Efforts to suppress dengue fever have succeeded in drastically reducing the number of people who get sick and die from the mosquito-borne virus—but in the process, they may have created a perpetual epidemic, health officials say.
“Usually dengue comes in three- to four-year cycles,” said Dr Chang Moh Seng of the World Health Organization. But as long as outbreaks of the disease are suppressed, “it will come back every year,” forcing health officials to fight it near-continuously.
So far this year dengue—sometimes called “bone-breaking fever” for its painful effects—has killed 46 people nationwide, 15 in Phnom Penh. “We are monitoring the situation very closely, and we expect more of an outbreak in July and August,” Chang Moh Seng said.
Last year for the first time, the WHO and the National Malaria Center treated 2.6 million water jars across the country with the insecticide Abate, which kills mosquito eggs and larvae before they can spread dengue. An education campaign and an insecticide-spraying program supplemented the Abate effort.
The results were dramatic. In 1998, during the last outbreak, 16,113 people got the disease; in 2001 only 10,264 did, according to WHO figures. In 1998, 461 people died. In 2001 there were just 185 deaths.
Ordinarily, dengue recurs about every three years because that’s how long humans’ immunity to the virus lasts. There were outbreaks in 1995 and 1998, and 2001 would have been a full outbreak year if not for the fact that “we were expecting it, so we stopped it with a lot of interventions,” said Dr Ngan Chantha, the malaria center’s dengue officer.
But the prevention of the outbreak was also the prevention of widespread immunity to the virus. Humans are as vulnerable to dengue this year as they were last year.
There are fewer cases this year (2,552) than there were by this time last year (3,646), but that could be due to the late onset of the rainy season this year.
“[This year], I would say that it is not a real outbreak, but an anomaly situation,” Ngan Chantha said. With 2,552 cases and 46 deaths this year, the figures are less than an outbreak year, but more than an off-year, he said.
In addition, there are strange trends. Typically a disease that relies on population density to spread, dengue is appearing in remote rural areas that have never seen outbreaks before.
Chang Moh Seng noted that sparsely populated Stung Treng province has 147 cases this year. That’s far less than the 374 cases in Phnom Penh or the 483 in Kandal province, but it is the highest per capita infection rate in the country.
Ngan Chantha attributed the change to a more mobile population. “More Cambodians are moving from rural areas to town for their jobs,” he said.
He also said last year saw an “unusual” surge in dengue cases in December—during the dry season, when dengue rarely appears. He speculated it might be because dengue has lately worsened in neighboring countries.
Judging from December, he said, it’s possible that dengue could become a year-round disease instead of a seasonal disease.
There is no vaccine that prevents dengue and no single drug that cures it. The best way officials have yet found to combat the disease is simply to prevent the tiger mosquitoes that carry it from breeding.
This year the first round of Abate distribution, in early May, sent 82,585 kg of the insecticide pills to 38 districts in 15 provinces. Another is to begin July 22, Chang Moh Seng said.
“We will keep treating it like this until we have other methods,” he said.
