Yellow Fever Virus: Infection, Transmission & Diseases

The yellow fever virus belongs to the so-called Flavi viruses and triggers the life-threatening infectious disease yellow fever. This is transmitted by mosquitoes of the genus Aedes (Africa) and Haemagogus (South America). It occurs in the tropical regions of Africa and South America. Depending on the severity of the disease, infection with yellow fever virus can be fatal.

What is yellow fever virus?

Yellow fever virus belongs to the genus Flavi virus. It is transmitted through the bite of the yellow fever mosquito. Both humans and monkeys can serve as hosts for the virus. For many monkey species, especially those living in Africa, infection is harmless, but for humans it can have fatal consequences. Direct transmission of the yellow fever virus from one human to another is not possible. Only the yellow fever mosquito can carry it from host to host and thus, in the worst case, trigger an epidemic. The name yellow fever comes from the disease’s ability to raise the patient’s body temperature to a fever. In addition, the virus causes liver failure, which can lead to jaundice. Because of the bleeding that begins throughout the body, yellow fever is one of the hemorrhagic fevers.

Occurrence, distribution, and characteristics

Yellow fever virus is the causative agent of yellow fever. It is transmitted to humans through the bite of an infected mosquito (Egyptian tiger mosquito). The disease spreads permanently only in certain regions, which are therefore called yellow fever endemic areas. These are found in South America and tropical Africa. Europe, Asia, Australia and Oceania are currently considered yellow fever-free areas. For every 200,000 yellow fever infections per year, there are approximately 30,000 deaths, of which about 90% occur in Africa. The WHO also assumes a high number of unreported cases, although every death resulting from yellow fever is reportable. There are two forms of yellow fever: urban yellow fever and jungle yellow fever, depending on where the infection occurs. The animals in which the viruses normally reproduce are monkeys that live in the jungle. The pathogens are transmitted there from one monkey to another by mosquitoes. If humans stay in the jungle, they also run the risk of becoming infected via the mosquitoes. The disease is called jungle yellow fever because of its prevalence and affects mostly young men, such as forest workers. In the case of urban yellow fever, on the other hand, a sick person becomes a source of danger for other people. If he or she is bitten by mosquitoes, there is a risk of an epidemic. From person to person, yellow fever is then spread across a particular region.

Diseases and symptoms

If the virus enters the body, it first replicates through the lymph nodes, gradually spreading throughout the body. In addition to the main target organ, the liver, it reaches other organs such as the spleen, kidneys, muscles, and bone marrow. In order to defend itself against the virus, various messenger substances are produced in the body. However, this production and release may occur in an uncontrolled manner, causing severe damage to the body and multi-organ failure. Symptoms of yellow fever develop with an incubation period of three to six days. In about 85% of cases, the disease takes a mild course with symptoms similar to those of influenza. These include chills, fever up to 40°C, aching limbs, muscle aches, headache, vomiting and nausea. Recovery occurs after only a few days. The remaining 15% of cases are characterized by a very severe course. Kidney and/or liver failure is a common symptom. The further course is often characterized by multiple organ failure with bleeding throughout the body. The severe disease progresses in two phases. The first phase is similar to the mild form of the disease, but in addition to the symptoms, other manifestations develop, such as diarrhea, vomiting of bile, severe thirst, overheated skin, bad breath, jaundice, bleeding from the palate, and a drying up of urine production. In the following 1-2 days, the patient undergoes a resting interval before the second phase breaks out.In addition to liver and kidney failure, this is also characterized by bloody diarrhea, bleeding from the skin and mucous membranes, high blood and fluid loss with the onset of shock and neurological disorders. In the worst case, death subsequently results from renal failure, circulatory arrest, and cardiac failure. Mortality in people suffering from the severe form of the disease is 50-60%. Infection with the yellow fever virus is not a mandatory death sentence. 85% of those affected contract the mild form of the disease and recover within a few days. Of the 15% who suffer from the severe form, about half survive. Medical science assumes that surviving patients form antibodies and are immune to yellow fever from that point on. There is no specific treatment for yellow fever and in some countries in Africa and South America the risk of infection is relatively high. Therefore, anyone traveling there should be sure to get vaccinated, a measure that is mandatory in some countries. In addition to vaccination, other measures should be taken to protect against mosquitoes. Since the yellow fever mosquito is active at night and during the day, constant protection with special mosquito repellents and mosquito nets is necessary. These prevent not only yellow fever but also other tropical diseases such as malaria and dengue fever.