Mumps

Mumps is a viral disease caused by the mumps virus. Initial symptoms are non-specific and include fever, headache, malaise, muscle pain, and loss of appetite. These symptoms are usually followed by painful swelling of the parotid glands, called parotitis, which is the most common symptom of infection. Symptoms typically occur 16 to 18 days after exposure to the virus and resolve within two weeks. About one-third of infections are asymptomatic.

Complications include deafness and a wide range of inflammatory conditions, of which inflammation of the testes, breasts, ovaries, pancreas, meninges, and brain are the most common. Testicular inflammation may result in reduced fertility and, rarely, sterility.

Humans are the only natural host of the mumps virus, an RNA virus in the family Paramyxoviridae. The virus is primarily transmitted by respiratory secretions such as droplets and saliva, as well as via direct contact with an infected person. Mumps is highly contagious and spreads easily in densely populated settings. Transmission can occur from one week before the onset of symptoms to eight days after. During infection, the virus first infects the upper respiratory tract. From there, it spreads to the salivary glands and lymph nodes. Infection of the lymph nodes leads to presence of the virus in blood, which spreads the virus throughout the body. Mumps infection is usually self-limiting, coming to an end as the immune system clears the infection.

In places where mumps is common, it can be diagnosed based on clinical presentation. In places where mumps is less common, however, laboratory diagnosis using antibody testing, viral cultures, or real-time reverse transcription polymerase chain reaction may be needed. There is no specific treatment for mumps, so treatment is supportive in nature and includes bed rest and pain relief. Prognosis is usually excellent with a full recovery as death and long-term complications are rare. Infection can be prevented with vaccination, either via an individual mumps vaccine or through combination vaccines such as the MMR vaccine, which also protects against measles and rubella. The spread of the disease can also be prevented by isolating infected individuals.

Mumps historically has been a highly prevalent disease, commonly occurring in outbreaks in densely crowded spaces. In the absence of vaccination, infection normally occurs in childhood, most frequently at the ages of 5–9. Symptoms and complications are more common in males and more severe in adolescents and adults. Infection is most common in winter and spring in temperate climates, whereas no seasonality is observed in tropical regions. Written accounts of mumps have existed since ancient times, and the cause of mumps, the mumps virus, was discovered in 1934. By the 1970s, vaccines had been created to protect against infection, and countries that have adopted mumps vaccination have seen a near-elimination of the disease. In the 21st century, however, there has been a resurgence in the number of cases in many countries that vaccinate, primarily among adolescents and young adults, due to multiple factors such as waning vaccine immunity and opposition to vaccination.

The incubation period, the time between the start of infection and when symptoms begin to show, is about 7–25 days, averaging 16–18 days. 20-40% of infections are asymptomatic or are restricted to mild respiratory symptoms, sometimes with a fever. Over the course of the disease, three distinct phases are recognized: prodromal, early acute, and established acute. The prodromal phase typically has non-specific, mild symptoms such as a low-grade fever, headache, malaise, muscle pain, loss of appetite, and sore throat. In the early acute phase, as the mumps virus spreads throughout the body, systemic symptoms emerge. Most commonly, parotitis occurs during this time period. During the established acute phase, orchitis, meningitis, and encephalitis may occur, and these conditions are responsible for the bulk of mumps morbidity.

The parotid glands are salivary glands situated on the sides of the mouth in front of the ears. Inflammation of them, called parotitis, is the most common mumps symptom and occurs in about 90% of symptomatic cases and 60-70% of total infections. During mumps parotitis, usually both the left and right parotid glands experience painful swelling, with unilateral swelling in a small percentage of cases. Parotitis occurs 2–3 weeks after exposure to the virus, within 2 days of developing symptoms, and usually lasts for 2–3 days, but it may last as long as a week or longer.

In 90% of parotitis cases, swelling on one side is delayed rather than both sides swelling in unison. The parotid duct, which is the opening that provides saliva from the parotid glands to the mouth, may become red, swollen, and filled with fluid. Parotitis is usually preceded by local tenderness and occasionally earache. Other salivary glands, namely the submaxillary, submandibular, and sublingual glands, may also swell. Inflammation of these glands is rarely the only symptom.

Outside of the salivary glands, inflammation of the testes, called orchitis, is the most common symptom infection. Pain, swelling, and warmness of a testis appear usually 1–2 weeks[12] after the onset of parotitis but can occur up to 6 weeks later. During mumps orchitis, the scrotum is tender and inflamed. It occurs in 10-40% of pubertal and post-pubertal males who contract mumps. Usually, mumps orchitis affects only one testis but in 10–30% of cases both are affected. Mumps orchitis is accompanied by inflammation of the epididymis, called epididymitis, about 85% of the time, typically occurring before orchitis. The onsent of mumps orchitis is associated with a high-grade fever, vomiting, headache, and malaise. In prepubertal males, orchitis is rare as symptoms are usually restricted to parotitis.

