By Dr. Colin Michie of AUC Medical School
Dr. Colin Michie has worked as a paediatrician in the United Kingdom, Africa, the Caribbean and the Middle East. He specialised in nutrition, haematology and infectious diseases. He is now the associate academic Dean for the American University of the Caribbean Medical School in Cupecoy, Sint Maarten. His enthusiasm is training medical students and health care teams to ensure they deliver better value health care.
Living on St. Maarten comes with some special risks. One of these is dengue fever, which becomes more common towards the end of the year. This illness involves all of us; it causes a significant burden on households, families and the economy. It may become a much larger problem in the future. We can help eliminate it if we work together on this island.
Dengue is the world’s most widely-spread mosquito-borne illness. Approximately half of the world’s population is at risk of infection and some 100 million individuals suffer with dengue each year. The global burden has increased steadily over 30 years because of the increasing range of Aedes mosquitoes. Dengue has become more common in Asia and the Americas and presently occurs in nearly 100 tropical and subtropical countries. Epidemics have become progressively larger. Why is this increased risk happening?
Changes in world climates along with movement into towns and cities have helped extend the numbers of humans exposed to Aedes species. Increased international trade and travel have also expanded the spread of mosquitoes. For instance, A. Aegypti can move throughout the tropics on boats and ships, while A. albopictus has spread to many countries in traded used vehicle tires. Climatic disasters, such as hurricanes, can increase the rates of illness too as following environmental damage, the Aedes mosquitoes often find more breeding sites.
Dengue is one of the flaviviruses, a family that also includes yellow fever, the Zika and the West Nile viruses. Dengue virus has four distinct serotypes or variants. Flaviviruses may have spread from Africa; there is an account of an outbreak of a dengue-like illness in Philadelphia in 1780 and in the diaries of sailors and travellers in the Caribbean at that time. The illness probably received its name from a West Indian Spanish word in 1828. Studies in Venezuela in the last century by Louis Daniel Beauperthuy, whose family is well known on this island, helped link mosquito bites with the fevers from flaviviruses such as dengue.
When a person is bitten by a mosquito infected with the dengue virus, it takes three to 15 days to develop symptoms. Early signs of illness include fever, fatigue and pain. This virus often causes pain behind the eyes and bone pain that gives dengue the nickname “break bone fever” or “breaker”. If the disease becomes severe, a rash or red spots on the skin will develop and there can be bleeding in the stomach and gut. Fluid can collect in the lungs and abdomen, as blood vessels become leaky. Those with any evidence of bleeding require hospitalisation and supportive treatment as dengue is potentially lethal.
A correct diagnosis of dengue fever requires a history, physical exam and checks such as blood tests and radiographs. Many people suffer severe disease with infection with one subtype of dengue; some folk do not even know they have been infected. However, those experiencing a second infection, particularly with serotype 2, are more likely to suffer with severe dengue.
Aedes mosquitoes will carry the dengue virus once they have taken a feed from an infected human. Only the female mosquito will bite humans, the males feed off plants. The mosquito has a range of about 400 metres around where it hatches from standing water. An adult Aedes female bites more, survives longer and can lay more eggs if the temperature is higher. She tends to bite more in the day and lay a batch of eggs after each feed. Here in the Caribbean, dengue is spread along with Zika, as well as chikungunya and sometimes other pathogens too. If the mosquito bites more often on hot days, the insects will bite more humans and are more likely to bite someone infected with dengue or Zika. Unless the insects are controlled, this means the virus will spread rapidly, causing an epidemic.
For example, Florida has had Aedes mosquitoes and susceptible people for many years, but only in 2009 did an outbreak of dengue occur when a traveller there, ill with dengue, was bitten. This year the dengue reporting centre in San Juan has observed that many Caribbean islands, including ours, are in a similar position: epidemics are a significant risk for all of us.
With an illness spread by mosquitoes, the obvious solution is to tackle the insects. Surely we can eliminate them! What strategies can we use? Aedes Aegypti was eliminated from large areas of the Americas in the 1950s and 1960s, as part of a campaign to eliminate yellow fever. This was carried out with insecticide sprays and public education. More recently in Vietnam, a biological control was introduced. Local leaders, together with schoolchildren, conducted clean-up campaigns and awareness events as well as putting a small crustacean, Mesocyclops, into household water tanks exposed water.
Mesocyclops eats the larvae of the mosquito that live in water. This strategy eliminated Aedes almost completely. In Cambodia, a new long-lasting insecticide-treated netting cover for household water storage containers has been tested by the World Health Organisation. They found that these treated nets, initially developed for malaria prevention, are useful in controlling Aedes too.
In theory we might also try to eliminate mosquitoes by genetic methods. If one introduced sterile male mosquitoes, or mosquitoes that cannot allow any viruses to remain alive within them onto an island, one can greatly reduce the successful breeding of these insects. A bacterial parasite of mosquitoes called Wolbachia that might assist by shortening their lifespan. An area of Australia that suffered with dengue has eliminated the disease for four years by employing Wolbachia-infected Aedes mosquitoes.
A crucial challenge is to make sure mosquitoes do not bite. In particular, they must not bite anyone who might have dengue, Zika or chikungunya. This link must be broken for us all to remain healthy on St. Maarten. We all need to pay close attention to our own mosquito protection – use mosquito deterrents, put nets on the windows and nets over a bed for anyone ill with a fever in the home. These sensible guidelines are those recommended by the Public Health authorities on the island.
Many hoped that a vaccine might help protect against dengue fever, but it appears these systems are useful only for those who have had an initial dengue illness, to protect them against a more severe second episode. Treatment of dengue is supportive: that is, no specific medications have been found to stop the virus multiplying in patients (or in mosquitoes). This situation may change because molecules that tackle the dengue virus and slow its multiplication have been extracted from the stinging nettle, the dandelion and a plant found on the islands east of Madagascar (Aphloia theiformis). These are being tested – a medicine may be developed in future.
There is good news too regarding mosquito repellents. These chemicals are effective and safe – they have to be reapplied to ensure they are effective. Many products contain the molecule DEET (N,N-Diethyl-3-methylbenzamide) but many herbal preparations can be employed for shorter term. One of these, nookatone, that gives the scent to grapefruit, is currently being trialled as a new repellent and mosquito killer by the CDC.
Here on Sint Maarten, the most important message is that we need to tackle mosquitoes. We can all stop them breeding by removing or covering water needed for their larvae. We can put nets over water tanks, our windows and doors to reduce breeding and biting too. If we are sick, we must rest and sleep under nets to reduce biting and the spread of virus to our family and neighbours. Make mosquitoes your enemy to make St. Maarten healthier this year!
