Recent Human Respiratory Viruses of Epidemic and Pandemic Potential
Respiratory viruses causing human epidemics and pandemics have been a recurrent occurrence since the dawn of time. Many of these emerging viruses originate in animals and are associated with animal-human contact. They are termed “zoonotic “ in origin. These have occurred through the complex interactions between humans and animals. The mode of transmission is often through the respiratory route but may not always be immediately apparent. These viruses typically cause sudden onset of illness, rapid human and human transmission and significant mortality. As such, these new and novel viruses typically instill widespread fear. Besides concerns about morbidity and mortality, such epidemic and pandemics can also have economic impact on the countries affected. For example SARS CoV pandemic that started in 2002 had been estimated to result in an economic cost of between USD 35 to 45 billion to Asian countries and Canada. Singapore was one of the countries affected with schools being closed and much of its economy temporarily grinding to a halt. In the more recent example of MERS CoV in South Korea, no official figures were available on the economic impact of this epidemic but it is likely to have been substantial. This has prompted the South Korean government to institute additional measures to instill confidence for its tourism industry.
In this short article, recent information on MERS CoV and avian influenza will be highlighted.
Middle East Respiratory Syndrome Corona Virus (MERS CoV)
MERS CoV was first reported in Saudi Arabia in June 2012 and cases have continued to be reported up to September 2015. The virus has probably been circulating in the Middle East much earlier. A retrospective investigation of a hospital respiratory infection outbreak in Jordan that occurred in February 2012 identified 2 confirmed cases of MERS CoV and 11 probable cases. Of note, ten of the probable cases were healthcare workers.
As of September 18th 2015, the number of confirmed cases worldwide was 1564 with at least 551 deaths. More than 20 countries have reported cases of MERS CoV. All the cases have originated from or had history of travel to Middle East. A few secondary cases have also occurred. The epidemic has been ongoing in the Middle East and with 17 new cases reported from Saudi Arabia in the week of September 11th to 17th 2015.
As alluded to earlier, many cases have been acquired in the health care setting probably through human to human transmission. This is best exemplified by the MERS CoV outbreak in South Korea that occurred recently from May to July 2015. The index patient was a Korean man who traveled to the Middle East and subsequently sought treatment at several medical centres in South Korea when he fell ill. The web of transmission with secondary and tertiary cases resulted in a total of 186 cases and 36 deaths. Strict measures were instituted and more than 16000 persons (including health care workers) were quarantined. This outbreak seems finally over with no new cases in South Korea since July 4 2015.
Although the dromedary camel is thought to be a possible source of MERS CoV, the exact transmission route remains uncertain. The current management is to raise awareness of MERS CoV and to institute strict infection control measures (however unpleasant) when faced with a possible case. Any lapse of such infection control measures will easily result in outbreaks within the country.
Influenza And Avian Influenza H7N9
The Influenza virus is familiar to all of us. The virus causes annual epidemics and strains of influenza A and B viruses continue to be circulating in the community. The medical advice is to get annual influenza immunization as the virus undergoes minor changes frequently. Pandemics that result in millions of cases globally occur approximately once every 20-30 years when a significant protein shift has occured in the influenza strain. The last pandemic in 2009 that started in Mexico was due to a H1N1 strain that had undergone reassortment. This virus remains in circulation.
Avian Influenza Viruses (AIV)
There are many influenza viruses that cause infection and circulate among birds and are called avian influenza viruses. These AIV may circulate between wild migratory birds and domestic poultry. They rarely cross species barrier to infect humans and other mammals. Continuous antigenic shift and drift occurs within this circulating pool of AIV and may result in novel viruses. Four such instances occurred in the past 100 years and resulted in pandemics. Influenza A H2N2 occurred in 1957, H3N2 in 1968 and H1N1 occurred twice in 1918 and 2009. The AIV that have received most attention in the past 10 years were H5N1 and H7N9.
Avian Influenza A H5N1
This first occurred in 1996 and caused 18 human infections with 6 deaths. The virus recurred in 2003 and has become enzootic in a number of avian species across wide geographic locations. 784 confirmed human cases have been reported in 16 countries with a total of 429 deaths. Case fatality rate has remained high at 55%. The number of human cases appeared to have declined in recent years and the recent focus has been on the H7N9 strain.
Avian Influenza H7N9
The novel influenza A H7N9 was first reported to infect humans in early 2013. From Eastern China it has spread rapidly to involve many other provinces in China and other countries. In contrast to H5N1, H7N9 does not result in mortality in domestic poultry and this makes control of this virus more difficult.
As of September 2015, there have been 677 cases reported with mortality of 36% in those less than 60 years of age and higher mortality of 59% in those aged 60 years and above. Cases in other countries have all been linked to travel to China. Family clusters of AIV have been reported and may have occurred as a result of common exposure to the virus or may be due to limited human to human transmission.
Although the number of human cases has declined in 2015, it is prudent to remain vigilant against H7N9. At present, no commercial vaccine is available against influenza H7N9.
Once again, novel respiratory viruses infecting humans have been a regular occurrence for most of our recorded history and we must expect that we will hear of such new viruses in the coming years.