Global heating driving spread of mosquito-borne dengue fever
Rising temperatures across Asia and the Americas have contributed to multiple severe outbreaks of dengue fever globally over the past six months, making 2019 the worst year on record for the disease. In 1970 only nine countries faced severe dengue outbreaks. But the disease, which is spread by mosquitoes that can only survive in warm temperatures, is now seen in more than 100 countries. There are thought to be 390 million infections each year. Thais Dos Santos, an expert adviser on the surveillance and control of arboviral diseases for the Pan American Health Organization, said dengue was reaching new areas as temperatures rise. “The hotter the climate, the better the mosquito is at breeding, but it is now coming up higher and higher to cities like BogotĂĄ and Mexico City that are well
above sea level, cities traditionally believed to be safe from the Aedes mosquito.� Across Asia the scale of outbreaks has been taking communities by surprise. In Pakistan one city hospital in Rawalpindi admitted more than 2,000 dengue patients in a single weekend in October. Across the country a record-breaking 47,000 people have been infected this year, almost double the previous high of 27,000 in 2011. Other Asian countries, including Bangladesh, Thailand, Cambodia and Laos, also had outbreaks, with a particularly severe situation in the Philippines, where 1,000 people died of the disease, including hundreds of children.
The Guardian Patients suffering from dengue fever receive medical treatment at an isolation ward at a hospital in Larkana, Pakistan. Photograph: Waqar Hussain/EPA Mosquitoes have also brought dengue to places where it has never been seen before, including the hills of Nepal, never warm enough in previous years for the disease to take hold. In Nepal there have been nearly 14,000 cases of dengue since mid-July. The previous record was 2,000 two years ago.
The dengue season in Kathmandu is usually short, mild and over by November. But Dr Dev Basu Pandey said he was still treating dengue patients on his ward at the Sukrara Tropical and Infectious Diseases hospital in the Nepalese capital. “Usually there would not be dengue still at this time of year,” he said. “But this year it is still warm, even once the sun has set. We have never seen an outbreak like this; people are afraid.” From 1976 to 2010, temperatures in western Nepal, the hill region of the country, have risen at an average of 1.2°C per year. The spread of the disease is also heavily related to human activity. The insects thrive in human habitation, laying their eggs in used tyres, flowerpots, tree holes and any water-filled container. As climate change alters monsoon and rain patterns in many countries, this creates the damp and warm conditions that, along with rapid urbanization, help the Aedes mosquitoes flourish. The Americas also broke records for dengue. More than 2.7 million cases were reported across the region by mid-November, the largest number since records began, and a 13% increase on the total yearly number in 2015, the previous record year. Over 2 million of the cases came from Brazil.Dos Santos said there were clear changes in dengue patterns. “Dengue is episodic so we would expect to see a larger than normal epidemic every two to five years but this year was unusual. We are seeing a shift in seasonality, seeing outbreaks start earlier. We are seeing a shift in seasonality, seeing outbreaks start earlier Thais Dos Santos, expert on arboviral diseases “Another unusual thing this year was a heavy circulation of the second type, dengue 2, which is well known for causing severe forms of the disease. We saw a lot of severe cases and death this year.”There is also another way that hotter temperatures help dengue spread. “After a mosquito bites an infected person and drinks up the virus in the blood, the virus needs to spend time in the mosquito’s gut before it can be passed on through a bite. As the outside
temperature increases ... the time for the virus to survive and be able to be passed on is reduced.”
