Chapter 24: Orthomyxo and Paramyxoviridae

The Orthomyxoviridae family, represented primarily by influenza viruses, possesses a distinctive segmented RNA genome enclosed within a lipid bilayer membrane studded with two critical surface glycoproteins. Hemagglutinin functions as the primary attachment molecule, binding to sialic acid residues on host cell surfaces to initiate viral entry, while neuraminidase acts as a sialidase enzyme that cleaves these sialic acid bonds, enabling progeny virions to exit infected cells and spread to neighboring tissues. The chapter contrasts antigenic drift, the gradual accumulation of point mutations in viral genes that produces seasonally circulating strains, with antigenic shift, the reassortment of genome segments between different influenza viruses that can generate novel subtypes capable of causing pandemic outbreaks. Additional topics include secondary complications such as Reye syndrome, a severe metabolic encephalopathy historically associated with aspirin administration during acute viral infection, and emerging zoonotic threats including the avian-derived H5N1 and H7N9 influenza strains. The second half addresses Paramyxoviridae, a family of non-segmented, negative-sense RNA viruses that share structural similarities with orthomyxoviruses but lack genome segmentation. Parainfluenza viruses typically cause croup and upper respiratory infections, while respiratory syncytial virus produces severe bronchiolitis particularly in infants. Mumps virus demonstrates tropism for salivary gland tissues, resulting in parotitis, and can involve the reproductive system causing orchitis. Measles virus progresses through distinct clinical phases, beginning with a prodromal period marked by fever, cough, coryza, and conjunctivitis, followed by the appearance of pathognomonic Koplik spots on the buccal mucosa, and culminating in a characteristic centrifugal maculopapular rash spreading from the head downward.