Vector Control, Saving Lives

Highlights of vector-borne disease history

1647-1650: Yellow fever was brought from Africa to Barbados on the slave ships. White settlers on the island had no immunity to the disease. Six thousand of them died during an outbreak lasting several years. Read more…

Yellow fever travelled from Africa on the slave ships. The mosquitos that carried the disease bred in the drinking water casks. Many on board fell ill or died, but the crew was often more susceptible than the slaves, many of whom were already immune. (Yellow fever provides immunity to its survivors.) Some passengers incubated the disease in their bodies, ready to unleash it on the population of their destination. Outbreaks of ‘yellow jack’ as the sailors called it, affected every port in the New World. Quarantines failed to work, as mosquitos flew off ships towards a mass of people on land.

1793: When ships arrived from the West Indies, residents of Philadelphia, USA, are struck down with an outbreak of yellow fever during a long hot summer. About 5,500 inhabitants die before winter begins when the mosquitos can no longer survive.

When people began to fall ill with yellow fever in the busy port city of Philadelphia, people thought it was a local problem. They blamed the hot weather or hailstones, poor living conditions in the slums, or even passing meteors. Others correctly identified that the ships were bringing in the disease, but did not understand the connection between the mosquito and yellow fever. What they could agree on was that it could not be passed between one person and another, as nurses who looked after patients did not automatically fall ill. People fled the city, closed shops and federal offices stopped their functions. When the outbreak came to its conclusion, the population was decimated: 5,500 people died out of a total 55,000. Outbreaks continued in the summers of many years to come and the residents would be filled with dread.

1802: The French send 29,000 soldiers and sailors to regain Haiti. Only 6,000 return, defeated. The Haitians have immunity to yellow fever while the French do not. French losses caused by yellow fever outnumber those caused by warfare.

Mosquito-borne disease, although no one knew it at the time, was the leading cause of illness and death for many thousands of European sailors and soldiers. The French lost 6,000 out of 29,000 soldiers and sailors to yellow fever when they tried to regain Haiti. From 1819 to 1936, nearly fifty percent of British soldiers sent to Sierra Leone died. The French army also lost 16 percent of their soldiers in Senegal. In the Bight of Benin, each year, one in ten of every lower class Englishman and one in twenty of upper class Englishmen died, mainly of malaria. In India, many of the empire’s British soldiers were killed by malaria.

1830s-1880s: Europeans begin to associate good health with cleanliness. Efforts to improve public sanitation mean mosquitos cannot breed as easily and malaria and yellow fever begin to reduce. No one knows the connection between mosquitos and disease.

People promoting public health strategies unknowingly reduced malaria and yellow fever around the world. In England, Elizabeth Chadwick published a book in 1842 to promote the construction of sewers and underground water systems. Clean water, clean streets and proper drainage became associated with good health. In the 1850s, work began in earnest to improve public sanitation and similar campaigns began across the rest of Europe. In America, the government built drainage systems and sewers and paved roads. In 1880, Memphis built a sanitation system, and the city never again suffered a yellow fever outbreak. People were building systems that meant mosquitos had nowhere to breed; but they had no idea of the significance of their actions. They thought at that time that the illness was connected only with dirt and waste.

1881: Initial attempts to build the Panama Canal result in spectacular failure; mosquito-borne disease causes the death of thousands of workers, costs investors the equivalent of $3 billion and ruins the reputation of French hero, Ferdinand de Lesseps.

When steam shovels and dynamite became the everyday tools of engineers, the effort of building a canal to connect the Atlantic and Pacific became a serious proposition. French hero and builder of the Suez Canal, Ferdinand de Lesseps negotiated the rights to cut the canal and raised the capital to execute the project. Construction began in 1881.  Promoters declared that the reputation for deadly disease in the region had no grounds and claimed that mosquitoes were nothing but a nuisance. The houses for construction workers, the barracks and even hospitals had no screens to protect inhabitants against the insect. The French built luxurious gardens in their complexes and constructed water filled pottery rings around the bases of trees to protect them from ants. These pottery rings were prime breeding sites for mosquito larvae.

