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What is a vector?

Precise definitions are important in science, because I said so (and other better reasons). In parasite ecology, the tricky definitions that students often mix up are things like parasite versus parasitoid and microparasite versus macroparasite. In fact, at least 20 visitors per week happen upon this blog because they want look up one of those terms.

But it isn’t just students and the general public who struggle with tricky definitions in parasite ecology. Within the field, we can’t even agree on what to call our discipline! (Disease ecology? Parasitology? Epidemiology?) And it has recently come to my attention that a term that I thought had bullet-proof definition is somewhat controversial among parasite ecologists.

In an awesome special issue of the Philosophical Transactions of the Royal Society B that just came out this month, there was a thought-provoking article entitled, “What is a vector?” The idea for the article came from a working group of the British Ecological Society’s ‘Parasites & Pathogens’ Special Interest Group, where participants unexpectedly found that they did not all use the same definition of “vector.” The article contained a whole list of possible definitions that the authors found in the literature, including this subset, which I have re-numbered for my own purposes:

Definition 1A: “Any organism (vertebrate or invertebrate) that functions as a carrier of an infectious agent between organisms of a different species.”

Definition 1B: “Any organism (vertebrate or invertebrate) or inanimate object (i.e., fomite) that functions as a carrier of an infectious agent between organisms.”

Definition 2: “Any organism that can transmit infectious diseases between humans or from animals to humans.”

Definition 3A: “Hosts that transmit a pathogen while feeding non-lethally upon the internal fluids of another host.”

Definition 3B: “Blood-feeding arthropods such as mosquitoes, ticks, sandflies, tsetse flies and biting midges that transmit a pathogen while feeding non-lethally upon the internal fluids of another host.”

I couldn’t believe that parasite ecologists differed so much with their working definitions, so I put them into a poll, and then I asked you guys to tell me which definitions you use. (Thanks for your participation!) To my great surprise, your answers were all over the place! No one really used Definition 2 (the “anthropocentric” definition), but all of the other definitions received some support. As of 21 March, the most popular definition was 1A, and 1A+B were more popular than 3A+B.

Wilson et al. (2017) do a great job of discussing the pros and cons of each definition, and they also take a stab at a possible mathematical definition (the “sequential” definition), so I’d recommend giving their paper a read for a lot more coverage than you’re going to get here. I’m just going to cover two points that were surprising to me.

First surprise: I did not expect that so many people would prefer 1B over 1A, because I don’t like including fomites in the definition of a vector. My primary reason for preferring IA is that we already have a term for inanimate objects that transmit infectious agents (i.e., fomites). Wilson et al. (2017) provide a good discussion on the utility of thinking about some fomites (e.g., drug needles) in pseudo-biological terms that would normally apply to vectors. But I think that I’d still prefer to call things like needles “fomites,” even if it’s helpful to think of their parallels with living vectors.

Second surprise: I did not expect that so many people would prefer 1A+B to 3A+B. As Wilson et al. (2017) discuss, the 1A+B definitions are broad; for instance, the wording suggests that we include intermediate hosts (i.e., snails infected by trematodes) as vectors! It also suggests that any host capable of interspecific transmission could be a vector. 

In the end,Wilson et al. (2017) suggested that parasite ecologists think carefully about their definitions of the term “vector,” and then they scored a closing home run with, “all vector definitions are wrong, but some are (we hope) useful.” WOMP WOMP.

Give the paper a read, and share your thoughts in the comments!

Reference:

Wilson, A. J., E. R. Morgan, M. Booth, R. Norman, S. E. Perkins, H. C. Hauffe, N. Mideo, J. Antonovics, H. McCallum, and A. Fenton. 2017. What is a vector? Phil. Trans. R. Soc. B 372:20160085.

POLL: What is a vector?

Next week, I’m going to write a post about vectors. This week, I want your input! So tell me: what is a vector? (I took these definitions from the literature, but I’m not going to tell you where they came from until next week.)

Definition 1A: “Any organism (vertebrate or invertebrate) that functions as a carrier of an infectious agent between organisms of a different species.”

