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.

S Car Go

Hi, Folks! I’m still traveling for the holidays, so I can’t do a full post this week. But here’s a punny cartoon! (This doesn’t have anything to do with symbionts, unless you count the fact that both snails and caterpillars have many symbionts.)

Also, remember to vote on the best Parasite Ecology cartoon of 2014! So far, no one has voted Gary for Snail President.

SCarGoGet it? 😛

Pond Ecologist Christmas Carols

Merry Christmas, Everyone!!

This year, I tried my hand at composing some Christmas carols for pond ecologists. (I suppose these will be on Volume 2 of Songs of the Pond Ecologist.) Feel free to suggest more in the comments!

1. Jingle Snails

2. Deck the Pond (“…fa la la la laa, la la la snails….”)

3. Trematodes Rock (“Trematodes, trematodes, trematodes rock….”)

4. Rudolph, the red-striped mollusk (“…had a very slimy trail….”)

5. Grandma got run over by a mollusk

7. You’re a mean one, Mr. Pinch (song about crayfish)

8. I’ll be slow for Christmas (“…you can collect me….”; song about snails)

I usually make cartoons digitally, but as a special Christmas gift to you, my dear audience, I’ve taken photos of some of my exquisitely crafted hand-drawn Christmas cards. Enjoy!



How can you tell if a snail is dead?

Summer is right around the corner, and I’m getting all excited to start working with our new REU students.  This is a good time to remind myself of the importance of clear communication.  I am continually surprised by how what I thought I said was not at all what I actually said when what I said was interpreted by someone else.  You see?  All very clear.

Relatedly, how can you tell whether a snail is dead?  Well, if you haven’t worked with snails for very long, I can tell you that it’s surprisingly difficult!  For instance, when I was an undergrad, my friend and I went out and collected dozens of snails, only to return to lab to find that we had collected dozens of empty snail shells that were full of mud.  Oops.

I now have some standardized tests for snail aliveness.  This isn’t on my office door yet, but its only a matter of time.


Last year, I apparently had a communication breakdown, and I didn’t explain how to identify the snail species that I wanted everyone to collect.  Unsurprisingly, I ended up with more than I bargained for on that collecting trip!  Here’s a cartoon that I made after that.  For the few snail nerds that will stumble upon this blog, have you ever noticed how deceivingly Helisoma-ish Physa snails can look?


Wicked Cool Host-Commensal-Parasite System

I really like symbionts.  I really, really like interactions among symbionts, and I especially like it when commensals/mutualists eat parasites.  

So, it is with great pleasure that I introduce to you this urchin-crab-snail system.  The common pencil sea urchin (Eucidaris galapagensis) is a host for parasitic snails (Sabinella shaskyi and Pelseneeria spp) and commensal crabs (Mithrax nodosus).  And the crabs eat the snails!

Sonnenholzner et al. (2011) did some neat field and lab work to figure out how fishing for urchin predators affects parasitism of urchins by snails in this cool system.  Hilariously, they sum up their findings in the first line of the discussion by saying that they “found that the enemy (fisher) of the enemies (fish and lobster) of the enemy (crab) of the urchin’s enemy (snail) was the urchin’s friend.”  Swag.

Here’s the quick (and simplified!) version of their results, but I highly recommend checking out the paper!

I drew this fishing pole myself.

Do you know of any other host-commensal-parasite systems?  Bonus points if you guess my FAVORITE system of all!


Sonnenholzner, J.I., K.D. Lafferty, and L.B. Ladah. 2011. Food webs and fishing affect parasitism of the sea urchin Eucidaris galapagensis in the Galapagos. Ecology, 92(12): 2276-2284.

The Oatmeal – Parasite Zombies

Are you getting tired of parasite zombie posts yet?  I hope not, because I’ve got a lot more where these have come from!  Starting with this really awesome (I mean, REALLY AWESOME) cartoon by The Oatmeal.   

Dicrocoelium dendriticumthe lancet liver fluke, is a parasite of livestock (e.g., cows, sheep).  Of course, Parasite of the Day has written about this awesome parasite before.  I highly recommend popping over there for more details.  And of course, check out the full cartoon from The Oatmeal!


A taste of The Oatmeal‘s brilliant cartoon about the lancet fluke.

Trematode Biomass = Beetle Biomass = True Bug Biomass = Odonate Biomass

One big interest in parasite ecology right now is the quantification of parasite biomass in ecosystems.  Ecologists have spent a lot of time looking at energy flow through ecosystems and quantifying how much of the total biomass in ecosystems can be found in each trophic level and taxonomic group.  Until recently though, no one had quantified parasite biomass.  But since parasites eat a little bit of just about everyone, they might add up to a big chunk of the total biomass, right?

The initial work regarding parasite biomass in ecosystems came from three California estuaries.  There are several cool papers that have already stemmed from that work, and if you’re interested, you should check out the publications from the UCSB Ecological Parasitology Lab.  Lately, the Johnson Lab has been tackling the parasite biomass question in ponds by studying trematode parasites, specifically.

Remember when I said that parasites eat a little bit of everyone?  Well, according to Preston et al. (2013), if you’re a first intermediate infected rams horn snail (genus Helisoma), you’re actually 25% trematode larvae, on average.  That’s NUTS.  That’s like a human having an entire leg made out of parasites.  Because ~30% of snails were first intermediate trematode infected in ponds, roughly 2-4% of the total biomass for that snail species was actually trematode parasites.  NEAT!

Overall, the dry trematode biomass in the ponds was ~0.1 g per meter squared.  That was roughly equal to the dry biomass of beetles..and the dry biomass of odonate larvae (dragonflies and damselflies)…and the dry biomass of hemipterans.  In other words, there were a lot of parasites!  Preston et al. (2013) estimated that ~0.1 g of cercariae (just one trematode stage) would be produced per meter squared per summer.  Like I mentioned in a previous post about how cercariae get neglected from inclusion in zooplankton studies, all those cercariae make a great food resource for hungry aquatic predators!


Larval trematode biomass is equal to that of odonate larvae in ponds. (I know what you’re thinking. You’re thinking that I spent <5 minutes making this picture. You are correct.)

Ok, one last neat thing.  Preston et al. (2013) found that there was a negative relationship between the size of the cercariae (which varies by trematode species) and the number of cercariae that were released by a snail within 24 hours.  Preston et al. (2013) weren’t the first to discover this, but I still think it’s a very cool trematode example of evolutionary trade-offs between producing many offspring and big (high quality) offspring.  If you’re going to produce relatively small cercariae, you can produce a lot of them.  But if you want to produce big, energy rich cercariae, you can only crank out a few per day.

So far, we only have data for systems where aquatic trematodes (in snails) are a huge component of the parasite community.  What ecosystem should we quantify parasite biomass in next?


Preston, D.L., S.A. Orlofske, J.P. Lambden,  P.T.J. Johnson. 2013. Biomass and productivity of trematode parasites in pond ecosystems. Journal of Animal Ecology.

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?