Yellow fever: Description
Yellow fever is caused by the yellow fever virus. It is transmitted to humans through the bite of infected mosquitoes. The disease only occurs permanently in certain regions of the world. These are known as yellow fever endemic areas. They are located in (sub-)tropical Africa and South America. Travelers to these destinations should find out in advance whether a vaccination against yellow fever is mandatory. Asia, Australia, Oceania, North America and Europe are currently considered yellow fever-free.
Tropical medicine experts estimate that there are around 200,000 cases of yellow fever and up to 60,000 deaths worldwide every year. Around 90 percent of these are in Africa. Every suspected case, every illness and every death caused by yellow fever must be reported. Nevertheless, the World Health Organization (WHO) estimates that there is a high number of unreported cases. This means that even more people may contract yellow fever, but these cases are either not reported or not recognized as such.
There are two forms of yellow fever: jungle yellow fever and urban yellow fever. The name depends on where and from whom you contract the disease.
Jungle yellow fever
City yellow fever
In contrast to this is urban yellow fever. In this case, a person suffering from yellow fever spends time with other people. If the carrier mosquitoes are still present, they can transmit the yellow fever virus from the sick person to other people. Direct infection from person to person is not possible (or only theoretically via direct blood contact, for example during blood transfusions).
Yellow fever: symptoms
Some infected people do not develop any symptoms at all. In this case, doctors speak of an asymptomatic course.
In other cases, the first symptoms of yellow fever appear around three to six days after infection (incubation period). The disease usually takes a mild course, which is similar to a flu-like infection. However, some patients also fall seriously ill with yellow fever – sometimes with a fatal outcome.
Yellow fever: mild course
Around 85 percent of those who contract yellow fever develop flu-like symptoms such as
- Fever up to 40 °C
- chills
- headache
- aching limbs
- muscle pain
- nausea
- vomiting
Yellow fever: Severe course
In around 15 percent of yellow fever patients, the disease takes a severe course, sometimes after a temporary slight improvement in the symptoms of the initial phase. This leads to the toxic phase of the disease. In addition to the symptoms of a mild course, the following yellow fever symptoms can develop:
- Vomiting of bile
- diarrhea
- severe thirst and overheated skin on the face and trunk (“red stage”)
- unpleasant bad breath
- Mild jaundice (icterus)
- Decreasing urine production
- bleeding on the palate
In very severe yellow fever, the main symptoms are bleeding and damage to the liver and kidneys (“yellow stage”). The following symptoms may occur:
- coffee grounds-like vomiting (hematemesis), tarry stools (melena) or bloody diarrhea
- Bleeding of the skin and mucous membranes
- Yellowing of the skin (icterus) due to acute liver failure
- Acute kidney failure with greatly reduced or absent urine production (oliguria, anuria)
- slowed heartbeat (bradycardia) – relative bradycardia in combination with fever is called Faget’s sign
- Neurological abnormalities such as speech disorders, apathy, convulsions and movement disorders
- Shock due to high blood and fluid loss (through bleeding, vomiting, diarrhea), characterized by low blood pressure
Due to the various organ hemorrhages in severe yellow fever, the disease is classified as a hemorrhagic fever (like dengue, Ebola, Lassa fever, etc.). Around half of people with this severe form of yellow fever die.
Yellow fever: causes and risk factors
A host is an organism whose cells the virus needs in order to multiply. Both humans and monkeys serve as hosts for the yellow fever virus. Monkeys are the natural reservoir of the virus. For many monkey species, especially the African ones, infection with the yellow fever virus is harmless. Only when a mosquito picks up the virus during a blood meal from a monkey and then bites a human does the virus reach the latter (sylvatic or jungle cycle).
If a person is infected, mosquitoes can pick up the virus from them and infect other people (urban or city cycle). This can trigger epidemics.
Spread of the yellow fever virus in the body
When the yellow fever virus enters the bloodstream via a mosquito bite, it first multiplies in the lymph nodes. It then spreads throughout the body via the lymph and blood. An important organ for the multiplication of the yellow fever virus is the liver, which can be particularly damaged by the disease. This also explains the frequently occurring yellowing of the skin and eyes (icterus). The virus also reaches various other organs such as the kidneys, spleen, bone marrow and muscles. Many organs can be damaged to such an extent that they can no longer function (properly). Doctors then speak of multi-organ failure, which can be life-threatening or even fatal.
Yellow fever: examinations and diagnosis
The travel history (travel history), fever, bleeding and a yellow coloration of the skin point the way to a diagnosis of yellow fever. If your doctor suspects yellow fever, he or she will ask you the following questions, among others, when taking your medical history:
- When exactly were you there?
- What did you do there?
- Are you in pain?
- Do you have a fever?
- Are your stools black in color?
- How long have you had the symptoms?
The interview is followed by a physical examination. For example, he will palpate your abdomen to determine whether your liver and spleen are enlarged. He will also measure your temperature and blood pressure. He will also take blood samples and have them examined in the laboratory. In the case of yellow fever, typical changes such as increased liver values, an accumulation of toxic metabolic products and possibly a coagulation disorder will be detected. Urine tests can also show kidney damage, for example excessive protein excretion (albuminuria).
Detection of yellow fever infection
After the first two to five days of the illness, the genetic material of the yellow fever virus (RNA virus) can be detected in the blood using a polymerase chain reaction (PCR). From around the fifth to seventh day of the illness, the patient has formed specific antibodies against the yellow fever virus. These can also be visualized in the blood (serological test).
Yellow fever: treatment
There is currently no specific treatment for yellow fever – there are no drugs or other therapies that can directly combat the yellow fever virus. The disease can therefore only be treated symptomatically. This means that only the signs of the disease can be alleviated.
A therapy with interferon alpha is currently being researched. It is showing initial success in infected monkeys.
Symptomatic treatment
Patients must be cared for in an intensive care unit, especially if the disease is severe. In a yellow fever endemic area where the Egyptian tiger mosquito is present, the patient must be isolated. In this quarantine, they cannot be bitten by mosquitoes so that they cannot transmit the virus to other people.
Yellow fever: vaccination
You can find out how to prevent yellow fever with a vaccination in the article Yellow fever vaccination.
Yellow fever: course of the disease and prognosis
If symptoms occur at all after infection, yellow fever is mild in most cases (85%) and is overcome after a few days. Of the approximately 15 percent of patients who fall seriously ill with yellow fever, around one in two dies – even if maximum intensive medical care is provided. Measured against all yellow fever infections, this means that around ten to 20 percent of those affected die.
Once you have survived a yellow fever infection, you are probably immune to yellow fever for life due to the antibodies you have developed, experts believe.
Preventing yellow fever
As there is no specific treatment and yellow fever is potentially life-threatening, vaccination is very important. Some African and South American countries make vaccination mandatory on entry and exit (and possibly transit). An epidemic spread can only be prevented if the majority (60 to 90 percent) of people in an area are vaccinated against yellow fever.
