They are a major cause of the common cold. They gained notoriety in 2002 due to SARS (severe acute respiratory syndrome) pandemic which had a case fatality rate of up to 50%. The virus has a high frequency of recombination. Its replicative cycle is unique in that it obtains its envelope from the endoplasmic reticulum and not plasma membrane. The receptor of CoV-SARS is angiotensin converting enzyme 2.
Clinical features: Common cold with nasal discharge, mild fever, cough, sore throat and malaise is seen. It also causes gastroenteritis in infants. SARS is a severe pneumonia with fever, cough, dyspnea with hypoxia resulting from pulmonary edema. Leukopenia and thrombocytopenia may be seen.
Laboratory diagnosis of Coronavirus infections: Infections can be detected by ELISA, electron microscopy of stool samples, PCR and cell culture.
Rotavirus
It is in the Reovirus family. It is the only medically important RNA virus that is double stranded. It has a wheel like morphology. Depending on the outer viral hemagglutinin it is divided into six serotypes. Antibody to haemagglutinin is protective from further infection. Beta adrenergic receptors are the receptors that the virus uses to attach to human cells. Calcium ions have an important role to play in the Rotavirus replicative cycle. The Rotavirus protein NSP 1 functions as an interferon antagonist.
Mechanism of Rotavirus diarrhea: It causes a watery diarrhea with minimal intestinal inflammation. It primarily infects intestinal villus enterocytes while crypts are spared. Several mechanisms cause diarrhea. There is malabsorption due to virus-mediated destruction of absorptive enterocytes, virus-induced downregulation of the expression of absorptive enzymes, and functional changes in tight junctions between enterocytes that lead to paracellular leakage. There is a secretory component of rotavirus diarrhea that is thought to be mediated by activation of the enteric nervous system and the effects of NSP4—the first described virus-encoded enterotoxin. Activation of Cl channels with consequent loss of water and Cl occurs. There is villus ischemia and alterations in intestinal motility. Rotavirus infection can spread extra intestinally also.
Clinical features: Patients are typically infants and children under 2 years of age. They present with vomiting, greenish yellow, non bloody, watery diarrhea. It may cause dehydration.
Laboratory diagnosis of Rotavirus diarrhea: Detection of Rotavirus in stool samples by electron microscopy, immunoassay or ELISA are preferred methods. IgM and IgG antibodies can be detected in blood by ELISA.
Norwalk Virus
It belongs to the family Calicivirus. Also called Norovirus. It causes outbreaks of viral gastroenteritis in long term healthcare facilities, restaurants (lettuce, fresh fruit and shellfish being the main culprits), schools and childcare facilities and in cruise ships. It presents as fever, abdominal cramps, nausea, vomiting, non-bloody diarrhea. Headache and photophobia can occur. Laboratory diagnosis is made by viral antigen detection in stool by RT PCR or electron microscopy. Antibodies can be detected by ELISA. Remember that no pus cells or RBCs are seen in stool microscopy.
Astrovirus
Human astroviruses (HAtVs) are positive-sense single-stranded RNA viruses. They are star shaped hence the name. Classic HAstVs contain 8 serotypes and account for 2 to 9% of all acute nonbacterial gastroenteritis in children worldwide. Infections are usually self-limiting but can also spread systemically and cause severe infections in immunocompromised patients. The diarrhea is mild, watery with fever and abdominal pain. It is a zoonosis.
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