The warmer weather has brought a lull in the number of infections reported in the press. It is a fact that, in the western world, almost all of our seasonal virus infections peak in the winter months. A combination of crowding, lower temperatures, lower levels of UV radiation (which inactivates viruses) and wet conditions or rather non-desiccating conditions, act together to favour virus survival and the cases of infection rise accordingly. Food borne bacterial infections show the opposite trend, peaking with the barbecue and salad season (Figure 1).
Figure 1. Google trends of the seasonal variation in infectious disease – Influenza in red and Salmonella (typical bacterial food poisoning) in blue. The data is for 2012 but is typical of any year except those with unusual events (like the 2009 pandemic).
But what has happened to the infections that were of such concern only a few months ago? Here is a quick update:
Norwalk like viruses
As predicted in these pages[1] the highly seasonal winter vomiting virus is no longer widely reported despite a very active season aided no doubt by the cold weather which extended to March and April. Isolated outbreaks still occur and cruise ships continue to be plagued by the virus as a result of their isolated and crowded status, a modern day cabin fever, but the general surge of cases is over for the time being. Six months from now will be a different story although you will be forgiven for thinking you have heard it all before.
MERS Coronavirus
The number of cases of the new coronavirus, whose epicentre of infection seems to be the Middle East, has declined. WHO reports a cumulative total of 96 cases and 46 deaths to date[2] but the incidence rate seems to have dropped although this could be, in part, because there were a number of cases in the system that were only confirmed once the causative virus was discovered. The source of the virus remains unknown: an animal reservoir is often suggested and a recent candidate has emerged from epidemiological tracing – the dromedary camel has shown the presence of antibodies to the virus which suggests they have been infected in the past. They obviously feature in the Middle-East but I am not yet convinced they are the source of infection: camel herders seem not to have been infected and surely those that race, milk or slaughter the animals would have received the virus. What about all the tourists who take camel rides and have their picture taken patting their steed, would they not have been obvious potential patients? We must wait for more data on how the virus gets into camels and the possible ways in which it could be transmitted further. I have suggested that intermittent contaminated food could be a source of infection although, as for other intermediate animals , there is no direct evidence for this. The possible routes of infection are summarised in Figure 2.
Figure 2. The possible routes to man for the MERS coronavirus. The current data is insufficient to suggest one is more likely than any other despite evidence for the virus circulating in camels.
Quite a lot of coverage has been devoted to the risk to travellers to the forthcoming Hajj. With such low numbers of infections and no real idea where the virus is, my stated view is that no special measures beyond those associated with large gatherings need to be taken.[3] I have an opinion piece on the virus in the August issue of the SGM’s Microbiology Today – take a look.[4]
Influenza H7N9
The incidence of H7N9 in China peaked in April and has since declined although sporadic cases continue to be reported. More than 130 people have been infected and about 1/3rd of them have died. This is the biggest breakthrough of H7 avian influenza to date, certainly the most serious and the combination of H7 and N9 is unique and may be part of the reason for the success of the virus in diverse hosts. To rehearse some well-known background, influenza A viruses are naturally distributed among avians and intermixing occurs all the time as the virus jostles within each host to make more of itself in the face of an immune response which tries to restrict it. Some of these combinations result in viruses that by chance are adapted to transmit and grow in mammalian hosts, typically pigs and man. The presence of such a virus in migratory birds is not really an issue as the chance of an encounter with man is very limited but if the virus gets into domestic poultry the opportunity for the virus to transmit to man goes up substantially and local infections, usually clustered, begin to be seen. While causing infections in people there is a finite chance that the virus will adapt to spread among people more easily and so trigger a pandemic although these events are rare (1957, 1968 and 2009 being the last true pandemics). In this case the H7N9 virus seems to have arisen by a three way mix of viruses in or around the Yangtze river delta during 2012 from where the virus moved into local domestic birds. The authorities’ action of closing live bird markets is presumed to have led to the sudden fall off in the number of cases but the virus itself is still around, just the opportunity to transmit to man has been made much less likely. As with the H5N1 virus, sporadic infections are likely to continue but as the virus has not shown an ability to transmit between people it is not thought currently to represent a pandemic risk.[5]
