Mumps virus

Background

• Presents with a prodrome followed by parotitis

Microbiology

• Enveloped single-stranded RNA virus in the genus Rubulavirus and family Paramyxovirus
• Only one serotype but 13 genotypes (A to M)
• Genome encodes eight proteins: hemagglutinin-neuraminidase (HN), fusion (F), nucleocapsid (NP), phosphoprotein (P), matrix (M), hydrophobic (SH) and L protein
  • P contains V and I proteins
• Irregular spherical shape with nucleocapsid enclosed by a three-layered envelope
  • Surface studded with glycoproteins: HN and F, which are the most important for immunity
  • Middle layer is lipid bilayer from the host cell
  • Inner layer in membrane protein

Epidemiology

• Worldwide distribution
• Epidemics every 2 to 5 years in unimmunized settings, with a peak between January and May
• Spread primarily by schoolchildren
• Outbreaks have happened amongst immunized people, suggesting that a third dose of MMR may be needed to confer ongoing immunity
• Before vaccination, it was the leading cause of viral encephalitis and a common cause of viral meningitis

Pathophysiology

• Acquired through virus (contact, droplet, fomites) entering nose or mouth, with tropism for endo/exocrine glands
  • Salivary, pancreatis, testicular
• Less infectious that measles or varicella
• Peak contagion is just before parotitis
• Immune response begins with antibodies against NP protein (S antigen), and may be detectable at presentation, but decline quickly over months
• Antibodies against HN protein (V antigen) follow, peaking at 2 to 4 weeks and persist for years
  • IgM antibodies fall within 2 to 6 months
• Neutralizing antibodies to HN and F are detectable during convalescence

Clinical Manifestations

• Incubation period of 16 to 18 days (range 2 to 4 weeks)
• One-day prodrome of low-grade fever, anorexia, malaise, and headache
• Earache and parotitis soon follow
  • Parotitis progresses over 2 to 3 days, with severe pain
  • The other parotid usually follows, but it can be unilateral
  • Stensen's duct is edematous and erythematous
  • Pain exacerbated by citrus
  • Can involve other salivary glands in 10%
• Temperature can be as high as 40º C
• Pain, fever, tenderness resolve, with parotid returning to normal within 1 week
• Can lead to sialectasia resulting in recurrent or chronic sialadenitis

Mumps epididymo-orchitis

• The most common extrasalivary gland manifestation, occuring in 20-30% of postpubertal men (bilateral in 15%)
• Occurs in first 1-2 weeks after parotitis
• Fevers up to 41º C, chills, headache, vomiting, and testicular pain
• Swollen warm testicles with scrotal erythema
• Fever resolves within 5 days, followed by slower resolution of the orchitis
  • Tenderness can sometimes last longer than 2 weeks
• Longterm testicular atrophy in 50%
  • If unilateral, no concerns
  • If bilateral, sterility is rare, and impotence is not a sequela

Mumps oopheritis

• In 5% of cases in postpubertal women
• May cause impaired fertility

Mumps meningitis

• Fever, headache, vomiting, and nuchal rigidity, with an aseptic CSF (lymphocyte-predominant more often than neutrophil-predominant)
  • Amylase may be elevated
• Onset usually after parotitis, but can be 1 week before or up to 2 weeks after
• Can also occur without parotitis
• Lasts 3 to 10 days, with complete recovery

Mumps encephalitis

• Non-focal encephalitis, high fever, altered LOC, seizures, paresis, aphasia, and involuntary movements, with an aseptic CSF
  • Fever can be up to 41º C
• Can occur concurrent with or up to 10 days after onset of parotitis
• Early-onset is from virus; late-onset is a postinfectious autoimmune demyelinating disease; but there is likely a continuum between these two extremes
• Gradually resolves over 1 to 2 weeks
• Can cause sequelae, including psychomotor retardation, seizures, and death

Other complications

• Cerebellar ataxia, facial palsy, transverse myelitis, Guillain-Barré syndrome, and poliomyelitis-lik syndrome
• Migratory polyarthritis, usually starting 10-14 days after parotitis and lasting up to 5 weeks
• Pancreatitis
• ECG changes with ST depression and T-wave changes, 1st degree heart block
  • Myocarditis is rare

Pregnancy

• Pregnant women who are infected have increased risk of spontaneous abortion in the first trimester, as well as low birth weight
• Not clearly related to any significant birth defects

Differential Diagnosis

• Infectious parotitis
  • Parainfluenza 3 virus, coxsackievirus, and influenza A
  • HIV infection (bilateral, parotid)
  • Staphylococcus aureus or Gram-negative bacilli
• Drugs (bilateral, mild)
• Metabolic disorders, including diabetes, malnutrition, cirrhosis, and CKD (bilateral, mild)
• Tumours, cysts, sialolithiasis, and stricture (unilateral)
• Eosinophilic parotitis, often as a drug reaction

Diagnosis

• Traditionally a clinical diagnosis
• CBC and diff are normal or mild leukopenia; amylase may be up from parotitis, or lipase from pancreatitis
• Can be diagnosed with serology or PCR
• ELISA for IgM, or a fourfold rise from acute to convalescent ELISA or HAI serologies, are diagnostic
  • HAI may be affected by parainfluenza
• PCR or culture detectable in saliva, though relatively low level after 5 days; also found in CSF
  • Can be detected in urine up to 2 weeks after onset

Management

• Symptomatic
• Immune globulin not helpful
• Post-exposure immunization may not be helpful, though in an outbreak situation, may consider giving an MMR booster
• Isolation for 5 days after onset of parotitis to reduce spread
• Reportable disease, public health may do outbreak investigation and consider booster MMR in high-risk populations

Prevention

• Live attenuated vaccine in the MMR is given at 12-15 months and again at 4-6 years
• Vaccine 65-70% effective, so need high vaccination rate to achieve herd immunity
• Titres positive for at least 10 years, but immunity wanes
• Contraindicated in pregnant women