Yellow fever is a viral disease spread by mosquitoes that has caused large epidemics in Africa and the Americas. Infection causes a wide spectrum of disease, from mild symptoms to severe illness and death. The "yellow" in the name is explained by the jaundice that affects some patients. Although an effective vaccine has been available since the 1940s, the number of people infected in recent decades has increased and yellow fever is now a serious public health issue again.

The disease is caused by the yellow fever virus, which belongs to the flavivirus group. In Africa there are two distinct genetic types (called topotypes) associated with East and West Africa.

The virus remains silent in the body during an incubation period of three to six days. There are then two disease phases. While some infections have no symptoms whatsoever, the first, "acute", phase is normally characterized by fever, muscle pain (with prominent backache), headache, shivers, loss of appetite, nausea and/or vomiting. Often, the high fever is paradoxically associated with a slow pulse. After three to four days most patients improve and their symptoms disappear.

However, 15% of patients enter a "toxic phase" within 24 hours. Fever reappears and several body systems are affected. The patient rapidly develops jaundice and complains of abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes and/or stomach. Once this happens, blood appears in the vomit and faeces. Kidney function deteriorates; this can range from abnormal protein levels in the urine (albuminuria) to complete kidney failure with no urine production (anuria). Half of the patients in the "toxic phase" die within 10-14 days. The remainder recover without significant organ damage.

Yellow fever is difficult to recognize, especially during the early stages. It can easily be confused with malaria, typhoid, rickettsial diseases, haemorrhagic viral fevers (e.g. Lassa), arboviral infections (e.g. dengue), leptospirosis, viral hepatitis and poisoning (e.g. carbon tetrachloride).

A laboratory analysis is required to confirm a suspect case. Blood tests (serology assays) can detect yellow fever antibodies that are produced in response to the infection. Several other techniques are used to identify the virus itself in blood specimens or liver tissue collected after death. These tests require highly trained laboratory staff using specialized equipment and materials.

The virus is constantly present with low levels of infection (i.e. endemic) in some tropical areas of Africa and the Americas. Thirty-three countries, with a combined population of 508 million, are at risk in Africa. These lie within a band from 15°N to 10°S of the equator.

There are 200,000 estimated cases of yellow fever (with 30,000 deaths) per year. However, due to underreporting, only a small percentage of these cases are identified.

Humans and monkeys are the principal animals to be infected. The virus is carried from one animal to another by the Aedes aegypti mosquito. (In South America, the Haemogogus also spread the virus.) The mosquito can also pass the virus via infected eggs to its offspring. The eggs produced are resistant to drying and lie dormant through dry conditions, hatching when the rainy season begins. Therefore, the mosquito is the true reservoir of the virus, ensuring transmission from one year to the next.

Three paths of infection:
There are three types of transmission cycle for yellow fever: sylvatic, intermediate and urban. All three cycles exist in Africa, but in South America, only sylvatic and urban yellow fever occur.

Sylvatic (or jungle) yellow fever: In tropical rainforests, yellow fever occurs in monkeys that are infected by wild mosquitoes. The infected monkeys can then pass the virus onto other mosquitoes that feed on them. These infected wild mosquitoes bite humans entering the forest resulting in sporadic cases of yellow fever. The majority of cases are young men working in the forest. On occasion, the virus spreads beyond the affected individual.

Intermediate yellow fever: In humid or semi-humid savannahs of Africa, small-scale epidemics occur. These behave differently from urban epidemics; many separate villages in an area suffer cases simultaneously, but fewer people die from infection. Semi-domestic mosquitoes infect both monkey and human hosts. This area is often called the "zone of emergence", where increased contact between man and infected mosquito leads to disease. This is the most common type of outbreak seen in recent decades in Africa. It can shift to a more severe urban-type epidemic if the infection is carried into a suitable environment (with the presence of domestic mosquitoes and unvaccinated humans).

