Yellow fever: the disease and the vaccine
Yellow fever is a disease caused by a virus, transmitted by mosquitoes and often deadly. But it can be prevented with a vaccine.
The YELLOW FEVER is a disease caused by virus , transmitted by mosquitoes , often deadly and in its severe form has liver dysfunction, kidney failure, coagulation disorders and shock.
The yellow fever virus
The yellow fever virus is from the Flaviviridae family, a small group (40–60 nm), with replication in the cytoplasm of infected cells. Because it is a single antigenically-preserved serum, the vaccine protects against all virus samples.
At the nucleotide sequence, the seven major genotypes are represented: West Africa (2 genotypes), East Central Africa and Angola (3 genotypes) and South America (2 genotypes). Most non-human primates are susceptible to infection and some species develop clinical manifestations.
Each mosquito female inoculates approximately 1,000 to 100,000 viral particles during the bite; from the dendritic cells initiate the replication, spreading through the lymphatic channels and regional lymph nodes, reaching several organs through the dissemination through the blood. During the viraemic phase (3–6 days) the infection can be transmitted from a new mosquito bite.
The disease appears abruptly after 3–6 days of the infected mosquito bite and is characterized in three classic stages: period of infection, period of remission and period of intoxication.
Symptoms
The period of infection is characterized by viraemia of 3–4 days, fever, general malaise, headache, photophobia, lumbosacral pain, myalgia, anorexia, nausea, vomiting, irritability and seizures. These are non-specific, indifferent signs and symptoms of other acute infections.
The remission period occurs after at least 48 hours of acute infection and is defined by the decrease in symptoms, especially fever. The patient recovers. Approximately 15% of individuals infected with the yellow fever virus develop into the third stage of the disease.
The intoxication period occurs 3–6 days after the onset of the disease, establishing itself by the return of fever, prostration, nausea, vomiting, epigastric pain, jaundice, oliguria and blood dysfunction. The viremia ends and the antibodies appear in the blood. This phase evolves into dysfunction and, subsequently, failure of various organs and systems resulting in high level of inflammatory cytokines released into the blood.
Diagnosis
The LABORATORIA diagnosis l Yellow fever is performed by serological tests (ELISA) detection of the viral genome through the “polymerase chain reaction” (PCR), virus isolation, and histopathology immunohistochemistry of biopsy or necropsy material.
The presence of IgM antibody in a single blood sample results in a presumptive diagnosis of the disease; confirmation is only established after IgG antibody titres obtained from paired samples of blood collected in the acute phase and convalescence of the disease have been established.
Rapid tests include PCR tests to detect the viral genome in blood and tissues and the serological examination to identify the IgM antibody. The isothermal amplification test — RT-LAMP has shown promise.
Differential diagnosis involves several diseases depending on the evolutionary stage. Examples are: viral hepatitis, influenza, dengue, malaria, leptospirosis, Q fever, and other viral diseases causing haemorrhage (Marburg virus, Ebola virus, lass fever).
Treatment Of Yellow Fever
Treatment consists of life support measures . There is no specific anti-viral medicine . The benefit of using hyperimmune globulin or monoclonal antibody is still uncertain. There are however some side effects some people may experience as a results of the vaccination.
Yellow Fever Vaccination
The live attenuated yellow fever vaccine was developed in 1936. There are six manufactured types of vaccines in the world, with an estimated annual production of 70–90 million doses.
The World Health Organization keeps stockpiled six million doses for emergency cases. Three million were used in Angola in 2016. Because of the low stock and seizure of yellow fever spread to other countries, especially Asia, WHO considered and approved the fractional use (1/5) doses (0.1 ml sub- in emergency conditions.
The estimated risk of disease and death from yellow fever in unvaccinated patients traveling to endemic areas is high (1/1000 and 1/5000, respectively). The cost of the yellow fever vaccine usually varies from region to region and even among states.
By 2015, the United States Immunization Practices Advisory Committee (Acip) has recommended that the single dose is adequate and sufficient for travelers. In July 2016, the WHO Assembly removed the need for the strengthening dose of international health standards.
The option for the fractional dose of the yellow fever vaccine was due to a circumstantial need, but it was supported by international agencies and based on scientific works of worldwide repercussion. The study with a dose of 0.1 ml concluded that the efficacy is similar to a dose of 0.5 ml and the protection durability is at least eight years.
Although the disease represents a major problem for national public health, researchers advance in discoveries, such as the understanding of the ecological corridors established by the Epidemiological Surveillance with the consequent preventive indication of vaccines, research of new specific viricidal drugs and the unusual performance of liver transplantation for the treatment of fulminant hepatitis caused by the Yellow Fever virus.
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