HMPV: It’s Entry Into Human Cells, Replication and Infection

Not long after the news of another viral outbreak in China which can cause similar disease related to the respiratory system as the COVID-19 started circulating, on Monday three cases were detected in India, two in Bengaluru and one in Gujarat. Yes, infections caused by the virus in focus, that is the HMPV (Human Metapneumovirus), have been detected in India. The harrowing memories of COVID are still afresh in our minds and a similar outbreak anywhere in the world naturally creates concerns, especially when the virus has knocked at our doors. 

Hang on! It may not be as deadly a situation as COVID, however exercising caution is the best way out. The ICMR is on tight monitoring of the situation with regular updated inputs from the WHO (World Health Organization) and assured the continuity of its vigilance throughout the year. The medical body has also urged the countrymen not to panic.

Pediatricians in Hyderabad have told media (The Hindu) that the virus is neither mysterious nor its presence in India is new. Every year, the HMPV is seen in India, specifically from around July, August to December, January.

It is not known whether China is witnessing the seasonal surge or a new mutated version of the virus is causing it. Winter generally sees rise in respiratory diseases. According to Chinese Center for Disease Control and Prevention, from 2009 to 2019, HMPV ranked eighth among the eight viruses that cause acute respiratory infections, with a positive rate of 4.1 per cent, which is far lower than the 28.5 per cent rate of the influenza virus. This was based on the monitoring data of respiratory infectious diseases in China. 

The union health ministry has said that there are no international travel histories associated with the cases that have been detected during surveillance. This indicates that the detected cases have no link to the outbreak of the viral disease in China. According to the ministry, the HMPV is already in circulation globally, including in India, and cases of respiratory illnesses associated with HMPV have been reported in various countries.

Infection Caused by HMPV

The CDC (Center for Disease Control and Prevention), USA says that the HMPV causes disease at the respiratory system. Normally, it attacks the upper respiratory tract UST), which comprises the nose, mouth, sinuses, pharynx (throat), and larynx (voice box). The UST plays crucial function in protecting the air pathways.

However, in severe cases, the HMPV can affect the lower respiratory tract (LRT) as well. The LRT comprises the lungs and the bronchi. Entering of the HMPV into the lungs can lead to pneumonia, while in bronchi, it causes bronchiolitis as well as pneumonia. 

Symptoms associated with HMPV infection include cough, fever, nasal congestion, and shortness of breath. However, this may progress to more worrying situation like bronchitis or pneumonia and are similar to other viruses that cause upper and lower respiratory infections.

In this situation, one may need critical hospital care and it is highly recommendable to see the doctors when more severe symptoms like shortness of breath and lowering of oxygen level is detected. Here comes the monitoring at the personal level—rush to the doctor immediately if these occur.

The estimated incubation period of HMPV is around three to six days. The incubation period is the time from start of infection to show the symptoms. The median duration of illness can vary based on severity.

There is no specific antiviral treatment for HMPV. But early detection can help manage symptoms and prevent complications. In addition, supportive care, such as oxygen therapy for severe cases, plays a crucial role. It is always advisable that  precautionary measures such as hand hygiene, mask-wearing, and avoiding crowded spaces, particularly during peak seasons be taken.

Notably, those at risk include children, elderly people and the immuno compromised individuals, meaning those who have a lower immunity.

The Entry to Human Cells

Can we recall a name that became widely popular breaking the boundaries of scientific naming? It was the S protein or the spike protein. It is a protein on the surface of SARS-CoV-2 virus (that caused COVID-19 pandemic) through which the virus can cling on to human cells.

The HMPV has proteins on its cell surface to disseminate similar function. A glycoprotein (G protein) on its surface attaches the virus to human cells. After the virus is attached to human cell surface, the next strategy of it is to fuse its cell surface along that of the human cell. This fusion of viral-human cellular surfaces is mediated by another protein named the F protein. It does so in the neutral pH environment that the host cell membrane maintains.

Notably, the F glycoprotein on HMPV is also a target for vaccine development against the virus. 
After the fusion, the virus releases its RNP (ribonucleoprotein) which serves as the template for the viral replication. The HMPV is also an RNA virus like that of the COVID. The HMPV viral RNA is single stranded.

Detection of HMPV in Laboratories

The HMPV was first detected in 2001 by Dutch scholars in nasopharyngeal aspirate samples of children with respiratory infections. Studies later showed  that the virus existed for at least 60 years, distributed all over the world as a common respiratory pathogen.

The technique of RT PCR (reverse transcription-polymerase chain reaction) is also applied in HMPV detection. This method amplifies viral RNAs collected from respiratory samples. It can even detect a meagre amount of RNA, making it suitable for early detection of infections.

The other important technique approach is the ELISA (immunofluorescence or enzyme-linked immunosorbent assays). This technique involves detecting viral antigens in respiratory secretions through.

This uses a combination of antibodies and enzymes to identify the target, producing colour change if the target is present.