Hepatitis A

Hepatitis A is an infectious disease that specifically targets the liver. It is caused by the Hepatitis A virus, often called HAV. This virus belongs to a group of five known hepatitis viruses, which include types B, C, D, and E. While the infection can be serious, many people experience few or no symptoms. This is especially true for children, as about 90% of them do not show signs of being sick. However, even if a child does not feel ill, they can still spread the virus to others.

The virus spreads primarily through the fecal-oral route. This happens when a person eats food or drinks water contaminated with infected feces. Undercooked or raw shellfish are common sources of infection. HAV can also spread through close personal contact or through blood in very rare cases. Once the virus is swallowed, it enters the bloodstream through the lining of the intestines or the throat. The blood then carries the virus to the liver.

Inside the liver, the virus begins a complex replication cycle. It enters liver cells, known as hepatocytes, and certain immune cells called Kupffer cells. The virus uses a process called endocytosis to enter these cells. Once inside, the virus releases its genetic material, which is a single strand of RNA. The cell's own machinery, specifically the ribosomes, then translates this RNA to make more viruses. The new virus particles eventually exit the host cell through a process called lysis. They are then secreted into the bile and released through the stool.

Symptoms of the disease typically appear during the incubation period. This is the time between exposure and the start of illness, which usually lasts two to six weeks. When symptoms do occur, they often last for about eight weeks. Common signs include fever, nausea, vomiting, diarrhea, and abdominal pain. Many people also experience jaundice, which is a yellowing of the skin or eyes. This happens because of hyperbilirubinemia, a condition where bile is not removed correctly from the blood. Instead, it is excreted in the urine, making it a dark amber color.

Scientists have used phylogenetic analysis to study the history of the virus. This research suggests that the human Hepatitis A virus may have originated in rodents. There are several different genotypes of the virus. Human cases are mostly caused by genotype I, specifically the IA subtype. Other versions of the virus are found in animals like bats, shrews, and hedgehogs. For example, a virus found in seals shared a common ancestor with human HAV about 1,800 years ago. Some human strains appear to have separated from simian, or monkey, strains about 3,600 years ago.

The impact of Hepatitis A is significant on a global scale. There are approximately 114 million total infections every year. Of these, about 1.4 million cases involve people showing symptoms. In developing regions with poor sanitation, the virus is very common. In these areas, about 90% of children are infected by age 10. In 2015, acute hepatitis A was responsible for 11,200 deaths.

Prevention is possible through vaccination and good hygiene. A vaccine developed by Maurice Hilleman was licensed in 1995. This vaccine is highly effective, protecting people in more than 95% of cases for over 25 years. Other ways to prevent the disease include frequent hand washing and cooking food thoroughly. Once a person has recovered from a Hepatitis A infection, they gain immunity for the rest of their life. Doctors can diagnose the infection by testing for specific IgM antibodies in the blood. They also look for high levels of the enzyme alanine transferase, which indicates liver cell damage.