This lesson will describe the structure and function of the virus that causes hepatitis A. After the lesson, there will be a few questions to test what you have learned.
What Is Hepatitis A Virus?
Viral hepatitis is the most common cause of hepatitis worldwide. Hepatitis A virus is very common in areas of the world with poor sanitation.
The disease is easily transmitted to others through blood, the oral-fecal route, anal sex, and contaminated food and water. The virus travels through the bloodstream to the liver where it infects hepatocytes, cells that form the liver’s main tissue and make up most of its mass.Like all viruses, the Hepatitis A virus is simple, made up of only nucleic acid and proteins, but it is still able to take over normal cell functions and convert them to make more viruses. This lesson will cover the structure, replication, and transmission of the virus that causes hepatitis A.
Viruses contain only what’s necessary to allow them to hijack the cells they infect.
The nucleocapsid, the core structure of all viruses, contains protein and nucleic acid. The protein portion of the nucleocapsid, called the capsid, coats and protects the virus genome. In some cases, the capsid is also involved in viral entry. Other viruses contain an envelope that aids in cell entry, but also protects the virus from the host immune system. The nucleic acid contains the genes of the virus and can be RNA or DNA. The genome only has the genes needed to shut down the host cell and make more viruses.
These features of Hepatitis A Virus (HAV) are described below.
Viral hepatitis is caused by a variety of viruses; each coming from different viral families. HAV differs from other hepatitis strains because it’s a member of the picornavirus family. Picornaviruses are small, non-enveloped, icosahedron-shaped viruses containing a single strand of RNA.
An icosahedron is a 3-dimensional figure with 20 faces resembling a soccer ball. The viral structure is further described below.
The capsid of HAV is made up of subunits called capsomeres as shown in Figure 2.
Each capsomere is made up of five protomers. Each protomer of HAV is made of three proteins; VP1, VP2, and VP3, which have a role in cell entry.
The genome of HAV is positive-sense, single-stranded RNA, written as ss(+)RNA.
Because the virus is positive-sense RNA, it can be used as mRNA and converted (translated) into protein upon entering the cell. The genome has an attached protein VPg that acts as a primer for copying the genome (replication).As shown in Figure 3, the genome is divided into three segments: P1, P2, and P3. The first segment encodes genes to make capsid proteins.
The other two segments make other genes needed for replication.
- Entry: The first step of viral replication is getting into the cell.
VP1 and VP3, found on the capsid, are involved in binding to receptors on the host cell’s plasma membrane. VP2 causes the virus to be engulfed by the membrane to enter the cell through endocytosis.
- Translation: Once the genome is released into the cytoplasm, the genome is translated into a large polyprotein. The genome can be translated into protein because it is an ss(+)RNA virus.
- Protein Production: The polyprotein is cleaved to make the capsid proteins, and other non-structural proteins.
- Genome Replication: The RNA-dependent RNA-polymerase (RdRp) creates a double-stranded RNA from the genome, which consists of both positive-sense and negative-sense RNA. The positive-sense strands serve to make more protein. The negative-sense strands, written as ss(-)RNA, are complementary to the positive-sense strands and serve to make more copies of the genome to be packaged.
- Viral Assembly: The capsid is assembled, and the genome is packaged.
- Viral Release: The virus is released through cell lysis, which disrupts the host cell’s membrane.
Viral Transmission and Symptoms:
The Hepatitis A Virus can be transmitted in the blood and feces of infected individuals.
If transmitted through the oral-fecal route, it is carried in the blood stream to the liver where it causes inflammation. Hepatitis A infections are common in areas with poor sanitation. People who are at risk of exposure include those who come into contact with contaminated food, healthcare workers, daycare workers, people who have unprotected anal sex, and IV drug users.The incubation period is between 15-50 days. During this time period, the infected person shows no symptoms but can transmit the disease. Children infected with the virus may not show signs of illness, but infected adults develop symptoms typically associated with hepatitis:
- Loss of Appetite
- Jaundice (yellowing of eyes and skin)
- Dark urine due to the excretion of bile
- Clay colored feces
There is no treatment or cure for HAV.
Most healthy individuals recover from the disease and develop a lifetime immunity. The elderly, immunocompromised, and people who have other Hepatitis (B, C, D, or E) viral infections have an increased chance of developing serious symptoms. The disease can be prevented by vaccination and improved sanitation.
Hepatitis A is caused by a virus in the picornavirus family. The viral capsid is non-enveloped and icosohedral-shaped containing a ss(+)RNA genome. The virus enters through the blood stream or by ingestion of feces-contaminated food or water and infects cells of the liver.
HAV cannot be cured, but it can be prevented by vaccination.