Yellow Fever: Causes, Symptoms, and Prevention

Introduction to Yellow Fever

Yellow fever is a tropical disease spread to humans by infected mosquitoes. Although most infections are mild, the disease can be severe and life-threatening. Found in Africa and South America, yellow fever is preventable by immunization, and travelers to those regions are advised to get the vaccine. In South America, sporadic infections occur mostly among forestry and agricultural workers due to occupational exposure in or near forests.

Transmission Cycles: Jungle and Urban Yellow Fever

There are two kinds of yellow fever, each spread by a different cycle of infection. Jungle yellow fever is mainly a disease of monkeys and is spread from infected mosquitoes to monkeys in the tropical rainforest. It is then transmitted to people who are bitten by mosquitoes that have been infected by monkeys. Jungle yellow fever is rare and usually occurs in persons who work in tropical rainforests.

Urban yellow fever is a disease of humans, spread by mosquitoes that have been infected by other people — usually by the Aedes aegypti mosquito. These mosquitoes have adapted to living among humans in cities, towns, and villages and are known to breed in discarded tires, flower pots, oil drums, and water storage containers close to human dwellings.

History and Classification of the Disease

It was Walter Reed and his assistants James Carroll, Aristides Agramonte, and Jesse Lazear, while working at experimental stations outside Havana, Cuba, who proved that the Aedes aegypti mosquito was the "vector" for the yellow fever virus. This work destroyed the popular myth that yellow fever was spread by direct contact with infected people or "contaminated" objects and thus focused efforts on the eradication of the Aedes mosquito. Urban yellow fever is the cause of most yellow fever outbreaks and epidemics.

Although a disease of the tropics, from 1793 to 1822, yellow fever was one of the most dreaded diseases in the port cities of the United States. It is thought to have entered America primarily as a scourge claiming the lives of those who voyaged to the West Indies, the nation's principal trading partner at the time.

Symptoms and Disease Progression

Yellow fever is recognized in historic texts stretching back some four hundred years. The term "yellow" derives from the jaundice that affects most patients. The first recorded outbreak occurred in the New World in 1648. Yellow fever is a member of the flavivirus family, group B arbovirus, a genus comprising more than sixty-eight arthropod-transmitted viruses, thirty of which are known to cause human disease. Other flaviviral infections include dengue, Japanese encephalitis, and tick-borne encephalitis. It is important to consider this group of viruses in the clinical differential diagnosis of central nervous system infection, hemorrhagic fever, and acute febrile illnesses with arthropathy. A vaccine has been available for over sixty years, yet during the last two decades, the number of people infected has increased, and the disease is once again a serious public health issue.

The virus remains silent in the body during an incubation period of three to six days, after which there are two disease phases. Although some infections produce no symptoms, the first — or "acute" — phase is generally characterized by fever, muscle pain with prominent backache, headache, chills, loss of appetite, nausea, and/or vomiting. The high fever is paradoxically associated with a slow pulse. Most patients improve after three to four days and symptoms disappear.

However, 15 percent of patients enter a "toxic phase" within 24 hours of apparent recovery, when fever reappears and several body systems are affected. Patients rapidly develop jaundice and complain of abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes, and/or stomach, with blood also appearing in the vomit and feces. Kidney function deteriorates; this can range from abnormal protein levels in the urine (albuminuria) to complete kidney failure with no urine production (anuria). Roughly half the patients who enter this toxic phase die within ten to fourteen days; the remainder recover without significant organ damage.