Dos Santos said he wants people to see how rising temperatures and climate change are beginning to harm human health. “We are detecting a shift to a younger population, most of the cases in Central America were in under-15s. Sadly, most severe cases and deaths were disproportionately affecting children. “We see children dying but how this is related to the climate is not always made explicit. [We need to do] a better job of connecting these constellation of ways in which the change in climate is affecting us.” Rachel Lowe, of the London School of Hygiene & Tropical Medicine, said the current situation with dengue spreading to new areas was caused by a multitude of factors, including poverty and slum-like living conditions. “This year’s outbreaks are unprecedented, part of 30 years of a global spread. We have seen dramatic increases in global temperatures and also unplanned urbanisation. There are a combination of factors that lead to dengue, including living conditions and the way people are forced to live without refuse
collection.”While many vaccine trials are under way, she and her colleagues are looking at preparing for outbreaks using early warning prediction systems. Related: Europe at risk from spread of tropical insect-borne diseases “We have a UK space agency-funded project to develop early warning in Vietnam and we are working with the Met Office and UNDP to develop seasonal forecast systems in Brazil and Equador. We are trying to improve preparedness so we can estimate six months in advance where you can go and target limited vector control and make sure health facilities know there is an epidemic under way. Researchers from LSHTM and Oxford University produced figures earlier this year to show that if warming continues globally the two main disease-spreading mosquitoes – Aedes aegypti and Aedes albopictus – will significantly expand their range. The models predict that by 2050, 49% of the world’s population will live in places where these species are established if greenhouse gas emissions continue at current rates, and if they are not curbed, even greater areas will be at risk. “If no action is taken to reduce the current rate at which the climate is warming, pockets of habitat will open up across many urban areas” said Moritz Kraemer, an infectious disease scientist at Boston Children’s Hospital and the University of Oxford, who co-authored the research. 02 Mosquito-borne dengue fever cases in Miami-Dade continue to mount No result found, try new keyword!Another case of locally transmitted dengue fever has been confirmed, bringing Miami-Dade’s total to 12 this year, the Florida Department of Health in Miami-Dade said this week. The uptick in cases ... 03 Dengue death toll reaches 123 over 11 months The death toll of Dengue Haemorrhagic Fever (DHF) has surged to 123 across Myanmar in 11 months, said figures released by the Ministry of Health and Sport, according to a Xinhua report.
The Public Health Department under the ministry said that 23,203 mosquitoborne disease infection cases were recorded in Myanmar's regions and states from Jan. 1 to Nov. 23 this year. During the period, the DHF infection mostly occurred in Kachin state, Yangon, Ayeyarwady, Mandalay and Sagaing regions.
Distribution and outbreaks of dengue
Before 1970, only 9 countries had experienced severe dengue epidemics. The disease is now endemic in more than 100 countries in the WHO regions of Africa, the Americas, the Eastern Mediterranean, South-East Asia and the Western Pacific. The America, South-East Asia and Western Pacific regions are the most seriously affected, with Asia representing ~70% of the global burden of disease. Cases across the Americas, South-East Asia and Western Pacific exceeded 1.2 million in 2008 and over 3.34 million in 2016 (based on official data submitted by Member States). Not only is the number of cases increasing as the disease spreads to new areas including Europe, but explosive outbreaks are occurring. The threat of a
possible outbreak of dengue fever now exists in Europe as local transmission was reported for the first time in France and Croatia in 2010 and imported cases were detected in 3 other European countries. In 2012, an outbreak of dengue on the Madeira islands of Portugal resulted in over 2000 cases and imported cases were detected in mainland Portugal and 10 other countries in Europe. Among travellers returning from low- and middle-income countries, dengue is the second most diagnosed cause of fever after malaria. The year 2016 was characterized by large dengue outbreaks worldwide. The Region of the Americas region reported more than 2.38 million cases in 2016, where Brazil alone contributed slightly less than 1.5 million cases, approximately three times higher than in 2014. 1032 dengue deaths were also reported in the region. The Western Pacific Region reported more than 375,000 suspected cases of dengue in 2016, of which the Philippines reported 176 411 and Malaysia 100 028 cases, representing a similar burden to the previous year for both countries. The Solomon Islands declared an outbreak with more than 7000 suspected. In the African Region, Burkina Faso reported a localized outbreak of dengue with 1061 probable cases. In 2017, a significant reduction was reported in the number of dengue cases in the Americas - from 2 177 171 cases in 2016 to 584 263 cases in 2017. This represents a reduction of 73%. Panama, Peru and Aruba were the only countries that registered an increase in cases during 2017. Similarly, a 53% reduction in severe dengue cases was also recorded during 2017. The post Zika outbreak period (after 2016) has seen a decline of cases of dengue and the exact factors leading to this fall are still unknown. WHO’s Western Pacific Region has reported dengue outbreaks in several countries in the Pacific, as well as the circulation of DENV-1 and DENV-2 serotypes.
