A pretty cool (pun intended) story’s cropped up about the influence of cold temperatures on the body’s response to Rhinovirus infection – one of the viruses that leads to the common cold.
In brief, the interferon response pathway (which counteracts virus infection inside cells) doesn’t work as well at temperatures colder than 37°C. When the research team at Yale University School of Medicine compared interferon production in mouse cells at 33°C and 37°C, they found far more at the hotter temperature. Conversely, when they looked at Rhinovirus growth, surprise surprise, the virus grew better in the low temperature / low interferon environment.
This is neat, because it suggests that cold temperature may actually increase your susceptibility to getting a cold, just like your family always told you. I say may, because this study was conducted in mouse cells using a mouse-adapted strain of Rhinovirus, but this is intriguing evidence that your nose may be a frosty gateway to virus misery.
This is also interesting given a wonderful talk I listened to by Cardiff University’s Prof. Ron Eccles at the Society of General Microbiology conference in Liverpool last year, in which he discussed the symptoms behind the common cold. Most notable to me was his mention of how the insides of our nostrils reciprocally expand and relax, due to the dilation of large veins in our nose. This expansion becomes exaggerated during infection due to inflammation, which results in a horrible blocked nose (here’s a cool paper – not Open Access, sorry).
Not only was this interesting enough, but he hypothesised that this could be a defence against virus infection. How? The blocked nostril steadily increases in temperature and the poor sucker of a virus has a hard time growing as it finds itself out of its comfort-zone. If this is true, perhaps it also gives the innate immune response the jump-start it needs to get to work.
This holiday season, many of us will have enjoyed the convenience of ordering presents online. Want gift, find gift, buy. Behind the scenes, your gift was found in a warehouse full of other stuff, selected by a person or machine, labelled with a delivery address and transported to the right place (hopefully your door!). But what if you’re a flu virus, hanging out inside a lung cell with only the gift of more flu on your wish list?
Down at the size of molecules, nobody is going to take your order. Influenza has a genome made out of 8 separate segments of RNA, each containing genes essential for building the virus. Imagine one long set of instructions, but chopped into eight pieces. If you have one complete piece of genetic material – like Poliovirus, for example – you have the easy task of sticking exactly one copy of RNA into one empty virus shell before leaving the infected cell. If you’re flu, you need to collect eight pieces: eight different pieces.
Headache. But just how good is flu at stuffing itself with RNA?1
According to an article published in 2013, flu doesn’t care much for finishing the full set. When Brooke and colleagues2 looked at infected cells using fluorescent antibodies to see the flu virus’ proteins, they found many in which at least one protein was missing. Some of these proteins included HA and NA, proteins sitting on the outside of the virus that act as both the key into cells and the knife to cut the virus free – in short, proteins the virus needs to spread between cells and people.
This observation is weird because it suggests that some flu viruses either don’t carry all eight RNA segments (after all, no HA gene segment in the incoming virus = no HA protein in the infected cell), or that individual genes just randomly fail to be read during infection.
To the last point, the authors infected the same type of cells with Vesicular Stomatitis Virus (VSV), which has a genome made from just one piece of RNA. When they looked at the production of two VSV proteins during infection, they couldn’t find cells in which one or the other was missing – failing to make proteins at random doesn’t look like a regular foible of virus infection. The authors are also trying to make the point here that unlike flu, VSV can’t misplace any of its genome, and perhaps this explains the difference between the viruses when it comes to gene expression3. Maybe. But these data are a bit weak for me – they’re different viruses. You wouldn’t infer dog behaviour by studying cats.
Much better evidence comes from flow cytometry experiments analysing many thousands of cells for the presence and absence of individual virus proteins. Not only is this approach higher throughput, but it also allowed the authors to study the effect of increasing the amount of flu used to infect the cells. When flu virus was outnumbered by cells, more than half of all infected cells were shown to have at least one flu protein missing. But by increasing the amount of flu so as to outnumber the cells, the vast majority then contained a full set of virus proteins4. Whilst a large number of individual flu viruses are sloppy enough to forget segments of their genome, when they gang up in the same cell, together they have everything they need.
Looking for proteins is cool, but viruses live and die on their ability to transmit themselves to new cells. When the authors looked at flu’s ability to spread to new cells5 they found that this happened just over a tenth of the time. By all accounts, this suggests that influenza is rubbish at its job of making more flu. And yet yearly flu epidemics suggest otherwise.
I think we virologists6 get hung up on a faux “physiological relevance” when we study viruses in a dish. We have the ability to infect cells with billions of virus particles, but as most people don’t get infected by a scientist wielding a pipette, we imagine the individual invaders staking their claim to just a few cells in our body. Therefore, when it comes to scientific thinking, experimenting with low virus concentrations appears to be inherently more “realistic” than high concentrations. This thinking is probably correct when a host is first infected, but once each hijacked cell begins to spew out hundreds of new viruses into the neighbourhood, co-infection of nearby cells with 10’s to 100’s of viruses is probably common. All this is to say, if flu needs more than one virus to infect a cell successfully, no problem.
