Yellow Fever

Yellow fever is the name given to a disease that is caused by the yellow fever virus. The virus is a member of the flavivirus group. The name of the disease is derived from the appearance of those infected, who usually present a jaundiced appearance (yellow-tinted skin).

The agent of infection of yellow fever is the mosquito. The agent was first identified in 1900 when the United States Army Yellow Fever Commission (also referred to as the Reed Commission after its leader, Walter Reed) proved that the mosquito species Aedes aegypti was responsible for spreading the disease. Until then, yellow fever was regarded as requiring direct person-to-person contact or contact with a contaminated object.

The disease has caused large outbreaks involving many people in North America, South America, and Africa, stretching back at least to the 1700s. At that time the disease was often fatal. The availability of a vaccine reduced the incidence and mortality of the disease considerably in the latter part of the twentieth century. However, since 1980 the number of cases of the disease has begun to rise again.

There are now about 200,000 estimated cases of yellow fever in the world each year. Of these, some 30,000 people die. Most researchers and health officials regard these numbers as underestimates, due to underreporting and because in the initial stages yellow fever can be misdiagnosed.

The yellow fever virus infects humans and monkeys—no other hosts are known. Humans become infected when the virus is transmitted from monkeys to humans by mosquitoes. This is referred to as horizontal transmission. Several different species of mosquito are capable of transmitting the virus. Mosquitoes can also pass the virus to their own offspring via infected eggs. This form of transmission is called vertical transmission. When the offspring hatch they are already infected and can transmit the virus to humans when they have a blood meal. Vertical transmission can be particularly insidious as the eggs are very hardy and can resist dry conditions, hatching when the next rainy season occurs. Thus the infection can be continued from one year to the next even when there is no active infection occurring in a region.

The different habitats of the mosquitoes ensures a wide distribution of the yellow fever virus. Some of the mosquito species breed in urban areas while others are confined to rural regions. The latter types were associated with the outbreak of yellow fever that struck workers during the construction of the Panama Canal in Central America in the nineteenth century. In South America a concerted campaign to control mosquito populations up until the 1970s greatly reduced the number of cases of yellow fever. However, since that time the control programs have lapsed and yellow fever has increased as the mosquito populations have increased.

Infection with the yellow fever virus sometimes produced no symptoms whatsoever. However, in many people, so-called acute (rapid-onset, intense) symptoms appear about three to six days after infection. The symptoms include fever, muscle pain (particularly in the back), headache, chills, nausea, and vomiting. In this early stage the disease is easily confused with a number of other diseases, including malaria, typhoid fever, hemorrhagic fevers such as Lassa fever, and viral hepatitis. Diagnosis requires the detection of an antibody to the virus in the blood. Such diagnosis is not always possible in underdeveloped regions or in rural areas that are distant from medical facilities and trained laboratory personnel.

In many people the acute symptoms last only a few days and recovery is complete. However, in about 15% of those infected, the disease enters what is termed the toxic phase: a fever reappears and several regions of the body become infected as the virus disseminates from the point of the mosquito bite. Disruption of liver function produces jaundice. Kidney function can also be damaged and even totally shut down. Recovery from this more serious phase of the infection can be complete; although half of those who are afflicted die.

Yellow fever appears in human populations in different ways. One pattern of appearance is called sylvatic (or jungle) yellow fever. As the name implies, this form is restricted to regions that are largely uninhabited by humans. The virus cycles between the indigenous monkey population and the mosquitoes that bite them. Humans that enter the region, such as loggers, can become infected.

Another cycle of infection is referred to as intermediate yellow fever. This infection is found in semi-urban areas, such as where villages are separated by intervening areas of farmland or more natural areas. Infections can spring up in several areas simultaneously. Migration of people from the infected areas to larger population centers can spread the infection. This is the most common pattern of yellow fever occurring in present day Africa.

The final pattern of yellow fever is that which occurs in fully urban settings. The large population base can produce a large epidemic. The infection is spread exclusively by mosquitoes feeding on one person then on another. Control of these epidemics concentrates on eradicating the mosquito populations.

Treatment for yellow fever consists primarily of keeping the patient hydrated and comfortable. Prevention of the infection, via vaccination, is the most prudent course of action. The current vaccine (which consists of living but weakened virus) is safe and provides long-lasting immunity. While side effects are possible, the risks of not vaccinating far outweigh the risk of the adverse vaccine reactions. For a vaccination campaign to be effective, over 80% of the people in a suspect region need to be vaccinated. Unfortunately few countries in Africa have achieved this level of coverage. Another course of action is the control of mosquito populations, typically by spraying with a compound that is toxic to mosquito larvae during breeding season. Once again, this coverage must be extensive to be successful. Breeding areas missed during spraying ensure the re-emergence of mosquitoes and, hence, of the yellow fever virus.

See also Transmission of pathogens; Zoonoses