YELLOW FEVER DEFINITION
An acute systemic (bodywide) illness caused by a virus called a Flavivirus. In severe cases, the viral infection causes a high fever, bleeding into the skin, and necrosis (death) of cells in the kidney and liver. The damage done to the liver from the virus results in severe jaundice which yellows the skin.Hence, the “yellow” in “yellow fever.”
Yellow fever once ravaged port cities in the United States. (A viewer writes in: “This disease was prevalent in the deep south, not just in the seaports. My grandparents lived through an epidemic of yellow jack in central Mississippi around 1900, and they were a long way from the seacoast.”)
Today yellow fever is most common in tropical areas of Africa and the Americas. The virus of yellow fever is transmitted in most cases by a bite of a mosquito. In urban settings, yellow fever may be transmitted from person to person by the Aedesaegypti mosquito. In the jungle, yellow fever is transmitted from monkeys to people by mosquitoes that breed in tree-holes in the rainforests. The diagnosis of yellow fever is made by observation or, if need be, by culturing the virus from a blood sample.
There is no cure for yellow fever, although antiviral medications may be tried. Non-aspirin pain relievers, rest, and rehydration with fluids decrease discomfort. The disease usually passes within a few weeks.
Yellow fever can be prevented by vaccination. The yellow fever vaccine is a live attenuated (weakened) viral vaccine. It is recommended for people traveling to or living in tropical areas in the Americas and Africa where yellow fever occurs. Because it is a live vaccine, it should not be given to infants or people with immune-system impairment.
The vaccine is based upon classic medical research done under Dr. Walter Reed. When yellow fever broke out among U.S. troops in Cuba in 1900, Dr. Reed, a member of the Army Medical Corps, headed a commission of physicians on yellow fever. They discovered that the fever was transmitted by the Aëdesaegypti mosquito which breeds near houses (and also transmit dengue). Reed’s team later showed that the mosquito injected a virus that caused the dread disease. Sanitary engineers eradicated the mosquito and freed Cuba of yellow fever in 1902 (the year of Reed’s death from appendicitis).
YELLOW FEVER CAUSES
Yellow fever is caused by a virus that is spread by the Aedesaegypti mosquito. These mosquitoes thrive in and near human habitations where they breed in even the cleanest water. Most cases of yellow fever occur in sub-Saharan Africa and tropical South America.
Humans and monkeys are most commonly infected with the yellow fever virus. Mosquitoes transmit the virus back and forth between monkeys, humans or both.
When a mosquito bites a human or a monkey infected with yellow fever, the virus enters the mosquito’s bloodstream and circulates before settling in the salivary glands. When the infected mosquito bites another monkey or human, the virus then enters the host’s bloodstream, where it may cause illness.
YELLOW FEVER PATHOPHYSIOLOGY
YF virus, an arbovirus, is the type species for the family Flaviviridae and is a single, positive-stranded, enveloped RNA virus. The envelope consists of a lipid bilayer containing an envelope glycoprotein and a matrix protein. The single RNA is complexed with a capsid protein. Viral strains from South America are closely related to those from West Africa. This observation supports the supposition that YF virus originated in West Africa.
The virus is transferred from the infected female mosquito’s salivary gland by means of saliva introduced into a bite wound during a blood meal. It replicates in local tissues and regional lymph nodes. The virus can then infect a feeding mosquito during the initial 3-4 days of the illness. No human-to-human transmission is known.
After the virus enters the bloodstream, hematogenous spread to the bone marrow, kidney (probable), liver (the main target), myocardium, and spleen ensues, and further replication occurs. Cerebral edema and cerebral petechial hemorrhages result from secondary factors. In the hemorrhagic diathesis that may follow, disseminated intravascular coagulation (DIC) involves decreased synthesis of coagulation factors, altered platelet function, and bleeding from the gastrointestinal (GI) mucosa and abdominal or pleural serosa.
Myocardial fiber injury occurs secondary to direct virus activity, with cloudy swelling and fatty change. Shock and death can result. Multiple organ insult involves the liver, kidneys, brain, and heart. Other consequences include hemorrhage and secondary effects of vasoactive cytokines. In an immune response, viral neutralizing antibodies are present by the end of the first week, and the virus is rapidly cleared. Immune response confers lifelong immunity. The role of immune response in pathogenesis has not yet been established.
YELLOW FEVER SYMPTOMS
During the first three to six days after you’ve contracted yellow fever — the incubation period — you won’t experience any signs or symptoms. After this, the infection enters an acute phase and then, in some cases, a toxic phase that can be life-threatening.
Once the infection enters the acute phase, you may experience signs and symptoms including:
- Muscle aches, particularly in your back and knees
- Sensitivity to light
- Nausea, vomiting or both
- Loss of appetite
- Red eyes, face or tongue
These signs and symptoms usually improve and are gone within several days.
Although signs and symptoms may disappear for a day or two following the acute phase, some people with acute yellow fever then enter a toxic phase. During the toxic phase, acute signs and symptoms return and more-severe and life-threatening ones also appear. These can include:
- Yellowing of your skin and the whites of your eyes (jaundice)
- Abdominal pain and vomiting, sometimes of blood
- Decreased urination
- Bleeding from your nose, mouth and eyes
- Slow heart rate (bradycardia)
- Liver and kidney failure
- Brain dysfunction, including delirium, seizures and coma
The toxic phase of yellow fever can be fatal.
YELLOW FEVER DIAGNOSIS
A presumptive diagnosis of yellow fever is often based on the patient’s clinical features, places and dates of travel (if the patient is from a non-endemic country or area), activities, and epidemiologic history of the location where the presumed infection occurred.
Laboratory diagnosis of yellow fever is generally accomplished by testing of serum to detect virus-specific IgM and neutralizing antibodies. Sometimes the virus can be found in blood samples taken early in the illness.
YELLOW FEVER TREATMENT
No antiviral medications have proved helpful in treating yellow fever. As a result, treatment consists primarily of supportive care in a hospital. This includes providing fluids and oxygen, maintaining adequate blood pressure, replacing blood loss, providing dialysis for kidney failure, and treating any other infections that develop. Some people receive transfusions of plasma to replace blood proteins that improve clotting.
If you have yellow fever, your doctor will likely recommend that you stay inside, away from mosquitoes, to avoid transmitting the disease to others. Once you’ve have yellow fever, you’ll be immune to the disease for the rest of your life.