Yellow fever is an acute viral infectious disease that is transmitted to humans through the bite of infected mosquitoes. Though many cases of yellow fever are mild and self-limiting, yellow fever can also be a life-threatening disease causing hemorrhagic fever and hepatitis (hence the term "yellow" from the jaundice it can cause).
This viral disease occurs in tropical areas of Africa and South America, and each year there are an estimated 200,000 cases of yellow fever worldwide, leading to approximately 30,000 deaths. An increase in the number of cases of yellow fever in the last few decades has led to campaigns aimed at improving public awareness and disease prevention for this re-emerging infectious disease.
Yellow fever is thought to have originated in Africa and was likely brought to the Americas on ships during the slave trade. Several significant yellow fever outbreaks have occurred throughout history, with the first documented outbreak occurring in the Yucatan peninsula during the 17th century.
During the late 18th century, a severe yellow fever outbreak struck New England and several North American port cities. The city of Philadelphia is though to have lost about one-tenth of its population during the 1793 yellow fever epidemic, causing many notable figures in American politics to flee the city. The last major yellow fever outbreak in North America occurred in New Orleans in 1905.
The theory that yellow fever is transmitted by a mosquito was first introduced by a Cuban physician, Dr. Carlos Finlay, in the late 19th century. It was not until 1900, using earlier research from Dr. Finlay as a foundation, that U.S. Army Major Dr. Walter Reed and his team proved that yellow fever was in fact transmitted by mosquitoes.
This groundbreaking idea was instrumental in leading to the subsequent control of yellow fever in various regions. The virus responsible for yellow fever was later isolated in the late 1920s, and this breakthrough discovery allowed Max Theiler to later develop the firstvaccine against yellow fever in the 1930s. This successful vaccine helped control and eliminate yellow fever from various countries in Africa and South America during the mid 20th century.
After infection with the yellow fever virus, many individuals will experience no clinically apparent manifestations of the disease (asymptomatic), while others will go on to develop a mild, self-limiting flu-like illness characterized by the following symptoms and signs:
Most patients with this initial acute phase of the disease improve after approximately three to four days. However, 15% of patients may enter a second phase of the disease which typically appears after a short remission of symptoms (about 24 hours) from the initial phase of the illness.
This toxic phase of the disease is more severe, at which time the high fever returns and more organ systems become involved.
In addition to the above symptoms, the following symptoms and signs may also develop:
- abdominal pain,
- bleeding from the gums, nose, eyes, and/or stomach,
- blood in the stool (black stool) and blood in the vomit (black vomit),
- easy bruising of the skin,
- kidney failure,
Yellow fever is caused by a virus. The yellow fever virus is a single-stranded RNA virus that belongs to the Flavivirus genus. After transmission of the virus occurs, it replicates in regional lymph nodes and subsequently spreads via the bloodstream.
This widespread dissemination can affect the bone marrow, spleen, lymph nodes, kidneys, and liver, in addition to other organs.
Tissue damage to the liver, for example, can lead to jaundice and disrupt the body's blood-clotting mechanism, leading to the hemorrhagic complications sometimes seen with yellow fever.
There is no specific curative treatment for yellow fever. Treatment is supportive and aimed at relieving the symptoms of the disease, including the pain and fever. As previously mentioned, the majority of patients who develop symptoms from yellow fever will experience a mild course of illness which will resolve on its own within three to four days.
Supportive measures implemented depend on the severity of the disease, and may include:
- oxygen administration,
- intravenous fluid administration for dehydration,
- medications to increase blood pressure in cases of circulatory collapse,
- transfusion of blood products in cases of severe bleeding,
- antibiotics for secondary bacterial infections,
- dialysis for kidney failure,
- endotracheal intubation (placement of a breathing tube) and mechanical ventilation in cases of respiratory distress.
Acetylsalicylic acid (Aspirin) and nonsteroidal anti-inflammatory drugs (NSAIDs) should be avoided because of the increased risk of bleeding.
For the first few days of illness, infected individuals should also be isolated indoors and/or under mosquito netting in order to prevent further mosquito exposure, thus eliminating the potential for further transmission of the disease.