An Ode to Quantifying Infection Risk in Addition to Prevalence

When you’re studying parasites (or symbionts or pathogens), the prevalence of the parasite in the host population is one of the easiest response variables to measure. That’s not to say that it is easy; there are certainly a variety of methodological difficulties that crop up, and it can be expensive to run lots of blood tests if you’re looking at seroprevalence. But getting a prevalence estimate is certainly a lot easier than pinpointing when each host becomes infected (e.g., via mark-recapture methods) and/or calculating the actual risk of infection (i.e., the rate that susceptible hosts become infected = force of infection). For that reason, we often use prevalence as a response variable, and hope that we can infer things about parasite transmission based on those data. Sometimes, it works out great! For instance, in 1854, John Snow (the physician, not the Brother on the Wall) mapped the locations of Cholera cases in London. By pinpointing an area of high incidence on the map, he found a water pump that was probably an important source of infection in the epidemic. But do areas of high disease incidence or prevalence always occur in areas of high disease exposure?

Littorina littorea, the common periwinkle, is an abundant and widespread marine snail that hangs out in the intertidal zone (various levels of exposure to the air with the tides) and the subtidal zone (almost never exposed to air). Periwinkles are hosts for a few different trematode species, but for today, we’ll just focus on Cryptocotyle lingua, which infects snails, then fish, then shorebirds. Snails get infected when they consume trematode eggs from shorebird feces. ‘Loitering’ shorebirds are 6-20 times more likely to hang out in the high intertidal zone than the low intertidal zone, and as a result, the density of shorebird feces in the high intertidal zone is 70 times higher than in the low intertidal zone (Byers et al. 2015). Therefore, it is not surprising that when uninfected ‘sentinel’ snails were placed in field cages in the high and low intertidal zones, snails were four times more likely to become infected in the high intertidal zone (Byers et al. 2015). In fact, the probability that an uninfected snail would become infected in the low intertidal zone was effectively zero. That makes sense, because bird guano was almost never found in that zone.

So, when Byers et al. (2015) went out and sampled periwinkles in the high and low intertidal zones, they found way higher prevalences of infection in the high intertidal zone, where infection risk was high, right? WRONG! The prevalence of infection was much higher in the low intertidal zone, even though snails do not become infected there! How could that be?

First, let’s back up and talk about an important selection pressure in the low intertidal zone: predation. There are extreme size-dependent predation pressures in that zone that pretty much prevent small/young snails from living there. So, the only snails in the low intertidal zone are bigger/older snails. Big/old snails are much more likely to be infected by trematodes than small/young snails, because they have had longer to be exposed and become infected. But we know that the big snails aren’t becoming infected in the low intertidal zone, so where are they coming from? It may be that young snails hang out in the high intertidal zone, escaping predation but experiencing high infection risk, until they are big enough to safely live in the low intertidal zone. Once big enough, the snails migrate to that low zone, which provides better foraging opportunities, and the high density of big, infected snails results in high prevalences of infection (76% infection!) in an area that has effectively zero risk of infection. Isn’t that neat?!

So, as Byers et al. (2015) point out, “disease risk and prevalence patterns need not be tightly coupled in space.” I think that’s important to remember when we’re deciding what response variables we want to consider in ecological and epidemiological studies.



Byers, J.E., A.J. Malek, L.E. Quevillon, I. Altman, and C.L. Keogh. Opposing selective pressures decouple pattern and process of parasitic infection over small spatial scale. Oikos.

Parasites and Snail Personality

When individual animals show consistent behavioral responses in different scenarios or in similar scenarios across time, and individuals vary in their responses, we can say that there are behavioral ‘types’ or personalities in that animal population. For instance, some animals might have aggressive personalities, where those individuals are consistently more aggressive than other individuals in the population. Personality is getting a lot of attention in disease ecology right now because particular animal behavioral traits (or suites of personality traits = behavioral syndromes) might increase an individual’s risk of becoming infected by a pathogen or the probability that an infected individual transmits a pathogen. For instance, I recently blogged about how Tasmanian devils that receive many head wounds in aggressive encounters (=lower social rank individuals) are less likely to become infected by Tasmanian devil facial tumor disease than individuals with fewer head wounds (=more aggressive/higher social rank individuals).

