An arbovirus transmitted by mosquito bites

Yellow fever is an infectious disease caused by an arbovirus: the amaril virus (of the genus Flavivirus), transmitted by various types of mosquitoes, including those of the genus Aedes (as well as Hemagogus or Sabethes in Latin America). The intermediate hosts, which act as amplifiers, are monkeys; mosquitoes serve as both vectors and reservoirs due to their ability to transmit the virus vertically to their offspring.
The virus is endemic in the tropical regions of Africa, Central America, and South America.
Humans can become infected in several ways:

• through bites from infected “wild” mosquitoes during a stay in the forest (“wild” cycle, predominant in Latin America and African forests)

• through bites from “domestic” mosquitoes (Aedes aegypti), following the introduction of the virus into urban areas from rural areas (“urban” cycle, responsible for major epidemics)

• through bites from “semi-domestic” mosquitoes, which infect both humans and monkeys (the “intermediate” cycle, most common in African savanna regions, which can cause small outbreaks in rural villages).

Yellow fever varies in severity and can be fatal. Reporting of the disease is mandatory, as is reporting to the WHO, as yellow fever falls under the International Health Regulations.
The yellow fever vaccine is effective.

In this context, Santé publique France’s priorities are:

• to analyze and monitor notifiable diseases, including yellow fever

• to promote preventive measures, particularly vaccination

A Possible Approach to Prevention

Possible prevention based on:

• Vector control in countries where yellow fever is endemic,

• Insecticide treatment in airplanes,

• Vaccination, which:

  • Provides effective, lifelong protection (no boosters needed) 10 days after injection in 90 to 95% of cases

  • Must be administered at an approved vaccination center

  • Is required starting at age 1, and available as early as 6 months

  • Is mandatory for travelers to French Guiana

  • Is recommended for any stay in an endemic intertropical area of Africa or Latin America, even in the absence of an administrative requirement

  • Is a live attenuated vaccine with certain contraindications, particularly for breastfeeding women and infants under 6 months of age (see vaccination schedule)

  • Is not recommended during pregnancy; however, if the stay or travel to an endemic area cannot be postponed, the decision to administer the vaccine must be carefully weighed due to the high fatality rate of the disease.

  • This vaccine may cause mild local or systemic side effects in about one-third of people receiving their first dose. It may also, much more rarely, cause serious or even fatal side effects.

Only one French territory is located in an amarile zone

Only French Guiana, where Aedes aegypti is present, is located in an amarile zone.

Clinical and laboratory diagnosis

After the infectious bite, the incubation period lasts 3 to 6 days. The infection remains asymptomatic in 50 to 85% of cases.

If symptoms appear:

• The onset of yellow fever is marked by a nonspecific flu-like syndrome, high fever, muscle and lower back pain, and flushing of the face and neck (the "red" phase). Minor bleeding may occur. In most cases, symptoms disappear after 3 to 4 days

• The “toxic” phase occurs in 15–25% of symptomatic individuals (and 10% of all infections) following a transient clinical improvement lasting a few hours to 1–2 days. It is characterized by the onset of:

  • jaundice,

  • bleeding from the nose and mouth,

  • upper gastrointestinal bleeding (from the esophagus, stomach, and duodenum) leading to abdominal pain, melena, and bloody vomiting (vomito negro),

  • renal failure (proteinuria and oliguria), ​​​​​

  • myocardial damage and neurological signs (seizures) may also be present.

• Approximately 50% of patients who enter the "toxic" phase die within 10–14 days. The remaining patients recover without sequelae.

• Biological confirmation of the diagnosis of yellow fever is based on:

  • viral identification (RT-PCR, virus isolation) in the blood, possible within the first 6 days after the onset of symptoms,

  • serology (detection of specific IgM antibodies).

Symptomatic treatment

There is no curative treatment for yellow fever. Symptomatic management consists primarily of correcting hypotension, supporting renal failure, and replacing lost electrolytes.