Urban yellow fever: Large epidemics can occur when migrants introduce the virus into areas with high human population density. Domestic mosquitoes (of one species, Aedes aegypti) carry the virus from person to person; no monkeys are involved in transmission. These outbreaks tend to spread outwards from one source to cover a wide area.

There is no specific treatment for yellow fever. Dehydration and fever can be corrected with oral rehydration salts and paracetamol. Any superimposed bacterial infection should be treated with an appropriate antibiotic. Intensive supportive care may improve the outcome for seriously ill patients, but is rarely available in poorer, developing countries.

Vaccination is the single most important measure for preventing yellow fever. In populations where vaccination coverage is low, vigilant surveillance is critical for prompt recognition and rapid control of outbreaks. Mosquito control measures can be used to prevent virus transmission until vaccination has taken effect.
Yellow fever vaccine is safe and highly effective. The protective effect (immunity) occurs within one week in 95% of people vaccinated. A single dose of vaccine provides protection for 10 years and probably for life. Over 300 million doses have been given and serious side effects are extremely rare.

However, recently a few serious adverse outcomes, including deaths, have been reported in Brazil, Australia and the United States. Scientists are investigating the cause of these adverse events, and monitoring to ensure detection of any similar incidents.

The risk to life from yellow fever is far greater than the risk from the vaccine, so those who may be exposed to yellow fever should be protected by immunization. If there is no risk of exposure, for example, if a person will not be visiting an endemic area, there is no necessity to receive the vaccine. Since most of the other known side effects have occurred in children less than six months old, vaccine is not administered to this age group. The vaccine should only be given to pregnant women during vaccination campaigns in the midst of an epidemic.

For more information about malaria:
World Health Organization (WHO)
Centers for Disease Control and Prevention

Dengue fever is primarily a disease of the tropics that is transmitted by the Aedes aegypti mosquito. Aedes aegypti is a day-biting mosquito that prefers to feed on humans. It is also transmitted by the Aedes albopictus (also called the "tiger mosquito"). Those infected with dengue can suffer from a spectrum of illnesses ranging from a viral flu to severe and fatal hemorrhagic fever (DHF).The dengue virus is passed back and forth between mosquitoes and humans and causes an extraordinarily painful ailment that exists in four known strains or serotypes. Dengue is especially dangerous to children, who generally have one infection, but if bitten again can get a more serious infection that can lead to dengue hemorrhagic fever (DHF). DHF causes severe internal bleeding, shock, and circulatory collapse, and is usually fatal to children.

Dengue fever is a very unpleasant illness with joint pains and fever, but if contracted a second time it can sometimes be fatal. Hence it is important to contol the mosquitoes that carry Dengue fever. These do not fly far, and usually breed close to or in houses, often in potplants, old tyres or tins and bottles that hold water. Removal of these breeding sites is the best protection against attack.

Dengue is increasingly becoming a plague of global proportions and may soon eclipse malaria as the most significant mosquito-borne viral disease affecting humans.

For more information about dengue fever:

World Health Organization (WHO)Centers for Disease Control and Prevention

Doctors typically diagnose dengue fever from a patient's history of symptoms and risk factors, including whether that person has been anywhere where dengue fever is found. The doctor looks for three signs: high fever, pain, and rash. These symptoms are known as the dengue triad. To help confirm the diagnosis, doctors may order blood tests or tests on spinal fluid to look for antibodies* to the virus.

No specific treatment exists for dengue fever, but the disease usually clears up on its own. In serious cases, supportive care in a hospital can make recovery easier or even save a person's life. It is important to drink plenty of fluids to avoid dehydration*. Over-the-counter medicine, such as acetaminophen (uh-see-teh-MIH-noh-fen), can relieve pain and fever while the disease runs its course.

*petechiae (puh-TEE-kee-eye) are tiny dots of blood beneath the skin that indicate bleeding from small vessels.

*antibodies (AN-tih-bah-deez) are protein molecules produced by the body's immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.

*dehydration (dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unreplaced loss of body fluids, such as through sweating, vomiting, or diarrhea.