Definition 1B: “Any organism (vertebrate or invertebrate) or inanimate object (i.e., fomite) that functions as a carrier of an infectious agent between organisms.”

Definition 2: “Any organism that can transmit infectious diseases between humans or from animals to humans.”

Definition 3A: “Hosts that transmit a pathogen while feeding non-lethally upon the internal fluids of another host.”

Definition 3B: “Blood-feeding arthropods such as mosquitoes, ticks, sandflies, tsetse flies and biting midges that transmit a pathogen while feeding non-lethally upon the internal fluids of another host.”

 

 

Visiting the UCSB Parasite Ecology Lab

[I don’t usually post about my own science adventures on this blog, but I’m going to make an exception today because I think that the topic could be broadly interesting for you guys. This is cross-posted from the UCSB Parasite Ecology Lab’s Adventure Science blog, which you should follow!]

The University of California Santa Barbara Parasite Ecology Lab seems to host a perpetual stream of visiting scientists – including me! I’m currently a PhD candidate at Virginia Tech, but in Fall 2016, I had the opportunity to be the Queen of the Salt Marsh Parasites while I visited UCSB for a semester. I had a great time, I learned a ton, and I highly recommend that you – yes you! – try to visit the UCSB Parasite Ecology Lab, too.

The first step in visiting is finding the funding to do so. I’m incredibly lucky to be supported by an NSF Graduate Research Fellowship, which pays my salary and tuition. NSF Fellows are also invited to apply for professional development opportunities, like the Graduate Research Internship Program (GRIP). The GRIP pairs fellows with mentors in federal agencies, and through their internships they work on “enhancing professional skills, developing networks, and preparing for a wide array of career options.”  Fortunately for me, one of my science heroes – Dr. Kevin Lafferty from the USGS – was on the mentor list last year, and in an extra stroke of good luck I was successfully funded through the GRIP to go work with him.

It’s a ~40 hour drive from Virginia to Santa Barbara, California, so I had the opportunity to see a good chunk of our beautiful country with my faithful and fuzzy copilot while traveling to the internship. (Here he is cheesing for a photo at Ozone Falls, Tennessee.)

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But when I reached Santa Barbara, I was disappointed to discover how hideous it was. There was unlimited sunshine and nice weather, and I could see the beach from my office window. And there was an authentic Mexican restaurant every 20 feet, including right near the salt marsh nature preserve where I did my field work (see next photo). It was rough. All I can say is that you’ll just have to learn to live with it if you visit.

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Though Santa Barbara legitimately lacked one of my all-time-favorite things (caves), it made up for it by having many of my other all-time-favorite things: snails, parasites, and mud. I have seen some Virginia ponds with high snail densities, but nothing I’ve seen compared to the number of California horn snails that I found in the Carpinteria Salt Marsh (see every bump in the mud in the next photo). The majority of those snails harbored first intermediate host trematode infections.

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And trematodes were my parasite of interest during my tenure at UCSB. A postdoc in the lab, Julia Buck, had recently found some interesting differences in the types of trematodes that infect male versus female snails. So I did some field and laboratory experiments to see if the foraging behaviors of male and females might explain sex-based trematode infection dynamics. For instance, some trematodes infect snails after the snails ingest trematode eggs from bird feces, so I did experiments to determine whether male or female snails were more likely to hang out near bird feces. You might say it was a “crappy” project.

Anyways, my visit was a blast, and I met a bunch of really amazing scientists from all career stages while I was there. I hope to go back soon, and maybe I’ll see you there when I do.

Soldier trematodes

Many animal species have fascinatingly complex social systems, but the pinnacle of sociality is relatively rare: the reproductive division of labor. Taxa that have separate castes of reproductive and non-reproductive individuals include the hymenopteran insects (ants, bees, and wasps), gall-forming aphids, termites, ambrosia beetles, sponge-dwelling shrimp, naked mole rats, and – because this is a Parasite Ecology blog – trematode parasites.

