Yellow fever: an overview

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

Yellow fever refers to quarantine diseases (especially dangerous), subject to international registration.

ICD-10 codes

• A95. Yellow fever.
  • A95.0. Forest yellow fever.
  • A95.1. City 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 anthropogenic).

On the American continent, mosquitoes of the genera Haemagogus and Aedes are referred to carriers of the causative agent of this disease , 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 a monkey, but there may also be rodents and hedgehogs. A. Africanus during activity during the dry season remains active in the crowns of trees and thus constantly circulates the virus. Usually people are infected, often visited by forests. The infected person, in turn, represents the source of the virus for the mosquito A. Aegypti, which lives next to the human dwelling. For reproduction mosquitoes use decorative reservoirs, barrels with water and other temporary containers of water; often attack a person. In the mosquito organism, the pathogen persists throughout the life of the insect, but the transovarial transmission of the pathogen does not occur. At an ambient temperature of 25 ° C, the mosquito becomes capable of transmitting the pathogen to a human 10-12 days after infection, and at 37 ° C it is 4 days. The virus that gets into the mosquito first multiplies in the tissues of the middle intestine, then through the hemolymph gets into almost all the organs of the insect, including the salivary glands; while the amount of the virus increases thousands of times compared with the original level. Pathological changes in mosquitoes occur at the cellular level, but do not affect the physiological indices and life span of the insect.

The development of the pathogen in the mosquito is affected by the volume of blood absorbed by it and the amount of the virus thus obtained (mosquito infection occurs only when a certain minimum number of virions enter it). If the virus circulates through the human-mosquito-human chain, then yellow fever becomes a typical anthroponosis. Viralemia in the patient develops at the end of the incubation period and in the first 3 days of the course of the disease. The virus transmitted by mosquitoes affects people of any age and sex. In endemic foci, adults suffer less often than children.

Yellow fever is periodically recorded in a number of countries in tropical Africa, in South America and Mexico. However, outbreaks of this disease can occur in virtually any territory where there are vectors of the virus: from 42 ° north to 40 ° south latitude. The virus spreads from endemic foci with the help of infected people and vectors. 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. The patient in the absence of a carrier is epidemiologically not dangerous. The A. Aegypti mosquito lives in subtropical areas. Congenital immunity to yellow fever a person does not possess. In those who have recovered, life-long immunity develops. The susceptibility of people to the virus is very high, while in the endemic areas, latent immunization of the local population with small doses of the virus often occurs.

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What causes yellow fever?

Yellow fever is caused by the RNA-containing virus Viceronhilus tropicus of the Flavivirus family Flaviviridae, which belongs to the group of arboviruses. The capsid has a spherical shape; dimensions of about 40 nm. In the environment, it is not very stable: it is rapidly inactivated at low pH values, by exposure to high temperatures and conventional disinfectants. Long lasts at low temperatures (in liquid nitrogen up to 12 years). An antigenic relationship with the viruses of dengue and Japanese encephalitis has been established. The yellow fever virus agglutinates the erythrocytes of geese, causes a cytopathic effect in Hela, KB, Detroit-6 cells.

What pathogenesis does yellow fever have?

Infection with yellow fever occurs when bitten by an infected mosquito. When infected, the virus enters the regional lymph nodes through lymphatic vessels, in which its replication takes place during the incubation period. After a few days, it enters the bloodstream and spreads throughout the body. The duration of the viremia period is 3-6 days. At this time the virus gets mainly into the endothelium of the vessels, the liver, kidneys, spleen, bone and brain. When the disease develops, a pronounced tropism of the pathogen to the circulatory system of these organs is revealed. As a result, there is an increased permeability of blood vessels, especially capillaries, precapillaries and venules. Dystrophy and necrosis of hepatocytes, defeat of the glomerular and tubular kidney systems develop. The development of thrombohemorrhagic syndrome is caused both by vascular damage and microcirculatory disorders, and by a violation of the synthesis in the liver of plasma factors of hemostasis.

What are the symptoms of yellow fever?

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

Yellow fever begins, as a rule, acutely without prodromal phenomena. High fever develops within the first 24 hours. The rise in temperature is accompanied by the symptoms of yellow fever: chills, pain in the muscles of the back and limbs and a severe headache. There is nausea, vomiting.

Usually the body temperature drops to normal values by the end of 3 days, but with a heavy current, fever can last 8-10 days. Then the temperature rises again, usually not reaching the original 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 vessels sclera, eyelid edema, swelling of the lips, puffiness of the face ("amaryl mask"). Characteristic photophobia and lacrimation. The mucous membrane of the mouth and tongue are bright red. Patients suffer from insomnia. Appears nausea, vomiting with an admixture of bile. Pulse reaches 100-130 beats per minute, good filling; in the future develops a bradycardia. Arterial blood pressure is normal, the heart sounds are slightly muted. They reveal an increase in the liver and spleen, and it is possible that they are painful on palpation. In a general blood test, neutropenia and lymphopenia are detected. ESR is not increased. Proteinuria is characteristic.

How is yellow fever diagnosed?

Diagnosis of yellow fever in most patients is based on the characteristic clinical manifestations (typical saddle-shaped temperature curve, expressed 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 abundance, attack activity on humans and other characteristics of vectors; as well as laboratory data (leukopenia, neutropenia, lymphocytopenia, significant albuminuria, hematuria, bilirubinemnia, azotemia, a significant increase in aminotransferase activity).

How is yellow fever treated?

Yellow fever of mild and moderate form requires the appointment of strict bed rest, careful care, sparing diet, abundant drinking; according to the indications - different chemotherapy drugs. Patients with severe disease are treated intensively. Assign cardiotropic drugs, blood transfusion and blood replacement fluids. When the kidneys are affected, hemodialysis is indicated. Recommendations for the use of heparin and corticosteroids are based only on theoretical data: no controlled studies have been performed.

How is yellow fever prevented?

Yellow fever is prevented through immunization of the population. For this purpose, yellow fever vaccination is used - two live vaccines, in particular a vaccine based on strain 17D, obtained by prolonged passage of the virus in a cell culture. The vaccine, which was created on the basis of the adapted Dakar strain adapted for serial mice, received less spread. This strain expresses residual virulence, therefore, when immunization is carried out, the human immune serum is first introduced.

What is the prognosis of yellow fever?

Yellow fever has a favorable prognosis with mild and moderate forms of yellow fever. In severe forms, the mortality rate is 25%. Even with severe forms after the 12th day of the disease, recovery comes . Elderly people do not tolerate the disease. Least mortality among children. Even during severe epidemics, it does not exceed 3-5%.