Simple versus Complex Life Cycles

When you first start learning about parasite ecology—and even decades later, when you’re an expert—parasite life cycles can be confusing! Parasite life stages have a plethora of fancy names, like L3 larva and miracidium, which can be difficult to remember. Furthermore, every parasite’s trajectory from immature to adult stages seems different than the last, and we don’t even know all the life cycle details for most parasite species. So, if you’re feeling confused by parasite life cycles, you’re in good company! You might never memorize all the complex life cycles that exist, but you can understand the general ecology and evolution of complex life cycles. We’ll cover the basics in this post.

Simple versus Complex Life Cycles:

Let’s start with simple life cycles, which are sometimes called direct life cycles or one-host life cycles. Monoxenic or homoxenous parasite species with simple life cycles only use a single host species in their life. The single host species is entered by infective stages of the parasite, and then the parasite grows and develops in the host before switching to reproducing within the host.

Direct life cycle parasites include parasites with fecal-oral transmission, like Ascaris lumbricoides, a roundworm that infects humans. Adult male or female roundworms live in the small intestine, where the female releases fertilized eggs into the environment through human feces. Eggs are ingested by people when they (usually accidentally) consume fecal matter. The parasite grows and develops in a series of larval stages in multiple tissues. The larvae eventually make their way to the respiratory system, where they are then coughed up and swallowed, which allows them to find their way to the small intestine to mature. This entire life cycle uses just a single host (humans), even as the parasite goes through many developmental stages inside and outside host.

This life cycle diagram for a simple life cycle parasite, Ascaris lumbricoides, depicts the parasite moving through several life stages from egg to larvae to adult, where the eggs exist in the external environment and the other life stages occur in the human host.

Now let’s move on to complex life cycles, which are sometimes called indirect life cycles or multi-host life cycles. Parasites with complex life cycles are indirectly transmitted from one host species to the next. These heteroxenic parasites need to use multiple host species in sequence to successfully develop and reproduce. Reproduction occurs in the final host in the life cycle, which is called the definitive host. The one or more hosts where parasite growth or development occur (but no reproduction occurs) are called intermediate hosts. Sometimes a parasite will not complete any life stages within a host, and instead only use the host for transportation; those transportation hosts are called paratenic hosts. The number of hosts needed to complete a life cycle is the life cycle length, and it must be at least two hosts long.

Ready for some examples? Schistosomiasis is caused by a parasite with a two-host life cycle, which uses humans as the definitive human host and snails as the intermediate host. Euhaplorchis californiensis is an example of a parasite with a three-host life cycle, which uses birds like herons as the definitive host, snails as the first intermediate host, and killifish as the second intermediate host. Life cycle lengths appear to have an upward limit, because most life cycles require four or fewer host species. Why do you think that is?

The life cycle of Euhaplorchis californiensis, a trematode that must sequentially infect three host species to complete its life cycle. This life cycle diagram came from The Ethogram Blog.

To get from one host to the next, parasites with complex life cycles can use a few different modes of horizontal transmission. Passive transmission occurs when the parasite just waits around for the next host, like when E. californiensis eggs in heron feces wait to be consumed by a salt marsh snail. Active transmission occurs when the parasite is free-living in the environment and moves around to seek out the next host, like when E. californiensis cercariae leave their snail host and swim around looking for a killifish to infect. And finally, complex life cycles often involve trophic transmission, where the parasite is consumed along with its intermediate host by the next host in the life cycle. Trophic transmission is used by all cestodes and acanthocephalans and many nematode and trematode species. Trophic transmission is probably so common in complex life cycles because predator–prey interactions are one of the most common ways that two (host) species might interact.  

Three ways to add hosts to a life cycle

You might have noticed that we need to make an important distinction between whether a parasite uses multiple host species in sequence, in parallel, or both. For example, E. californiensis does both: it must infect birds, snails, and killifish in that order to complete its life cycle, so it uses multiple host species in sequence. But it can also use multiple host species at a given life stage; in particular, E. californiensis can successfully infect and reproduce in more than one bird species. The number of host species that a parasite can successfully use for any given life stage is quantified as host specificity. Many complex life cycle parasites have high host specificity for some parts of their life cycle (e.g., they can only infect a single snail species as a first intermediate host) and low specificity for other parts of their life cycle (e.g., they can infect many bird species as definitive hosts).

