Mumps Virus (MuV)

Mumps virus (MuV) is a member of the family Paramyxoviridae and the organism that causes the acute infectious disease mumps. MuV is an enveloped, single-stranded negative-sense RNA virus that is highly neurotropism-causing, and can be the cause of encephalitis, meningitis, orchitis, parotitis and more. And while overpopulation and vaccine efficiency have slowed the growth of mumps, outbreaks of the disease have been breaking out everywhere in recent years as virus evolution and vaccine efficacy have retreated.

Figure 1. Schematic Representation of Mumps Virus (Source: Wu H, et al., 2021)

Basic Characteristics

MuV is an enveloped negative-sense RNA virus from the Paramyxoviridae family. It has seven genes in its 15,384-nucleotide single-stranded RNA genome: nucleocapsid protein (N), phosphoprotein (P), matrix protein (M), fusion protein (F), small hydrophobic protein (SH), hemagglutinin-neuraminidase (HN), and large protein (L). The virus enters the upper airways through respiratory mucus before spreading to other organs via blood.

Figure 2. Overview of the Shared and Variable Sequences of the F and HN Proteins in Mumps Virus (Source: Gouma, S., et al., 2018)

It's infected, usually, from breathing airway drops. The virus strikes epithelial cells of the upper respiratory tract, which reproduces and spreads into the bloodstream to parotid glands, testes, ovaries and brain. Parabola swelling and rheumatism are the most frequent clinical symptoms. But beyond the parotid glands, MuV invades other tissues and organs and causes orchitis, meningitis and encephalitis.

Mumps Vaccine

It wasn't until the 1960s that the mumps vaccine was introduced as a preventive measure. Vaccinations had slashed mumps in the grand total, especially in the developed world where the disease almost never occurs. In 2001, mumps rates had decreased 99.9%, falling below 0.1 per 100,000 per year. Yet even where the vaccine rate is high, outbreaks of mumps still occur, particularly in young people and adults.

We have no idea what's responsible for the vaccine's lower efficiency, but it may be because of loss of immunity, virus strain mutations and vaccine escape. Researchers also noted that the vaccine strain may in rare instances cause aseptic meningitis, making vaccine safety uncertain. Mumps was also halted in Japan, for instance, after the vaccine strain was found to be unsafe, so mumps is relatively widespread in Japan.

The 1960s invention of the mumps vaccine swiftly found its way into routine immunizations. Vaccination has diminished the disease, most notably in the industrialized world, where it is practically eradicated. But the vaccine was no longer effective, and strain mutations of the virus caused recurrences around the globe. The MMR (measles, mumps, rubella) vaccine is a live vaccine derived from dead measles, mumps and rubella viruses and is the world's most widely administered mumps vaccine. MMR has been a childhood vaccine of choice since the 1960s, and has cut mumps rates by an order of magnitude. Despite the strong protection offered by the MMR vaccine, immunity does wear off over time, especially in adults. In the wake of poor results with the MMR vaccine, countries across the globe began prescribing boosters for teenagers and adults. That would enhance protection and keep people safe longer, especially those at high risk, so as to avoid more outbreaks of mumps.

As mumps hit the headlines, the vaccine has become the object of new attention. Researchers are also trying to improve the vaccine's efficiency and safety, to curb issues such as virus strain changes and immune escape. As immunity declines, new vaccine therapies could include boosters. Furthermore, improved understanding of MuV's history and virus changes could, in principle, motivate new vaccines.

In short, MuV is a neurotropic virus that not only causes parotitis, but a wide range of fatal complications, particularly central nervous system infections. While mumps vaccination has substantially decreased the incidence of the disease, problems such as diminished vaccine effectiveness and virus mutations remain a huge concern. Future research should improve vaccine safety and effectiveness, create new vaccine strains, and look at how MuV gets virulent to develop better prevention strategies.

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