That aside, what I find most interesting about this work is the idea that influenza infects as a swarm of viruses by necessity. Flu, like many RNA viruses, is renowned for the sloppy reproduction of its genetic code. What we have here is not only a virus that creates a population of mutants in its host, but a virus that appears to be critically dependent on co-infection of cells to successfully complete its life cycle.
Some final thoughts on this study. I’d have liked to see some effort to address the impact of defective interfering particles (DIPs) on these results. DIPs are viruses with a large deletion in one or more of their genome segments and crop up in lab stocks of flu. Cells infected with DIPs would appear just the same as the cells studied in this work: missing one or more flu proteins. While I doubt DIPs can explain the high frequency of cells observed to lack flu proteins, the suggestion that influenza often fails to package its entire genome is extraordinary and thus needs all the evidence it can get – including ruling out a significant contribution from other known phenomena.
Also, regarding the infectious foci assays, it would be interesting if the relatively rare clusters of infected cells the authors observed were actually the result of co-infection themselves – just how often can a flu virus go in alone and be successful?
This study looked at flu infection of cells in a dish, but what’s the story in a complex species like a rodent or human? More on this next time.
I’m only covering one article today (and another related piece soon!), but I’ve no desire to sell the field short: there’s a wealth of research into flu genome packaging that I can’t get into now. Here’s a great open access review if you want to get your teeth into this area: Hutchinson et al. 2010.↩
Authors! This would be way more compelling if you could have looked at more than 2 VSV proteins, and also quantified the analysis with flow cytometry to make a better point. #nitpick↩
I’d like to bring you a number here, but this isn’t directly quantified in the paper. Check out the flow cytometry in Figure.3D to see how the top right hand quadrant of the graph (NA and HA co-expression) increases in intensity as multiplicity of infection (the number of viruses per cell) increases.↩
The authors used an infectious foci assay here, whereby they looked at cell layers with fluorescent antibodies to flu proteins again. After 15 hours they could see either single infected cells or clusters. In the clusters, the virus had colonised the neighbouring cells around the original victim.↩
I wrote a piece on the MERS coronavirus a while ago for the Cambridge University BlueSci magazine. The story has moved on since (with an increased number of cases in the Middle East, and the first infections recorded in the US, including human-to-human transmission), but it’s still a useful round-up of the beginning of the MERS story.
The full piece can be found at the BlueSci website (and in print), but here’s an excerpt I particularly enjoyed writing, even if it does stretch the term ‘animal reservoir’ to near breaking point:
When using the phrase ‘animal reservoir’, most imagine the animal kingdom spreading parasites and viruses via malarial mosquitoes, rabies-infected dogs, and Dustin Hoffman chasing a screeching ebolavirus- riddled capuchin monkey in the 1995 film, Outbreak. Not camels.
I aim to give Ebola virus and the current West African outbreak a proper write-up in the near future, but here’s a quick link to the Associated Press blog. The blog has an interesting piece about the stigma suffered by the virus survivors in Guinea.
It is perfectly understandable that the population is terrified of the disease, and to many, contact with the survivors of the virus is deemed too much of a personal risk. One of the most interesting parts of the piece is the way that health officials are trying to fight this public perception:
The most powerful tool to combat stigma is the way health care workers treat a discharged patient, said Corinne Benazech, the representative in Guinea for Doctors Without Borders in Guinea.
“The patient never leaves alone,” she said of when Ebola survivors leave their isolation wards, and health care workers individually shake hands with the survivor.
Discharged patients receive a certificate from the minister of health that states they are no longer contagious, said Tom Fletcher, an infectious disease physician with the World Health Organization who is working in Guinea.
There is a fascinating clinical addendum to surviving ebola: the virus can linger in surviving individuals. Immunologically protected sites within the body (parts of the body where the immune system cannot fully function) appear to clear the virus more slowly. Ebola virus has been found in such regions, including the testes, the anterior chamber of the eye (the fluid behind the cornea) and potentially the mammary glands (the discussion of this paper [open access] is useful here). Whilst the virus can persist in these sites, it is eventually destroyed. Condom usage and breast milk alternatives are used to prevent spread to partners and mother to child transmission.
Importantly, from the same paper cited above:
“The absence of EBOV [Ebola virus] in the urine, low prevalence on the skin, and rapid clearance from the saliva in surviving patients provides some reassurance that the risk of secondary transmission from casual contacts, fomites, or the sharing of toilet facilities in the home after discharge from the hospital is minimal.”
Bottom line: casual contact with surviving ebola patients is not risky.
