Dengue is reported to be the fastest growing vector-borne disease in the world. In recent years, dengue has spread extensively in North and South America and in Mexico it increased 600 percent between 2001 and 2007. The disease has spread to Hawaii and along the border in Texas. The disease is expected to increase in northern Australia with climate change (see Climate Change Health Check on this page)

Dengue fever is caused by an RNA flavivirus spread by the bites of mosquitoes particularly in the urban areas of the tropics. Symptoms include fever, headache, rash, severe pains in the muscles and joints, and pain behind the eye. The fever form of the disease is rarely fatal, while the related dengue haemorrhagic fever is a severe disease with death in approximately 5 percent of cases. Dengue hemorrhagic fever occurs most often in children younger than 15 years old. It also occurs most often in individuals who were previously infected with simple dengue fever.

The dengue flavivirus occurs in four different serotypes, DEN-1, DEN-2, DEN-3, and DEN-4. Contracting one form of dengue fever provides lifelong immunity from that serotype, but not from the others. Humans contract dengue fever from bites of infected female mosquitoes of the genus Aedes . Aedes aegypti is the primary vector in most regions. When a female Aedes mosquito bites a person infected with dengue, the virus incubates in the insect body for 8-11 days, after which the mosquito can spread the disease to other humans for the remainder of its life span (15-65 days). Once the virus enters a human, it circulates in the bloodstream for two to seven days, during which time the virus can be spread to other humans. Aedes albopictusis was originally the primary vector of dengue fever, and remains a major vector in Asia. The species has recently spread to Central America and the US, where it is a secondary vector of the disease . Aedes aegypti is primarily urban, and Aedes albopictusis rural, thereby increasing the ecological range of habitats in which people can become infected. Humans are the primary reservoir for the virus.

Terminator Mosquitoes to Control Dengue?

This article from Prof. Joe Cummins and Dr. Mae-Wan Ho can be found on http://www.i-sis.org.uk/ These authors ask Why use a dangerous, costly, and untested high tech option when safe, effective, and affordable alternatives are available?

Millions of transgenic mosquitoes are to be released into the fishing village of Pulau Ketam off Selangor, Malaysia, as part of an international series of field trials to fight dengue fever. The Malaysian field trials will be undertaken by the Health Ministry’s Institute of Medical Research (IMR) in collaboration with Oxitec Ltd., a spin-off biotech company from the University of Oxford in the UK.This follows the reported success of confined laboratory trials conducted under the supervision of the IMR over the past year.

The technique, which has won Oxitec the Technology Pioneers 2008 award at the World Economic Forum, involves releasing transgenic male Aedes mosquitoes carrying a ‘killer’ gene to mate with wild female mosquitoes, which causes (nearly) all their progeny to die. This is a variant of the Sterile Insect Technique (SIT) that has been successfully used in wiping out other insect vectors in the past, though the sterile males were created by X-irradiation, and not by transgenesis.

The release of sterile males is considered “environmentally benign”, as only female mosquitoes bite and suck blood and transmit the disease-causing virus; not the male mosquitoes..

Environmental groups fear that releasing the transgenic mosquitoes may affect the ecosystem and cause further damage. But there has been remarkably little informed reporting on the nature of the potential hazards involved.

Cummins and Ho state, “The most glaring aspect of the proposed release is that the lethally acting transcription activator tTAV has a rather ill-defined action. The information presently available does not tell us what is killing the target animals. Even though a homologous tetracycline-repressed gene was not toxic to mice upon its activation, the killing toxin in the mosquito should certainly be identified before released to the environment is contemplated”

What are the alternatives to genetic control of mosquitos?

Cummins and Ho state “There are recent effective and affordable alternatives to in controlling mosquitoes that spread dengue fever and other diseases. Extracts from the paradise tree Melia azedarach showed promising larvicide and oviposition deterrent effects on the mosquito. Essential oil from mullila and zedoary plants also proved effective in treating mosquito larvae. Euphoriaceae extracts, particularly Euphorbia tirucalli can be applied as an ideal larvicide against Aedes aegypti.

A study in Thailand surveyed water-filled containers where Aedes mosquitoe pupae were found. Large water containers held 90 percent of pupae in rural areas and 60 percent in urban areas. Covering and treating such large containers should greatly reduce the mosquito population. Bacillus thuringiensis israelensis VectoBac proved effective in treating water jars to combat dengue mosquitoes in Cambodia. In Cuba, studies on the social and environmental determinants of Aedes aegypti confirmed that the greatest risks were associated with failure to treat stored water, and water in flower vases for religious practices. Efforts to reduce infestation should therefore focus on preventive practices.

These low tech practices may prove much more effective than the expensive high technology solutions, which are also far from safe”.

The references for this information can be found at

http://www.i-sis.org.uk/Terminator_Mosquitoes_to_Control_Dengue.php

David Shearman