Thirty people died from malaria during the project’s planning stage. As the project continued, more and more deaths of malaria and yellow fever plagued the construction effort. In the year of 1885, the deaths of 1,200 workers made it hard to deny the extent and horror of the problem. In some parts of the rain-soaked jungle, two out of every three Europeans died from yellow fever or malaria. Thirty percent of the workforce was in hospital at any one time. The canal project managers connected ill health to the morality of each victim and considered drinking, gambling and embezzlement as causes of death. The connection between the mosquito and disease was still unknown.

However, it was not just mosquito-borne disease that made the project impossible. Poor study of the region’s geology and hydrology and landslides also contributed to the defeat of the engineers and constructors. An estimated 30,000 people died during the attempt and then five years later De Lesseps died. Following legal proceedings and financial scandal, De Lesseps’ son went to jail and his colleague, the great French engineer Eiffel, was forced to pay an enormous fine.

1897: Young British scientist Ronald Ross is the first person to demonstrate that the malaria parasite enters the human blood stream through a mosquito bite.  The mosquito is not just an insect; it is a vector of disease.

In the late 1870s and early 1880s, scientists in Europe and the United States competed vigorously to see who could unravel the secrets of malaria; a disease with clearly devastating impact on human health. In 1893, Theobald Smith demonstrated that a tick transmitted a malaria-like infection among cattle in the American west. This showed for the first time that blood-feeding creatures were capable of transmitting disease to mammals. In 1878, in a colonial outpost in Algeria, French Doctor Charles Lavaran was the first person to see malaria parasites in the blood of an infected person. However, it was a young British scientist Ronald Ross, with the help of Patrick Manson, an expert in parasitology and tropical medicine who first demonstrated the connection between mosquitos and malaria. Ross - recently returned from Bangalore - and Manson both agreed with a theory currently circulating: that mosquitos might carry malaria.  Under Manson’s guidance, Ross returned to India in an attempt to observe malaria’s development in mosquitos. After experimenting for three years, he finally discovered, inside a mosquito’s belly, parasites that were identical to those found in patients with malaria. The connection between mosquito and malaria had been confirmed. A mosquito with malaria parasites infects a human through its bite. When another mosquito without malaria parasites, bites an infected human, it then acquires the parasites and can transmit them to someone else. Ross however was still confused that some mosquitos seemed to acquire and transmit malaria while others did not. Later, other scientists found which particular mosquitos (various anophelines) acquire malaria and also observed the parasite’s life cycle.

1900: Following Ross’s discovery of the connection between the mosquito and malaria, entomologists worldwide begin to plot military style attacks on the mosquito. One of the most aggressive and successful campaigns takes place in New Jersey, USA under the command of a man called John Smith.

In 1900 John Smith, Head of Entomology for the State of New Jersey, USA, began one of the world’s most aggressive anti-mosquito campaigns with huge success. With only a basic understanding of the diversity of mosquitos inhabiting the area, how they survived and how they might be fought, he began his attack. He identified several Anopheles mosquitos that appeared to transmit malaria and discovered they were breeding in particular sites; mainly the huge brackish marshland north of the city of Newark, now called the Meadowlands.

A particularly aggressive mosquito Ochlerotatus sollicitans bred in the brackish water of the marshes. These mosquitos attack in hundreds at any time of day, sometimes descending onto a person’s head and upper body. People were literally driven indoors. In 1900 Smith’s view of ‘controlling’ mosquitos rather than extermination was remarkably sophisticated. Yet at the time journalists and the public ridiculed him for even suggesting it. Smith planned a ferocious campaign involving a motorized ditch digger to slice drainage channels through thousands of acres of marsh. He organised ‘mosquito brigades’; groups of people who cut ditches and poured oil over the waters where larvae developed. Barges dredged sand from riverbeds and pumps filled the swamps with the sand. The effects were dramatic. Everywhere they applied these techniques, the mosquito population was drastically cut.

