Where will the next bat virus spillover?

A few weeks ago, I went spelunking for the first time. Since then, I’ve had bats on the brain! And that means that for a few weeks, Parasite Ecology is going to be all bats, all the time.

In a previous post, I discussed potential characteristics of bats that might make them “good” at “sharing” novel viruses with humans. There are many hypotheses out there, and probably all of the proposed important bat characteristics play a role for spillover of some viruses in some places at some times. There’s still a lot of research that needs to be done there, so investing in bat research is still a high priority. For today, we’ll just talk about one characteristic: for whatever reason, bats are hosts for a huge diversity of viruses, so there are a lot of viruses for them to potentially transmit to humans.

Even if bats are highly likely to share novel viruses with human populations, that sharing could never happen if bats and humans didn’t directly or indirectly interact. Therefore, there are also lots of hypotheses out there about which anthropogenic activities lead to high rates of interaction between humans and bats. For instance, areas with many people, areas with many domesticated animals that interact with bats and people, and areas where people are particularly likely to encounter bats (e.g., by eating them as bushmeat) might be especially likely to experience spillover of bat viruses into human population.

So, wouldn’t it be awesome if we had a map that showed where these drivers of bat virus spillover were particularly prominent, so that we could predict areas where spillover is most likely to occur? Why yes, yes it would be awesome. And just such a map was recently created by Brierley et al. (2016).

Here’s the just of it: using spatial regression techniques, Brierley et al. (2016) came up with a list of drivers that were good at predicting the total number of viruses shared between humans and bats in 1 decimal degree-sized grid blocks all over the world. They found that bat host diversity and annual rainfall were important drivers, and they suggested that these were links between virus diversity and the potential for virus spillover. They also found that things like human population sizes, the number of domesticated pigs, and the use of bats as bushmeat were important drivers, suggesting that anthropogenic activities are also important to spillover.

Interestingly, the areas where risk is high due to the high diversity of bat viruses (South America) are not the same as the areas where the risk is high due to high human-bat interaction rates (Sub-Saharan Africa). This suggests that when we think about preventing spillover of bat viruses into human populations, we probably need different plans for regions with different drivers. That’s not necessarily a new idea, but now we have a great map to show us which areas need which kinds of prevention!

This cartoon is not intended for people eating bats because they have few sources of protein in their lives. Obviously, eating bats isn’t a decision for them, it’s a necessity. But those of us in positions of relative power can work towards alleviating the socioeconomic situations that push people towards the consumption of bushmeat. And if you do have a choice, don’t eat bats!!



Brierley, L., M.J. Vonhof, K.J. Olival, P. Daszak, and K.E. Jones. 2016. Quantifying global drivers of zoonotic bat viruses: a process-based perspective. The American Naturalist.

Is zombie transmission transmission frequency or density dependent?

For reasons that I cannot explain, visitors have started to stumble upon my blog by googling the question, “is zombie infection frequency or density dependent?” Maybe there’s a really awesome educator out there using that example in class. Or maybe the zombie apocalypse has started and people are secretly beginning to plan for the end. Either way, this is a neat question that I’m willing to speculate about!

First, we need to decide what kinds of zombies we are talking about. Let’s assume we’re looking at World War Z type zombies, where infection is transmitted via bites/saliva/fluid transfer. Let’s say that the zombies are highly mobile and thus the human and zombie populations are well-mixed. Also, let’s assume that zombies don’t really have a contact structure, like humans do, because they’ve lost any kind of social system that they had a humans.

Given those assumptions, I would expect disease-relevant contacts to increase with host density. So, if I had to pick between density dependent and frequency dependent transmission, I’d expect density dependent transmission. But don’t forget that there are nonlinear contact functions, too. Those might work better, because even a tireless biting machine can only bite so many people per day.

When might zombie transmission be frequency dependent? FD transmission would be appropriate if larger populations covered larger areas, so that host density was constant. I suppose that could happen if humans were dispersing as much as possible and running away from zombie-packed areas. What do you think?

The unexpected consequences of intercourse with an old lady (beetle)

Recently, I posted about one of my favorite symbiont-host systems: mites that are sexually transmitted among their lady beetle hosts. Because that system is so empirically tractable, studies on mites and lady beetles are increasing our understanding of STIs by leaps and bounds. Today I want to talk about another really cool thing that we just learned by studying lady beetle sex.

