CLINICAL MATTERS

A lesser-known virus or bacteria causes an outbreak somewhere in the world and is reported on the news. People think that another terrible pandemic is upon us and the fake news on social media goes into overdrive, causing widespread panic. This scenario is unfortunately becoming commonplace in this age of information, or more accurately, misinformation. The phenomenon isn’t new but is occurring with some concerning regularity since the emergence of the SARS-CoV-2 Omicron variants of concern. It previously happened when there was a rise in RSV (respiratory syncytial virus) cases, and when Mycoplasma (an atypical bacteria) infections increased. The latest pathogen to cause dread in the “susceptible population ”is the virus known as the human metapneumovirus, or hMPV for short, after reports of an outbreak in China.

hMPV is not a new virus. The reason it sounds new to many people is that, up until the last few years, it has been very difficult to diagnose. hMPV is just one of the many respiratory viruses that cause the common cold. These include different strains of rhinoviruses, enteroviruses, non-SARS coronaviruses, adenoviruses, and parainfluenza viruses. Unlike bacteria which can be readily grown in the laboratory, viruses require special techniques for detection, and it takes a long time to grow and isolate viruses in culture.

Traditionally, viruses have been diagnosed retrospectively using antibody tests which only become positive after the infection has run its course. Following the increased availability of PCR because of the Covid-19 pandemic, new multiplex PCR panels that can detect many viruses simultaneously are now part of mainstream medicine .These were previously prohibitively expensive and required fully equipped research labs. With advances in technology and automation, these tests have become easier to perform and the costs have gone down substantially. From a public health perspective, the use of these panels and subsequent genomic sequencing of viral RNA and DNA has been a huge boon in diagnosing and monitoring outbreaks. This logically means that we are now able to identify and monitor many different viruses, including those that are not as well known to the public. What we used to collectively call “common cold viruses” are now able to be named and differentiated with the new tools we have. Unfortunately, this leads to an artificial rise in the number of confirmed cases of each virus, just because we can now detect them. Without proper context, it can easily look like an epidemic of a new virus.

hMPV was definitively identified for the first time in 2001 using new molecular techniques. It is, however, estimated to have been circulating in the human population for more than 50 years based on serological data. This highlights the difficulty in diagnosing these elusive viruses despite hMPV causing up to 10 percent of acute respiratory illnesses in otherwise healthy children. hMPV is an RNA virus and generally causes mild disease. It can cause pneumonia in young children with immature immune systems and in elderly and immunocompromised persons. It is usually milder than the flu and most people recover without any problems or complications.

hMPV is most closely related to the avian metapneumovirus. It likely jumped from birds or another intermediate host to humans a long time ago. This is why it is considered to be of low pandemic potential because the virus has already adapted to humans and there is at least partial immunity to severe disease. In addition, it does not have the extreme genetic variability or recombination potential seen with either influenza or SARS-CoV-2. Almost all children by the age of five years old have been exposed to hMPV. In contrast, SARS-CoV-2 jumped to humans at the end of 2019 and there was no immunity to it, causing severe disease in a disproportionate number of people.

Signs and symptoms of hMPV are difficult to distinguish from other respiratory viruses that cause upper respiratory tract infections. These include a runny nose, sneezing, and congestion. Many people develop headaches and muscle pains. Fever may or may not be present but can be high grade, especially in more severe diseases. There are no specific antivirals that have been shown to be effective against hMPV, although candidate drugs are being studied. There is no approved vaccine, but several vaccine candidates including a combination mRNA-based vaccine for RSV and hMPV are in early clinical trials.

In the absence of effective treatment and a working vaccine, the prevention of hMPV is similar to that of other respiratory viruses. Wearing a medical-grade mask or respirator can decrease the risk of infection. Good hand hygiene and proper sneezing and cough etiquette can also decrease the risk of transmission. Staying home rather than going to work or school when sick can decrease the number of sick contacts and aid in faster recovery. Diagnosis of hMPV is through the use of PCR-based respiratory virus panels which are usually only done for patients at risk of severe disease due to the relatively high cost. Several hMPV antigen tests have been developed but are not available in the Philippines.

In the meantime, the hMPV outbreak in China seems to be winding down. While cases did increase, the healthcare system was not overwhelmed and there was no indication of more severe disease compared with the past seasons. The overall number of cases does not seem to be different from the range of other seasonal hMPV outbreaks in the past. hMPV outbreaks are also currently occurring in other countries especially in the Northern Hemisphere, with the United States reporting a recent uptick in cases. The US Centers for Disease Control and Prevention notes that the increasing trend in cases is not a cause for concern and is still within seasonal projections. In the Philippines, the Department of Health reported 284 confirmed cases of hMPV for 2024, which made up 5.8 percent of samples that tested positive for respiratory viruses. This shows that this is not a new virus even in the Philippines. The bottom line is that not everything unfamiliar is necessarily new, and increases in cases need to be considered within the context of increased testing. Before we click and share and run off like headless chickens, it is best to fact check and ask the experts.