People began to work freely outdoors, local development accelerated, properties and neighbourhoods grew up in low-lying areas. Local economies benefitted and land became more valuable. Throughout the area, local people formed drainage committees, becoming competitive and proud of their efforts to control malaria.

Knowledge of the success spread. Public health officials in New York City followed suit and attacked mosquito breeding areas, recommending screening for houses and quinine treatment for patients. Malaria was deemed under control in the area by 1905.

1904: The USA, under President Theodore Roosevelt, takes over work on the Panama Canal. William Crawford Gorgas is a hero for eliminating yellow fever in Cuba, largely by breaking the chain of infection and reducing mosquito breeding grounds. He is hired to eliminate yellow fever and malaria from the canal zone.

The Panama canal zone was a strip of land forty-five miles long and fifteen miles wide. Most of it was uninhabited by local communities. In order to maintain a healthy workforce to build the canal, mosquitos had to be eliminated. Gorgas was fastidious in his methods. He ordered the nice water receptacles in French gardens to be removed and ordered groups of men to go out and destroy any containers that might capture and hold rainwater. He spread oil on larvae breeding sites and sent soldiers to fumigate homes and swat mosquitos. He also installed screens where yellow fever and malaria patients were being cared for so that infections could not be passed on. By 1906, yellow fever had disappeared from the canal zone and malaria was at a much lower level (the mosquito that passed on malaria was harder to kill off). It took ten years for the USA to finish building the canal and only two percent of the workforce was in hospital at any one time.

1930: In Brazil, entomologist Raymond C. Shannon finds an unusual mosquito for the region - Anopheles gambiae - a malaria vector. It probably arrived from West Africa on a destroyer ship used to deliver mail. World travel has opened up possibilities for vectors to move across countries and continents.

When the world’s travel connections increased - ships using the Panama canal and more air travel - the problem of vectors passing from one region to another also increased. The West African mosquito had probably arrived in Brazil via a ship and found a perfect breeding ground: a system of dykes installed in local hayfields. Early on, officials misunderstood the problem and refused to flood the dykes with salt water as suggested. The local population experienced a small malaria outbreak. Ten months later, malaria turned into an epidemic with ten thousand people falling ill. The Government carried out an aggressive anti-mosquito campaign to fumigate houses and drain or spread oil on breeding areas. It seemed to have worked; from then on outbreaks remained small and controllable. But in 1938, a shockingly fast and virulent malaria epidemic swept several states in Brazil. It quickly infected 100,000 people and killed up to 20,000. If it spread, it had the potential to reach all of the Americas.

A man called Fred Soper was put in charge of a 4,000 strong emergency workforce to fight the disease. He attacked the problem with military authority, his antimalarial brigades able to inspect and spray every property with insecticides and larvicides. They used ‘Paris Green’, a spray that killed larvae in water. Soper realized the problem caused by increased travel and set up posts on the roads to rid cars and trucks of insects. Ships and planes were fumigated. In just one year, Soper managed to contain the problem to just two small inland towns. This proved that malaria could be fought on a large scale.

The threat of vectors moving from country to country, continent to continent is still real. Asia has always been free of yellow fever, but the dreadful possibility always lingers that it could be introduced to a dense population of hundreds of millions with no immunity.

1943: DDT (dichloro-diphenyl-tichloroethane) is invented. An insecticide used initially to de-louse prisoners and refugees of World War II and therefore fight typhus outbreaks, the eradication of mosquitos becomes a real possibility. The battle against malaria is declared almost won.

