Yellow Fever - Overview

Yellow fever is an acute natural focal transmissible viral disease characterized by liver damage, hemorrhagic syndrome, and a severe cyclical course.

Yellow fever is a quarantine disease (especially dangerous) subject to international registration.

ICD-10 codes

• A95. Yellow fever.
  • A95.0. Forest yellow fever.
  • A95.1. Urban yellow fever.
  • A95.9. Yellow fever, unspecified.

Epidemiology of yellow fever

Currently, yellow fever is divided into two epidemiological types:

• endemic yellow fever (jungle or zoonotic);
• epidemic yellow fever (urban, or anthroponous).

In the Americas, the carriers of the pathogen of this disease include mosquitoes of the genera Haemagogus and Aedes, and in Africa - a number of species of the genus Aedes, among which the main role is played by A. africanus and A. simpsoni. The main source of the pathogen is monkeys, but rodents and hedgehogs can also be involved. A. africanus remains active in tree crowns during the dry season and thus constantly ensures the circulation of the virus. People who often visit forests due to the nature of their work usually become infected. An infected person, in turn, is a source of the virus for the mosquito A. aegypti, which lives near human dwellings. Mosquitoes use decorative ponds, barrels of water and other temporary water containers for reproduction; they often attack humans. The pathogen remains in the mosquito's body throughout the life of the insect, but transovarial transmission of the pathogen does not occur. At an ambient temperature of 25 °C, a mosquito becomes capable of transmitting the pathogen to a human 10-12 days after infection, and at 37 °C - 4 days. The virus that enters the mosquito first multiplies in the tissues of the midgut, then through the hemolymph enters almost all the organs of the insect, including the salivary glands; in this case, the amount of the virus increases thousands of times compared to the initial level. Pathological changes in mosquitoes occur at the cellular level, but do not affect the physiological indicators and lifespan of the insect.

The development of the pathogen in the mosquito is affected by the volume of blood it absorbs and the amount of virus it receives (the mosquito becomes infected only when a certain minimum number of virions enters it). If the virus circulates along the chain "human-mosquito-human", then yellow fever becomes a typical anthroponosis. Viremia in the patient develops at the end of the incubation period and in the first 3 days of the disease. The virus transmitted by mosquitoes affects people of any age and gender. In endemic foci, adults get sick less often than children.

Yellow fever is periodically recorded in a number of countries in tropical Africa, South America and Mexico. However, outbreaks of this disease can occur in almost any area where there are virus carriers: from 42° north to 40° south latitude. The virus spreads from endemic foci with the help of infected people and carriers. In the presence of conditions for the spread of the pathogen (virus carriers, a large number of carriers and susceptible people), yellow fever can take the form of an epidemic. A patient in the absence of a carrier is not epidemiologically dangerous. The A. aegypti mosquito lives in subtropical regions. People do not have innate immunity to yellow fever. Those who have recovered from the disease develop lifelong immunity. Human susceptibility to the virus is very high, while in endemic areas latent immunization of the local population with small doses of the virus often occurs.

[ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ]

What causes yellow fever?

Yellow fever is caused by the RNA-containing virus Viceronhilus tropicus of the genus Flavivirus of the family Flaviviridae, which belongs to the group of arboviruses. The capsid has a spherical shape; the size is about 40 nm. It is unstable in the environment: it is quickly inactivated at low pH values, exposure to high temperatures and conventional disinfectants. It is preserved for a long time at low temperatures (in liquid nitrogen for up to 12 years). Antigenic relationship with dengue and Japanese encephalitis viruses has been established. The yellow fever virus agglutinates goose erythrocytes, causes a cytopathic effect in Hela, KB, Detroit-6 cells.

What is the pathogenesis of yellow fever?

Yellow fever is acquired through the bite of an infected mosquito. When infected, the virus penetrates the regional lymph nodes through the lymphatic vessels, where it replicates during the incubation period. After several days, it enters the bloodstream and spreads throughout the body. The viremia period lasts 3-6 days. During this time, the virus mainly enters the vascular endothelium, liver, kidneys, spleen, bone marrow, and brain. As the disease progresses, the pathogen exhibits a pronounced tropism for the circulatory system of these organs. This results in increased vascular permeability, especially capillaries, precapillaries, and venules. Dystrophy and necrosis of hepatocytes, and damage to the glomerular and tubular systems of the kidneys develop. The development of thrombohemorrhagic syndrome is caused by both vascular damage and microcirculatory disorders, as well as impaired synthesis of plasma hemostasis factors in the liver.

What are the symptoms of yellow fever?

The incubation period for yellow fever is usually 3-6 days (rarely up to 10 days).

Yellow fever usually begins acutely without prodromal symptoms. High fever develops within the first 24 hours. The rise in temperature is accompanied by yellow fever symptoms: chills, muscle pain in the back and limbs, and severe headache. Nausea and vomiting occur.

Usually, the body temperature drops to normal values by the end of the 3rd day, but in severe cases, the fever can last 8-10 days. Then the temperature rises again, usually without reaching the initial high values. At the onset of the disease, typical symptoms of yellow fever occur: hyperemia of the face, neck and upper body, pronounced injection of the scleral vessels, swelling of the eyelids, swelling of the lips, puffiness of the face ("amarilla mask"). Photophobia and lacrimation are characteristic. The mucous membrane of the mouth and tongue are bright red. Patients suffer from insomnia. Nausea and vomiting with an admixture of bile appear. The pulse reaches 100-130 beats per minute, good filling; bradycardia develops later. Blood pressure is normal, heart sounds are slightly muffled. An enlarged liver and spleen are detected, they may be painful upon palpation. A general blood test reveals neutropenia and lymphopenia. ESR is not increased. Proteinuria is characteristic.

How is yellow fever diagnosed?

The diagnosis of yellow fever in most patients is based on characteristic clinical manifestations (typical saddle-shaped temperature curve, pronounced symptoms of hemorrhagic diathesis, kidney damage, jaundice, enlarged liver and spleen, bradycardia, etc.). In this case, the presence of an endemic focus is taken into account; species composition and numbers, activity of attack on humans and other characteristics of carriers; as well as laboratory test data (leukopenia, neutropenia, lymphocytopenia, significant albuminuria, hematuria, bilirubinemia, azotemia, significant increase in aminotransferase activity).

How is yellow fever treated?

Mild and moderate yellow fever requires strict bed rest, careful care, a gentle diet, plenty of fluids; various chemotherapy drugs are used as indicated. Patients with severe forms of the disease undergo intensive treatment. Cardiotropic drugs, blood transfusions and blood substitutes are prescribed. In case of kidney damage, hemodialysis is indicated. Recommendations for the use of heparin and corticosteroids are based only on theoretical data: no controlled studies have been conducted.

How is yellow fever prevented?

Yellow fever is prevented by immunizing the population. For this purpose, a yellow fever vaccine is used - two live vaccines, in particular a vaccine based on the 17D strain, obtained by long-term passage of the virus in cell culture. A vaccine created on the basis of the Dakar strain adapted by serial passages on mice has become less widespread. This strain has residual virulence, so when performing the vaccination, human immune serum is first administered.

What is the prognosis for yellow fever?

Yellow fever has a favorable prognosis in mild and moderate forms of yellow fever. In severe forms, the mortality rate reaches 25%. Even in severe forms, recovery occurs after the 12th day of illness. Elderly people tolerate the disease poorly. The lowest mortality rate is among children. Even during severe epidemics, it does not exceed 3-5%.