You’ve probably seen photos of the insane phenotypic differences between castes in some species; for instance, the difference between a queen fire ant and a worker fire ant (amazing photo by Alex Wild):

queenfire

Or between queen, worker, and soldier termites (photo from here):

termitecastes

Trematode rediae have equally obvious caste differences, where reproductive rediae are huge and full of developing offspring, whereas soldier rediae are tiny with relatively large pharynxes (photo from here). And they don’t just look different; these castes are also spatially segregated, and they have unique behaviors. Reproductive individuals tend to hang out in the host snail’s gonads, while soldier trematodes tend to hang out in the mantle. And reproductive rediae rarely attack rediae from other trematode species, whereas soldier rediae readily attack invading species.

soldiertrems

But despite these differences between reproductive and soldier rediae, the reproductive division of labor in first intermediate host trematode colonies wasn’t discovered until a few years ago. And until January (Garcia-Vedrenne et al. 2017), soldier rediae had only been documented in one trematode superfamily: the Echinostomatoidea.

It would not have been surprising if echinostomoids were the only trematodes to have soldier rediae, because echinostomoids are known for their ability to “fight” other trematode species. For instance, in a well-studied salt marsh system, echinostomoids sit at the top of a trematode dominance hierarchy, where they can successfully invade and conquer a California horn snail infected by a different trematode species, and they can successfully fight off invasions of their snail by other trematode species. But we now know that at least four species of heterophyid trematodes, which fall in the middle of that dominance hierarchy, also have a distinct soldier caste (Garcia-Vedrenne et al. 2017)!

tremdominance

This is a pretty big addition to our existing knowledge of these systems, and it makes one wonder how many other trematode species have undocumented soldier castes. Check out the paper to learn more!

Reference:

Garcia-Vedrenne, A.E., A.C.E. Quintana, A.M. DeRogatis, C.M. Dover, M. Lopez, A. Kuris, and R.F. Hechinger. 2017. Trematodes with a reproductive division of labour: heterophyids also have a soldier caste and early infections reveal how colonies become structured. International Journal for Parasitology, 47(1): 41-50.

Parasite Valentines

I usually blog about STIs on Valentine’s Day. But this year, I’ve decided to spread the parasite love in a different way – with a few awesome links and a valentine. If you want to help spread the parasite love, you are more than welcome to send me parasite valentines via Twitter. I’ll be waiting.

Why killer viruses are on the rise

An awesome new symbiont was recently discovered. It’s a beetle that pretends to be an ant’s butt.

Superspreaders played a big role  in the Ebola epidemiciencyst

What’s on my reading list

The flu and job search committee duties have contributed to me falling behind in reading and blogging. In an attempt to get back on track, I’m posting this week’s reading list (in no particular order).

  1. Pepin, et al. (2017). Inferring infection hazard in wildlife populations by linking data across individual and population scales. Ecology Letters.
  2.  Cizauskas et al. (2017). Parasite vulnerability to climate change: an evidence-based functional trait approach. Royal Society Open Science.
  3. Betini et al. (2017). Why are we not evaluating multiple competing hypotheses in ecology and evolution? Royal Society Open Science.
  4. Weinersmith et al. (2017). Tales from the crypt: a parasitoid manipulates the behaviour of its parasite host. Proceedings of the Royal Society B.
  5. Venesky et al. (2017) The thermal mismatch hypothesis explains host susceptibility to an emerging infectious disease. Ecology Letters.
  6. Garcia-Vedrenne et al. (2017). Trematodes with a reproductive division of labour: heterophyids also have a soldier caste and early infections reveal how colonies become structured. International Journal for Parasitology.

What am I forgetting?

Compiling List of Jobs in Parasitology and Parasite Ecology

Hello Readers!

Jobs in parasitology and/or parasite ecology have been recurring topics on the blog in the past few months. For instance, are there government jobs for people studying wildlife parasites? And are you more likely to find a job as a “parasitologist” or as a “disease ecologist?” I’ve also received several recent emails from students looking for parasite-related research opportunities, and I wasn’t as helpful as I wish I could have been. So, I’m happy announce that I’m going to start compiling a list of parasite-related jobs for all career stages on this blog. If you know of a relevant job that should be added to the list, you can post it in the comments or send it via email to dinoverm (@) gmail.com.

Please share widely!