Ticks suck moose dry

Like many New Englanders, moose aren’t particularly good at personal grooming. (I can say that because I’m a Mainer!) While deer and elk groom off winter ticks, moose do not, so moose are spending their winters covered in tens of thousands of engorged ticks. These tick populations consume an astounding volume of blood, so calves and even adult moose are being effectively sucked dry.

To give the moose some credit, they do try to groom off their ticks by scratching and biting their own fur, rubbing on trees, etc. Those behaviors aren’t effective at tick removal, though, and instead the moose end up rubbing off their dark outer hairs, leaving behind just their pale, broken hair shafts and bald patches. As a result, “Ghost Moose” are running around New England forests in freezing winter temperatures wearing nothing but their skivvies, trying vainly to produce enough blood to keep their own machinery running.

We’d expect to find that are these tick-infested moose are dying, and that appears to be the case. Estimating moose population sizes is not particularly easy, but it looks like New England moose populations are declining in some states. Additionally, scientists have found high mortality rates in radio-collared moose, especially during the later spring months when ticks are heavily feeding. And when the fresh moose corpses are found, they’re covered in engorged winter ticks.

But winter ticks on moose were documented forever ago in places like southern Canada, so why are they suddenly an issue for moose in New England? Climate change. New England winters haven’t exactly been a walk in the park in the past decade or two, but winters have been getting shorter, and shorter winters are probably better for winter ticks. Here’s what people think is happening: first, substantial snow pack isn’t accumulating until much later in the season, which gives ticks more time to find and attach to a moose host before the vegetation and ticks are buried under the snow. And then that snow pack disappears earlier in the spring, which means that when engorged winter ticks bail off their moose hosts in the spring, the ticks have an easier time finding places to lay eggs.

There is a bunch of potentially interesting parasite ecology here – like, probably at least one PhD dissertation project to be had. If you’re interested, here are some books and articles about winter ticks and a moose that you should check out:

This paper.

This book.

This Boston Globe article.

This post about the Isle Royal moose population.


Climate Change and Host-Parasite Temporal Mismatches

Climate change is (appropriately) a big topic in the ecological literature.  We have a pressing need to understand how climate change is going to affect individuals, populations, communities, and ecosystems.  And of course, we need to know how climate change is going to alter disease dynamics, so that we can be ready to control/manage important parasites and pathogens if we need to.

As I’ve discussed before, at one point, there was a lot of concern that the geographic ranges of diseases (like malaria) were going to increase dramatically with climate change.  In 2009, Lafferty published a paper where he pointed out that warmer isn’t always better for parasites.  He argued that we’re probably more likely to see range shifts than range expansions.  Similarly, a few months ago, I posted about a paper by Pickles et al. (2013) that supported Lafferty’s hypothesis.  Pickles et al. (2013) used models to show that in some parts of its range, the adult meningeal worm is likely to go extinct because its hosts will not be there, and in other places, the adult meningeal worm will be able to invade new areas along with its hosts.  In total, the parasite’s range is expected to shift, but not to increase.

The Pickles et al. (2013) paper was about how hosts and parasites might end up having a mismatch in space, so that parasites suddenly can’t find their hosts in areas that used to have hosts.  There has also been a lot of interest in how climate change might cause temporal mismatches among interacting species (e.g., predators and prey, plants and pollinators).  These mismatches result from climate-induced changes in the phenology of species.  And now we ask: will climate change cause temporal mismatches between hosts and parasites?

Paull and Johnson (2014) performed a mesocosm experiment to consider how one aspect of climate change – increased temperature – might affect disease dynamics in a trematode system.  They studied a particularly important trematode (Ribeiroia ondatrae) that causes mortality and severe limb malformations in some amphibian species.  The internet is rather devoid of good diagrams of the life cycle of R. ondatrae, but here’s a link to one such copyrighted diagram.  (If asked very nicely, I might cartoon one myself.)  Briefly, the adult trematode uses a heron as the definitive host.  The adult trematode releases eggs in the aquatic environment (e.g., pond), and those eggs hatch into free-living miracidia that swim around looking for a snail to infect.  After the parasite has sufficient time to develop and asexually reproduce in the snail, the snails release another free-living parasite stage called a cercaria.  (You guys should be familiar with cercariae at this point because they’re my go-to cartoon parasite.)  Cercariae swim around until they find a tadpole to infect, and then they encyst in the tadpole’s tissues.  If the tadpole lives long enough, it metamorphoses into an adult frog – possibility with limb malformations – and gets eaten by a heron.  Etc.