This brings up an important question: how do host species get “added” to a parasite’s life cycle? We assume that parasite species start by infecting just one host species and then complex life cycles evolve from those simple life cycles. There are two ways that a host species is thought to be added in sequence, thereby increasing the life cycle length: upward incorporation and downward incorporation.

In upward incorporation, a new definitive host that is a predator of the original definitive host is added to the life cycle. Parasites that can infect the new predator without being digested are selected for because they have avoided a source of mortality. They also likely have higher adult body sizes, life spans, and fecundity inside the new definitive host, because the new definitive host species should be larger and longer-lived, on average, than the old definitive host species. After the new definitive host species is added to the life cycle, the parasite then represses reproduction in the old definitive host species, which is now used as an intermediate host. There is an upper limit to how long the life cycle can be made using upward incorporation, because there are only so many trophic levels in a given food web.

In downward incorporation, a new intermediate host that consumes free-living stages of the parasite and is consumed by the definitive host is added to the life cycle. (Yes, parasites are often eaten by predators!) This again reduces parasite mortality, because the parasites can now infect the new intermediate host instead of being digested. Since fewer parasite stages are lost to mortality and more make it to the definitive host via trophic transmission, overall transmission rates increase. Both downward incorporation and upward incorporation are thought to have led to the evolution of complex life cycles for some parasite species.  

This diagram from Parker et al. (2015) shows how a host is added to higher trophic level in upper incorporation and a lower trophic level in downward incorporation.

There is also lateral incorporation, where host species are added in parallel, making the parasite species less host specific (more of a generalist) for a given life stage. Parasites can benefit from infecting more host species in a given life stage whenever that makes them more likely to be able to find a host that they can successfully infect that can continue their life cycle. However, there are also some likely costs associated with being a generalist, instead of specializing on just one host resource.

Final thoughts

In summary, some parasite species have complex life cycles and some have simple life cycles. Some parasite species are highly host specific at every life stage and others are host generalists that seem to infect nearly everything. There’s a lot of variability in parasite life cycles, but in general, they can be described by their life cycle length and the transmission modes that parasites use to get from one host to the next. Beyond that, each life cycle diagram is just a bunch of fancy terminology.

If you’re new to parasite ecology and thinking about life cycles for the first time, I have a question to leave you with: how do you think scientists have figured out all these life cycles? If you found a new species of larval trematode in a fish, how would you figure out its life cycle?

References:

Parker, G.A., Ball, M.A. & Chubb, J.C. (2015). Evolution of complex life cycles in trophically transmitted helminths. I. Host incorporation and trophic ascent. J. Evol. Biol., 28, 267–291.

A Global Plan for Parasite Conservation

Why should we conserve parasites?

If you’re a long-time follower, you probably already know why we should conserve parasites. But for those of you who are new, welcome, and please enjoy this short journey into posts from the past!

Parasitism is a common consumer strategy in the natural world; so much so that 40-50% of all animals might be parasites! That’s perhaps millions of parasitic animal species spread across 15 phyla, including animals as diverse as ticks, intestinal worms, and bot flies. There are also parasitic plants and fungi. Parasites might have especially high extinction risks, because they are at risk from both primary extinction pressures, like the direct effects of climate change, and secondary extinction, or co-extinction, when their host species decline or disappear. If conservation efforts are supposed to conserve all species based on their intrinsic value, then parasite species should be a large target for conservation activities.

But maybe you’re more of a utilitarian, and you want to know what parasites do for ecosystems and for us. The short answer? A lot, and probably a lot more than we know. We know the most about parasite species that harm people, harm our domestic species, and threaten wildlife species, but those parasite species are just drop in the bucket of global parasite biodiversity. We haven’t discovered and described most of those other, relatively benign parasite species, even in groups that we know provide important ecosystem services, like the parasitoid wasps that provide pest control. And some parasite species have already gone extinct due to human activities—science didn’t even give them a name before we didn’t have them any moa. All of this is to say that we do not know everything about parasites, so we do not know exactly what a world without parasites would look like.

