Vector control

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The need for vector control

History is littered with deaths of large numbers of people caused by vector-borne diseases. These were largely initiated by the movement of humans to new areas by migration, military campaigns and for trade, by both land and ship.

These movements carried African diseases to Europe and tropical Asia, central Asian diseases to Europe, and Old World diseases to the New World, as sailing ships travelled further afield.

Advances in chemistry provided novel products such as DDT in the 1940s and malathion in the 1950s to control insects, and also anticoagulants in the 1940s to control rodents. Malaria was eliminated in the US by the use of DDT and virtually eliminated in many countries. Many other vector-borne diseases were brought under control by concerted vector control campaigns using these new tools. Large scale outbreaks of diseases caused by rodents virtually died out.

Abandoned programmes

The early successes, however, brought complacency, budget cuts and disbanding of vector-control programmes. This allowed resurgence of once decimated mosquito populations and the diseases they carried, and spread of new diseases. This has been compounded by growing resistance of insect vectors to commonly used modern insecticides, making some programmes ineffective. The use of insecticides also brought other problems: “The focus of vector control on insecticides meant that environmental management and other alternative methods were underexploited or even forgotten.” (WHO, 2012).

According to WHO, continuous vector surveillance and control should be maintained for many years. This requires strengthening and maintaining of expertise and capacity for entomological surveillance, including for insecticide resistance. “Closing down such programmes and allowing the expertise and staff to disperse has proved disastrous.”

WHO has called for broadening the mandate of national malaria elimination programmes to control other vector-borne diseases such as dengue, yellow fever, leishmaniasis, Rift Valley fever and chikungunya. This would help to maintain the technical expertise in entomology and vector control (WHO Regional Office for Africa, 2015).

Lessons from history

Malaria

Malaria, caused by the protozoan parasite Plasmodium, is thought to have originated in apes in Africa or Southeast Asia and spread to humans when agriculture started (Wikipedia, 2016). It is recorded in ancient Egypt, Greece, Rome and China, and had spread as far north as Scotland when the Romans invaded, killing over 40,000 soldiers in AD 71-79 (IVCC, 2016). It spread to the Americas with European settlers and their slaves in the 16th century — although genetic studies show one, less virulent, malaria strain had arrived via the Pacific thousands of years earlier.

The new malaria strain had devastating consequences for the South American natives and the settlers, and is thought to have been one driver of the slave trade. This introduced malaria-resistant African slaves to tropical and subtropical colonies where malaria became endemic, while in cooler areas, European settlers were cheaper sources of labour (Standley, 2012).

Malaria has been eliminated in many countries in recent times. It was endemic in Italy and the Mediterranean basin up to the 1940s when vector control measures were implemented using the newly discovered DDT to spray inside houses.

Malaria remains the single biggest vector-borne killer worldwide and is still present in 99 countries, putting 3 billion people at risk, although around 90% of infections are in sub-Saharan Africa.

Dengue

WHO recently rated dengue as the most important mosquito-borne viral disease (WHO, 2012). The worldwide cases of dengue have increased by a factor of 30 over 50 years and more countries are reporting their first outbreaks.

Figure 1. Average number of dengue cases reported to WHO 1955–2010.
Source: WHO, 2012.

Contracting dengue results in an average of 14.8 working days lost when not hospitalised and 18.9 days lost if hospitalised, costing US$1491 per patient, if non-fatal (WHO, 2014). In 2002 over 90,000 cases of dengue were reported in Rio de Janeiro alone; in 2015 there were 1.6 million ‘probable’ cases of dengue throughout Brazil, according to PAHO (PAHO, 2015).

In 1958 Brazil was declared free of the Aedes aegypti mosquito, the main vector for dengue, after an extensive vector-control programme using the basic measures available at the time (including spraying inside houses with DDT).

The cooler climate dengue vector, Aedes albopictus, has been recorded in many countries in Europe, providing a potential source of outbreak of several diseases that it can carry. France and Croatia had autochthonous cases of dengue in 2010 and the Portuguese island of Madeira had an outbreak of dengue in 2012/13, thought to be spread by A. aegypti. This infected over 2000 people (ECDC, 2013), some of whom were tourists who returned to their home countries around Europe while infected.

Yellow fever

Yellow fever was another mosquito-borne disease carried on slave ships to the Americas in the 17th century. While the African slaves had some immunity to the disease, many settlers and soldiers of invading European armies died from the disease, including over half of 12,000 British soldiers sent to take Cartagena in Columbia in 1742 and 23,000 out of 29,000 French soldiers sent to retake Haiti in 1802 (IVCC, 2016).

The first attempt to build the Panama Canal failed when the company went bankrupt in 1889, partly due to deaths of workers from yellow fever and malaria — 22,000 out of 40,000 died. The legs of the hospital beds were placed in cans of water to prevent insects crawling up them, but they provided breeding grounds for the mosquitoes that infected the patients. This was before the link between mosquitoes and malaria was discovered by Ronald Ross in 1897 (Wikipedia, 2016). During the second construction project (1905-10) yellow fever was eliminated and malaria was also greatly reduced through vector control (CDC, 2016).

Yellow fever is one of the few tropical diseases for which a vaccine is available.

New diseases

Since 2015, the increased global range of A. aegypti coupled with a degradation in municipal pest control operations and increased urbanisation has been responsible for a Zika outbreak in the new world. This spread extensively in Brazil and to over 40 countries where it had not been detected before.

Climate change and the continuing spread of vector mosquitoes such as A. albopictus and A. aegypti have seen increasing incidences of these and other tropical diseases in developed countries. Europe and the US have both had an increase in West Nile virus infections and Zika is seen as a threat in southern areas of the US. Florida reported the first autochthonous cases in July 2016.

The US has implemented a vector control programme for Zika in 2016, including the CDC awarding Rentokil North America a contract for pest control, community outreach, surveillance and inspection, and educational support.

All the previously endemic areas in the Mediterranean basin are still vulnerable to diseases borne by mosquitoes, especially Aedes species, as shown by an outbreak of Chikungunya in Italy in 2007, when 205 cases were identified (Rezza, et al, 2007) and in southern France in 2010 when there were two autochthonous cases.

Bibliography

Rezza, G. 2016. Dengue and other Aedes-borne viruses: a threat to Europe? Eurosurveillance, Volume 21, Issue 21, 26 May 2016.

IVCC. History of vector control. 2016. (link (accessed July 2016).

Wikipedia. (2016) History of Malaria. (link accessed 14 October 2016).

Wikipedia. 2016. History of the Panama Canal. (link (accessed August 2016)

WHO. 2012. Global strategy for dengue prevention and control 2012-2020. Geneva. ISBN 978 92 4 150403 4.

WHO Regional Office for Africa. 2015. Handbook on vector control in malaria elimination for the WHO Africa Region. Brazzaville. ISBN: 978-929023306-0.