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Yellow Fever


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Yellow fever is one of the most dangerous infections. It occurs in the regions of tropical Africa and Latin America. It is transmitted by the female mosquito Aedese aegypti and some other species. This viral fever is also known as the Yellow Jack. Let us learn more about this acute viral hemorrhagic infection in the following paragraphs.


What is Yellow Fever?

Yellow fever is a disease transmitted by mosquitos that can lead to death. The disease often leads to a toxic phase where the liver gets damaged and leads to jaundice. Thus, this classic symptom is the reason the disease is called as yellow fever. 

This viral disease can spread as a sudden epidemic. This has prompted many countries to make it compulsory for people to get vaccinated before they travel in high-risk regions.


Transmission of Yellow Fever

Yellow fever is transmitted by the bite of Aedes aegypti virus. Other mosquitoes too, like the Aedes albopictus can transmit the virus to humans.The virus is ingested by the mosquito after a blood meal from an infected person or primate. Monkeys, too are hosts for the Arbovirus that causes yellow fever. The ingested viruses are in high concentration, reach the mosquito's stomach and starts infecting the epithelial cells to replicate.The virus then reaches the blood system of the mosquito and finally the salivary glands. The next time the mosquito bites another person, the saliva containing the virus is transmitted to the bloodstream. Soon, the virus replicates within the host body, leading to symptoms of yellow fever.


Classification of Yellow Fever

Based on the infectious cycle, yellow fever has been classified into three types.

These are as follows:

Urban Cycle

The urban cycle is the one where the virus is transmitted only by the the Aedes aegypti mosquito to humans or other primates. This cycle is no longer found in South America. 

Jungle Cycle

The jungle cycle is also called as a sylvatic or forest cycle. The mosquitos usually affect the non-human primates. In Africa, the disease is asymptomatic in the affected primates. In South Africa, the disease can affect humans only when they get infected in the jungle and pass it on to humans through urban cycle. The main reason for the difficulty to eradicate yellow fever, is the jungle cycle.

Savannah Cycle

The third cycle called the Savannah cycle is seen in Africa. It is between the jungle and urban cycle and thus also called the intermediate cycle. This is one of the most common reasons for yellow fever transmissions in Africa. There are different mosquitoes of the genus Aedes that lead to transmissions of the virus in healthy humans.


Causes of Yellow Fever

The main cause of yellow fever is the virus belonging to the Flaviviridae family. The virus enters through the blood through the bite of the female Aedes aegypti mosquito. It can also be passed on by other mosquito species. The infection can also be passed by the mosquito to the eggs, through eggs to the larvae and finally the mature female mosquito passes the virus to primates or humans after a blood meal.


Symptoms of Yellow Fever

Yellow fever symptoms begin to appear after an incubation period of 3 to 6 days. Most of the cases of yellow fever the symptoms are mild and the phase is called as acute phase.

The symptoms in this phase include: 

  • Body ache
  • Chills
  • Loss of appetite

These symptoms last for about 3 to 4 days. In about 15% of the cases, the disease enters the toxic phase. The symptoms in this case are more severe and may be fatal.

The symptoms include: 

  • Yellowing of the skin and eyes due to liver damage, that is, jaundice
  • Bleeding from the gastrointestinal tract
  • Bleeding from the mouth, nose, eyes
  • Delirium, seizures or coma due to brain dysfunction 
In some cases, the toxic phase proves to be fatal. Those who survive yellow fever, normally do not develop any permanent organ damage and have lifelong immunity to the disease.


Diagnosis of Yellow Fever

During the early phase of yellow fever, it is very difficult to diagnose the disease. It is because the disease is quite similar to other diseases such as typhoid, malaria, dengue, etc. The diagnosis is usually made by taking a medical history. The patient will likely be asked if he/she had travelled in the past few weeks. A blood sample will be taken and tests such as ELISA or PCR will be conducted. These tests will help detect presence of antigens and antigens that are specific to the virus.


Treatment of Yellow Fever

There is no specific treatment or antiviral drug for yellow fever. Treatment given is specific to the symptoms exhibited. These include maintenance of blood pressure, providing oxygen and fluids, dialysis in case of kidney failure, etc.


Vaccination for Yellow Fever

The only way to prevent yellow fever is vaccination. Those living or travelling to Africa or South American regions that come under high-risk for yellow fever, are advised vaccination. The vaccine is given about 10 to 15 days before one travels to these regions. The vaccine provides immunity against yellow fever for about 10 years. Some who get vaccinated may develop low-grade fever, soreness at site of injection as well as fatigue as an effect of the vaccine. Infants under the age of 9 months and older patients over the age of 6 years should proceed with caution before they get vaccinated. 

When one travels to sub-Saharan Africa or regions of South America, they should make sure they are vaccinated. Also, one should protect themselves from mosquitoes, by using a mosquito repellent; wear long sleeved shirts and long pants. One should restrict outdoor activity, when the mosquitoes are most active, that is after sunset. Application of skin repellent containg DEET is the most effective way to repel mosquitoes.

Yellow fever tends to be more severe in non-native people, that is, travelers from other countries. Make sure you follow all the rules laid down by WHO to prevent yellow fever. For further details, speak to your healthcare provider.

Written by: healthplus24.com team
Date last updated: November 26, 2013

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