Hepatitis A is an acute infectious disease of the liver caused by the hepatitis A virus.
Many cases have little or no symptoms especially in the young. The time between infection and
symptoms, in those who develop them, is between two and six weeks. When there are symptoms
they typically last eight weeks and may include: nausea, vomiting, diarrhea, yellow skin, fever,
and abdominal pain. Around 10–15% of people experience a recurrence of symptoms during
the six months after the initial infection. Acute liver failure may rarely occur with
this being more common in the elderly. It is usually spread by eating or drinking
food or water contaminated with infected feces. Shellfish which have not been sufficiently
cooked is a relatively common source. It may also be spread through close contact with
an infectious person. While children often do not have symptoms when infected they are
still able to infect others. After a single infection a person is immune for the rest
of their life. Diagnosis requires blood testing as the symptoms are similar to those of a
number of other diseases. The hepatitis A vaccine is effective for prevention.
Some countries recommend it routinely for children and those at higher risk who have
not previously been vaccinated. It appears to be effective for life. Other preventative
measures include hand washing and properly cooking food. There is no specific treatment,
with rest and medications for nausea or diarrhea recommended on an as needed basis. Infections
usually resolve completely and without ongoing liver disease. Treatment of acute liver failure,
if it occurs, is with liver transplantion. Globally around 1.5 million symptomatic cases
occur each year with likely tens of millions of infections in all. It is more common in
regions of the world with poor sanitation and not enough safe water. In the developing
world about 90% of children have been infected by age 10 and thus are immune by adulthood.
It often occurs in outbreaks in moderately developed countries where children are not
exposed when young and there is not widespread vaccination. In 2010, acute hepatitis A resulted
in 102,000 deaths. World Hepatitis Day occurs each year on July 28 to bring awareness to
viral hepatitis.
Signs and symptoms Early symptoms of hepatitis A infection can
be mistaken for influenza, but some sufferers, especially children, exhibit no symptoms at
all. Symptoms typically appear 2 to 6 weeks after the initial infection. 90% of children
do not have symptoms. The time between infection and symptoms, in those who develop them, is
between two and six weeks with an average of 28 days.
The risk for symptomatic infection is directly related to age, with more than 80% of adults
having symptoms compatible with acute viral hepatitis and the majority of children having
either asymptomatic or unrecognized infections. Symptoms usually last less than 2 months,
although some people can be ill for as long as 6 months:
Fatigue Fever
Nausea Appetite loss
Jaundice, a yellowing of the skin or whites of the eyes due to hyperbilirubinemia
Bile is removed from blood stream and excreted in urine, giving it a dark amber colour
Diarrhea Clay-coloured feces
Virology
Following ingestion, HAV enters the bloodstream through the epithelium of the oropharynx or
intestine. The blood carries the virus to its target, the liver, where it multiplies
within hepatocytes and Kupffer cells. Virions are secreted into the bile and released in
stool. HAV is excreted in large quantities approximately 11 days prior to appearance
of symptoms or anti-HAV IgM antibodies in the blood. The incubation period is 15–50
days and mortality is less than 0.5%. Within the liver hepatocytes the RNA genome is released
from the protein coat and is translated by the cell's own ribosomes. Unlike other picornaviruses
this virus requires an intact eukaryote initiating factor 4G for the initiation of translation.
The requirement for this factor results in an inability to shut down host protein synthesis
unlike other picornaviruses. The virus must then inefficiently compete for the cellular
translational machinery which may explain its poor growth in cell culture. Presumably
for this reason the virus has strategically adopted a naturally highly deoptimized codon
usage with respect to that of its cellular host. Precisely how this strategy works is
not quite clear yet. There is no apparent virus-mediated cytotoxicity
presumably because of the virus' own requirement for an intact eIF4G and liver pathology is
likely immune-mediated. Structure
The hepatitis A virus is a picornavirus; it is non-enveloped and contains a single-stranded
RNA packaged in a protein shell. There is only one serotype of the virus, but multiple
genotypes exist. Codon use within the genome is biased and unusually distinct from its
host. It also has a poor internal ribosome entry site. In the region that codes for the
HAV capsid, there are highly conserved clusters of rare codons that restrict antigenic variability.
Genotypes One serotype and seven different genetic groups
have been described. The human genotypes are numbered I-III. Six subtypes have been described.
The simian genotypes have been numbered IV-VI. A single isolate of genotype VII isolated
from a human has also been described. Genotype III has been isolated from both humans and
owl monkeys. Most human isolates are of genotype I. Of the type I isolates subtype IA accounts
for the majority. The mutation rate in the genome has been estimated
to be 1.73–9.76 x 10−4 nucleotide substitution per site per year. The human strains appear
to have diverged from the simian ~3600 years ago. The mean age of genotypes III and IIIA
strains has been estimated to be 592 and 202 years respectively.
