of A. aegypti and a prior self-reported case of dengue was water storage practices.
Because fresh water is scarce, the islands’ inhabitants store water in containers which attracts mosquitoes to settle there. The paper suggests that focusing on sealed water storage would be a useful addition to vector control in addition to the current campaign to clean up and remove trash. “Having people simultaneously deal with water scarcity, storage and distribution is a consistent problem,” Ryan notes.
In a counter-intuitive twist, they also found that homes with air-conditioning were more positively associated with the presence of A. aegypti, possibly because of puddles created from condensed water, or possibly because limited use of air-conditioning may lead to opening windows (which may not have screens) in the evenings for natural cooling.
But one of the more complicated findings was that human movement played an important role in dengue transmission, because people who travel between islands, or to the mainland and back, may be exposed to the dengue virus and bring it back with them to their local communities. “These are very small and somewhat geographically-isolated populations,” Ryan says. But visitors outnumber permanent residents by seven to one, due to the Galapagos Islands’ main economic engine: tourism.
With so many people flowing through the island chain, the opportunities to import or export a case of dengue are vast. The proportion of locally-acquired dengue cases versus cases imported from the mainland is not known, but periodic outbreaks suggest local circulation. Given the complex dynamic of a high-volume tourism bumping up against a small local population, Ryan believes that public health officials need to retool their approach.
Says Ryan, “Our findings suggest that public health officials could develop targeted interventions that increase people’s knowledge of dengue transmission while changing their behaviors to prevent exposure to mosquito bites.” New interventions might include: more training for local clinicians to know what to look for in possible mosquito-borne illness infections so that local authorities can quickly mobilize to mitigate a potential dengue, Zika or chikungunya outbreak; and posting highly visible informational signs at ports of entry to warn travelers of the risk of mosquito-borne diseases, and that they should wear protective clothing and insecticide.
The authors also recommend a higher level of community involvement in vector control campaigns while conceding that the nearly two-hour boat ride which separates the islands is a barrier to enacting a coordinated and uniform vector-control and surveillance program. Future research, they write, might help to strengthen surveillance of tourism entry routes to mitigate dengue being reintroduced to the islands. “Authorities need to identify the major routes of entry where people enter these islands and have focused, targeted vector reduction and mosquito surveys in those areas,” Ryan says. “Anyone coming to a clinic with fever or signs of a mosquito-borne illness in these islands should be given a full work up.”
The findings from this study have important implications for forming new health policies and providing localized information that will help reduce future outbreaks. Senior author, Anna Stewart-Ibarra of Upstate Medical University, says, “We need more research dedicated to understanding the health and well-being of tens of thousands of local residents and hundreds of thousands of tourists who visit the Galápagos Islands each year, and how these health outcomes interact with the unique local ecosystems and wildlife populations.”
How many Galapagos Islands residents are sickened by dengue?
- At the study site of Puerto Ayora, on Santa Cruz, there are an average of 40 cases per 10,000 people during the arbovirus season, but this can spike to 250 or more cases per 10,000 during an outbreak.
- At the study site of Puerto Baquerizo Moreno, on San Cristobal, the incidence rate spiked to 1,410 dengue cases per 10,000 people in a 2010 outbreak. Since then, a total of 25 cases were confirmed between 2014 and 2018.
- The islands of Floreana and Isabela have no reported dengue cases.
When does dengue strike?
- Generally, dengue cases occur in May – July following a rainy season that occurs in February – April. Cases spike in June.
By: DeLene Beeland