Mumps

Mumps is caused by the mumps virus, an enveloped single-stranded RNA paramyxovirus with one serotype and humans as its only reservoir. That single serotype is why vaccination works well and why lifelong immunity normally follows infection.

Transmission is by respiratory droplets, direct contact with saliva, and contaminated surfaces. The virus first replicates in the upper respiratory epithelium and local lymph nodes, then spreads through the blood to tissues with a particular affinity for glandular and nervous tissue — the salivary glands, testes, ovaries, pancreas, meninges and, less often, the inner ear.

The incubation period is 16 to 18 days, with a range of about 12 to 25. Infectivity runs from roughly two days before parotid swelling to five days after, which is the basis of the standard exclusion period. Around a third of infections cause no parotitis at all — they are subclinical or present as a non-specific respiratory illness — and these still transmit.

Incidence fell dramatically after the introduction of MMR vaccination. Outbreaks continue to occur, however, in settings with close contact and large numbers of young adults — universities and residential institutions in particular. Two factors explain this: incomplete vaccine coverage, and waning immunity, with effectiveness declining over the years since the second dose. This is why mumps in a vaccinated young adult is not a reason to discard the diagnosis, though disease is usually milder.

A prodrome of one to two days precedes the gland swelling: low-grade fever, headache, malaise, myalgia and anorexia.

Parotitis is the defining feature, present in most symptomatic cases. Swelling is bilateral in about 70%, though often sequential rather than simultaneous, with the second side following a day or two after the first. The swollen gland pushes the earlobe upward and outward and obscures the angle of the jaw — a useful distinction from cervical lymphadenopathy, which does not. Pain is worsened by anything that stimulates salivation, so sour foods are characteristically intolerable, and the parotid duct opening may be red and swollen. Trismus and difficulty chewing are common. Swelling peaks over one to three days and settles within about a week.

Submandibular and sublingual glands are involved less often. Fever is usually modest and the illness is generally self-limiting in children.

Complications are more frequent and more severe in adolescents and adults than in young children.

Orchitis affects 15–30% of post-pubertal males, typically four to eight days after parotitis, and is unilateral in most cases. It causes marked testicular pain, swelling and fever. Some degree of testicular atrophy follows in about half, but infertility is uncommon, though it is a frequent and disproportionate fear. Oophoritis occurs in around 5% of post-pubertal females, causing pelvic pain, and does not impair fertility.

Aseptic meningitis is common, with CSF pleocytosis in a substantial minority of all cases though symptomatic in perhaps 10%; it is usually self-limiting. Encephalitis is rare but serious. Sensorineural hearing loss is usually transient and unilateral, but permanent deafness occurs rarely and was historically a leading cause of acquired childhood deafness. Pancreatitis presents with epigastric pain and vomiting. Infection in the first trimester of pregnancy is associated with an increased rate of spontaneous miscarriage, but mumps is not teratogenic.

In an outbreak, a clinical diagnosis of acute bilateral parotitis with a compatible exposure history is usually sufficient. Laboratory confirmation is nonetheless important for public health purposes, since mumps is a notifiable disease in most jurisdictions.

RT-PCR on a buccal or parotid duct swab is the most sensitive test and is best taken within the first three days of swelling; a urine or CSF sample can also be used. Serology detects mumps-specific IgM, but it is less reliable in vaccinated individuals, in whom IgM is frequently absent or transient — paired acute and convalescent IgG is more useful there.

Serum amylase is commonly raised from the salivary glands and does not by itself indicate pancreatitis; lipase distinguishes the two. Lumbar puncture is reserved for suspected meningitis. Testicular ultrasound may be needed in orchitis to exclude torsion, which is the critical alternative.

Mumps is generally self-limiting in the immunocompetent. Risk of severe or complicated disease rises in adolescents and adults and in immunocompromised people. Because presentation in vaccinated individuals is often atypical and milder, diagnosis is more easily missed in that group — with the consequence that transmission continues in exactly the settings where outbreaks take hold.