GENEVA (IDN) – A form of insect birth control – Sterile Insect Technique (SIT) – that sterilises male mosquitoes using radiation will soon be tested as part of global health efforts to control diseases such as dengue, chikungunya and Zika.
The technique involves the mass production, sex-separation and sterilisation of male mosquitoes by exposing them to low doses of radiation. Sterile males released into the wild mate with wild female mosquitoes of the same species, resulting in the production of unviable eggs that lead to a decline in wild mosquito populations.
This well-established technology has a proven successful track record for agricultural and veterinary pests.
The Special Programme for Research and Training in Tropical Diseases (TDR) and the International Atomic Energy Agency (IAEA), in partnership with the Food and Agriculture Organisation of the United Nations (FAO), and the World Health Organisation (WHO) have developed a guidance document for countries that have expressed interest in testing SIT for Aedes mosquitoes.
“Half the world’s population is now at risk of dengue,” said Dr Soumya Swaminathan, WHO Chief Scientist. “And despite our best efforts, current efforts to control it are falling short. We desperately need new approaches and this initiative is both promising and exciting.”
Dengue outbreaks are currently occurring in several countries, notably on the Indian sub-continent. Bangladesh is facing the worst outbreak of dengue since its first recorded epidemic in 2000. The South Asian nation has seen the number of cases rise to over 92,000 since January 2019, with daily admissions peaking at more than 1,500 new dengue patients in hospitals in recent weeks and is one of the countries to express interest in SIT.
Diseases transmitted by mosquitoes such as malaria, dengue, Zika, chikungunya, and yellow fever account for about 17 percent of all infectious diseases globally, claiming more than 700,000 lives each year, and inflicting suffering on many more. The 2015 outbreak of Zika in Brazil was linked to an increase in the number of babies being born with microcephaly.
SIT was first developed by the U.S. Department of Agriculture and has been used successfully to target insect pests that attack crops and livestock, such as the Mediterranean fruit fly and the New World screwworm fly. It is currently in use globally in the agriculture sector on six continents.
The guidance on using the technique to control diseases in humans recommends adopting a phased approach that allows time to test the efficacy of the sterilised insects. Epidemiological indicators monitor the impact of the method on disease-transmission. It also provides recommendations on mass production of the sterile mosquitoes, government and community engagement, measuring the impact of the technique, and assessing cost-effectiveness.
“Countries seriously affected by dengue and Zika have shown real interest in testing this technology as it can help suppress mosquitoes that are developing resistance to insecticides, which are also negatively impacting the environment,” said Florence Fouque, a scientist at TDR.
According to the WHO, there has been a rapid spread of dengue and continued dengue outbreaks of epidemic potential since the beginning of 2019. The Global Burden of Disease Study has reported that dengue is increasing at a higher rate than any other communicable disease, with a 400 percent increase in just 13 years (2000–2013).
“Dengue continues to be neglected, with a lack of resources at country level and declining research funds in recent years,” said Dr Mwelecele Ntuli Malecela, Director, WHO Department of Control of Neglected Tropical Diseases. “We need to turn the tide and refocus efforts on building country capacity and working across sectors using locally adapted interventions to implement sustainable dengue prevention and control.”
While countries including Bangladesh, Brazil, the Philippines and others in Africa and Latin America are confronting major outbreaks, other countries in South-East Asia and in many tropical islands have recorded increasing numbers of cases. Climate change is considered to have amplified the distribution of Aedes aegypti, the vector responsible for transmitting dengue, chikungunya, yellow fever, zika virus and other arboviral diseases.
Other factors including rapid unplanned urbanisation, increased humidity, devolved vector control services and movement of people and goods have also facilitated the spread of the disease.
“With extended rainy seasons and variations in climatic conditions, dengue is spreading to more temperate countries,” said Dr Raman Velayudhan, head of WHO’s Vector and Ecology Management unit. “Locally adapted vector control measures such as household water storage management and, more importantly, engaging individuals and communities to manage their immediate environment are essential to controlling the spread of the disease.”
Late 2018, WHO identified dengue as one of 10 threats to global health in 2019. Around 390 million people are at risk of the disease in more than 128 countries, and an average of 96 million infections occur annually, including among asymptomatic carriers.
Dengue is implicated with rapid urbanisation and associated substandard planning, inadequate drainage and irregular water supply in outlying suburbs where poor water storage practices favour the proliferation of vector breeding sites.
Proper storage of potable water – by placing tightly fitting lids on containers used for water storage and removing vessels and other items that can hold rain or standing water – as well as management of the immediate environment are essential as the mosquito that transmits dengue breeds in water containers, empty cans, bottles and used tyres.
Since 2009, WHO has been collaborating with the Singapore Cooperation Programme in the Western Pacific Region to assist other countries in dengue prevention and control.
Recently, Singapore has seen many dengue outbreaks and is keen to share its experience with other countries. The Environmental Health Institute of Singapore, a public health laboratory at the National Environment Agency, conducts research, surveillance and evidence-based risk assessments on vector-borne diseases and other infectious diseases of environmental concern.
“We offer courses every 1-2 years and, since 2009, we have trained more than 400 participants on dengue case management, laboratory diagnosis and vector surveillance and control,” said Dr Ng Lee Ching, Director, Environment Health Institute, National Environment Agency, Singapore. “We share our experience in inter-epidemic surveillance and vector control, and promote the integration of epidemiological, laboratory and field data and clinical management for coordinated effort.”
The workshops have stimulated the creation of a cross-border support network among Asian countries to share expertise and monitor the dengue serotypes circulating in the region. UNITEDengue, a network for cross border sharing of dengue surveillance information and knowledge on dengue control, contributes to strengthening regional capacity in response to new needs at short notice and is completing a survey of trained participants to identify areas of further skills development.
In addition to SIT, several other promising new vector control tools are being studied to minimise vector populations and prevent dengue transmission. While some have the potential to become entomologically effective, they are not currently recommended by WHO because data on their impact on the pathogen or the disease are absent.
Examples of these tools include Wolbachia spp. bacteria, which when introduced into populations of Aegypti are expected to reduce the ability of mosquitoes to transmit arboviruses to humans. Laboratory results show that Wolbachia infection reduces viral replication of dengue, chikungunya and Zika viruses within Aedes mosquitoes, and eliminates or substantially delays the appearance of virus in mosquito saliva – reducing its competence for transmitting dengue viruses. More studies are under way in several countries.
Spatial repellents – which are designed to interrupt human-vector contact through vector behaviour modification induced by airborne chemicals and have the potential to offer protection from the bites of vectors and nuisance pests – are also being studied.
These repellents are designed to release volatile pyrethroid into the air and prevent human–vector contact in the treated space. This method targets Anopheles, Aedes and Culex spp., with claims to protect all age groups and populations in countries endemic for mosquito-borne diseases from daytime, early evening or late-night biting by mosquitoes in enclosed and semi-enclosed structures. [IDN-InDepthNews – 18 November 2019]
Photo: This drone releases sterile male mosquitoes to help reduce transmission of dengue, Zika, and chikungunya. Credit:TDR/F. Fouque
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