In a recent study, Seaman and Briffa (2015) set out to determine (1) whether snails have personalities and (2) whether snail personality traits are related to trematode infection status. The personality trait that they considered was “re-opening” time, which was a measure of how reluctant the snail was to re-open it’s operculum after being poked by the investigator. I’m not ashamed to say that this reads like an excerpt from the description of my dream job: “All observations were carried out by a single observer who had practised touching snail’s feet with a consistent level of pressure.”

Seaman and Briffa (2015) found that snails did have consistent responses to the mock predation encounters, where individual snails took consistently more or less time than average to re-open their operculum. Additionally, snails that were first intermediate hosts for trematodes (i.e., castrated snails) had longer re-open times, on average.

Because Seaman and Briffa (2015) used uninfected and infected field snails – as opposed to experimentally infecting their snails – it is unclear whether the differences in snail opening times is driven by infection, or whether the differences in opening times somehow affects the probability of becoming infected. Even if infection is driving the different mean response times, it doesn’t mean that the parasites are manipulating the snails. For instance, it might just be that infected snails are showing a sickness response, which makes them act sluggish. (Heh, get it?) Or it could be that the trematodes are manipulating the snails to make them behave more cautiously, thereby decreasing the probability that the host (and parasites!) gets eaten by a predator.

Because there are multiple potential mechanisms at work, I couldn’t decide on the dialogue of this cartoon. So, pick your favorite!





Seaman, B., and M. Briffa. 2015. Parasites and personality in periwinkles (Littorina littorea): Infection status is associated with mean-level boldness but not repeatability. Behavioural Processes.


I finally took a picture of a great blue heron (Ardea herodias) today!  I see them frequently when I first arrive at ponds, but they spook easily, so they’re usually long gone before I get my camera out.


“You’re interrupting my frog-eating.”

I also saw what might have been a green heron (Butorides virescens) yesterday.  I’m not a very good birder (yet), so I’ll have to see it again before I can be confident in my identification.  Here’s a photo of a green heron that someone else took:

Green Heron in flight (Photo by Alex Lamoreaux)

Green Heron. Photo by Alex Lamoreaux.

And now, for the parasites!  One cool thing about having this blog is that I’m motivated to do things that I might not normally do, but that I probably should do.  For instance, I went to the Natural History Museum host-parasite database and looked up the parasites of the great blue heron.  There are a lot of potential parasites!

One of those parasites is the trematode Ribeiroia ondatrae.  You might have heard of this parasite because the metacercariae stage of the trematode encysts in larval amphibians, and is known to cause limb malformations in frogs.

A frog with limb malformations caused by the trematode parasite Ribeiroia ondatrae. Photo credit: Nature

Because Ribeiroia can cause these limb malformations, the parasite has been studied extensively to determine whether it is one partial cause of the worldwide decline in amphibian populations.  The jury is still out on that one!  I’ll probably talk more about Ribeiroia in the future, but for now, here’s a bit more information from the Parasite of the Day blog.

Any one else have heron pictures to share?  Or perhaps better yet, do you know of any other cool heron parasites?

Zombie Snails

Today is my one month blog anniversary!  Yay!  In honor of this occasion, I decided to post about something very fun:  ZOMBIE SNAILS.

Animals Animated Gif on Giphy

(GIF from giphy)

The parasite in question is the trematode Leucochloridium paradoxum, which is also called the green-banded broodsac.  The adult trematode lives in birds, and parasite eggs are shed in the birds’ feces.  Snails consume the eggs, and get infected with the intermediate parasite stages.  If you’re interested in trematode life cycles, you’ll find the details of the sporocyst to cercariae stages BIZARRE.  Check out this blog, if you’re interested.

Anyways, the parasite does two things to manipulate the snail.  First, it makes the snail hang out in the open, exposed to predation.  Second, it turns the snail’s antennae into psychedelic, pulsating, caterpillar-like beacons of yum for birds.  Birds nip off the antennae full of cercariae (infective parasite stages), get infected, and the life cycle continues.  Awwwwwesome.

You really need to watch this short video about zombie snails before you can consider your life complete.

So far, I haven’t gotten any comments on my blog.  My goal this month is to get some feedback from my wonderful audience.  To break the ice, I’m going to start with a poll.  So, how do you feel about zombie snails?