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Germs that infect humans

Virus responsible for yellow fever

Virus responsible for yellow fever


Microorganism: the virus responsible for yellow fever is a flavivirus belonging to the family Flaviviridae.

Disease: yellow fever

Occurrence of the disease

History: yellow fever was the first human disease attributed to a virus. Large epidemics affected tropical America in the 17th, 18th and 19th centuries making it the "most feared disease of the Americas."

Current situation: between 1986 and 1991, 20,000 cases were reported in Nigeria, including 4,000 deaths. Since 1942, there have been no urban yellow fever epidemics due to the mosquito A. aegypti in America.

Forecast: many cities in America suffer from a re-infestation of the mosquito A. aegypti, which could possibly result in an epidemic. However, in 1998, no cases were reported in Canada.

Mechanism of action of the microorganism: this virus is capable of reproducing inside many different tissues of the Aedes aegypti mosquito (commonly called the Asian tiger mosquito) as well as in human or monkey tissues. The virus travels to the lymph nodes, then moves on to the liver, the spleen, the kidneys, and the heart.

Symptoms of the disease: fever, muscular pain, nausea, vomiting. Jaundice may also appear, moderate at the beginning of the disease and more severe towards the end. Fever and jaundice have given the disease its name. The majority of cases reach this point and then recovery follows. In certain cases, however, a kidney lesion develops that can be fatal.

Geographical distribution of the microorganism: the majority of the tropical countries of Central and South America, as well as Africa.

Incubation period: about three to six days

Contagious period: human blood can infect mosquitoes shortly before the appearance of fever and during the first three to five days of the disease. Then follows an incubation period of nine to 12 days in the mosquito, after which the mosquito is contagious for the rest of its life.

Transmission: the disease is transmitted by a mosquito called Aedes aegypti. There are two cycles of transmission: the urban cycle (epidemic) and the jungle cycle. In the urban cycle, the virus spreads from person to person by mosquito bites. The blood of an infected patient contains the virus. When the mosquito bites a human, it ingests virus along with the blood. The virus then travels to the intestinal tissues of the insect, reproduces and then moves to the salivary glands of the mosquito. By biting other humans, the insect injects them with the virus. The jungle cycle works essentially in the same way, except that the insects bite wild animals such as monkeys instead of humans.

Hosts: humans, monkeys, and mosquitoes

Discoverer of the microorganism: Walter Reed in 1901

Treatment: no treatment exists for this disease

Geographical distribution of the microorganism: The virus is found especially in Central and South America, and in Africa.

Prevention: vaccine

Vaccine: the vaccine used is in fact the attenuated virus. It is effective in 99% of cases. Immunization persists in the body for about 35 to 40 years but a booster is suggested for people travelling to high-risk locations.

Vaccination is often required to enter endemic zones. Although yellow fever is widespread in South America and in Africa, it has never reached Asia, even though Asians also get bitten by A. aegypti. The reason for this phenomenon is unknown.