Dengue Outbreak: A Taste of Things to Come?

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Frank Howarth knows about dengue. “It’s horrible,” the Bishop Museum entomologist says. “Breakbone fever – it’s well named.” Howarth contracted the disease years ago in Laos, but memories of the suffering remain clear and strong.

Howarth is encountering the disease once more – this time in Hawai`i, where, since September, increasing numbers of cases of the viral disease transmitted by mosquitoes have been confirmed, with many more suspected. As one of the state’s most respected insect experts, Howarth is now studying what could have set off this most recent round of infections – the first in Hawai`i since the 1940s.

“This 50-year hiatus in transmission is not from the absence of disease in the islands,” he insists. “Travelers are coming back to the state all the time with the disease.” But the local strain of the Asian tiger mosquito, Aedes albopictus, present in Hawai`i since about 1902, is a “poor transmitter of the disease,” he continues, so Hawai`i was spared an outbreak of dengue. Now, though, “with this outbreak moving so fast, getting out of hand, it suggests a new vector is in town.”

It could be that a new strain of Ae. albopictus has arrived in Hawai`i. Elsewhere, Ae. albopictus has been known to transmit the virus that causes dengue, although Aedes aegypti is the more common carrier. But Howarth suspects a relative newcomer to the islands might be to blame: Ochlerotatus japonicus, a mosquito so closely related to Aedes species that it has often been described as Aedes japonicus.

Ochlerotatus japonicus has been implicated in the establishment and spread of the West Nile Virus along the Eastern seaboard,” he says. “It came in with imports of Lucky Bamboo,” the small potted plants popularized by the feng shui trend in home decorating. “If Ochlerotatus was able to go from Japan to New York City in 1998, it could well have stopped in Hawai`i for a visit.”

So far, the Department of Health has been unable to identify any new vectors. According to George Kitaguchi of the Department of Health’s Vector Control Branch, “we don’t have japonicus here – not yet.”

Dengue in Hawai`i

The first recorded case of dengue in Hawai`i occurred in 1893, when it was known locally as “boohoo fever,” a name arising out of the emotional depression that frequently accompanies the disease. According to the state Department of Health, the first widespread epidemic of dengue appeared in 1903, when about 30,000 people were infected. Another epidemic lasted from 1912 to 1915. Not until 1943 did the virus reappear in an outbreak that infected about 1,500 people – and killed three — before it ran its course in 1945. Reports in the Hawai`i Medical Journal suggest that the infection may have been transmitted from Fiji, which was experiencing an epidemic at the time.

The epidemic in the 1940s led to an effort, supported by federal funds, to wipe out the suspected vector, Aedes aegypti. The DOH’s Kitaguchi says Ae.aegypti was eliminated from most areas, although that mosquito still remains in parts of Moloka`i and the coast of West Hawai`i from Kawaihae to Captain Cook. In the meantime, where Aedes aegypti was eradicated, the void was quickly filled by Aedes albopictus.

Since then, cases of dengue have been reported in the state – including 17 in the last five years — but all were among travelers who contracted the disease elsewhere. With an incubation period of from three to 14 days after initial exposure to the virus through a mosquito bite, an infected person can easily carry the virus from one place to another unaware that he or she is a carrier. From about one day before the first symptoms occur – usually fever, severe headache behind the eyes, and debilitating joint pain – and for the next week, the infected person can pass the virus to a mosquito host. The mosquito passes the virus on to anyone it bites for the rest of its life, usually no more than three weeks. Some researchers also believe the female mosquito can transmit the virus to eggs, so that the next generation of mosquitoes is also infectious.

Arthropod-Borne Viruses

Four distinct strains of the dengue virus have been identified. The first to be identified, DEN-1, is also known as the Hawai`i strain and was responsible for the 1943 outbreak. It was isolated and identified by Dr. Albert Sabin, who at the time was stationed in Hawai`i as a physician with the U.S. Army Medical Corps. Sabin would go on to invent the oral vaccine for polio.

Exposure to any one of the dengue virus strains confers immunity to reinfection by the same strain. However, if a person once infected becomes re-infected with a different strain, there is an increased likelihood that the virus will be expressed as a much more severe form of the disease, dengue hemorrhagic fever, which has been compared to ebola in its symptoms, or dengue shock syndrome, characterized by extremely low blood pressure. Both conditions can be fatal and require immediate medical attention.

Dengue is a type of viral disease known as an arbovirus, a group of more than 400 diseases so named because they are borne by arthropods. It is also one of 69 identified pathogens included in the Flavividae family and is in the genus that includes such other nasty diseases as yellow fever, St. Louis encephalitis, and West Nile virus. Not all arboviruses are flaviviruses: examples of the former include Lyme disease, plague, and Hanta virus. Nor are all flaviviruses – named after the Latin word for yellow ( flavus) – carried by arthropods; for example, hepatitis C, a flavivirus, is transmitted through body fluids.

Collateral Damage

In the 1943-45 dengue outbreak, the Aedes aegypti mosquito was eradicated through a program of spraying with DDT that was coordinated by the U.S. Army. Today, with DDT having been removed from the arsenal of available weapons to combat insect pests, the state Department of Health is trying to control mosquitoes by spraying with a pyrethrin-based insecticide. Pyrethrins are synthetic chemicals that mimic a compound that occurs naturally in chrysanthemums; generally they are non-toxic to humans and other terrestrial vertebrates, although lethal to fish.

