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Samoa has officially declared a dengue fever outbreak - more than 250 cases have now been reported - but it’s far from the first time a Pacific Island country has been hit with the mosquito-borne disease.
If you’re planning on travelling to the Pacific this winter, here’s what you need to know.
Dengue is a common, mosquito-borne, viral disease, widespread throughout the tropical and subtropical world. It is spread by mosquitoes that bite during the day and thrive where humans live. There are four different variants of the virus and infection with each variant gives long-term protection against that variant. However, it may make subsequent illness due to one of the other three dengue variants much more serious.
It’s the most common cause of illness with fever in travellers returning from Southeast Asia, Latin America and the Caribbean, considerably higher than the risk of typhoid or hepatitis A. Cases imported into NZ fluctuate and increase when there are outbreaks in our popular tourist destinations.
The main way to avoid dengue is to prevent mosquito bites. The Culex mosquitos, common in NZ, do not transmit dengue virus and bite at night. The Aedes mosquitoes, which transmit dengue fever, are daytime feeders and have two peak times of biting activity in the day: 2 to 3 hours after dawn and mid-to-late afternoon. However, they may feed all day indoors or on overcast days.
People should regularly apply effective insect repellent (such as those that contain DEET or picaridin) and should wear clothes that cover as much skin as possible. In addition, they should take steps to prevent mosquitoes entering their homes, using screens on windows and doors, and should try to reduce the ability of mosquitoes to breed close to homes by removing and emptying any water-holding containers.
Two dengue vaccines (Dengvaxia and Qdenga) are available overseas but are not available in NZ. They’re mainly for use in children living in high-risk areas, who have already had one episode of dengue.
The risk of dengue can vary widely by season and year. Outbreaks occur when a dengue variant arrives in a region after having been absent for many years, and people who have not previously been infected with that variant become infected and sick.
Fortunately, Aotearoa New Zealand does not have the type of mosquitoes that transmit dengue viruses so it’s important we prevent them from becoming established here.
Global cases have increased more than six-fold since 2000 and it is expected cases will continue to increase with climate change and increased urbanisation. Parts of southern Europe now regularly see dengue cases.
Since the 1970s, dengue has caused outbreaks in the Pacific. Up to 20% of the population have been affected in some outbreaks. Pacific Island countries and areas are vulnerable as they have high levels of mosquitoes and dengue viruses can readily be introduced by the arrival of people who were infected with dengue elsewhere.
Between 2012 and 2021, there were 69 outbreaks of dengue fever among the Pacific Islands. (Zika and chikungunya are other mosquito-spread virus illnesses present in the Pacific). The most recent dengue outbreak in Samoa during 2017-2018 resulted in more than 3000 cases, mainly in children aged 5-9 years.
Unfortunately, there were five deaths during that outbreak. Currently, about 30 cases of dengue fever a week are being reported in Samoa and, since November 2023, there have been about 250 confirmed cases, a significant increase above usual numbers.
After a bite by an infected mosquito, there is an incubation period of 5-7 days, during which the virus multiplies in cells of the immune system. Dengue infection may be asymptomatic, but commonly causes an illness with fever, pain behind the eyes, bone, joint and muscle pain, and sometimes rash, vomiting and diarrhea.
These symptoms usually last 7-10 days. In up to 5% of infections, most often in people who are experiencing their second episode of infection, serious complications, such as bleeding or shock, can arise. A blood test can confirm the diagnosis of dengue. There is no antiviral treatment for it, and hospital care may be required for those with severe symptoms.
Dr Joan Ingram is the medical adviser for the Immunisation Advisory Centre and Associate Professor Mark Thomas is an infectious diseases doctor in the University of Auckland’s Faculty of Medical and Health Sciences.
