Insect-borne diseases pose immense health & economic threats
Malaria is thought to have first spread to human populations in central Africa 5-10,000 years ago. This coincided with the adoption of slash-and-burn agriculture in which patches of forest were cleared for crop planting. Genetics research points to human infection with the Plasmodium falciparum parasite as originating from a single transfer between an infected western gorilla and a human (Liu et al, 2010).
Malaria was recorded in the Nei Ching, the Chinese Cannon of Medicine, in 2700 BCE and had spread to Egypt by the time of pyramid building (2700–1700 BCE). The Greek historian Herodotus wrote in the 5th century BCE that an inscription on a 26th century pyramid recorded the amount spent on garlic for workers to ward off mosquitoes. The Sanskrit text on medicine and surgery, the Sushruta Samhita, which is dated to around the first millennium BCE, also describes what is thought to be malaria and attributed it to the bite of insects.
The Chinese plant Qing Hao (Artemisia annua) was recorded in the texts of Ge Hong, a minor government official in Southern China and alchemist, as a treatment for malaria in the fourth century AD.
The disease was present in Greece well before the 5th century BCE and in Italy was first introduced to Sicily around the fifth century. It spread to western central Italy between 400–100 BCE either from the islands of Sardinia and Sicily or directly from Africa. Hippocrates (460–370 BCE) recognised fevers with different period — every third or fourth day — and used the term ‘miasma’ to describe fumes that arise from the ground that can cause illnesses.
Greek colonies along the southern coast of Italy suffered from malaria, especially around the coastal marshlands. By the time of the Roman Empire it was endemic in the marshlands around Rome. The Roman scholar Marcus Terentius Varro (116-27 BCE) wrote:
“Precautions must also be taken in the neighbourhood of swamps, both for the reasons given, and because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious diseases.”
Malaria is thought to have contributed the decline of several city states in the ancient world and even of the Roman Empire itself in the fifth century AD.
The spread of malaria through Italy took at least 1500 years because it was dependent on the spread of a suitable mosquito vector. As the authors noted, the Plasmodium parasites would have been transported across the whole of the Roman Empire by infected soldiers, slaves, administrators and tradespeople. The local species of mosquito, however, were not efficient vectors of malaria. The disease only reached north eastern Italy when the anthropophilic species of mosquito, Anopheles sacharovi, colonised the coastal areas.
Treatments for malaria in medieval Europe included bloodletting, induced vomiting, limb amputation and trepanning (cutting a hole in the skull). The belief that bad air, or mal aria in medieval Italian, caused malaria lingered on in western medicine up to the late 19th century.
Malaria was widespread in Europe up to the 18th century, but declining in the western areas. It was still common in the north and north eastern areas of Europe in the 19th century and only disappeared in the 1930s apart from a few outbreaks during World War II.
It was even present in the northern areas of Sweden, Finland and Russia north of the Arctic Circle, where the summer temperatures are below 16°C — which is thought to be the limit for sporogony of the plasmodium parasite. A study by Huldén et al (2005) found the population of infected mosquitoes was maintained by the female mosquitoes hibernating in houses during the winter.
Malaria was endemic in Britain and caused high levels of mortality in the 15th to 19th century around inland and coastal marshlands. It was transmitted by the mosquito species Anopheles atroparvus, which is still present across Europe. High death rates were also reported in Holland at the end of the 19th century. In both Britain and Holland the parasite is thought to have been the species Plasmodium vivax, which now does not cause a lethal infection.
The presence of this mosquito species is not thought to present a threat to the spread of malaria at present. There have been over 52,000 cases of imported malaria in Britain since 1953, but none has resulted in local transmission of the disease.
The disease is not thought to have been present in the Americas until introduced by European colonists and their West African slaves in the 16th century. As malaria reached Virginia and South Carolina the European plantation workers, who had come as indentured servants, became infected in large numbers. The large plantations in British America introduced African slaves, who had greater immunity to the parasite.
In South America Spanish Jesuit missionaries in Peru learned from the Quechua tribe of the use of the bark of Cinchona species to treat fevers. They introduced the ‘Peruvian bark’ (also called Jesuit bark) to European medicine and first transported it to Spain and Italy in 1632. The trees harvested for the bark were named by Linnaeus as Cinchona after the Countess of Cinchon, the wife of the Viceroy of Peru, who was cured of a fever by the bark.
The name first appeared in the second, revised edition of his Genera Plantarum in 1742, as the last entry. Linnaeus, however, concluded in his thesis for his medical degree at the University of Harderwijk in Holland that intermittent fevers (malaria) were caused by small clay particles!
In 1877 British medical officer Patrick Manson, while in Amoy in China, discovered that a Culex species of mosquito could carry the human filarial roundworm, Wuchereria bancrofti. This was the first evidence that mosquitoes could transmit human parasites. After returning to England, in the early 1890s he developed a mosquito-malaria hypothesis in which he proposed that the parasite was transmitted between humans by mosquitoes.
In November 1894 he described his hypothesis to Ronald Ross, a British Army surgeon from the Indian Medical Service who was on leave in London. Manson also taught Ross a technique for detecting different stages of malaria parasite in blood samples. Ross then returned to India to test the hypothesis.
French army doctor Charles Laveran was the first to observe and identify malaria parasites in the blood of patients, in 1880 at a Military Hospital in Algeria. He observed a pigmented spherical body with filiform elements and flagellating male gametocytes. He was awarded the Nobel Prize for Physiology or Medicine in 1907.
American physician Albert King proposed in 1881 that malaria was transmitted by mosquitoes, presenting to the Philosophical Society of Washington in 1882 and writing in an article in the September 1883 issue of The Popular Science Monthly. He was not believed, however.
In 1886 Italian doctor Camillo Golgi showed that there were two species of malaria parasite, one causing fever with tertian periodicity (every other day) and one with quartan periodicity (every third day). These cycles coincided with the release of merozoites into the blood. He was awarded the Nobel Prize for Physiology or Medicine in 1906.
The two malaria parasites were named Plasmodium vivax and Plasmodium malariae by Italians Giovanni Grassi and Raimondo Filetti in 1890, while the more malignant tertian parasite was named P. falciparum by American William Welch in 1897. These became the accepted names of the parasite species.
Ronald Ross, back in Secunderabad in India, had conducted a fruitless two-year search looking for malaria parasites in mosquitoes. Finally, in 1897 he fed a new batch of mosquitoes, later identified as Anopheles species, with the blood of an infected patient. On 20 August he saw pigmented bodies in their stomach walls that could only have come from the human blood. This was the missing link in Manson’s hypothesis that showed mosquitoes could be infected by biting an infected human. Ross called 20 August ‘Malaria Day’, which is now adopted as World Mosquito Day.
The following year he showed that mosquitoes could transmit the bird malaria parasite (Plasmodium relictum) between birds, with a spore phase that developed in the mosquito. He also showed that the parasites developed in the stomach wall of the bird and in the mosquito were stored in the salivary gland. For this work Ross was awarded the Nobel Prize for Physiology or Medicine in 1902.
The complete cycle of infection in humans was proven in 1899 when a team at Rome University, including Giovanni Grassi, Amico Bignami, and Giuseppe Bastianelli collected Anopheles claviger mosquitoes in Rome and fed them on patients with malaria. These mosquitoes were then sent to Patrick Manson at the London School of Tropical Medicine where they were allowed to feed on a volunteer, his 23 year old son Patrick. He developed malaria 14 days later, but was given quinine to cure it.