Paull and Johnson (2014) put snails in 36 mesocosms (miniature ponds) in August.  Half of the mesocosms had greenhouse lids that caused them to be heated to 3˚C warmer than the other mesocosms.  Then they added R. ondatrae eggs to the mesocosms so that the eggs could hatch into miracidia and infect the snails.  For the next year, they checked every few weeks to quantify 1) snail mortality, 2) what percentage of snails were infected, and 3) how many cercariae the infected snails were releasing per night.  Also, starting in April, they added one bullfrog tadpole to each mesocosm each month, let the tadpoles hang out for two days getting infected, and then dissected them to quantify infection.  Then in May, they added 20 chorus frog tadpoles to each mesocosm and left them until they started metamorphosing.  Then they quantified survival, limb malformations, and infection of the chorus frog tadpoles.  They also quantified a bunch of other stuff, but all that is less important to the story.

When the mesocosms weren’t heated, infected snails started releasing cercariae in late spring.  In the heated mesocosms, infected snails started releasing cercariae nine months earlier, in the fall!  The warmer temperatures vastly increased the developmental rates of the parasites.  Furthermore, more second-generation snails got infected in the heated treatment.  So, did this increase in parasite development rates and the percentage of infected second-generation snails result in more tadpole infection?  Actually, no!  First of all, more adult snails died in the heated treatment and there were fewer total second-generation snails in that treatment, too.  So, the total number of shedding snails was actually lower in the heated treatment.  Second, in the unheated treatment, snail infection peaked just when the tadpoles were available in the spring, whereas snail infection in the heated treatment rapidly declined in the spring.  The total number of cercariae was the same in both treatment groups, but due to the temporal mismatch and increased snail mortality, tadpole infection actually declined under the warmer temperature scenario.

Sometimes, I'm the only one who understands my cartoons....

Sometimes, I’m the only one who understands my cartoons…. Yes, those are soldier cercariae.

So, this was a very cool study.  Climate change experiments are really difficult to tackle because running long-term experiments with frequent sampling events is hard work!  And trying to manipulate multiple species in biologically relevant ways can be tricky.  In this case, the results of this experiment would likely be very sensitive to the timing of the input of R. ondatrae eggs.  Paull and Johnson (2014) did just one pulse of eggs in the fall.  If eggs also enter the system in the early spring, infection dynamics might be quite different under the +3˚C warming scenario.

Ok!  I’m ready to see more work like this.


Paull, S.H., and P.T.J. Johnson.  2014.  Experimental warming drives a seasonal shift in the timing of host-parasite dynamics with consequences for disease risk. Ecology Letters.

Climate Change and Parasite Range Shifts

Time flies during field season!  I apologize for falling a bit behind in my weekly post schedule.  I could tell you exciting field stories about killer bees and torrential downpours, or less exciting stories about endless rows of snails waiting to be dissected, but instead, let’s talk about climate change and parasites.

There has been a lot of talk about potential shifts in the ranges of parasites/pathogens as the global climate changes.  For instance, will we see increases in range sizes of tropical parasites/pathogen as they move into warming temperate regions?  I tend to think about a 2009 paper by Kevin Lafferty when I think about range changes of parasites/pathogens, where he argued that for a given parasite, certain regions may become favorable, but others will also become unfavorable.  Therefore, we might expect range shifts, rather than range expansions.

I also blogged about a 2012 Ecology Letters paper that I liked where Mordecai et al. (2013) showed that transmission of malaria via mosquitoes should have a unimodal relationship with temperature.  That is, as Lafferty (2009) suggested, some places where malaria is currently endemic should become too hot for malaria as the global temperature increases.

And now, for a new paper!  This one is open access, so you can check out here if you’re interested.  As I’ve blogged about before, many parasites are “ontogenetic niche specialists.”  That is, because of their complex life cycles, parasites end up occupying multiple specific niches during one ‘generation.’  This makes parasites vulnerable to secondary extinction, because if one host goes extinct in an area, the parasite cannot complete the life cycle.  Pickles et al. (2013) explored this concept as it relates to climate change.  That is, if the ranges of the hosts change with the climate, what happens to the range of the parasite?  Each host might increase the size of its range, but if the hosts’ ranges don’t overlap, the parasite might actually lose some of its range.  Pickles et al. (2013) call this an “ecological mismatch.”