But we do know that parasites play important roles in ecosystems. For example, parasite biomass is a large and important part of food webs. Within food webs, parasites link many species together in ways that we might not even expect, like the nematomorphs that cause crickets to jump into streams, where the crickets are eaten by endangered Japanese trout. Every non-parasitic species that you can think of evolved with parasites and interacts with parasites, which is why sex and immune systems evolved. In humans, immune systems might totally freak out in the absence of parasites, leading to auto-immune disorders. While no one wants to conserve detrimental human parasites, a few relatively benign parasites might be good for people and other species, too. Parasites are so central to the biology and ecology of non-parasitic species that some question whether we can even conserve hosts without their parasites: if we brought back mammoths from extinction, but couldn’t bring back mammoth parasites, would we really have brought back mammoths?      

What steps do we need to take to conserve parasites?

There are strong arguments for conserving parasites, but unfortunately, we are not conserving parasites yet. In fact, in some cases, we are driving parasites to extinction when we try to conserve other species, like when we delouse or deworm host species brought into captivity. Given how little we know about most parasite species and how little we are currently doing to conserve them, what immediate steps can we take to conserve parasite biodiversity?

We suggest that 12 steps should be taken in the next decade to conserve parasite biodiversity. Some of these steps will appeal most to researchers interested in fundamental science and people who want to participate in community science programs, because they involve data collection and synthesis. For instance, we need more research about how parasite biodiversity responds to changes in host biodiversity. Other steps are geared more towards practitioners, because they involve risk assessment and prioritization and conservation practice, like creating ways to assess parasites’ extinction risks and building red lists of threatened parasite species. And everyone can enjoy and be involved with the steps related to education and outreach, like including parasite-themed lessons in K-12 and college education.

If you’re interested in learning more about the 12 steps in The Global Parasite Conservation Plan, check out our recently published paper! This was a wonderful group effort from an international team of researchers, many of whom you might have seen at our ESA Organized Oral Session in 2018. And for a bunch of new papers about parasite conservation, check out our whole “Parasite Conservation in a Changing World” special issue that was just published in Biological Conservation!

This was, of course, a shameless plug for my own research, but it was for a good cause. Let’s save the parasites.

Do fungi have parasites?

Parasite ecologists spend copious time studying parasitic fungi. For instance, we’re interested in controlling the fungal pathogens responsible for the wildlife diseases that have decimated populations of amphibians, bats, and snakes. And we’re fascinated by the Cordyceps fungi that manipulate the behavior of ants and other insects. But how often do we study parasites that infect fungi (i.e., host = fungus)? Before I tackle this question, here’s a little backstory:

Last week, I went grocery shopping and bought some baby portabella mushrooms. I was feeling lazy, so I bought them pre-sliced and packaged in a cardboard box, which had an open top and was clearly labelled “sliced baby portabella mushrooms”. When I was checking out, the adult human bagging my groceries picked up the box and asked, “Are these vegetables?”

Yes, a piece of my soul died. But the educator inside me immediately announced, without distress or pause, “Oh, no, they aren’t. We generally eat three types of organisms: (1) Animals, where meat comes from, (2) plants, where vegetables come from, and (3) fungi, where mushrooms come from.” And while the woman nodded, seeming to confirm this information from some previous memory, a different, dark voice in my head added, “…and they all have worms.”

Fortunately, some intelligent internal filter kept me from saying the last bit out loud. But as I made my way to my car, I became increasingly concerned that even though I could tell you what kinds of parasites infect most plant and animal host taxa, and I knew that fungi must have parasites, I didn’t know which parasites infected fungi.

I did some googling as soon as I arrived home, and I learned that fungi have fungal, bacterial, and nematode parasites. Larval flies in mushroom gills can also be considered parasites of fungi. But overall, I didn’t find much information about parasites of fungi in my (admittedly not exhaustive) search. It might be that (1) I gave up too soon, (2) we don’t use classical parasitological terms for parasites of fungi, and/or (3) we study parasites of fungi less than those of animals and plants.

If you’re an expert on the parasites of fungi, please share your wisdom with us!

Best parasite ecology cartoon of 2017?

Happy New Year!

Before we leave 2017 behind us, let’s take a walk down memory lane, and re-visit some of the blog’s best parasite ecology cartoons. At the end, you can vote on your favorite 2017 cartoon.

