Yellow Fever: Mode of Transmission, Causes and Symptoms

A certain species of mosquito can bite a person and transmit the virus that causes yellow fever. The tropics and subtropics of Africa and South America are home to these mosquitoes and yellow fever. When mosquitoes bite primates carrying the virus get infected.

The symptoms of the yellow fever virus might vary. Some individuals could be symptom-free. When it's at its worst, it can be fatal but only exhibit minor flu-like symptoms. You can experience symptoms similar to the flu, including aches, pains, and fever, or you might start bleeding and get liver damage. About three to six days pass before symptoms appear.

Read this blog further to get a quick overview of this important topic for PREVENTIVE AND SOCIAL MEDICINE and ace your NEET PG exam preparation.

Mode of Transmission of Yellow Fever

The mode of transmission of yellow fever is via 3 cycles:

1. The Sylvatic cycle also known as the Jungle cycle
2. The intermediate cycle 
3. Urban cycle

• Non-human primates (such as monkeys) and mosquito species found in the forest canopy help spread the virus via the sylvatic) cycle which is also known as the jungle cycle. The disease can be transmitted to humans by the mosquitoes of monkeys when people visit or work in the jungle.
• People who live or work close to rainforest borders in Africa are at risk of contracting a virus from mosquitoes via the intermediate cycle which is also known as the savannah cycle. The virus can be transmitted in this cycle by mosquitoes from monkeys to humans or from humans to monkeys.
• In the urban cycle, the virus is spread from individuals to urban mosquitoes, especially Aedes aegypti. The virus usually enters the urban environment through a human who acquired it in the savannah or jungle.

Favorable Conditions for Transmission

• If the population is unvaccinated.
• If Vector aedes aegypti found.
• If Climate conditions are favorable
• If Reservoir monkeys are susceptible.

Causes of Yellow Fever

Yellow fever is caused by a flavivirus. It is spread through the bite of an Aedes aegypti mosquito. The mosquito spreads the disease after biting an infected person or monkey. For the rest of its life, an infected mosquito will continue to spread infection. It has a Hunchback appearance with white stripes on the black body.

                                      AEDES AEGYPTI

Symptoms of Yellow Fever

Yellowish discoloration of the skin and fever are two of yellow fever's most obvious symptoms. Hepatitis, the illness that causes the yellowing, damages the liver. Initial yellow fever symptoms might occur anywhere between three and six days after a mosquito bite whereas some people experience no symptoms at all.

Yellow fever infections often have three stages. Most people experience the first phase of symptoms for three to four days and then these symptoms. The initial stage is typically non-specific and difficult to separate from other viral infections.

Yellow fever's earliest signs and symptoms include:

• Chills and a fever
• Symptoms that resemble the flu, such as headaches, nausea, and muscular aches.
• Remission follows, and it lasts for 48 hours. Patients become better. Most people get better.

Unfortunately, 15% to 25% of patients experience a third, more dangerous phase of illness. Viral hemorrhagic fever, which has internal bleeding (hemorrhaging), a high fever, and harm to the liver, kidneys, and circulatory system, may ultimately manifest. According to estimates from the World Health Organisation, half of all individuals who experience this severe stage of infection survive.

The following are examples of yellow fever third-phase symptoms:

• Jaundice (liver disease), which results in the skin and eyes turning yellow
• Hepatitis (liver inflammation)
• Hemorrhaging (bleeding) internally
• Blood in the vomit Shock
• Failure of multiple organ systems resulting in death

Diagnosis of Yellow Fever

You should consult a doctor right once if you get any symptoms after visiting a region where yellow fever is prevalent. Your doctor will order tests to see if the virus is in your blood in addition to interviewing you about your symptoms and the areas you've been.

Treatment of Yellow Fever

Yellow fever cannot be treated with antiviral drugs instead, supportive care is provided in a hospital. This includes giving fluids and oxygen, monitoring blood pressure, replenishing lost blood, starting kidney dialysis if there is kidney failure, and treating any secondary infections. 

To replenish the proteins that help in clotting in some individuals, plasma transfusions may be administered. Mosquitoes should not be near the sufferer. The patient will get the illness through a mosquito bite and then spread it to others.

Due to the possibility of bleeding, they should avoid using aspirin and non-steroidal anti-inflammatory medicines (NSAIDs).

Vaccine for Prevention of Yellow Fever

17D vaccine- strain is used. It is stored at a temperature of  +5 to -30⁰C. Yellow fever vaccine cannot be stored under cold chain temperatures in India.

The type of vaccine is the Reconstituted vaccine. It should be kept on ice away from light. The cold physiological saline diluent is added to the vaccine. It does not follow an open vial policy and It should be used within half an hour.

These vaccines are given via the Subcutaneous route with a Dose of 0.5ml at the insertion of the deltoid. Immunity starts after 7-10 days and lasts lifelong. Never give the yellow fever vaccine and cholera vaccine together. There has to be a gap of 3 weeks.

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Vector control

Vector control is done via the Aedes aegypti index (AAI) which is calculated as the Number of Aedes mosquitoes found within 400 mts of an area. It is also known as the house index which includes the number of houses showing the breeding of Aedes aegypti. The seaports and airports must be kept free from breeding Aedes aegypti up to 400 meters. The AAI is kept as <1(0r zero) for all international connection airports and seaports.

Quarantine: A traveler without a valid international certificate of vaccination is placed in quarantine in a mosquito-proof ward for 6 days.

Prevention of Yellow Fever

If you plan to go to any areas where yellow fever is known to exist, obtain the vaccine around three to four weeks before your departure. A single dose usually offers lifetime protection. Other advice is:

• Use a DEET-containing insect repellent.
• Put on socks, long trousers and long-sleeved shirts. Wearing clothing that has been treated to repel mosquitoes is recommended.
• If the lodge doesn't have air conditioning or window screens, think about utilizing a bed net.
• At times when mosquitoes are active, stay inside. However, one of the mosquitoes that transmit yellow fever eats throughout the day, so typically this implies sunset to morning.

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