Female mosquito (Aedes aegypti). Credit: USDA photo by Stephen Ausmus

Samoa’s dengue outbreak – Expert Reaction

Samoa will fumigate schools and hospitals in an effort to kill mosquitoes carrying the potentially life-threatening dengue virus.

The Pacific Island country has declared a dengue outbreak with over 200 confirmed cases, and health officials estimating many more have already been infected.

The SMC asked experts to comment.

Dr Joan Ingram, Medical Advisor, Immunisation Advisory Centre (IMAC), comments:

“Dengue is a common mosquito-borne viral disease widespread throughout the tropical and subtropical world.

“Cases of dengue have increased over six-fold since 2000. It is expected that cases will continue to increase with climate change and urbanisation.

“Dengue is spread by common day biting mosquitoes (various Aedes species) which thrive where humans live. There are four different dengue viruses, and infection with one gives long-term protection from that virus but may make the illness following one of the other three dengue viruses more serious. The risk of dengue can vary widely by season and year. Outbreaks with many cases occur when one of the viruses arrives in a region after being absent for years, meaning a large part of the population are susceptible to it and may become sick at the same time. Fortunately, Aotearoa New Zealand does not have the appropriate mosquitoes and it is important we avoid them becoming established here.

“Since the 1970s dengue has caused outbreaks in the Pacific with up to 20 percent of the population affected in some outbreaks. Pacific Island countries are vulnerable as they have high levels of mosquitoes and dengue viruses can be introduced by infected people.

“Between 2012 and 2021 there were 69 outbreaks of dengue fever among the Pacific Islands. (Zika and chikungunya are other mosquito-spread viruses present in the Pacific). Currently, approximately 30 cases of dengue fever per week are being reported in Samoa, and since November 2023 there have been around 220 confirmed cases which is a significant increase over average numbers.

“After an infected mosquito bite there is an incubation period of 5 to 7 days (maximum 10). Dengue infection may be unnoticed, or a mild illness or significant illness with fever, pain behind the eyes, bone, joint and muscle pain, and sometimes rash, vomiting and diarrhea. These symptoms usually last 7 to 10 days. In up to 5% of infections (most often after a second infection), serious complications such as bleeding or shock can arise. A blood test is required to diagnose dengue. There is no antiviral treatment for dengue, but hospital care may be required for those with complications to provide support.

“The main way to avoid dengue is mosquito bite avoidance. Aedes mosquitoes (unlike the malaria-transmitting Anopheles) are daytime feeders, with two peak times of biting activity in the day: two to three hours after dawn and mid-to-late afternoon. However, they may feed all day indoors or on overcast days. People should regularly apply effective repellent as well as using clothing to cover up. In addition, they should take steps to reduce mosquitoes indoors such as screens on windows and doors and in the environment by emptying any water holding containers.

“Two dengue vaccines (Dengvaxia and Qdenga) are available overseas but not in New Zealand. They are mainly for use in risk areas for children who have already had one episode of dengue, although Qdenga has been approved by the European Medicines Agency.”

No conflict of interest.

Professor Cameron Simmons, Director of the Institute of Vector-Borne Disease at Monash University; and Director of Global Delivery the World Mosquito Programme, comments:

“Samoa, like most other countries in the Western Pacific, has a long history of being impacted by dengue outbreaks. This most recent surge in dengue case numbers is troubling because the case numbers will inevitably increase, stressing the health system and hurting the wellbeing of patients and their families.

“We know that insecticides and environmental clean up campaigns can help a little to control the outbreak, but they are short-lived interventions that will not stop future outbreaks in Samoa.

“A safe, effective and self-sustaining solution exists. The Wolbachia method, developed by the non-profit World Mosquito Program, is the only proven self-sustaining method to prevent dengue outbreaks. We hope Samoa will join its neighbours in Fiji, Vanuatu and Kiribati in piloting the Wolbachia method.”

Conflict of interest statement: “I’m the Director of Global Delivery at the World Mosquito Program.”

Dr Mark Thomas, Associate Professor in Infectious Diseases, University of Auckland, comments:

“Dengue fever is caused by a viral infection transmitted by the bite of infected female Aedes mosquitoes. The Aedes mosquitoes that transmit dengue fever are not present in New Zealand, but are widespread in tropical countries around the world.

“The illness has been recognised for hundreds of years, and may be asymptomatic, or cause fever with muscle and joint pain (known colloquially as “breakbone fever”), and particularly in young people, may cause dengue haemorrhagic fever and dengue shock syndrome. Patients with these two conditions have a rash, and may die due to low blood pressure.

“Death is very rare in tourists with dengue fever, but the last epidemic of dengue fever in Samoa during 2017-2018 was associated with five deaths.

“There is no vaccine available in New Zealand against dengue fever. Prevention of disease is dependent on reducing mosquito bites by wearing clothes that reduce skin accessibility, and effective insect repellent.

“There is no specific treatment for dengue fever. Medical care involves managing the complications of the illness.

“People can have more than one episode of dengue fever, because there are different variants of the virus, and immunity to one variant does not protect against infection with another.”

No conflict of interest declared.

Associate Professor Dianne Sika-Paotonu,

Immunologist; Associate Dean (Pacific), Division of Health Sciences, University of Otago Wellington, comments:

“Dengue Fever is a viral illness that can be passed on after being bitten by an infected female mosquito. Dengue Fever is common in tropical and subtropical areas, and there are four different Dengue virus types.

“These Dengue viruses can be spread through bites from the Aedes mosquito species (mainly Aedes aegypti, but also the Aedes albopictus mosquito). Mosquitoes can also become infected with Dengue after biting an infected individual.

“According to the CDC, approximately half of the world’s population live in areas with Dengue Fever risk, with infants and pregnant women at most risk from severe Dengue.

“Many people who are infected with Dengue Fever can be asymptomatic, or have mild symptoms. Some however can develop severe Dengue which can be deadly and become life threatening very quickly. Severe episodes of Dengue fever usually require hospitalisation.

“Symptoms can occur approximately 4-10 after infection, and can last for 2-7 days. Common symptoms can include a high fever, severe headache, nausea, vomiting, headache, rash, aches and pains (eye, muscle, joint and bone pain) and swollen glands.

“The medicines given for Dengue Fever are only pain medications. A blood test is used to confirm diagnosis. There is a higher risk of ending up with severe Dengue if someone has previously had Dengue Fever before.

“According to the CDC the best way to prevent Dengue is to avoid mosquito bites and by controlling mosquitoes. Mosquito bites can be prevented by using insect repellents, mosquito nets and appropriate clothing.

“Mosquitoes will lay their eggs in or near pools of water. Removing areas of still or standing water from around the home and covering areas where water can pool, will help stop mosquitoes from laying their eggs. Keeping mosquitoes out of the home for example using screens etc will also be beneficial.

“Dengue Fever outbreaks occur regularly in many different parts of the world and include North and South America, Asia, Africa, the Middle East, and the Pacific Islands. According to the WHO, the incidence of Dengue Fever has increased over the past decades.

“Since most individuals infected with Dengue Fever may be asymptomatic or have mild symptoms, many cases can go unreported.”

No conflict of interest declared.