Paramyxoviridae

Paramyxoviridae are enveloped viruses with a helical nucleocapsid containing a linear, non-segmented, negative-sense single-stranded RNA genome. Because the genome is negative-sense, these viruses carry their own RNA-dependent RNA polymerase (RdRp) to transcribe mRNA upon entry. Key members include: Measles virus (causes measles with the classic triad of cough, coryza, conjunctivitis, plus Koplik spots and a cephalocaudal rash; can cause SSPE years later); Mumps virus (parotitis, orchitis, aseptic meningitis, pancreatitis); Parainfluenza virus (major cause of croup/laryngotracheobronchitis in children, presenting with a barking cough and stridor); and Respiratory Syncytial Virus (RSV, leading cause of bronchiolitis and pneumonia in infants, presents with wheezing).

A hallmark feature of paramyxoviruses is their ability to cause cell-to-cell fusion via fusion (F) proteins on the viral envelope, leading to multinucleated giant cells (syncytia) — a classic histologic finding, especially with RSV and measles (Warthin-Finkeldey giant cells in measles). Other envelope glycoproteins include hemagglutinin-neuraminidase (HN) in some members (parainfluenza, mumps) that mediate attachment and release, though measles and RSV differ in their attachment protein composition (measles has H protein without neuraminidase activity; RSV lacks both H and N proteins, hence its name).

Clinically, these viruses are relevant in pediatrics, immunization schedules (MMR vaccine for measles and mumps), and complications: measles can cause subacute sclerosing panencephalitis (SSPE) years after infection, mumps can cause orchitis (risk of infertility) and pancreatitis, and RSV is treated with supportive care and prevented with palivizumab (monoclonal antibody against F protein) in high-risk infants. Diagnosis often involves PCR, viral culture, or serology. Prevention through vaccination (MMR) has dramatically reduced measles and mumps incidence, making vaccine hesitancy a growing concern for outbreaks.