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.

Predator vs. Parasite vs. Parasitoid vs. Mutualist– A Simple Classification Scheme

One of the most frequently accessed posts on this blog defines the terms “predator,” “micropredator,” “parasite,” and “parasitoid” and then presents a classification scheme for differentiating among those natural enemies (Lafferty and Kuris 2002). If you haven’t read that post yet, I recommend taking a look before you read this one. I obviously fancy the dichotomous key presented in the previous post quite a bit. However, it is not be the best classification scheme for all situations. For example:

  1. Check out Britt Koskella’s comments on the previous post – it is difficult to classify the bacteriophages that she studies using that classification scheme.
  2. The previous key may be difficult to use for educational purposes. For instance, it requires explaining castration and trophic transmission, which are concepts that might be unnecessarily complicated for explaining the distinction between parasites and predators to non-specialists.
  3. The previous key only deals with natural enemies, so we can’t use it to explain how mutualists and commensalists fit into this group of trophic relationships.
  4. The previous key doesn’t show how relationships can vary with life stages, ecological conditions, and environmental conditions.
  5. The previous key doesn’t have any pictures of snails on it. (Priorities.)

Therefore, having an additional classification scheme that specializes in some of the aforementioned areas would be useful. And – you guessed it – one was recently(ish) published (Parmentier and Michel 2013)! This classification scheme uses two continuous variables to designate relationships: the relative duration of the association (RDA) and the fitness effects on the ‘host.’

The relative duration of association ranges from 0 to 1, where 0 means that the ‘symbiont’ (including predators) spends none of its lifetime associated with the ‘host’ (or prey), and 1 means that the symbiont spends all of its lifetime associated with the host. For instance, predators and micropredators have RDA’s close to zero – a lion spends only a small portion of its life with a single zebra prey. Conversely, an adult trematode parasite (the worm in the orange section of the figure) will spend that entire life stage in association with a single host. The RDA is the Y axis on the figure below.

In recent months, we’ve talked a lot about the fitness effects that symbionts have on their hosts. Briefly, symbionts may have both negative and positive effects on hosts, and it is the net effect that determines how we classify the relationship. However, the net effect can vary with ecological and environmental conditions (see here, here, and here). Therefore, whenever we place a point on this graph, we need to remember that it might slide left or right as conditions vary.

SymbiontClassification

Parasitoid wasps spend the entirety of their larval life stage in the host, and they ultimately kill the host. In this figure, parasitic castrators – like the trematodes that castrate snails – end up in the same region as the parasitoids. And this is where bacteriophages and Cordyceps fungi would fall out, too. However, like we discuss in the previous post, the term “parasitoid” is probably not a good one for this group, because that term is usually used to refer specifically to the unique life cycles of parasitoid wasps. In this figure, it means any parasite that reduces host fitness to zero.

Predators like lions, frogs, and crayfish also reduce prey fitness to zero. However, micropredators and herbivores (e.g., mosquitoes and cows) are special classes of predators that do not kill their prey. Then there is a group of animals that consume plants, but are probably more appropriately classified as parasites because they spend the majority of their life span (or a single life stage) on a single host plant. Therefore, things that eat plants will typically fall out somewhere between the aphids and the micropredators. (I should note that herbivores can have more than minimal impacts on host plant fitness, but many grazers have small impacts.) Similarly, Mark Siddall – the man who went on a quest for the hippo ass leech – doesn’t like classifying leeches as micropredators, because some spend most of their time on a single host. Therefore, most leeches would also fall out somewhere along that line between aphids and mosquitoes. Except, yaknow, the ones that are actually predators, and fall out near the lions.

On the mutualism side of things, we have symbionts like pollinators, which are only very briefly associated with each host; they’re like micropredators, but with positive fitness effects on their hosts. And then there are symbionts that are associated with single hosts for most of their lifespans, like ants on Acacia trees or guard crabs on corals. But again, remember that those relationships might shift left on the X axis as conditions vary.

Speaking of which, finding an example of a commensalist is hard. I used the example of epibionts on hermit crab shells, which help protect the hermit crabs from some predators but make the hermit crabs more susceptible to other predators. The net effect is unclear, but the positive and negative effects might balance out to a zero net effect.

So, there you have it. I think this figure should be really useful, especially as a general framework.

References:   

Lafferty, K.D., and A.M. Kuris. 2002. Trophic strategies, animal diversity and body size.  TREE 17(11): 507-513. (Direct link to PDF download)

Parmentier, E., and L. Michel. 2013. Boundary lines in symbiosis forms. Symbiosis 60: 1-5.

Emerging Infectious Diseases

Next week, I’m going to talk about the role of livestock, wildlife, and the environment in emerging infectious diseases (EIDs) of humans. This week, I want to talk more generally about emerging infectious diseases.

Let’s start with the most straightforward part: “infectious.” EIDs are caused by some kind of transmissible pathogen. Therefore, heart disease and obesity are not EIDs, even though there are major epidemics of these diseases in some countries. (As a side note, there are some cool papers that relate the spread of non-infectious diseases, like obesity, through social networks to the spread of memes.) And “disease” means that there is pathology or fitness decreases experienced by the hosts as the result of a pathogen.

There are two ways that infectious diseases can be “emergent.” First, an emerging pathogen can be novel to a naïve or highly susceptible host population, meaning that it never existed in that population or species before. For instance, the newest emerging fungal pathogen of salamanders in Europe (Batrachochytrium salamandrivorans) exists in populations of relatively resistant salamanders in Asia, but has not previously existed in European salamanders (Martel et al. 2014). B. salamandrivorans was likely introduced into Europe via the pet trade, and European salamanders are highly susceptible to the pathogen.

Pathogens can also be considered emergent when they have existed in a population previously (i.e., endemic pathogens), but the pathogens weren’t noticed by humans until recently and/or infection rates or mortality rates recently increased due to some change in ecological or environmental conditions (e.g., changing amounts of forest fragmentation and the re-emergence of Lyme disease).  Next week, I’ll go into much more detail about how disease emergence depends on ecological and environmental conditions.

Finally, why should we care if a pathogen causing an EID is novel to the focal host population or endemic to the population? Because the control measures that we use will depend on whether the pathogen is novel or endemic. For instance, targeting the trade of salamanders originating in Asia appears to be the best option to stop the spread of B. salamandrivorans, and that would not be the case if B. salamandrivorans were endemic to salamanders all over the world.

WrongWorm References:

Martel, A., et al. 2014. Recent introduction of a chytrid fungus endangers Western Palearctic salamanders. Science 346(6209): 630-631.