USMLE® Step 1 style questions USMLE
A 23-year-old woman came to the emergency department for vomiting and diarrhea for the last two days. Her sibling had similar symptoms a few days ago. Physical examination reveals generalized abdominal tenderness. Stool testing is positive for a naked virus with positive sense single-stranded RNA. The virus-causing disease in this patient belongs to which of the following families?
Contributors:Evan Debevec-McKenney, Marisa Pedron
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, 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 only 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 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: fluids, fields, flies, and fingers.
Norovirus is a single-strand RNA virus of the calicivirus family, which is known to cause a very contagious form of gastroenteritis. It is transmitted via the fecal-oral route and droplets of vomit. Affected people commonly present with diarrhea, vomiting, and abdominal pain. Other symptoms may include fever, headache, and muscle aches.
Treatment focuses on supportive management, with the administration of oral rehydration solutions or IV fluids. Its spread can be limited by applying preventive measures, which are proper handwashing, sanitizing surfaces, and having effective water and sanitation systems.