The microbiology of zombies, part V: beware the bite?

Now that seemingly the flu outbreak storyline has been wrapped up on The Walking Dead (unsurprisingly, but disappointingly, with their ineffective treatments proving to be miracle cures), there’s still one more zombie microbiology topic I’d like to cover: what’s up with the bite, and is it the cause of death? I said previously:

“We know the pathogen can certainly be spread by bites and then cause zombification that way…”

but one commenter disagreed, noting:

“I don’t think we have evidence for that from the show. I think it clearer that zombie bites cause death, and there doesn’t seem to be evidence that the agent that causes death also causes zombieism (or vice versa). In Walking dead, any death is a sufficient condition for becoming a zombie. I would guess that zombies cause death because of a massive polymicrobial infection/sepsis.”

So, could death be due to massive sepsis (an overwhelming immune response to infection, which can lead to organ failure and death) via the bite, rather than the introduction of a specific zombie pathogen? It’s certainly not the first time I’ve seen that argument. Even the Zombie Research Society has put that forth as a hypothesis (and Matt Mogk has written of it in his book as well). However, I don’t buy it for a few reasons.

First and foremost, human bites simply aren’t that deadly. Even in a study of patients presenting to emergency rooms (which are probably the most serious of bites), none were found to have sepsis. Well, you might say, maybe more would have had this if antibiotics weren’t available, which would be the case with TWD (well, except now they have them, but I digress…) A Medscape article addresses this, noting that prior to the antibiotic era, up to 20% of bites caused amputation of a finger–but still, a local nasty infection, not necessarily sepsis. Even in a 1936 NEJM paper studying bites, only 2 deaths are noted, and both are in “delayed cases”–individuals who waited 5 days post-bite to present to the hospital. In these cases, the cause of death is indeed listed as “extensive sepsis.”

However, it should be noted that hand bites in particular—the subjects of those papers and articles above– seem to be rather nasty. Per Medscape again, “… most human bite injuries occur on the hands, and hand wounds from any cause have higher infection rates than do similar wounds in other anatomic locations.” So, these papers focus on the worst types of bites (hand injuries) at the most severe locations (presenting to emergency rooms), and thus should be considered likely a worst-case scenario for our potential infectees: that, even if bitten, a minority of them would have serious complications, and a minority of those might perish of sepsis. This doesn’t match up with what we see in the show.

You might argue that the process of zombification would modify/increase the nasty bugs living in the mouth. I agree–rotting in this manner certainly could alter the presence and types of organisms that would be present in the mouth, and therefore possibly make them more deadly and more likely to result in a sepsis death of the bitten. However, even accounting for rapid reproduction rates for microbes (mostly bacterial when you’re talking about sepsis and oral germs), this doesn’t seem to be a satisfactory answer, as one can quickly die and be reanimated and immediately have the potential for a deadly bite. It could also be argued then that therefore *everyone*, living or dead, would also possess this quality in that case–it shouldn’t matter if the bite is from a zombie or from a living person; the result should be the same (sepsis and zombification).

Further, in the human bite literature, there are two types of bites typically described: occlusive bites and clenched-fist injuries. The former is probably what you think of when you think zombie bites: mouth open, teeth coming together on the skin, chomp, chomp, chomp. Clenched-fist injuries are what happens when someone strikes another person’s teeth with, as the name suggests, their clenched fist, often scraping the knuckles: basically, a punch that strikes the teeth/mouth. While on the Walking Dead the former universally mean death (except in the case of really quick amputation of the bitten part, like we saw with Hershel’s leg), we’ve seen many examples of the latter—how many fistfights has Rick alone gotten into now? Not to mention, scenes like this:

 So if one is going to support the “polymicrobial infection as a result of bites” scenario for zombification, the issues of living biters need to be explained away as well.

Others have argued along similar lines regarding bites and sepsis, suggesting that the zombie bite is analogous to what happens to the prey of the Komodo dragon:

“Animals that escape the jaws of a Komodo will only feel lucky briefly. Dragon saliva teems with over 50 strains of bacteria, and within 24 hours, the stricken creature usually dies of blood poisoning. Dragons calmly follow an escapee for miles as the bacteria takes effect, using their keen sense of smell to hone in on the corpse.”

The problem with that analogy is that it’s based on a myth. That’s not what really happens: the dragon actually has venom, as I noted way back in 2005 (and Ed Yong updated recently, both based on the work of Bryan Greig Fry). It’s not their bacteria that kill their prey, but their venom. Do zombies suddenly become venomous? Doubtful. So, another idea shot down.