If you want to delve even further into the past, you can also check out some of the previous best-of-the-best winners: 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. In 2015, the winner was “Bring out yer dead (prairie dogs),” a Monty Python reference tied to a cool prairie dog plague paper. And in 2016, the winner was my cartoon rendering of Frogald Trump.

Here are the cartoons that I’ll open the voting for this year:

(1) Ticks suck moose dry

(2) Parasite valentine

(3) Orange amphipod zombie apocalypse

(4) Parasites and de-extinction

(5) This terrifying clown isopod

Here’s the poll! You can only vote for one cartoon.

 

12 Days of Parasite Ecology Christmas

Happy Holidays, Everyone! I already spread some parasite love this season by giving Parasite Rex and a mistletoe ornament as a white elephant gift, but I feel like I have even more to give. So here’s my first and best take on a parasite ecology Christmas carol. You can click through the links to learn more about each system, should you so choose.

On the 12th day of Parasite Ecology Christmas, my true love sent to me:

12 nematomorph-infected crickets leaping

11 male crabs doing ladies’ dancing

10 Indian pipe plants piping

9 parasitoid wasps a-drumming (photo from here)

8 ants a-milking (I snuck in a mutualism! Deal with it!)

7 leeches a-swimming

6 cuckoos a-laying

5 TROPHOZOITE RINGS

(One hundred and) 4 Galapagos lice on birds

3 French T. gondii infections (cartoon from here)

2 turtle acanthocephalans

And a partridge that was covered in fleas!

The partridge lives in the Pear tree -but what lives on the partridge?
well the flea Ceratophyllus (Emmareus) garei is one such species #animalsofchristmas #museumadvent. check out @NHM_Digitise for amazing high res images of fleas #christmas pic.twitter.com/67Z6IinMX0

— NHM_Fleas (@NHM_Fleas) December 15, 2017

If you’re looking for more Christmas-themed parasite topics, you can check out my mistletoe-themed version of Twas the Night Before Christmas or my take on Santa’s bizarre roof top behaviors. See y’all next year!

Saving endangered vultures might save human lives

In the NCEAS SNAPP Ecological Levers for Health working group, we’re collecting examples of local or regional interventions that can have direct, measurable benefits for human health (via reduced infectious disease) and the environment – win-win solutions. The case studies that we’ve collected thus far are so cool that we just can’t wait to share them! So today I’m going to share a story about vulture conservation and human infectious disease. The bite-sized, Tweetstorm version of the story is available at @parasiteecology.

Let’s start with the obvious: vultures are crazy awesome birds. They have a gross/creepy reputation because they’re a bit funny looking, they eat dead stuff, and they have some odd habits, like defecating on their own legs to increase evaporative cooling. But they also have some animal superpowers: they can smell things that are kilometers away, they can fly despite being huge, and their stomach acids are so brutal that they can literally eat anthrax for breakfast.

But even superbirds have their Kryptonite. In the past few decades, millions of vultures have died after consuming human-sourced poisons. One such poison is Diclofenac, an NSAID that is used in veterinary medicine. Because a single carcass is typically visited by many vultures, contamination with the drug in discarded livestock carcasses can have huge impacts on vulture populations. And it did. For instance, in India, populations of three vulture species (Gyps indicus, G. tenuirostris, and G. bengalensis) plummeted by 97-99% in just one decade!! Globally, the majority of vulture species are facing extinction (critically endangered, endangered, or threatened), but the vulture extinction crisis in India is especially notable.

With local and global conservation efforts and funding already stretched thinly over thousands of endangered species, why should we care about vulture conservation, specifically? Well, for starters, vultures have been spiritual and cultural icons forever. Ever seen a Western movie? Watched the Jungle Book? Gone for a walk or a drive in the wilderness? Yeah, life without vultures would be weird. It’d also smell terrible. As obligate scavengers, vultures’ unique adaptations allow them to find carcasses much sooner than many facultative scavengers (e.g., dogs, raccoons, rodents). And vultures tend to pick carrion bones clean – and might even eat the bones! – whereas other scavengers often only eat specific tissues. That means that in a world without vultures, putrefying carrion would be more common. And not just “in the wild.” Many cities around the world have rudimentary waste management, at best, and vultures are a major player in waste removal/reduction.

