Pathogen Characteristics and Historical Background
The yellow fever virus is an RNA-containing microorganism belonging to the family Flaviviridae and serving as the type species of the genus Flavivirus. The pathogen's name derives from the Latin word flavus, meaning "yellow," reflecting a specific symptom of the disease — marked jaundice.
This is a historically significant infectious agent. It was the first discovered virus capable of causing disease in humans. The pathogen was discovered in Cuba in 1901 by a mission led by W. Reed, which marked the beginning of the global study of all arboviral infections.
The pathogen is characterized by several pronounced types of tissue tropism:
- Vasotropism (the ability to damage blood vessels of internal organs).
- Viscerotropism (destruction of parenchymal organ cells).
- Neurotropism (effects on nervous system structures).
Despite its high aggressiveness within the body, the virus exhibits low stability in the external environment and is rapidly inactivated.
Epidemiology and Transmission Routes
Yellow fever is endemic to regions of Africa, as well as South and Central America. The geographical boundaries of the disease's spread are strictly limited by the habitat range of its vectors — from 42° north latitude to 40° south latitude. The main mechanism of infection is vector-borne (via mosquito bites).
Two main epidemiological forms of the infection are distinguished:
- Jungle (sylvatic) cycle. This has a zoonotic, natural focal character. Monkeys serve as the reservoir, and the pathogen circulates via the "monkey — mosquito" chain. Humans become involved in the epidemic process accidentally when bitten by an infected insect in the jungle.
- Urban cycle. This is the main driver of mass epidemics. The reservoir of infection is an infected human, and transmission is carried out by synanthropic mosquitoes of the species Aedes aegypti via the "human — mosquito — human" chain.
For a mosquito to become infectious, the virus must actively replicate in its body and reach a critical concentration in the salivary glands. In addition to bites, a contact transmission route is possible: infection occurs when blood from an infected or deceased person contacts damaged skin or mucous membranes of a healthy person.
Pathogenesis of the Infection
The development of yellow fever proceeds through several consecutive stages determined by the biological properties of the virus:
- Inoculation and incubation. Following a mosquito bite, the virus enters regional lymph nodes, where it actively replicates. This stage takes 3 to 6 days and corresponds to the incubation period.
- Viremia. The pathogen enters the systemic circulation en masse. The phase of viral circulation in the blood lasts about 3–4 days.
- Dissemination. Hematogenous spread of the infection throughout the body occurs. Due to vasotropism, the virus damages vascular walls, manifesting as a severe hemorrhagic syndrome.
- Target organ damage. The pathogen actively invades the liver, kidneys, spleen, bone marrow, and brain.
Severe pathomorphological changes develop in the affected tissues. Marked dystrophy and extensive necrosis of hepatocytes are observed in the liver. Systemic damage to both the glomerular and tubular apparatus occurs in the kidneys. If the patient survives the acute phase, a robust and durable post-infection immunity is established.
Diagnosis, Treatment, and Prevention
No specific etiotropic drugs have been developed for treating the disease; therefore, prevention and timely diagnosis play a pivotal role. Laboratory confirmation includes the following methods:
- Virological. Aims to isolate the pathogen. The specimen is the patient's blood (strictly no later than the 3rd–4th day of illness during viremia) or liver tissue in fatal cases. Cell cultures or newborn white mice are used for cultivation.
- Serological. Evaluates rising antibody titers in paired sera using HI (hemagglutination inhibition), CF (complement fixation), neutralization, or ELISA tests. This method is especially important for atypical presentations of the infection.
- Rapid diagnostics. Performed via ELISA to quickly detect viral antigen or specific immunoglobulin M (IgM) classes, which serve as markers of active disease.
Specific prevention is the only reliable method of protection. A live attenuated vaccine (strain 17D) is used for this purpose. Immunization is mandatory for individuals residing in endemic areas or traveling to high-risk regions. Robust immunity is established starting from the 10th day after administration and lasts for at least 10 years. During outbreaks, immediate mass vaccination of the population is carried out, taking medical contraindications into account.