This is a rapid ‘need to know’ spotlight on current infectious disease threats. It is not intended to be an exhaustive list of cases and outbreaks but a focus on reasons for concern about specific infectious diseases currently posing threats to global public health.
Waiting in the wings: Aedes mosquitoes and flavivirus epidemics
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Flavivirus outbreaks are being reported around the world. 42 countries have reported Zika virus outbreaks since the beginning of 2015. A large outbreak of yellow fever in Angola which was reported in January in the Angolan capital city, Luanda, has now spread to many provinces in the country and has sparked local transmission in Angola’s northern neighbour, the Democratic Republic of Congo. Cases have also been exported to China - which has never had an outbreak of yellow fever – but local transmission has not occurred there to date. Outbreaks of yellow fever have also been reported in Uganda.
The international community has called for accelerating the research on Zika disease. The virus has been known since 1947 but this is the first time that outbreaks of this magnitude have been observed. The International Zika Summit, held at the Institut Pasteur, Paris on April 25-26, brought together researchers and public health professionals to share the latest research findings on Zika virus. The causal link between the infection by Zika virus and neurological malformations including microcephaly has now been established. Prominent among discussions was the role of flavivirus-transmitting mosquito species.
With urbanization and the colonization of urban setting by Aedes mosquito species, “old” viruses bring new challenges.
The increase in travel has also permitted rapid spread of infected mosquitoes across continents. In the past, the slave trade which shipped people from Africa to South America brought yellow fever to South America. The International Health Regulations have always recommended special precautions such as disinsection in planes to reduce transportation of mosquitoes between countries. Despite these precautionary measures, Aedes species have established themselves in environments and regions well beyond their original tropical forest ecological niche.
Aedes aegypti could be called the ‘dogs’ of the mosquito world because they have become so ‘domesticated’ that almost every aspect of their biology is dependent on humans. They prefer to live in and around human homes - many are born, breed and die in human homes - and biting aggressively, during daylight hours. A female will bite several people before she considers her blood meal adequate for egg-laying. Another Aedes species able to transmit flavirviruses, Aedes albopictus, has the advantage of producing eggs able to survive cooler temperatures so can live in more temperate regions. As a result Aedes are competent urban vectors for not only Zika virus but other flaviviruses such as yellow fever.
With ever greater urbanization and climatic phenomena (the El Nino effect and global warming) producing warmer and wetter environments, these mosquito species are thriving. As their numbers rise, so too does the probability that more humans will be bitten by those carrying flaviviruses. The current Zika virus and yellow fever epidemics are a symptom of the increased risk of vector-borne epidemics due to much greater vector density, especially in urban settings.
- Controlling mosquitoes especially in urban settings is challenging. It is everybody’s business.
- In addition to government vector control programmes, individuals can protect themselves and their families by preventing mosquito breeding- covering all water containers, removing any discarded containers, scraping the sides of water containers and using larvicide to destroy any eggs- and using residual insecticide or sprays to kill adult mosquitoes.