Yellow fever is an acute natural focal viral infection characterized by a severe course with a predominance of intoxication, jaundice and hemorrhagic syndrome. Yellow fever is among the most dangerous infections. Yellow fever is spread by vector-borne pathways, its carriers are mosquitoes. The incubation period of yellow fever is about a week. her clinic includes severe intoxication up to disorders of consciousness and cardiac activity, hemorrhagic syndrome, hepatosplenomegaly, jaundice of the sclera. Treatment of a patient with yellow fever is carried out exclusively inpatient in the department for particularly dangerous infections.
Yellow fever virus – RNA-containing, belongs to the genus Flavivirus, is stable in the external environment. It tolerates freezing and drying well, dies within 10 minutes when heated to 60 ° C, and is also easily inactivated by ultraviolet radiation and disinfectant solutions. It does not tolerate an acidic environment. The reservoir and source of infection are animals – monkeys, marsupials, rodents and insectivores. A person can become a source of infection only if there is a vector.
The disease spreads by a transmissible mechanism, the carriers of the virus are mosquitoes. In the Americas, yellow fever is spread by mosquitoes of the genus Naetadodes, in Africa – Aedes (mainly of the species A. Aegypti). Mosquitoes breed near human habitations, in barrels with water, artificial stagnant reservoirs, flooded basements, etc. Insects are contagious from 9-12 days after the bite of a sick animal or person at a temperature of 25 ° C and after 4 days at 37 ° C. Transmission of the virus at an ambient temperature of less than 18 ° C is not carried out.
In case of ingestion of blood containing the pathogen on areas of damaged skin or mucosa, it is possible to implement a contact pathway of infection (when processing carcasses of sick animals). People have a high natural susceptibility to infection, long-term immunity is formed after the transfer. The disease belongs to quarantine (due to the special danger), cases of yellow fever epidemics are subject to international registration.
Outbreaks of the disease can occur in any areas of the vector’s distribution area, mainly occur in tropical countries. The spread of fever from the focus of the epidemic is carried out during the relocation of patients and the movement of mosquitoes during the transportation of goods. Yellow fever epidemics develop in the presence of three necessary conditions: carriers of the virus, vectors and favorable weather conditions.
The virus enters the blood during blood sucking from the digestive system of the carrier and during the incubation period is reproduced and accumulates in the lymph nodes. In the first days of the disease, the virus spreads through the body with blood flow, settling in the tissues of various organs (liver and spleen, kidneys, bone marrow, heart muscle and brain) and affecting their vascular system and causing inflammation. As a result of trophic disorders and the direct toxic effect of the virus, necrotic destruction of the parenchyma occurs, increased permeability of the vascular wall contributes to hemorrhage.
Yellow fever symptoms
The incubation period of yellow fever is a week (sometimes 10 days). The disease proceeds with the change of successive phases: hyperemia, short-term remission, venous stasis and convalescence. The hyperemia phase begins with a sharp rise in temperature, increasing intoxication (headache, body aches, nausea and vomiting of central origin). With the progression of intoxication syndrome, disorders of central nervous activity may occur: delirium, hallucinations, disorders of consciousness. The patient’s face, neck and shoulder girdle are puffy, hyperemic, numerous injections of sclera take place, the mucous membranes of the mouth, tongue, conjunctiva are bright red. Patients complain of photophobia and lacrimation.
There are toxic disorders of cardiac activity: tachycardia, followed by bradycardia, hypotension. The amount of daily urine decreases (oliguria), a moderate increase in the size of the spleen and liver is noted. Subsequently, the first signs of developing hemorrhagic syndrome (hemorrhages, bleeding) appear, the sclera acquire a jaundiced hue.
The hyperemia phase lasts about 3-4 days, after which a short-term remission occurs (lasting from several hours to a couple of days). The temperature normalizes, the general well-being and the objective condition of patients improves. With abortive forms of yellow fever, recovery subsequently occurs, but most often after a short-term remission, the body temperature rises again. In general, the febrile period is usually 8-10 days from the onset of the disease. In severe cases, after a short-term remission, a phase of venous stasis occurs, manifested by pronounced pallor and cyanotic skin, rapidly developing jaundice, petechiae, ecchymatosis, purpura are common. Hepatosplenomegaly occurs.
The condition of patients worsens significantly, hemorrhagic symptoms are pronounced, patients note vomiting with blood, melena (tar–like feces is a sign of intense intestinal bleeding), nosebleeds, internal hemorrhages may occur. Oliguria usually progresses (up to anuria), bloody impurities are also noted in the urine. In half of the cases, the disease progresses with the development of severe lethal complications. With a favorable course, a prolonged period of convalescence occurs, the clinical symptoms gradually regress. It is possible to preserve a variety of functional disorders with significant destruction of tissues. After the transfer of the disease, lifelong immunity is preserved, there are no repeated episodes.
Severe yellow fever can be complicated by infectious and toxic shock, renal and hepatic insufficiency. These complications require intensive care measures, in many cases lead to death on the 7-9 day of the disease. In addition, encephalitis may develop.
In the first days, a general blood test shows leukopenia with a shift of the leukocyte formula to the left, a reduced concentration of neutrophils, platelets. Subsequently, leukocytosis develops. Thrombocytopenia is progressing. Hematocrit grows, the content of nitrogen and potassium in the blood increases. A general urinalysis notes an increase in protein, erythrocytes and cells of the cylindrical epithelium are noted.
A biochemical blood test shows an increase in the amount of bilirubin, the activity of liver enzymes (mainly AST). The pathogen is isolated in specialized laboratories, taking into account the special danger of infection. Diagnostics is performed using a bioassay on laboratory animals. Serological diagnostics is performed by the following methods: IHR, CBR and ELISA.
Yellow fever is treated inpatient in an infectious diseases department specialized for the treatment of particularly dangerous infections. Etiotropic therapy of this disease has not been developed at present, treatment is aimed at maintaining immune functions, pathogenetic mechanisms and relieving symptoms. Patients are shown bed rest, semi-liquid easily digestible food rich in calories, vitamin therapy (vitamins C, P, K). In the first days, plasma transfusion of convalescent donors can be performed (the therapeutic effect is insignificant).
During the fever period, patients are transfused blood in an amount of 125-150 ml every 2 days, drugs based on bovine liver extract are prescribed, iron intramuscularly to compensate for blood loss. In complex therapy, anti–inflammatory drugs (corticosteroids, if necessary), antihistamines, hemostatics, cardiovascular drugs can be prescribed. If necessary, reanematological measures are carried out.
Prognosis and prevention
The prognosis for yellow fever in the case of a mild or abortive course is favorable, with the development of a severe clinic, it worsens. Complications of infection in half of cases lead to death.
The prevention of the disease provides for the control of population migration and cargo transportation in order to exclude the possibility of importation of yellow fever from the epidemic focus. In addition, the destruction of yellow fever vectors in populated areas is being carried out. Individual prevention involves the use of means of protection against insect bites. Specific prevention (vaccination) consists in the management of a live weakened vaccine. Immunoprophylaxis is indicated for people of any age planning a trip to endemic regions 7-10 days before departure.