What causes yellow fever

What is Yellow Fever?

Yellow fever is a serious, potentially life-threatening viral disease that is transmitted through the bite of an infected mosquito. The name "yellow fever" comes from the jaundice that affects some patients, causing their skin and eyes to turn yellow. While there is no specific cure for yellow fever, it is preventable through vaccination and can be managed with supportive medical care.

The Culprit: The Yellow Fever Virus

Yellow fever is caused by the yellow fever virus, which belongs to the Flaviviridae family of viruses. This family also includes other well-known viruses like Dengue, Zika, and West Nile virus. The virus circulates in a cycle involving mosquitoes and primates (including humans). In jungle areas, the virus primarily cycles between mosquitoes and monkeys, with humans becoming infected incidentally when they enter these enzootic (endemic in animal populations) areas. In urban settings, the virus can spread rapidly among humans via Aedes aegypti mosquitoes, which are also vectors for other arboviruses.

Transmission: The Mosquito Vector

The primary vector for yellow fever transmission is the mosquito, particularly species within the Aedes genus, with Aedes aegypti being the most significant one in urban epidemics. These mosquitoes typically bite during the day, especially at dawn and dusk. When an infected mosquito bites a person, it injects saliva containing the virus into the bloodstream. The virus then replicates within the human body, and if the person develops a high enough viral load, they can become a source of infection for other mosquitoes when these mosquitoes bite them.

The Disease Cycle

Yellow fever has three main transmission cycles:

1. Jungle (Sylvatic) Cycle:

This cycle occurs in tropical rainforests. The virus is maintained in a cycle between mosquitoes that feed on infected non-human primates (monkeys). Humans entering forested areas can be accidentally bitten by infected mosquitoes and become infected. This is the ancestral cycle and can lead to sporadic human cases.

2. Intermediate (Savannah) Cycle:

This cycle occurs in semi-arid and humid savannas. Mosquitoes infect both monkeys and humans. This can lead to larger outbreaks than the jungle cycle as human populations are often denser in these areas.

3. Urban Cycle:

This is the most dangerous cycle for widespread epidemics. It occurs in densely populated urban areas. The virus is transmitted by Aedes aegypti mosquitoes that predominantly feed on humans. An infected person can introduce the virus into a susceptible urban population, leading to rapid transmission and large-scale outbreaks.

Symptoms and Stages of Infection

After being bitten by an infected mosquito, there is an incubation period of 3 to 6 days before symptoms appear. The infection often progresses through three stages:

1. Initial Phase (Infection Phase):

Most people infected with yellow fever virus have no symptoms or mild symptoms. Mild symptoms can include sudden onset of fever, chills, headache, backache, nausea, vomiting, fatigue, and loss of appetite. This phase usually lasts for 3 to 4 days and then the patient may recover.

2. Remission Phase:

After the initial symptoms subside, patients may enter a remission period where they feel better for a short time, lasting from a few hours to a day. However, approximately 15% of infected individuals progress to a more toxic phase.

3. Toxic Phase:

This is the most severe stage and is characterized by the return of high fever, accompanied by jaundice (yellowing of the skin and whites of the eyes), abdominal pain with vomiting (sometimes with blood, referred to as "black vomit"), facial flushing, dehydration, lethargy, and impaired kidney and liver function. Bleeding can occur from the mouth, nose, eyes, and stomach. In this toxic phase, the mortality rate can be as high as 20% to 50%.

Risk Factors

The primary risk factor for contracting yellow fever is travel to or living in areas where the virus is endemic, primarily in tropical and subtropical regions of Africa and South America. Within these regions, factors that increase risk include:

• Lack of vaccination.
• Engaging in activities that increase exposure to mosquitoes in endemic areas (e.g., working or camping in forests, visiting rural areas).
• Living in close proximity to mosquito breeding sites, especially in urban areas with poor sanitation and inadequate mosquito control.

Prevention and Control

The most effective way to prevent yellow fever is vaccination. The yellow fever vaccine is safe and highly effective, providing lifelong immunity for most individuals after a single dose. Travelers to endemic areas are often required to have proof of vaccination (an International Certificate of Vaccination or Prophylaxis - ICVP, often called the "yellow card").

Other preventive measures include:

• Mosquito bite prevention: Using insect repellent containing DEET, picaridin, or IR3535, wearing long-sleeved shirts and long pants, and staying in accommodations with screens on windows and doors or using air conditioning.
• Mosquito control: Reducing mosquito breeding sites by eliminating standing water around homes and communities.

While supportive care is crucial for those who become ill, preventing infection through vaccination and mosquito control remains the cornerstone of yellow fever management.