Transmission The virus spreads by the fecal–oral route
and infections often occur in conditions of poor sanitation and overcrowding. Hepatitis
A can be transmitted by the parenteral route but very rarely by blood and blood products.
Food-borne outbreaks are not uncommon, and ingestion of shellfish cultivated in polluted
water is associated with a high risk of infection. Approximately 40% of all acute viral hepatitis
is caused by HAV. Infected individuals are infectious prior to onset of symptoms, roughly
10 days following infection. The virus is resistant to detergent, acid, solvents, drying,
and temperatures up to 60 °C. It can survive for months in fresh and salt water. Common-source
outbreaks are typical. Infection is common in children in developing countries, reaching
100% incidence, but following infection there is lifelong immunity. HAV can be inactivated
by: chlorine treatment, formalin, peracetic acid, beta-propiolactone, and UV radiation.
In developing countries, and in regions with poor hygiene standards, the rates of infection
with this virus are high and the illness is usually contracted in early childhood. As
incomes rise and access to clean water increases, the incidence of HAV decreases. In developed
countries on the other hand, the infection is contracted primarily by susceptible young
adults, most of whom are infected with the virus during trips to countries with a high
incidence of the disease or through contact with infectious persons.
Diagnosis
Although HAV is excreted in the feces towards the end of the incubation period, specific
diagnosis is made by the detection of HAV-specific IgM antibodies in the blood. IgM antibody
is only present in the blood following an acute hepatitis A infection. It is detectable
from one to two weeks after the initial infection and persists for up to 14 weeks. The presence
of IgG antibody in the blood means that the acute stage of the illness is past and the
person is immune to further infection. IgG antibody to HAV is also found in the blood
following vaccination and tests for immunity to the virus are based on the detection of
this antibody. During the acute stage of the infection, the
liver enzyme alanine transferase is present in the blood at levels much higher than is
normal. The enzyme comes from the liver cells that have been damaged by the virus.
Hepatitis A virus is present in the blood and feces of infected people up to two weeks
before clinical illness develops. Prevention
Hepatitis A can be prevented by vaccination, good hygiene and sanitation.
There are two types of vaccines: one containing inactivated hepatitis A virus, and another
containing a live but attenuated virus. Both provide active immunity against a future infection.
The vaccine protects against HAV in more than 95% of cases for longer than 25 years. In
the US the vaccine was first phased in 1996 for children in high-risk areas, and in 1999
it was spread to areas with elevating levels of infection.
The vaccine is given by injection. An initial dose provides protection starting two to four
weeks after vaccination; the second booster dose, given six to twelve months later, provides
protection for over twenty years. Vaccination programmes
The vaccine was introduced in 1992 and was initially recommended for persons at high
risk. Since then Bahrain and Israel have embarked on eradication programmes. Australia, China,
Belarus, Italy, Spain and the United States have started similar programmes. The incidence
of hepatitis A where widespread vaccination has been practised has decreased dramatically.
In China and the United States the incidence of hepatitis A has decreased by 90% since
1990. In the United States vaccination of children
is recommended at 1 and 2 years of age. It is also recommended in those who have not
been previously immunized and who have been exposed or are likely to be exposed due to
travel. Treatment
There is no specific treatment for hepatitis A. Sufferers are advised to rest, avoid fatty
foods and alcohol, eat a well-balanced diet, and stay hydrated.
Prognosis In the United States in 1991 there was a low
mortality rate for hepatitis A of 4 deaths per 1000 cases for the general population
but a higher rate of 17.5 per 1000 in those aged 50 and over. The risk of death from acute
liver failure following HAV infection increases with age and when the person has underlying
chronic liver disease. Young children that are infected with hepatitis
A typically have a milder form of the disease, usually lasting from 1–3 weeks, whereas
adults tend to experience a much more severe form of the disease.
Epidemiology
Globally, symptomatic HAV infections are believed to occur in around 1,4 million people a year.
There is however likely tens of millions of infections in all. In 2010, acute hepatitis
A resulted in 102,000 deaths which is slightly up from 99,000 in 1990. Developed countries
have low circulating levels of hepatitis A virus while developing countries have high
circulating levels of hepatitis A virus. Most adolescents and adults in developing countries
are have already had the disease and are thus immune. Adults in mid level countries may
be at risk of disease with the potential of being exposed.
Countries There were 30,000 cases of hepatitis A reported
to the CDC in the US in 1997 but the number has since dropped to less than 2,000 cases
reported per year. The most widespread hepatitis A outbreak in
the 2003 United States hepatitis outbreak afflicted at least 640 people in north-eastern
Ohio and south-western Pennsylvania in late 2003. The outbreak was blamed on tainted green
onions at a restaurant in Monaca, Pennsylvania. In 1988, more than 300,000 people in Shanghai,
China were infected with HAV after eating clams from a contaminated river. In June 2013,
frozen berries sold by US retailer Costco and purchased by around 240,000 people were
the subject of a recall, after at least 158 people were infected with HAV, 69 of whom
were hospitalized. References
External links