Tom Fletcher from the WHO:
“These people should be celebrated, really, as opposed to stigmatized.”
“We are facing an epidemic of a magnitude never before seen in terms of the distribution of cases in the country,” said Mariano Lugli, the co-ordinator of Médecins sans Frontières’ project in Conakry, the capital of Guinea.
The organisation said on Monday it had been involved in dealing with nearly all other recent Ebola outbreaks, mostly in remote parts of central African nations, but Guinea is fighting to contain the disease in numerous locations, some of which are hundreds of miles apart.
You don’t need an overwhelming number of sick individuals to hinder virus containment. If the sick are dispersed across the country a concentrated relief effort is going to be less effective. Neighbouring countries are trying to prevent the virus entering in the first place, with Senegal closing its Southern border into Guinea. However, note the following from ProMED-mail’s Jack Woodall:
“Senegal has closed its southern border with Guinea.” Anyone who has lived in Africa knows this is impossible. Extended families live on both sides of the frontiers and people travel across unhindered on local footpaths.
The New Dawn Liberia reports that six people have been taken ill with ebolavirus in Liberia, of which five have since died. It appears that the cases originate from Guinea, but came to Liberia for hospital treatment.
A note to add to my previous post regarding contagion and spread: it’s not just the countries in which the outbreak originated that need to be watchful.
A nice roundup of the current ebolavirus outbreak in Western Africa in New Scientist.
Debora MacKenzie also raises an interesting question about whether an ebolavirus outbreak in an urban environment would give the virus a chance to mutate and become more contagious. Whilst an increased number of susceptible people would provide the virus with the ‘resources’ to mutate (i.e. more infections > more mutations > evolutionary selection for advantageous mutations), the comparison with avian influenza is a stretch. Avian Flu is a strain of a readily contagious, airbourne virus. Ebola transmission occurs through contact with infected body fluids; close physical contact is a requirement.
HIV, which is also mentioned in the article, provides a nice counter-point: it also spreads through contact with infected body fluids, but is of course now endemic in a number of countries. The difference to Ebola? Initial HIV symptoms are far more insidious and Flu-like. People spread the virus because they don’t know they have it. Ebola is a different story altogether. Haemorrhagic fevers are overtly scary diseases that hit around a week after infection. The ability for health enforcement to intervene in such an outbreak is very high because you can readily find out who is infected and separate them from susceptible, uninfected people.
So yes, the virus could evolve to more contagious in an urban environment, but the inefficient route of transmission the virus relies upon means that this would likely be a very slow process.
Curiouser and curiouser. When most people think ‘killer viruses’ they think Ebola. The current haemorrhagic fever outbreak in Guinea started in February 2014 and has so far claimed 59 lives. A crucial aspect of ebolavirus outbreaks is determining what strain of virus is responsible. Unfortunately, signs currently point to this outbreak being caused by Ebola Zaire (ProMED), the strain with the highest case fatality rate (up to 90%).
The ebolavirus is spread between people by close contact with infected bodily secretions. Disease surveillance is key for interrupting its transmission, such as by isolating infected individuals and protecting healthcare workers, who are often the next victims of the virus. So when signs of the disease hit the Guinea capital of Conakry, a coastal city home to just under 2 million people, health officials have been quick to act.
Tests have now shown that two fatalities suffering with Ebola-like symptoms at the time of death in Conakry were not infected with the virus. Whilst this may suggest that the outbreak has not reached the capital, it is nevertheless worrying that an unknown haemorrhagic fever may be lurking.
Of note, it now also appears that ebolavirus has spread into neighbouring Sierra Leone (ProMED), though we shall have to wait for tests to positively identify this disease.
National Geographic is running a story about a team from Aix-Marseille University that has discovered a previously unknown giant virus similar to Pandoravirus, buried for over 30,000 years in Siberian permafrost. If that wasn’t cool enough (pun… intended), they could get the virus to infect amoeba back in the lab.
Whilst it makes me slightly sad that the article descends into “BUT WHAT ABOUT THIS RISK TO HUMANS?”, I understand. The idea of a rare virus that destroys all humanity isn’t exactly hard to find in our culture. And whilst I do agree with the researchers (who made sure to test that the virus didn’t infect animal or human cells) that you have to be open to the possibility of finding a virus that is dangerous, it seems a shame to focus on that as the main story.
Quote from the article:
“The idea would make a great movie but is extremely unlikely unless the virus came from a frozen human being who possibly died from a virus that is no longer in circulation. A very small proportion [of the viruses on Earth] represent viruses that can infect mammals and an even smaller proportion pose any risk to humans.” – Edward Mocarski of Emory University.
Viruses are absolutely everywhere. We should be careful when we go looking for them, but we shouldn’t only be interested in them if they can infect us.