Astrovirus

Template:ToLCleanup An astrovirus is a type of virus that infects humans. They have been poorly studied due to the fact that they do not grow in culture. Astroviruses belong to the family Astroviridae along with a virus known as mamastrovirus. They were first described in the year 1975 using electron microscopes during an outbreak of diarrhoea. Astrovirus has a non-segmented, single stranded, positive sense RNA genome within a non-enveloped icosahedral capsid.

Effects

Members of a relatively new virus family, the astroviridae, astroviruses are now recognised as a major cause of gastroenteritis in children and adults. They are non-enveloped viruses displaying cubic symmetry, approximately 28-30nm in diameter. The genome consists of a positive sense single strand of RNA encoding a varying array of structural proteins.

A study of intestinal disease in the UK, published in 1999 determined incidence as 3.8/1000 patient years in the community (95%CI, range2.3-6.4), the fourth most common known cause of viral gastroenteritis. Studies in the USA have detected astroviruses in the stools of 2-9% of children presenting symptoms; illness is most frequent in children of less than two years, although outbreaks among adults and the elderly have been reported. Early studies carried out in Glasgow demonstrated that a significant proportion of babies excreting virus particles, 12%, did not exhibit gastrointestinal symptoms, and seroprevalence studies carried out in the US have shown that 90% of children have antibody to HastV-1 by age 9, suggesting that (largely asymptomatic) infection is common. There is, as with most viral causes of gastroenteritis, a peak of incidence in the winter.

Symptoms include diarrhoea, headache, malaise and nausea, vomiting being a less common complaint, and are usually milder than those experienced with rotavirus infection, and as a result dehydration is less severe. Incubation takes 3-4 days, and symptoms persist for less than 5 days in uncomplicated cases.

Diagnosis is achieved using electron microscopy; immune amplification is not necessarily required due to large numbers of virus particles in the stools. ELISAs, immunofluorescence, and nucleic acid based techniques have all been used.

Treatment may not be required, rehydration using oral rehydration therapy helps maintain electrolyte balance. There is potential for development of vaccination as it appears infection proffers some short-term protection against viruses of the same serotype.

Differential diagnosis

Astrovirus infection must be differentiated from other causes of viral, bacterial, and parasitic gastroentritis.

Organism			Age predilection	Travel History	Incubation Size (cell)	Incubation Time	History and Symptoms			Diarrhea type8				Food source	Specific consideration
							Fever	N/V	Cramping Abd Pain	Small Bowel	Large Bowel	Inflammatory	Non-inflammatory
Viral	Rotavirus		<2 y	-	<102	<48 h	+	+	-	+			+	-	Mostly in day cares, most common in winter.
	Norovirus		Any age	-	10 -103	24-48 h	+	+	+	+			+	-	Most common cause of gastroenteritis, abdominal tenderness,
	Adenovirus		<2 y	-	105 -106	8-10 d	+	+	+	+			+	-	No seasonality
	Astrovirus		<5 y	-		72-96 h	+	+	+	+			+	Seafood	Mostly during winter
Bacterial	Escherichia coli	ETEC	Any age	+	108 -1010	24 h	-	+	+	+			+	-	Causes travelers diarrhea, contains heat-labile toxins (LT) and heat-stable toxins (ST)
		EPEC	<1 y	-	10†	6-12 h	-	+	+	+			+	Raw beef and chicken	-
		EIEC	Any ages	-	10†	24 h	+	+	+		+		+	Hamburger meat and unpasteurized milk	Similar to shigellosis, can cause bloody diarrhea
		EHEC	Any ages	-	10	3-4 d	-	+	+		+	+		Undercooked or raw hamburger (ground beef)	Known as E. coli O157:H7, can cause HUS/TTP.
		EAEC	Any ages	+	1010	8-18 h	-	-	+		+		+	-	May cause prolonged or persistent diarrhea in children
	Salmonella sp.		Any ages	+	1	6 to 72 h	+	+	+		+	+		Meats, poultry, eggs, milk and dairy products, fish, shrimp, spices, yeast, coconut, sauces, freshly prepared salad.	Can cause salmonellosis or typhoid fever.
	Shigella sp.		Any ages	-	10 - 200	8-48 h	+	+	+		+	+		Raw foods, for example, lettuce, salads (potato, tuna, shrimp, macaroni, and chicken)	Some strains produce enterotoxin and Shiga toxin similar to those produced by E. coli O157:H7
	Campylobacter sp.		<5 y, 15-29 y	-	104	2-5 d	+	+	+		+	+		Undercooked poultry products, unpasteurized milk and cheeses made from unpasteurized milk, vegetables, seafood and contaminated water.	May cause bacteremia, Guillain-Barré syndrome (GBS), hemolytic uremic syndrome (HUS) and recurrent colitis
	Yersinia enterocolitica		<10 y	-	104 -106	1-11 d	+	+	+		+	+		Meats (pork, beef, lamb, etc.), oysters, fish, crabs, and raw milk.	May cause reactive arthritis; glomerulonephritis; endocarditis; erythema nodosum. can mimic appendicitis and mesenteric lymphadenitis.
	Clostridium perfringens		Any ages		> 106	16 h	-	-	+		+		+	Meats (especially beef and poultry), meat-containing products (e.g., gravies and stews), and Mexican foods.	Can survive high heat,
	Vibrio cholerae		Any ages	-	106-1010	24-48 h	-	+	+	+			+	Seafoods, including molluscan shellfish (oysters, mussels, and clams), crab, lobster, shrimp, squid, and finfish.	Hypotension, tachycardia, decreased skin turgor. Rice-water stools
Parasites	Protozoa	Giardia lamblia	2-5 y	+	1 cyst	1-2 we	-	-	+	+			+	Contaminated water	May cause malabsorption syndrome and severe weight loss
		Entamoeba histolytica	4-11 y	+	<10 cysts	2-4 we	-	+	+		+	+		Contaminated water and raw foods	May cause intestinal amebiasis and amebic liver abscess
		Cryptosporidium parvum	Any ages	-	10-100 oocysts	7-10 d	+	+	+	+			+	Juices and milk	May cause copious diarrhea and dehydration in patients with AIDS especially with 180 > CD4
		Cyclospora cayetanensis	Any ages	+	10-100 oocysts	7-10 d	-	+	+	+			+	Fresh produce, such as raspberries, basil, and several varieties of lettuce.	More common in rainy areas
	Helminths	Trichinella spp	Any ages	-	Two viable larvae (male and female)	1-4 we	-	+	+		+		+	Undercooked meats	More common in hunters or people who eat traditionally uncooked meats
		Taenia spp	Any ages	-	1 larva or egg	2-4 m	-	+	+	+			+	Undercooked beef and pork	Neurocysticercosis: Cysts located in the brain may be asymptomatic or seizures, increased intracranial pressure, headache.
		Diphyllobothrium latum	Any ages	-	1 larva	15 d	-	-	-	+			+	Raw or undercooked fish.	May cause vitamin B12 deficiency

