Explainer: what is parechovirus?

Most infections by this family of viruses are acquired in childhood and are usually mild. gabi menashe/Flickr, CC BY-SA

An outbreak of parechoviruses in Queensland is causing concern because of the severity of the illness among infants younger than three months old. Dozens of cases have been detected in Australia since late last year and 11 are confirmed in the latest outbreak.

More than 100 cases of infection by the group of viruses were reported in New South Wales in the last months of 2013, most prominently in western Sydney. All of those infected in the latest Queensland outbreak are babies.

The clinical picture of parechovirus infection is variable. Fever is prominent, as is general irritability, skin rash, both rapid breathing and heart rate, and diarrhoea.

More severe and even fatal disease are usually due to central nervous system involvement, causing meningitis and encephalitis, and even a sepsis-like illness.

A nasty family

Viruses from this family were first described in the United States in 1956. Known as echoviruses at the time, they were reclassified as parechoviruses in the 1990s, and have since been detected worldwide.

Parechoviruses are part of the Picornaviridae family, so named because they are small (“pico”) RNA viruses.

This large virus family causes many important animal and human diseases – over 100 enteroviruses from the group have been responsible for large outbreaks of hand, foot and mouth disease, the common cold and other respiratory tract infections, gastroenteritis, skin eruptions, meningitis or encephalitis, and polio.

Transmission is mostly by the faecal-oral route, droplets (through sneezing, for instance, or coughing) or contaminated surfaces. And outbreaks are often more prominent in summer.

Most infections are acquired in childhood and are usually mild, but severe and even fatal disease can occur in both healthy and immuno-compromised individuals (people with transplants for instance, or those taking drugs for autoimmune diseases that suppress their immune system).

Testing for the viruses

Parechoviruses and the diseases they cause are not new, but their identification has increased with the use of molecular testing. Nucleic acid detection, for instance, is much more sensitive and quicker than virus isolation, and other, more traditional laboratory techniques.

As the clinical picture of parechovirus infections can be difficult to distinguish from other infections, especially in very young children, this test has become the “gold standard” for confirming parechovirus’ presence.

The range of human samples used for testing include cerebrospinal fluid, respiratory tract swabs, skin vesicle fluid, and blood and stool samples. Their use is dependent on how the illness is manifesting (diarrhoea will require a different test to meningitis, for instance).

One problem with nucleic acid detection is it doesn’t account for viral genetic variability, meaning it’s difficult to develop a diagnostic test that captures all strains. This is one reason why parechoviruses have been under-diagnosed in years gone by.

Researchers at the Queensland Paediatric Infectious Diseases (QPID) laboratory ae reported to have developed a test for the virus strains in circulation in Australia, and there are thought to be at least four. Similar tests are available elsewhere in Australia.

Sequencing the RNA of parechoviruses isolated from human infections has led to at least 16 genotypes being described. Molecular analyses also allow better understanding of how the viruses cause disease, how more reliable diagnostic tests might be designed, and how antiviral drugs or vaccines might be developed.

Hope for better treatment

There are no antiviral drugs for parechovirus infections: treatment is essentially supportive.

Laboratory confirmation of infection does allow unnecessary antibiotics and other treatments to be discontinued, reducing treatment-related adverse effects and cost. It also helps doctors give better advice about disease prognosis.

There are no parechovirus vaccines, although the recent successful trials of enterovirus 71 vaccines and the longstanding success of polio vaccines offer hope.

The parechovirus story is another example of how molecular methods have increased the diagnostic rate of common clinical syndromes in children, such as respiratory infection, encephalitis and gastroenteritis.

The fact that the causative agents of a significant proportion of these syndromes remain undiagnosed means there’s plenty more investigative work to do.