After a drop in the number of cases in 2017-18, a sharp increase in cases is being observed in 2019. In the Western Pacific region, increase in cases have been observed in Australia, Cambodia, China, Lao PDR, Malaysia, Philippines, Singapore, Vietnam. DENV-2 was reported in New Caledonia and DENV-1 in French Polynesia. Dengue outbreaks have also been reported in Congo, Côte d’Ivoire, Tanzania in the African region; Several countries of the American region -Brazil, Colombia, Nicaragua and Honduras have also observed an increase in the number of cases. Bangladesh, Nepal, Sri Lanka, Thailand, parts of India have also recorded increase in dengue cases in South-East Asian region and Pakistan and Sudan have also reported increase in the Eastern Mediterranean region. An estimated 500 000 people with severe dengue require hospitalization each year, and with an estimated 2.5% case fatality, annually. However, many countries have reduced the case fatality rate to less than 1% and globally, decline in case fatality have been recorded between 2010 and 2016 with significant improvement in case management through capacity building at country level.
Human-to-mosquito transmission Mosquitoes can become infected from people who are viremic with DENV. This can be someone who has a symptomatic dengue infection, someone who is yet to have a symptomatic infection (they are pre-symptomatic), but also people who show no signs of illness as well (they are asymptomatic) [11]. Human-to-mosquito transmission can occur up to 2 days before someone shows symptoms of the illness [5, 11], up to 2 days after the fever has resolved [12]. Risk of mosquito infection is positively associated with high viremia and high fever in the patient; conversely, high levels of DENV-specific antibodies are
associated with a decreased risk of mosquito infection (Nguyen et al 2013 PNAS). Most people are viremic for about 4-5 days, but viremia can last as long as 12 days [13].
Other modes of transmission The primary mode of transmission of DENV between humans involves mosquito vectors. There is evidence however, of the possibility of maternal transmission (from a pregnant mother to her baby). While vertical transmission rates appear low, with the risk of vertical transmission seemingly linked to the timing of the dengue infection during the pregnancy [14-17]. When a mother does have a DENV infection when she is pregnant, babies may suffer from pre-term birth, low birth weight, and fetal distress
Dengue Dengue should be suspected when a high fever (40°C/104°F) is accompanied by 2 of the following symptoms during the febrile phase:
severe headache pain behind the eyes muscle and joint pains nausea vomiting swollen glands rash.
Severe dengue A patient enters what is called the critical phase normally about 3-7 days after illness onset. It is at this time, when the fever is dropping (below 38°C/100°F) in the patient, that warning signs associated with severe dengue can manifest. Severe dengue is a potentially fatal complication, due to plasma leaking, fluid accumulation, respiratory distress, severe bleeding, or organ impairment. Warning signs that doctors should look for include:
severe abdominal pain persistent vomiting rapid breathing bleeding gums fatigue restlessness blood in vomit.
If patients manifest these symptoms during the cirtical phase, close observation for the next 24–48 hours is essential so that proper medical care can be provided, to avoid complications and risk of death.
Prevention and control The proximity of mosquito vector breeding sites to human habitation is a significant risk factor for dengue as well as for other diseases that these species transmit. At present, the main method to control or prevent the transmission of dengue virus is to combat the mosquito vectors. This is achieved through:
Prevention of mosquito breeding in and around houses and places of work, schools and health facilities: o Preventing mosquitoes from accessing egg-laying habitats by environmental management and modification;
o o o
Disposing of solid waste properly and removing artificial manmade habitats Covering, emptying and cleaning of domestic water storage containers on a weekly basis; Applying appropriate insecticides to water storage outdoor containers;
Personal protection from mosquito bites: o
o
Using of personal household protection measures, such as window screens, repellents, insecticide treated materials, coils and vaporizers (these measures must be observed during the day both inside and outside of the home (eg: at work/school) because the primary mosquito vectors bites throughout the day); Wearing clothing that minimises skin exposure to mosquitoes is advised;
Community engagement: o Educating the community on the risks of mosquito-borne diseases; o Engaging with the community to improve participation and mobilization for sustained vector control;
Reactive vector control:
Emergency vector control measures such as applying insecticides as space spraying during outbreaks may be used by health authorities;
Active mosquito and virus surveillance: Active monitoring and surveillance of vector abundance and species composition should be carried out to determine effectiveness of control interventions; Prospectively monitor prevalence of virus in the mosquito population, with active screening of sentinel mosquito collections;
In addition, there is ongoing research amongst many groups of international collaborators in search of novel tools and innovative strategies that will contribute in global efforts to interrupt transmission of dengue, as well as other mosquito-borne diseases. The integration of vector management approaches is encouraged by WHO to achieve sustainable, effective locally adapted vector control interventions.