In this lesson, we'll discuss the parts of the yellow fever virus' structure and what functions those parts perform during infection and replication. We'll also learn about symptoms and treatment of the disease that the yellow fever virus causes.
What Is the Yellow Fever Virus?
Imagine coming down with the flu. You get a fever and your body starts to ache. You might feel nauseous or have spells of vomiting. Your head hurts and you shiver and curl up under a blanket, even though your face is covered in cold sweat. Normally, you consume soup or tea or both, perhaps some pain reliever, and recover in a few days. Your immune system fights off the flu virus and you're back to your old healthy self.
However, this isn't always how the situation plays out. Some viruses are far more deadly than those that cause the flu. The yellow fever virus causes acute hemorrhagic disease in which your fever worsens and your internal organs start to bleed, despite the initial symptoms resembling those of the flu.
Risk areas for yellow fever
Yellow fever virus is transmitted through mosquito bites and is mostly prevalent in Africa, although some cases have been recorded in tropical South America. Today, we'll examine the parts of the yellow fever virus' structure and their functions in causing these life-threatening symptoms.
Yellow Fever Virus Structure
Although they are so small they cannot be seen through the lenses of an optical microscope, viruses can be quite deadly. All viruses have a similar structure; all viruses have a genome and a nucleocapsid, and some have an envelope, too.
The nucleocapsid is a protein coat that protects the genome of the virus. A genome is the genetic material of the virus. Some virus' genomes are composed of DNA, while others are made of RNA. Yellow fever virus is a single-stranded RNA virus, meaning only one strand of RNA comprises its genome. Like those of some other viruses, yellow fever virus' envelope is a flexible coat made of lipids, or fats, that cover the nucleocapsid.
Structure of an enveloped virus
Yellow fever virus is a type of flavivirus, a small, enveloped virus whose diameter ranges in size from 40 to 60 nm. That means flaviviruses are 40,000,000 times smaller than a centimeter, which is about the width of a small screw.
Yellow fever virus particles under a transmission electron microscope
Yellow fever virus has three main types of proteins: E, M, and C proteins. The function of the virus' E proteins is to attach the virus to receptors on host cells; they initiate the biggest immune response from the host. The M proteins appear to keep the E proteins functional during the assembly of new viruses, but M proteins have been studied less than E proteins. C proteins are found in the virus' nucleocapsid.
All flaviviruses have a positive-sense RNA genome, meaning the RNA is in the correct order for the host cell to copy directly to produce protein. It doesn't need to be copied into other forms to be used as instructions for protein.
Infection Route and Replication
Viruses are specific to their hosts. Not all viruses infect all organisms. Yellow fever virus specifically infects humans, particularly liver cells, or hepatocytes. Once yellow fever viral particles reach the liver through the bloodstream, their E proteins attach to the hepatocytes. A hepatocyte brings the virus inside it in a sphere called an endosome, or membrane-bound vesicle. The virus then breaks out of the endosome by fusing its membrane with the endosomal membrane. The virus' RNA genome is then freed into the cell, where host cell machinery copies it to produce protein, creating more viruses.
You might be wondering why a host cell would make more of the thing that is killing it. The virus is clever and tricks the host cell into thinking that the viral RNA is host cell RNA. The cell unknowingly copies the viral RNA in the rough endoplasmic reticulum (ER), a structure made of membranes that produce proteins. From the ER, new viruses bud out, cloaking themselves in the cell's ER membrane. Soon, the host cell contains so many viruses that the host cell dies and the viruses escape to infect other host cells.
Since the viruses look a lot like the host, their camouflage improves. When the new viruses leave the cell, only the E and M proteins in their membranes tip off the host that the viruses aren't part of itself. The virus particles circulate throughout the host, looking for new cells to infect.
Infection of a liver cell with yellow fever
Symptoms and Treatments
The symptoms of yellow fever start out looking like the flu, as described earlier in the lesson. Most patients recover from this phase, but some enter a second phase with more serious consequences. In the second phase, which occurs about a day after the initial symptoms disappear, liver cells die and the resulting liver failure results in jaundice. Jaundice is a yellowish color in the skin and eyes due to a change in the skin's normal color.
The whites of the eyes become yellow during jaundice
Yellow fever is a virus that causes hemorrhagic disease, or one that results in bleeding. Soon after the appearance of jaundice, internal bleeding occurs and kidney function deteriorates, causing patients to begin bleeding through the nose and mouth. About 50% of patients recover from this stage, while the other 50% die within 10 to 14 days. Unfortunately, no treatment exists for yellow fever. People traveling to Africa or South America are strongly advised to get the yellow fever vaccine, which can prevent infection by the yellow fever virus.
In summary, yellow fever virus causes acute hemorrhagic disease in which your fever worsens and your internal organs start to bleed, despite the initial symptoms resembling those of the flu. It's a flavivirus - which is a small, enveloped virus whose diameter ranges in size from 40 to 60 nm - with a positive-sense single-stranded RNA genome, which means the RNA is in the correct order for the host cell to copy directly to produce protein.
A nucleocapsid made of C proteins surrounds the RNA genome, and a lipid envelope studded with E and M proteins, which help the E proteins, covers the nucleocapsid. E proteins are used for attachment to host cells. The viruses then enter the host cells by becoming encapsulated in an endosome, or membrane-bound vesicle. Next, the viruses release most of their RNA genomes into the cell. The cell unknowingly copies the virus' RNA, making more viral particles. The host cell fills with so many viruses that it dies, and the new viruses are released. Because the new viral particles are cloaked in the host cell's endoplasmic reticulum (ER) membrane, which is a structure made of membranes that produce proteins, they're camouflaged from the host's immune system.
The virus infects liver cells, resulting in jaundice, and eventually causes internal bleeding and kidney failure. Although there is no treatment, travelers can be given a yellow fever vaccine to prevent infection.
Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.