Yellow Fever

Yellow fever is an acute viral hemorrhagic disease that is endemic in tropical areas of Africa and Central and South America. The “yellow” in the name refers to jaundice (yellowing of the skin due to liver damage) that can affect some patients. Yellow fever was historically endemic in the United States, and outbreaks in the 18th and 19th centuries caused some 100,000 to 150,000 deaths. In 1945 a yellow fever vaccine was developed and that, in combination with improvements in mosquito reduction methods, practically eliminated yellow fever from the United States. The reintroduction of the Aedes mosquitoes in the United States and increased global trade and travel has resulted in occasional yellow fever cases in the US since 1999.

Transmission

Yellow fever is spread to people when an infected female Aedes mosquito bites them. Not surprisingly, the primary vector is the Aedes aegypt, the yellow fever mosquito. However, Aedes albopictus, the Asian tiger mosquito, is also responsible for the spread of yellow fever. Mosquitoes acquire the virus when they feed on a person who is viremic (has enough of the disease in their blood to make them contagious) and then feeds on another person, thus transporting the infection to them. Like many mosquito-borne illnesses, yellow fever was first transferred from monkeys to humans via mosquito bites. Once in the human population, the virus is spread from human to human via mosquito bites. Since most Americans are not vaccinated against yellow fever, an outbreak is possible in an urban area as the virus is often brought in from abroad. Mosquitoes capable of transmitting yellow fever are well entrenched in the United States, and it can spread quickly in a densely populated urban environment.

Signs and Symptoms

Many people who are infected with yellow fever either have no symptoms or mild symptoms and will generally recover without medical treatment. Symptoms for those that manifest will typically take about 3-6 days to appear. Symptoms include fever, headache, chills, back pain, fatigue, loss of appetite, nausea, and vomiting and may only last a few days. However, a small percentage of patients progress to a severe form of the disease with high fever, jaundice, internal bleeding, bleeding from the eyes nose or mouth, shock, organ failure, and death. Roughly half of those who develop a severe illness will die.

Treatment and Vaccine

There is a vaccine for yellow fever. Since yellow fever is not common at present in the US, Americans generally only get the vaccine when visiting countries where it is endemic. A single dose of the vaccine proves lifelong protection. No antiviral drug exists for those who contract yellow fever, but supportive hospital care helps improve patient outcomes.

Mosquitos that transmit yellow fever

Aedes aegypti, the yellow fever mosquito, has a black body with a white violin or lyre shape design on its thorax. These mosquitoes do not fly far from their breeding grounds and usually take their blood meals in the early morning or late afternoon around dusk.

Aedes albopictus, better known as the Asian tiger mosquito, is black with white “tiger” stripes and on its legs head and thorax. It has been adapting to our colder regions by diapausing (a period of suspending its development) in the winter.

Both Aedes aegypti and Aedes albopictus are floodwater mosquitoes, meaning that they lay their eggs in containers that will flood with water. When the rainwater reaches the eggs, under the right circumstances such as time of year and temperature, the eggs will hatch. These mosquito species also practice “skip oviposition,” in which they lay eggs in many different places. The eggs hatch into larvae and live in the water before emerging as an adult mosquito. They cannot survive without water, and this is why it is essential to “tip and toss” any standing water on your property during mosquito season.