Awesome things about this paper:

  1.  The first author’s last name is Pickles.  I personally don’t like pickles, but that last name deserves bonus points.
  2. COOL PARASITE SYSTEM!  The adult meningeal worm (Parelaphostrongylus tenuis) infects white-tailed deer as a definitive host, free-living parasite larvae are shed in the deer feces, slugs or snails eat the parasites and become infected (L2 and L3 larvae), and then deer later eat the gastropods and become infected with the adult worms.  How often do deer accidentally eat gastropods?
  3. They used a site that I hadn’t heard of for data collection.  www.mammalparasites.org has the distributions of known mammal parasites!  Check it out!

Pickles et al. (2013) modeled range shifts of the hosts and P. tenuis under a few different climate scenarios.  In general, the model predicted expansion of the ranges of all of the hosts.  However, the range of P. tenuis shifted without expanding by much.  In some places, like the increasingly dry Great Plains, they predicted that P. tenuis may be extirpated.   And in other places, like the warming Alberta, they predicted that P. tenius may invade with the white-tailed deer.  This seems to work well with Lafferty’s (2009) predictions about range shifts as opposed to range expansion.

Finally, Pickles et al. (2013) pointed out that this may be a big deal for other ungulates, like elk and caribou.  P. tenuis can infect several ungulates besides white-tailed deer, but can’t successfully reproduce in those ungulates.  However, P. tenuis infection causes morbidity and mortality in those other species, whereas the parasite is relatively benign in white-tailed deer.  So, ungulates in these currently P. tenuis-free regions may be in for a rude surprise. Maybe as ecological mismatch between deer and P. tenuis increases, we’ll see co-evolution between P. tenuis and other ungulates?  Dun, dun, dun!


Pickles, R.S.A, D. Thornton, R. Feldman, A. Marques, and D. L. Murray.  2013.  Predicting shifts in parasite distribution with climate change: a multitrophic level approach.  Global Change Biology, 19: 2645-2654.

Do parasites like it hot?

Today, I’m writing about a 2013 Ecology Letters paper entitled “Optimal temperature for malaria transmission is dramatically lower than previously predicted.”

Ro is the number of new disease cases that arise when just one infected individual is added to a susceptible population.  This is an important parameter for disease transmission models.  (Check out the wikipedia page for more information.)

Many factors affect Ro.  For malaria (Plasmodium falciparum), most of these factors are related to mosquito and parasite biology.  These are life history traits like the rate at which mosquitos bite people, the competence of the mosquito vector, and the mortality rate of the adult mosquitoes.  Mordecai et al. (2013) knew that these life history traits often don’t have linear relationships with temperature.  Instead, these life history trait rates tend to increase with temperature until some optimum, and then decline.


As it turns out, many biological responses have these nonlinear relationships with temperature because above some optimal temperature, enzymes/proteins become denatured. Photo Credit.


Using nonlinear estimates of the relationships between temperature and life history traits from the literature, Mordecai et al. (2013) mathematically modeled Ro.   They found that Ro peaked at 25°C, which is 6°C less than the peak Ro predicted if temperature is assumed to linearly affect life history traits and Ro.   Neat!


If you assume that temperature has a linear effect on life history traits, peak Ro occurs at 31°C (dotted line). If you allow for nonlinear relationships, peak Ro occurs at 25°C (solid line). Stylized results from Mordecai et al. (2013). 

You might be wondering why this 6°C difference is important.  First of all, the 6°C change equates to going from 77°F to 87.8°F, which is a big change!  Imagine which of those temperatures you’d prefer to work in on a summer day.

Second, temperature is one component of the global climate that is expected to change with climate change.  When people try to model how the range of malaria might shift with climate change, they need to know how mosquitos and the malaria parasite (and thus Ro) respond to various temperatures.  And the answer is:  that response is nonlinear, and the peak Ro is around 25°C! 


Predicted temperature increases around the world for 2070-2100. Photo Credit.

What do you think?  Modeling predicted range shifts is complicated business, and temperature is just one consideration.  Do you think malaria might end up in your continent/country/state/city?


Mordecai, E.A., K.P. Paaijmans, L.R. Johnson, C. Balzer, T. Ben-Horin, E. de Moor, A. McNally, S. Pawar, S.J. Ryan, T.C. Smith, K.D. Lafferty.  2013. Optimal temperature for malaria transmission is dramatically lower than previously predicted.  Ecology Letters 16(1): 22-30.