8Small bowel diarrhea: watery, voluminous with less than 5 WBC/high power field

Large bowel diarrhea: Mucousy and/or bloody with less volume and more than 10 WBC/high power field
† It could be as high as 1000 based on patient's immunity system.

The table below summarizes the findings that differentiate inflammatory causes of chronic diarrhea

Cause	History	Laboratory findings	Diagnosis	Treatment
Diverticulitis	Bloody diarrhea Left lower quadrant abdominal pain Abdominal tenderness on physical examination Low grade fever	Leukocytosis Elevated serum amylase and lipase Sterile pyuria on urinalysis	Abdominal CT scan with oral and intravenous (IV) contrast	bowel rest, IV fluid resuscitation, and broad-spectrum antimicrobial therapy which covers anaerobic bacteria and gram-negative rods
Ulcerative colitis	Diarrhea mixed with blood and mucus, of gradual onset. Signs of weight loss Rectal urgency Tenesmus Blood is often noticed on underwear Different degrees of abdominal pain	Anemia Thrombocytosis A high platelet count Elevated ESR (>30mm/hr) Low albumin	Endoscopy	Induction of remission with mesalamine and corticosteroids followed by the administration of sulfasalazine and 6-Mercaptopurine depending on the severity of the disease.
Entamoeba histolytica	Abdominal cramps Diarrhea Passage of 3 - 8 semiformed stools per day Passage of soft stools with mucus and occasional blood Fatigue Intestinal gas (excessive flatus) Rectal pain while having a bowel movement (tenesmus) Unintentional weight loss	cysts shed with the stool	detects ameba DNA in feces	Amebic dysentery Metronidazole 500-750mg three times a day for 5-10 days Tinidazole 2g once a day for 3 days is an alternative to metronidazole Luminal amebicides for E. histolytica in the colon: Paromomycin 500mg three times a day for 10 days Diloxanide furoate 500mg three times a day for 10 days Iodoquinol 650mg three times a day for 20 days For amebic liver abscess: Metronidazole 400mg three times a day for 10 days Tinidazole 2g once a day for 6 days is an alternative to metronidazole Diloxanide furoate 500mg three times a day for 10 days must always be given afterwards.

References

Caul, E. O. 1996. Viral gastroenteritis: small round structured viruses, caliciviruses and astroviruses. Part II. The epidemiological perspective. J. Clin. Pathol. 49:959-964

Caul, E. O. 1996. Viral gastroenteritis: small round structured viruses, caliciviruses and astroviruses. Part I. The clinical and diagnostic perspective. J. Clin. Pathol. 49:874-880

Treanor, J. J., R. Dolin. 2005. Astroviruses and picobirnaviruses. 2201-2203, in: Mandell, Douglas and Bennett’s Principles and practice of infectious diseases (6th Ed.). Mandell G. L., Bennett J. E., Dolin R. (Editors). Elsevier Churchill Livingstone.

Viral Gastroenteritis. 67-72, in: Notes on Medical Virology (11th Ed.) M. C. Timbury. 1997. Churchilll Livingstone.

Kapikan A. Z., R. M. Chanock. 1989. Viral Gastroenteritis. 293-340, in: Viral infections of humans (3rd Ed.) Evans A. S. (Ed.) Plenum Medical Book company

Wheeler J. G., D. Sethi, J. M. Cowden, P. G. Wall, L. C. Rodrigues, D. S. Tompkins, M. J. Hudson, P. J. Roederick. 1999. Study of infectious intestinal disease in England: rates in the community, presenting to general practice, and reported to national surveillance. Br. Med. J. 318:1046-1050