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You’ve probably had the stomach flu, at least once, right?
Chances are, it was due to norovirus,
which is one of the most common causes of viral gastroenteritis.
Sometimes it’s also called the Norwalk virus,
after the town in Ohio, where a big outbreak in 1968
allowed scientists to isolate the virus.
There are several genogroups of norovirus,
但只有GI GII GIV亚型会致人生病
but only groups I, II, and IV can cause disease in humans.
Norovirus is part of the caliciviridae family of viruses.
They are naked viruses surrounded by an icosahedral capsid,
which is a spherical protein shell made up of 20 equilateral triangular faces.
And they’re “naked” because the capsid isn’t covered by a lipid membrane.
They’re also single strand RNA viruses.
This means that their RNA is actually mRNA
and the host cell ribosomes use this mRNA to make one long polyprotein chain,
which is then broken into smaller pieces by viral proteases.
This all happens in the cytoplasm of the host cell,
since that’s where ribosomes are found,
and it results in several viral proteins.
The exact role of each specific viral protein is still a bit of a mystery,
but we do know that they mainly affect the small intestine.
The small intestine has lots of tiny ridges and grooves,
each of which projects little finger-like fibers called villi.
And in turn, each villus is covered in teeny tiny little microvilli.
This is called the brush border.
All of this gives the small intestines plenty of surface area to absorb nutrients.
Norovirus blunts the villi and shortens microvilli,
and this disrupts the ability of the brush border to absorb certain nutrients,
specifically fat and a simple sugar called D-xylose.
It also lowers the activity of alkaline phosphatase and trehalase,
which are digestive enzymes produced by brush border cells.
Under a microscope,
intestinal cells infected by Norovirus
have an intact mucosa and epithelium,
but there are a lot of lymphocytes in the lamina propria layer of the mucosa.
Intercellular spaces are also larger,
because tight junction proteins that keep the cells together are damaged.
Norovirus is very contagious
and it’s primarily transmitted from person to person via the fecal-oral route.
In other words, you catch it
by ingesting stool particles of someone who is sick.
This can happen if infected stool
ends up in the water supply or on agricultural fields,
if flies land on it, and transfer stool particles to other places,
or by touching contaminated surfaces.
You can summarize it as the four Fs:
流体 田地 苍蝇 手指
fluids, fields, flies, and fingers.
As a result, norovirus can end up in food and drinking water,
and is often spread via uncooked foods like leafy vegetables and shellfish.
It can also be spread by droplets of vomit.
The gist of it is,
Norovirus spreads quickly in close-quarter residence settings.
This is why many cruise ships have made the news because of norovirus outbreaks.
在养老院 医院 学校 军营 运动队和监狱里
Outbreaks are also common in nursing homes, hospitals,
schools, military barracks, athletic teams, and prisons.
Ok, now symptoms usually show up
after a 12 to 48 hour incubation period,
and they last for 48 to 72 hours.
They include acute onset, nonbloody vomiting,
which is linked to slower movement of food through the stomach,
and watery diarrhea, from the malabsorption of fat and D-xylose
and decreased brush border enzymatic activity.
也可能会有腹痛 肌肉痛 头痛和高烧的症状
There might also be abdominal pain, myalgia -or muscle pain, headaches, and fever.
Complications include chronic gastroenteritis,
which can happen in immunocompromised individuals,
dehydration, and malnutrition.
Some people may remain asymptomatic,
but all individuals who are infected with Norovirus
can potentially shed the virus in their stool
for up to four weeks afterward.
Diagnosis of norovirus infection can be done
with reverse transcriptase polymerase chain reaction
or RT-PCR, where a stool sample is tested for viral RNA.
Treatment for Norovirus is mostly supportive, with oral rehydration or IV fluids.
Additionally, antiemetics can be used to control vomiting.
Finally, there’s currently no vaccine for norovirus.
But preventive measures include proper water and sanitation facilities,
能阻断4种传播途径中的3种：流体 苍蝇 田地
which block fecal-oral transmission by fluids, flies, and fields – 3 of the 4 F’s.
Hand washing with antiseptic soap and water
and sanitizing surfaces that would’ve come into contact with stool
can prevent transmission by fingers.
Finally, it’s also recommended to avoid close contact with people who have symptoms.
All right, as a quick recap…
Norovirus is a single-strand RNA virus from the calicivirus family.
It’s a very contagious cause of viral gastroenteritis
that is transmitted via the fecal-oral route, droplets of vomit,
and contaminated food and surfaces.
It mainly affects the small intestine
and causes symptoms like acute onset vomiting and watery diarrhea.
It’s usually self-limiting,
with symptoms disappearing within 48 to 72 hours.
Since there’s no vaccine,
the best prevention for norovirus is proper handwashing, sanitizing surfaces,
and having effective water and sanitation facilities.