To me, the most convincing scenario, and the one that seems to jibe with both the idea that everyone is infected and with the little we know about the epidemiology of the outbreak, is that the immune system keeps the “zombie virus” under control while one is still alive and healthy. When one dies, the virus is allowed to replicate unchecked, resulting in both zombification/reanimation as the infection proceeds unabated throughout the body. The virus would also replicate (probably within the salivary glands) in order to enable transmission to the next bite victim. A zombie bite then introduces a large amount of this virus right into the bloodstream of the target, which overwhelms the body’s defenses and is responsible for both death and subsequent zombification—like rabies virus on steroids—and the cycle perpetuates itself.

Bottom line is that with the sepsis model, you have to explain more anomalies than with a virus-death model. You’d need to postulate immediate changes in the oral microbiome that aren’t readily accounted-for, but would be responsible for the 100% fatality rates upon receiving a bite (but ONLY a zombie bite, and not a live-human bite), while with the novel zombie virus model you get a bit more carte blanche to account for the transmission and certain death. That seems a much better explanation to me.

 Works Cited:

Welch CE. Human bite infections of the hand. NEJM, 215:901. 1936.

Talan DE et al. Clinical Presentation and Bacteriological Analysis of Infected Human Bites in Patients Presenting to Emergency Departments. CID, 37:1481. 2003.

See also:

Part I: the microbiology of zombies

Part II: ineffective treatments and how not to survive the apocalypse

Part III: “We’re all infected”

Part IV: hidden infections

The microbiology of zombies, part IV: hidden infections

(As previously, spoilers abound)

So on this week’s Walking Dead soap opera, we find that Daryl/Michonne’s group is still out and about searching for medical supplies. Back at the prison, the food situation is dire (apparently all the food stores were in the cell block where the infection broke out), so Rick and Carol head out to look for both medicines and food from the local ‘burbs. During their outing, discussion ensues of Carol’s attempt to stop the prison’s apparent influenza outbreak by killing two people who, at that point, were the only ones showing symptoms of disease. Rick decides he can’t trust her, and ends up banishing her from the group.

Carol said multiple times that she was trying to do the right thing, to protect the rest of the group from those who were sick and was only trying to end the outbreak. However, here’s where some knowledge of infectious disease would have helped her. Every disease has an incubation period: the time when the microbe is multiplying in your body, but you’re not showing any physical disease symptoms yet. This can be short–as little as perhaps a few hours for something like Salmonella food poisoning. It can be extremely extended, as I mentioned with rabies virus in my previous post, where the incubation period can be months to years. With influenza, the typical incubation period is 2 days, but it can be as short as 1 or as long as 4-5. The kicker is that a person who’s incubating flu can still spread it even before they show symptoms of the illness. So just because Karen and David were the only ones actively coughing and looking miserable, Carol was mistaken in her assumption that they were the only ones infected, and that she could stop the outbreak by snuffing them.

This is the difference between two similar concepts, quarantine and isolation. People who have been *exposed* to an infectious agent, but are not yet showing any signs of illness, can be quarantined to keep them away from others due to their *potential* to spread a disease. Those who are already showing signs and symptoms are placed into *isolation* to keep them from spreading it–they’re a known quantity. The prison group has used primarily isolation to keep the infection from spreading: they’re putting the ill in the Death Row cell blocks as an isolation area, and those who are well can roam around as they choose. (Maggie, for instance, hasn’t been sent to quarantine even though she clearly was exposed to the illness by being in such close contact with Glenn).

However, one thing that the group hasn’t yet determined (probably because no one has recovered as of yet) is how long they’re going to keep anyone who gets better in the isolation area. Though adults usually stop releasing influenza virus even before their symptoms are completely gone, kids can shed the virus for a long time: up to two weeks after their symptoms started according to one study (and others have found similar results). So while right now they have the healthy young children segregated from everyone else for their own protection, in theory, if Lizzie (the flu-infected child currently in held in isolation) gets well and is released back to the healthy kid’s room, she could simply re-start the outbreak there, among the most susceptible. 

This is why disease eradication is so difficult, and why it’s been accomplished for so few pathogens to date: many pathogens can spread on the sly, even when people don’t know they’re sick. For influenza, even if it’s knocked down in this group (and of course, it soon will be one way or another–at some point, the susceptible hosts in the prison will be exhausted, either by infection & recovery or by death), there is always another reservoir of disease out there. It may be other humans. Darryl/Michonne’s group finally made it to the veterinary school mentioned two episodes ago, and the zombies they ended up fighting there had clinical signs that looked an awful lot like the survivors had seen at the prison: blood that had come from the eyes and nose. Had flu been circulating there as well? It’s a vet school, pigs could certainly be housed (there were a number of animal cages, and could easily be an outdoor space for livestock somewhere). So pigs could be serving as a reservoir. Flu can also come from a number of other animals–most notably, birds, who don’t even have to appear sick to transmit the infection to people.

Infections can be sneaky and unseen, as this group should well know.

See also:

Part I: the microbiology of zombies

Part II: ineffective treatments and how not to survive the apocalypse

Part III: “We’re all infected”