Yellow fever is a viral disease, historically responsible for large scale epidemics in Africa and the Americas.
It can be recognized from historic texts stretching back 400 years. 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 for 60 years, the number of people infected over the last two decades has increased and yellow fever is now a serious public health issue. As such South African Port Health Authority requires Yellow Fever vaccinations for returned travellers from 42 countries globally.
Mode of Infection
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. South America has two different types, but since 1974 only one has been identified as the cause of disease outbreaks.
The virus remains silent in the body during an incubation period of three to six days followed by 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. After three to four days most patients improve and their symptoms disappear.
However, 15% 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; ranging 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 and can easily be mistaken for other diseases. A laboratory analysis is required to confirm a suspect case.
The virus is constantly present with low levels of infection (i.e. endemic) in some tropical areas of Africa and the Americas with periodic outbreaks in these areas. Until the start of this century, yellow fever outbreaks also occurred in Europe, the Caribbean islands and Central and North America. Even though the virus is not felt to be present in these areas now, they must still be considered at risk for yellow fever epidemics.
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. In the Americas, yellow fever is endemic in nine South American countries and in several Caribbean islands. Bolivia, Brazil, Colombia, Ecuador and Peru are considered at greatest risk.
Although yellow fever has never been reported from Asia, this region is at risk because the appropriate primates and mosquitoes are present.
Humans and monkeys are the principal animals to be infected. The virus is carried from one animal to another by a biting mosquito. 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. Control programmes successfully eradicated mosquito habitats in the past, especially in South America. However, these programmes have lapsed over the last 30 years and mosquito populations have increased thus increasing the risk of spreading the virus.
There is no specific treatment for yellow fever, consequently generally accepted medical treatment principles are applied. Whereas dehydration and fever can be corrected with oral rehydration salts and paracetamol, bacterial infection can 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.
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.
Since most of the known side effects have occurred in children less than six months old, vaccine is not administered to this age group or to pregnant women.
A vaccination certificate is required for entry to many countries, as well as returning to home countries after visiting endemic areas.