Could pyrethrins have harmful effects on non-target species, including native insects such as damselflies?

Barry Brennan, an entomologist at the University of Hawai`i-Manoa, says there would definitely be a concern “if you use something that’s persistent.” Pyrethrins, however, break down quickly after they are applied.

“There’s some concern about aerial spraying in general being able to penetrate the forest canopy so you may not be able to get to where your target pest is anyway,” he continues. For that reason, “I think probably it is much better to target resources on elimination of breeding places. In my opinion, pesticides should be a backup, supplementary type of thing.”

Dan Polhemus, an entomologist with the Smithsonian Institution in Washington and expert in Hawai`i insects, is not quite as sanguine. If permethrins are sprayed on a stream, he says, this “would have the potential to kill aquatic life, both vertebrate and invertebrate, in the stream reaches below the point of application.” In turn, this “could pose a threat to native stream animals,” he adds. Although the Department of Health is aware of the fact that permethrins should not be used near streams, “private landowners may not bother to read the cautions and might assume that any aquatic habitat should be treated. Thus, there is a clear potential for impact to streams.”

He goes on to note that two species of native damselflies – both petitioned for listing as endangered under the Endangered Species Act – occur on the Hana Coast, near areas where dengue fever has been confirmed. One, Megalagrion nesiotes, is known to have just a single remaining population at East Wailuaiki Stream. “The second species, Megalagrion pacificum, is known from 10 remaining populations statewide, four of which occur on windward East Maui at Haipuaena, Pi`ina`au, Hanawi, and Makapipi streams.

“Private landowners may not bother to read the cautions and might assume that any aquatic habitat should be treated. Thus, there is a clear potential for impact to streams.” – Dan Polhemus, entomologist

“Since Makapipi Stream flows through Nahiku, the initial focus of the Maui dengue outbreak, there is a clear potential for its M. pacificum population to be affected by mosquito control efforts, should these include the stream corridorÉ Similar concerns involve the Pi`ina`au population, which is adjacent to the habitations and taro fields at Ke`anae. The populations at Haipuaena and Hanawi are much more removed from human habitation and would likely only be affected were a large-scale aerial spraying campaign to be undertaken (which seems unlikely, given prevailing public sentiment.)”

Double Whammy

The U.S. Centers for Disease Control describes the mosquito that appears to be implicated in the Hawai`i outbreak, Aedes albopictus, as “a maintenance (occasionally epidemic) vector of dengue viruses in parts of Asia and É a competent vector of several other viruses under experimental conditions.” Among the other viruses that Ae. albopictus can carry are Eastern equine encephalitis, Cache Valley fever, and West Nile virus, all of which can cause disease in humans.

Says the DOH’s Kitaguchi, “There’s nothing to stop these other diseases from being picked up by local mosquitoes,” in the same way they apparently picked up the virus for dengue fever. Or, as Howarth puts it, “we have the double whammy of having the vector and are just waiting for the disease to arrive.”

And the arrival of more diseases seems almost certainly to be a question of when, and not if. The dengue now in Hawai`i appears to have come from Tahiti, but it also could have come from Samoa, Palau, Macau, Hong Kong, Central America, or a dozen other places where dengue fever is rampant. In recent years, the Centers for Disease Control reports, millions of people worldwide have been infected with dengue, with several thousands dying.

The range of West Nile virus in the continental United States has been expanding rapidly; last month, it was detected as far west as Missouri and Arkansas. One of the chief means of the spread of that disease has been through infected birds. To head off – or at least delay – the virus’ arrival in Hawai`i, David Duffy, professor and unit leader of the Pacific Cooperative Studies Unit at the University of Hawai`i at Manoa, has a modest proposal: “I hope the state will ban the import of birds before we have a similar experience with West Nile” as the state has had with dengue, he said. “Both dengue and West Nile virus have the same mortality rates, but WNV has a much greater hysteria quotient.”

“I hope the state will ban the import of birds before we have a similar experience with West Nile.” – David Duffy, biologist
Not only does West Nile virus cause disease among humans, it also – and more frequently – affects birds, most times fatally. Dennis LaPointe of the U.S.G.S. Biological Resources Division in Volcano, Hawai`i, suspects Hawai`i’s native birds would be susceptible. The virus seems to hit crows especially hard, he noted, “so I’m sure `alala would be affected.” Other birds in which the virus has been found include hawks, jays, mallards, pigeons, and herons.

The mosquito that most commonly transmits the disease, Culex pipiens, has a close relative, Culex quinquefasciatus, or the southern house mosquito, the most abundant species of mosquito in the islands. To date, it is notorious for transmitting avian malaria among native birds, but LaPointe suspects it would be an efficient vector for West Nile virus as well.

For more information on dengue fever:

[url=http://www.cdc.gov/ncidod/dvbid/dengue/facts.htm]http://www.cdc.gov/ncidod/dvbid/dengue/facts.htm[/url]

— Patricia Tummons

Volume 12, Number 5 November 2001

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