Mumps is caused by the mumps virus (MuV), scientific name Mumps orthorubulavirus, which belongs to the Orthorubulavirus genus in the Paramyxoviridae family of viruses. Humans are the only natural host of the mumps virus. MuV's genome is made of RNA and contains seven genes that encode nine proteins. In MuV particles, the genome is encased by a helical capsid. The capsid is surrounded by a viral envelope that has spikes protruding from its surface. MuV particles are pleomorphic in shape and range from 100 to 600 nanometers in diameter.

The mumps virus is mainly transmitted by inhalation or oral contact with respiratory droplets or secretions. In experiments, mumps could develop after inoculation either via the mouth or the nose. Respiratory transmission is also supported by the presence of MuV in cases of respiratory illness without parotitis, detection in nasal samples, and transmission between people in close contact. MuV is excreted in saliva from approximately one week before to eight days after the onset of symptoms, peaking at the onset of parotitis, though it has also been identified in the saliva of asymptomatic individuals.

Many aspects of the pathogenesis of mumps are poorly understood and are primarily inferred based on observations from the clinical features of the disease and experimental infections in laboratory animals. These animal studies may be unreliable for various reasons, including unnatural methods of inoculation. Following exposure, the virus infects epithelial cells in the upper respiratory tract that express sialic acid receptors on their surface. After initial infection, the virus spreads to the parotid glands, causing the signature parotitis. It is thought that shortly after infection the virus spreads to lymph nodes, in particular T-cells, which results in the presence of viruses in the blood, called viremia. Viremia lasts for 7–10 days, during which MuV spreads throughout the body.

Even though MuV has just one serotype, significant variation in the quantity of genotype-specific sera needed to neutralize different genotypes in vitro has been observed. Neutralizing antibodies in the salivary glands may be important in restricting MuV replication and transmission via saliva, as the level of viral secretion in saliva inversely correlates to the quantity of MuV-specific IgA produced. The neutralizing ability of salivary IgA appears to be greater than serum IgG and IgM.

Mumps is found worldwide. In the absence of vaccination against mumps, as occurred before vaccination was introduced, there are between 100 and 1,000 cases per 100,000 people each year, i.e. 0.1% to 1% of the population are infected each year. The number of cases peaks every 2–5 years, with incidence highest in children 5–9 years old. According to seroconversion surveys done prior to the start of mumps vaccination, a sharp increase in mumps antibody levels at age 2–3 was observed. Furthermore, 50% of 4–6 year olds, 90% of 14–15 year olds, and 95% of adults had tested positive to prior exposure to mumps, indicating that nearly all people are eventually infected in unvaccinated populations.

According to Chinese medical literature, mumps was recorded as far back as 640 B.C. The Greek physician Hippocrates documented an outbreak on the island of Thasos in approximately 410 B.C. and provided a fuller description of the disease in the first book of Epidemics in the Corpus Hippocraticum. In modern times, the disease was first described scientifically in 1790 by British physician Robert Hamilton in Transactions of the Royal Society of Edinburgh. During the First World War, mumps was one of the most debilitating diseases among soldiers. In 1934, the etiology of the disease, the mumps virus, was discovered by Claude D. Johnson and Ernest William Goodpasture. They found that rhesus macaques exposed to saliva taken from humans in the early stages of the disease developed mumps. Furthermore, they showed that mumps could then be transferred to children via filtered and sterilized, bacteria-less preparations of macerated monkey parotid tissue, showing that it was a viral disease.

In the 21st century, mumps has reemerged in many places that vaccinate against it, causing recurrent outbreaks. These outbreaks have largely affected adolescents and young adults in densely crowded spaces, such as schools, sports teams, religious gatherings, and the military, and it is expected that outbreaks will continue to occur. The cause of this reemergence is subject to debate, and various factors have been proposed, including waning immunity from vaccination, low vaccination rates, vaccine failure, and potential antigenic variation of the mumps virus.

The word "mumps" is first attested circa 1600 and is the plural form of "mump", meaning "grimace", originally a verb meaning "to whine or mutter like a beggar". The disease was likely called mumps in reference to the swelling caused by mumps parotitis, reflecting its impact on facial expressions and the painful, difficult swallowing that it causes. "Mumps" was also used starting from the 17th century to mean "a fit of melancholy, sullenness, silent displeasure". Mumps is sometimes called "epidemic parotitis".