With India in the grip of a deadly dengue outbreak, a new study suggests the virus has also become endemic in neighboring China where it can persist year-round, increasing the risk of more frequent and severe outbreaks.
The authors of the comprehensive study, published on October 12 in the American Journal of Tropical Medicine and Hygiene (AJTMH), say that southern China may suffer more often from dengue outbreaks similar to the one which hit Guangdong Province last year, sickening more than 40,000 people within two months. A key reason for this is that unlike previously characterized, dengue is no longer an imported disease, but has become endemic in the East Asian nation, said the US and Chinese researchers.
"Current data suggests a strong likelihood that dengue can persist in China - in some cases up to six to eight years," said Rubing Chen, an evolutionary virologist at the University of Texas Medical Branch, Galveston, and lead investigator of the study. "Further, we found a surprisingly complex and diverse mix of viral subtypes represented in China, a factor that can mean greater risk of epidemic dengue in the future."
'Ideal for dengue propagation'
Dengue is a notifiable disease in China. Since 1978, outbreaks have been frequently reported in southern provinces such as, Hainan, Guangxi, Fujian, Zhejiang and especially Guangdong, which has seen the largest number of cases.
Referring to official data, the World Health Organization (WHO) says the number of dengue cases in the world's most populous country has been rising in the past few years: 223 cases were reported in 2010, 120 cases in 2011, 575 cases in 2012, 4663 cases in 2013, and 46,864 cases in 2014. No deaths occurred from 2010 to 2013, whereas six deaths occurred in 2014.
"With its location in tropical and subtropical regions and hot, humid climate, as well as highly populated communities… southern China is ideal for dengue propagation and maintenance," said the authors of the study.
Dengue fever, a viral disease first recognized during outbreaks in the 1950s, is spreading rapidly as the range of the mosquitoes that transmit it - Aedes aegypti and Aedes albopictus - expands throughout the world. A female mosquito gets infected by ingesting the virus from a patient when taking a blood meal (usually during the day time), and can then transmit the virus - around 10 days later - when feeding on another person.
How deadly is dengue?
Estimates suggest that around 80 percent of people do not get ill from the infection, but the other 20 percent develop dengue fever, also known as "break bone fever" with severe joint pains, headache, nausea and often easy bleeding. Professor Arjen Dondorp, the deputy director of Mahidol-Oxford Tropical Medicine Research Unit (MORU) in Bangkok, told DW. In a small number of cases, complications occur, notably a plasma leak syndrome that may be severe enough to result in shock and can be fatal and less frequently life-threatening bleeding from the gut.
As the authors of the report explain, dengue infections are similar to influenza in that an individual can become infected over and over by different strains of the virus. But what makes the mosquito-borne disease particularly dangerous is that people who have had previous infections are more likely to have severe symptoms if they catch the disease multiple times.
For the study, Chen and her colleague Guan-Zhu Han, an evolutionary biologist based at Nanjing Normal University, evaluated all dengue virus sequences from China available in the public database GenBank - about 1,000-4,000 samples each for the four dengue serotypes (DENV 1-4), for a total of almost 10,000.
"Even within the same year, a person can catch dengue more than once if distantly related variants are circulating in the same region," said Chen. "That's why we become concerned about public health when many variants are found, as was the case in our study."
The new study found that the same grouping of Guangdong DENV strains persisted yearly from 2006-2014. However, given the incomplete information on strains in other Southeast Asian countries such as Indonesia, Laos and Vietnam, it is possible that these strains were imported each year from the same location, said Chen. But it is also possible that southern China is now part of an integrated pattern of dengue circulation that includes all of Southeast Asia and southern China, the expert added.
A 'substantial threat'
Regardless of the exact transmission pattern, the researchers stress that China is facing a "substantial dengue threat, with potential invasion into broader areas of the country," as sporadic cases have been identified in several provinces in recent years.
"The combination of a hot, humid climate ideal for breeding mosquitoes and large population centers in southern China combine to make this area a particular concern for public health officials," said the authors.
A similar view is shared by Bridget Wills, professor of Tropical Medicine at the University of Oxford. The scientist explains that in these provinces the climate is generally favorable, suitable vectors are present, and the existence of major urban centers - such as Guangzhou with a population in excess of 10 million, most of whom are likely non-immune - could readily facilitate explosive DENV transmission if conditions were right and a new viral serotype or genotype were introduced.
In this context, Wills also referred to a recent paper by Shengie Lai et al, describing the epidemiology of dengue in China between 1990 and 2014. The research indicates that the area affected has expanded considerably in the 21st century, with both imported and indigenous dengue increasing steadily, particularly in the southern provinces bordering other Asian countries that are already endemic for dengue.
"In common with neighboring countries affected by dengue, China is coming to terms with the urgent need to enhance disease and vector surveillance, improve vector control activities, promote public health awareness of dengue and its potential complications, and generally strengthen capacity for public health systems in the region," Wills, who is also vice-director of Oxford University Clinical Research Unit in Vietnam, told DW.
Other factors increasing the risk of infection may be related to urbanization, international travel, and perhaps global warming, said the WHO. So far this year, however, there has been a 94 percent drop in the number of dengue-related cases in Guangdong compared to last year, according to the provincial health ministry.
In many areas of the world, dengue has now become endemic, meaning that the virus exists at low levels year-round, and infections occur annually during the rainy season when mosquito populations increase rapidly. India, for instance, is currently in the grip of its worst dengue outbreak in years, with more than 6,500 confirmed cases and at least 25 deaths in the capital New Delhi this season.
Dengue now infects over 50 million people annually worldwide, with 500,000 severe cases of dengue hemorrhagic fever and 20,000 deaths, according to the WHO. Around 75 percent of the infections are thought to occur in the Asia-Pacific region.
In fact, the reported global incidence has increased 30-fold over the last five decades and the annually reported number of cases in the Western Pacific Region has more than doubled compared to those reported a decade ago, said the WHO.
No drugs available
There are no specific drugs effective at killing the virus, so the treatment is supportive care. "Giving intravenous fluids to the patients with shock is life-saving, where it is dangerous to give either too little or too much fluids. Blood transfusions may be needed if the patient has a bleed, usually from the gut," said Dondorp.
The health expert said that creating effective drugs had proven to be difficult, but recently a vaccine was developed, which provided around 60 percent protection against the symptomatic disease.
Experts point out that the main tool currently available to tackle dengue is the vigorous control of mosquito-breeding sites.
These include water storage tanks on roofs, gutters, drip pans from air conditioners, flower pots, water storage vessels in homes, and discarded tires as well as plastic containers. Furthermore, mosquitoes can find hidden breeding sites in underground drains that people cannot see.
Donald Shepard, a health economics professor at US-based Brandeis University, therefore recommends a series of steps to reduce the number of infections. "Trash should be removed. Containers should be sealed to keep out mosquitoes, cleaned to allow drainage, changed at least once a week, or treated with temephos or another product to stop mosquito breeding," he said, adding that a careful implementation of these steps needed commitment from the public and government.