But in a world overrun by carrion, the stench would be the least of our problems. Carcasses repulse us because they are hotspots of disease risk – sources of exposure to anthrax, botulism, and other infectious agents. And more subtly, abundant carrion might also increase populations of animal reservoirs for disease, like rodents and feral dogs. For instance, when vulture populations drastically declined in India in the 1990s – and carrion availability hypothetically increased – the feral dog population increased by millions, despite ongoing sterilization programs. We can’t be sure that vulture declines caused the increase in feral dog populations, because many other things changed in India during that same period (e.g., urbanization). But vulture declines are one possible driver of increased feral dog populations, and during the same period, the risk of feral dog bites increased, as did the number of human deaths due to rabies (Markandya et al. 2008).

Rabies kills 59,000 people per year – mostly in rural Asia and Africa, where access to treatment is limited – and almost all of these human rabies infections come from feral dog bites. Asia and Africa are also the hotspots of global vulture declines, and this spatial correlation suggests that adding an ecological intervention – in the form of vulture conservation – to ongoing dog sterilization and public health interventions might be a successful way to reduce rabies transmission. But interventions won’t be supported by the public and policy makers unless they are demonstrably cost-effective. So Markandya et al. (2008) figured out the economic cost of rabies in India and the cost of vulture conservation in India, and concluded that the benefits of reduced rabies outweighed the costs of vulture conservation. This could be a practical win-win!

But how do we conserve vultures? In addition to captive rearing programs to immediately buffer vulture populations, the most important conservation action was to switch from the lethal vet med Diclofenac to a vulture-friendly vet med, like Meloxicam. India, Nepal, and Pakistan all banned Dicofenac in 2006, and since then, vulture population declines seem to have slowed or even reversed (see below – Prakash et al. 2012)! But because the vulture populations are so small, the most recent populations estimates are admittedly rather uncertain, so these trends should be viewed cautiously.

If Diclofenac is banned, why aren’t vulture populations growing like crazy? For starters, vultures are K-selected species, so their populations grow slowly even under the best conditions. And despite the ban, Diclofenac is still readily acquired, so some contaminated carcasses are still finding their way into the food chain. It’s also possible that the sheer number of feral dogs in India is hampering vulture recovery, if the vultures are being outcompeted by dogs for available carrion.

Since Indian vulture populations haven’t rebounded yet – they’ve only (hopefully) stopped declining – we wouldn’t actually expect that available carrion, dog populations, and the incidence of human rabies have decreased. So it’s too soon to say whether this ecological intervention successfully reduced human infectious disease, as predicted. To further complicate measuring the public health success of this intervention, rabies isn’t a notifiable disease in India, so human rabies cases and deaths often go unreported. Therefore, if the human health impacts of vulture conservation in India are ever going to be decisively evaluated, some intensive surveying of vulture populations, dog populations, and human rabies cases will be required in the near future.

In conclusion, education/policy initiatives for vulture conservation are predicted to be #Levers4Health – mutually beneficial solutions for human infectious disease and conservation. But enacting these interventions can be tricky, and measuring their long term success might be prohibitively difficult. We’ll be eagerly awaiting more news and data on vulture conservation, feral dog populations, and human infectious diseases from Asia and Africa.

Do you know of other examples of potential win-win solutions for reducing human infectious diseases and advancing conservation goals? If so, we’d love to hear about them! You can let us know in the comments, on Twitter, or by email!

If you’d like to learn more about the vulture conservation crisis and it’s impacts on human health, check out these references:

Balmford, A. 2013. Pollution, politics, and vultures. Science 339: 653-654.

Buechley, E.R, and Ç.H. Şekercioğlu. 2016. The avian scavenger crisis: Looming extinctions, trophic cascades, and loss of critical ecosystem functions. Biological Conservation 198: 220-228.

Gangosa, L., R. Agudo, J.D. Anadón, M. de la Riva, A.S. Suleyman, R. Porter, and J.A. Donázar. 2012. Reinventing mutualism between humans and wild fauna: insights from vultures as ecosystem services providers. Conservation Letters 6(3): 172-179.

Green, R.E., J.A. Donazar, J.A. Sanchez-Zapata, and A. