While the World Health Organization and scientists around the world are focusing on identifying the microbe that will cause the next pandemic, the “world’s most deadly animal” (according to the US CDC- Centers for Disease Control and Prevention) is spreading its wings. The humble mosquito is a highly efficient ‘vector’ for disease- it can transmit a number of debilitating and also fatal infections to humans. There are at least six potentially deadly diseases causing almost three-quarters of a million deaths worldwide each year:
- Malaria - by far the deadliest is transmitted by the anopheles mosquito and causes around 250 million infections and up to half a million deaths, mainly in sub-Saharan Africa and many in children.
- Zika Virus - transmitted mainly by the aedes mosquito, the virus was associated with a large outbreak of miscarriages and congenital malformations centered in Brazil.
- Yellow Fever - a potentially deadly disease also transmitted by the aedes mosquito species, predominantly affecting people in sub-Saharan Africa and South America.
- Dengue - another aedes-transmitted infection that is endemic in over 100 countries and causes upwards of 400 million infections every year, strains on healthcare systems, and occasional deaths reported.
- Chikungunya - yet another widespread aedes-transmitted infection that causes several million cases a year, that can neither be prevented by vaccines nor treated by specific drugs.
- West Nile Virus - spread by the culex mosquito and usually causes no symptoms but can cause severe disease in those who are vulnerable through depressed immune systems.
The mosquito takes a ‘blood meal’ from an infected human using its highly efficient mouthparts that can pierce human skin without being noticed and then moves on to bite several others, thus facilitating transmission of disease. Of the 3,500 different species of mosquito, less than 10% of these are important in causing human disease. Eradicating mosquitoes may seem like a great idea to prevent these diseases, but apart from this being a monumentally difficult task, the insect plays a key role in many ecosystems and food chains.
Traditional Approaches and Innovations in Mosquito Control
Traditional means of modifying habitat, by removing breeding sites in standing water, widescale spraying of insecticides, and providing physical barriers to biting, such as window screens and mosquito nets, are vital in reducing infection. New trap designs using light, sounds, or attractive odors are being developed to lure and capture mosquitoes more effectively. Other novel methods for controlling mosquito populations have proved effective in recent trials in designated areas. Technology to modify specific genes in a population of the insect can lead to widespread infertility or a specific block in transmitting pathogens- there is always a fear when gene editing is used, but scientists are carefully evaluating ethical and environmental considerations. 4
A more palatable method, in the eyes of the public, involves the introduction of a specific bacterium called Wolbachia into wild populations of mosquitoes that also reduces fertility and disrupts transmission. This is a naturally occurring bacterium in up to half of all insects, but it needs to be artificially introduced into mosquitoes. Following a very large, randomized control trial in Indonesia that led to a 77% reduction in dengue cases and an 86% reduction in hospitalizations, further releases are planned for El Salvador, Colombia, and Australia. A massive factory is due to open in Brazil so that scientists can scale up manufacturing and deployment. Once the mosquitoes are released, they mate with wild populations and spread the harmless bacteria. The beauty of the process is that you only need one release because the bacteria then cycles through generation after generation. This technique is inexpensive and scalable and is proving highly effective after years of studies.
Global Spread and Emerging Threats
While it is a sad fact that the overwhelming majority of mosquito-borne infections that lead to deaths are in lower-income tropical countries, there is a gradual shift of mosquito populations north and south into more temperate regions as the global temperature rises. There is also a change in rainfall patterns that provide suitable habitats for breeding. This is resulting in traditionally tropical diseases being identified in countries in Europe and the US, potentially reaching millions of people with no previous exposure and no underlying immunity.
In Europe, cases of West Nile virus have been on the rise in recent years, with outbreaks reported in Italy, Greece, and France. In the US, the Asian tiger mosquito, a major vector for dengue and chikungunya, has established itself in many states, and cases of these diseases have been reported locally.
Mosquito-Borne Disease Outbreaks in the US
There was widespread media coverage of the impact of mosquito-borne disease on the states of Florida and Texas, primarily due to a cluster of locally acquired malaria cases- there have been plenty of cases of malaria unwittingly brought back from malaria-endemic countries, but local transmission is a rare occurrence in the US. While other vector-transmitted diseases like West Nile virus were also present, the malaria cases sparked significant concern and public health response. These cases resulted from local mosquitoes biting infected persons who had returned from endemic countries with malaria and going on to bite and hence infect other people with no history of travel.
Between May and July 2023, eight cases of plasmodium vivax malaria were reported, seven in Florida and one in Texas- this was the first such outbreak in the continental US in 20 years. The CDC collaborated with state health authorities to investigate the cases, conduct mosquito surveillance and control activities, and implement public outreach and education campaigns. It is also important that health professionals are aware in order that unusual diseases are considered in differential diagnoses. All patients received appropriate treatment and recovered, and there have been no further reports of local transmission since July. Surveillance programs are now up and running in both Florida and Texas and are being implemented in other southern states.
Effect of the COVID-19 Pandemic on Malaria Control in Africa
An epidemiological analysis of global malaria with special reference to the effect of the COVID-19 pandemic on malaria control in Africa has noted disturbing conclusions. Globally, malaria cases and deaths increased in 2020, with the most serious situation observed in Africa, especially West Africa. There were a number of contributory factors, mainly involving disruption to control measures locally, delayed diagnosis and clinical care, and a lack of availability of malaria medication.
Implications for Travelers: Malaria Risks and Awareness
This resurgence of malaria also has major implications for travelers. For all malaria cases imported into the UK in 2021, for which there was a recorded travel history, 94% of patients acquired their malaria in Africa [UKHSA]. Clearly, as international travel recovers further, the need to ensure protection against malaria for all those potentially exposed and to maintain awareness of malaria as a possible diagnosis in returned travelers is greater than ever.
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