In order for symbionts to be maintained in host populations over the long term, there needs to be transmission of the symbiont from one host generation to the next. In temperate regions, many animals have seasonal population cycles where adults from multiple generations don’t overlap and/or adults and their offspring don’t temporally overlap. For instance, many dragonflies lay eggs in the summer/fall and then die. The eggs hatch into aquatic larvae, which then overwinter and don’t emerge until the next year. In that case, adult dragonflies from the first generation never see the adult dragonflies from the second generation, so those dragonflies wouldn’t be able to pass on STIs between generations.

Coccipolipus hippodamiae lady beetles overwinter as adults, not larvae. They emerge in the spring, and in May-June they have lots of sex and lay lots of eggs. Eventually, the overwintered adults die. Also, at some point, the new cohort emerges from the eggs and individuals develop from larvae to sexually mature adults.

Interestingly, not all Coccipolipus hippodamiae lady beetle populations have mites, and the presence or absence of mites in a population is consistent over time (Pastok et al. 2015). Populations north of 61°N tend to be mite-free, and populations south of 61°N tend to have mites. But lady beetles in populations north of 61°N can become infected by mites and transmit them in the laboratory, so it doesn’t seem like there is a physiological/biological reason why northern and southern populations differ in mite infection (Pastok et al. 2015).

Instead, that division by latitude suggests that ecology (specifically, phenology) might play an important role in intergenerational mite transmission. And sure enough, in August, southern mite populations have adults from both the overwintered lady beetle generation and the new lady beetle generation (Pastok et al. 2015). And at least some of the new adults have already had sex when the old generation is still present, which means that sexual contacts could be happening between the generations. In contrast, in the northern populations, the overwintered lady beetles die sooner and the new lady beetles mature later, so there is no overlap among generations (Pastok et al. 2015). It looks like that generation gap prevents sexually transmitted mites from persisting in those northern populations! SO. COOL.

So, phenology plays a really important role in symbiont transmission when symbionts are sexually transmitted. But that’s not all! For instance, symbionts that require multiple host species may also be very sensitive to host phenology, especially when there are mismatches in the phenologies of various host species (here and here). As host phenologies continue to change in response to the changing global climate, the role of host phenology in symbiont transmission will remain a huge area for future research.



Pastok, D, M-J Hoare, J.J. Ryder, M. Boots, R.J. Knell, D. Atkinson, and D.D. Hurst. 2015. The role of host phenology in determining the incidence of an insect sexually transmitted infection. Oikos.

Links, news, and paper highlights: January 2016

I’m trying to do a better job of keeping up with parasite ecology and epidemiology related news this year. Here’s some recent work that might be of interest:


Tasmanian devils have TWO types of infectious cancer!

Romans were wormy, despite relatively good hygienic practices.

The West African Ebola outbreak is over.

It looks like the mosquito-borne Zika virus is the likely culprit of the rapid increase in microcephaly in infants born in Brazil.

Paper highlights:

Pertussis, also known as whooping cough, kills tens of thousands of children per year, despite high global vaccination coverage. Additionally, developed countries with high pertussis vaccine coverage – like the United States – have experienced bigger outbreaks in recent years. Many hypotheses have been suggested to explain the “resurgence” of pertussis: (1) there is waning immunity to the vaccines and adults act as bacterial reservoirs; (2) the new acellular vaccine isn’t as good as the previous whole-cell vaccine; (3) the vaccines protect against infection but not transmission; and (4) there isn’t really a resurgence; we’re just better at detecting pertussis now than we used to be. A recent paper argues that all of those commonly held views are wrong and proposes some new hypotheses. Cool stuff!

Antibiotic resistance is a huge challenge facing global medicine. We usually assume that when bacteria evolve resistance to a given antiobiotic, the mutation that provides resistance is costly. Because we assume that those resistance mutations are costly, we also assume that if we stop using an antibiotic, the bacteria populations will evolve back to their susceptible state by acquiring compensatory mutations that restore the function(s) lost by resistance mutations. But resistance mutations vary in how costly they are. Some aren’t costly at all. And there are only so many compensatory mutations that can restore a given function. So, we can’t necessarily expect a resistant population to revert to susceptibility, whether a compensatory mutation pops up in the population or not. Furthermore, there are many other possible mutations that can reduce or eliminate any cost of resistance just by increasing overall bacterial fitness, without actually returning lost functions. We might be overlooking the importance of those “generally beneficial mutations” in the evolution and subsequent loss of antibiotic resistance in bacterial populations. Check it out.