DDT seemed the perfect solution. It killed insects and seemed to cause no harm to other living things. It stayed useful for months at a time, so did not have to be reapplied very often. Fred Soper was an advocate and with his history of nearly eradicating the malaria vector in Brazil through spraying insecticides, he was well placed to promote it. In Sardinia the use of DDT was a success. Malaria had long been endemic and in 1946 alone there were more than 10,000 cases in a population of 1.2 million. Armies of people sprayed homes, streets and fields; 256 tons of DDT was spread over Sardinia. Malaria declined then eventually disappeared.

But the good news was tempered by one thing: although malaria had disappeared, the mosquitos had not. The mosquitos had developed resistance to DDT after just five years. In Greece, the same thing happened. In 1951, following a massive campaign using DDT, sprayers in one village noticed that the mosquitos came back after just a few days. The mosquito was capable of adapting to DDT very quickly. The weakest ones were being killed off and only the strongest were left, resistant and ready to breed resistant offspring.

DDT was only going to be successful as a major weapon against disease, if it could be applied quickly enough to disrupt the transmission of malaria and break the cycle. If people with malaria could be treated during this intermission, then when the mosquitos came back, there would be no malaria parasites for them to transmit. There would only be the nuisance of mosquitos.

1958: DDT is shipped to countries in the Southern hemisphere struggling with mosquito-borne disease. The urgency of the matter requires total commitment and loyalty from everyone in the field of tropical medicine. Time is of the essence as mosquitos develop resistance to DDT quickly.

DDT must work fast to stop the mosquitos carrying parasites returning and the authorities gave themselves five years to eradicate malaria. By 1960, the US Agency for International Development and the World Health Organization were working with 60 countries employing a spray campaign. Early success in island countries such as Taiwan, Zanzibar, Sri Lanka and Jamaica was encouraging. There was also fast success in India, where in 1961 malaria cases accounted for only one percent of hospital admissions, compared to a previous 10 percent.

But success did not last for long. DDT resistance began to appear in mosquitos in several countries. By the end of the 1960s, DDT resistant-mosquitos and malaria returned to many of the countries that had enjoyed their short disappearance. By 1969, Sri Lanka had half a million cases of malaria.

DDT worked in wealthier countries such as Europe and the USA. People kept mosquitos at bay because they could afford good housing and screens, and owned and developed land that was previously likely to breed mosquitos.

A more serious problem in poorer countries was people’s new vulnerability to disease. When a population is free from malaria for a period of time, they lose their immunity. If malaria should return, people are even more vulnerable to the disease than before. For example, in Sri Lanka, following a DDT spray campaign in the early 1960s, malaria cases dropped to eighteen nationwide. However, DDT resistance quickly set in. By 1977, a virulent strain of malaria killed 501 Sri Lankans, a higher death rate than before the DDT spray campaign began.

1962: Rachel Carson publishes Silent Spring; a powerfully written book arguing that DDT is not safe. The reaction is immediate in several US states: DDT is banned. A nation-wide ban follows ten years later.

When Rachel Carson’s book Silent Spring hit the shelves, it caused outrage.

Carson’s engaging and populist style meant the book appealed to many ordinary people, not just scientists. Carson used the scientific evidence of many researchers to argue that DDT can kill animals, cause bird populations to decline and lead certain pests to proliferate. Workers who handled the chemical suffer health problems and exposed fish got liver cancer. She also found evidence of DDT in mother’s breast milk and in the bodies of babies. Several US states immediately banned the use of DDT as a pesticide and for crops. In 1972, the USA banned it outright.

But there was a problem. DDT was and is the most effective means of reducing malaria incidences, particularly in developing countries. DDT is cheap, effective, easily stored and transported and relatively safe for the person spraying. It does not have to be applied very often and provides the best means of protection possible. But how could the USA promote DDT through its aid programmes if DDT was a banned chemical at home?

In 2000, a worldwide ban on DDT nearly ensued but it was stopped at the last minute. Today, DDT is still produced in China and India and available globally for use uniquely in anti-malarial efforts.