Death, sex, light sabers, and parasites

Arthropods have a bunch of really cool symbiotic bacteria that are vertically transmitted from parent to offspring. Some of these symbionts reduce their hosts’ susceptibility to infection by parasites and parasitoids. Because host susceptibility is very important to parasite transmission, symbiotic bacteria that reduce host susceptibility can influence epidemics of parasites and parasitoids in arthropod populations. However, arthropods have other symbiotic bacteria that don’t have any measureable effect on arthropod susceptibility to parasites and pathogens. Can those other symbiotic bacteria still influence interactions between arthropods and their natural enemies?

One of the cool things about vertically transmitted symbionts is that they are often male-killers (that’s cool for me, but maybe not for the male arthropods). Male arthropod embryos infected with the bacteria die, and this produces female biased sex ratios. This is just what happens when lady beetles are infected with Spiroplasma bacteria, where lady beetle populations infected by Spiroplasma are 74% female (Ryder et al. 2014). So…what does this have to do with parasite epidemics? Well, lady beetles are also infected with sexually transmitted mites, and the dynamics of epidemics of sexually transmitted parasites might be very sensitive to sex ratios.

To explore this possibility, Ryder et al. (2014) made a really pretty model to explore how female biased sex ratios might influence parasite epidemics. They found that female biased sex ratios result in “male first” epidemics, where mite epidemics happened faster in the male population than the female population. Then Ryder et al. (2014) observed a bunch of mite epidemics in areas with and without male-killing bacterial symbionts. And guess what: in the areas with male-killing symbionts, there were male-first mite epidemics, while in areas without male­-killing symbionts, male and female lady beetles had similar epidemic dynamics. Beautiful!

There is a lot more cool stuff in the paper, so go check it out! You should be at least a little bit jealous of how much data they had from experiments and field surveys. I know I am.


[My nerdiest cartoon yet? Perhaps.]


Ryder, J.J., M-J. Hoare, D. Pastok, M. Bottery, M. Boots, A. Fenton, D. Atkinson, R.J. Knell, and G.D.D. Hurst. 2014. Disease epidemiology in arthropods is altered by the presence of nonprotective symbionts. The American Naturalist 183(3): E89-E104.

Parasite ecology cartoon feedback

The main goal of this blog is to communicate recent symbiont ecology science to people in the field and to students and non-scientists outside of the field. Judging by the feedback that I’ve received already, the cartoons that accompany (most of) my posts are one of the main draws for scientists visiting the blog. They’re also the most important selling point for educators using my posts in their classes and other educational material. I have a few years of cartoon experimenting under my belt, but it is still difficult to guess which cartoons will be crowd pleasers. So, if you’re a regular visitor and/or you’re an educator using my cartoons for educational purposes, I’d greatly appreciate it if you could take a moment to give me some cartoon feedback. Thank you in advance!

First, you can visit last week’s post and vote on the best parasite ecology cartoon from 2015. I really use that feedback to think about what kinds of cartoons to make in the future.

Second, you can post in the comments of shoot me an email to tell me what you like and/or what could be improved to make my cartoons more accessible to students.

One recent experiment has been embedding movie/TV references in my cartoons. The downside of this is that not everyone will get all of the references. (I fear I’m getting old….) Stay tuned next week for my best and most timely movie reference yet!

Forrest Gump


House, MD


Home Improvement




Monty Python and the Holy Grail






Best parasite ecology cartoon of 2015?

Happy New Year!

It’s the first day of the new year, which means that you get to vote on the best parasite ecology cartoon from last year! In 2013, the winner was “Social Networking in Lemurs,” a cartoon about this study that painted lice on lemurs to infer lemur contacts. In 2014, the winner was “Oldest Trick in the Book,” a romantic cartoon about a snail who was castrated by trematodes. Which 2015 cartoon was best?! I’m opening up the voting for these candidates:

Dispersal is like a box of chocolates


Tethered love


Tastes like chicken


Crappy relationships:


House (finch), M.D.:


When snails feel sluggish:


The host plant is always greener on the other side:


Hasta la vista, Biomphalaria:


You don’t guano know:


Bring out yer dead (prairie dogs)!: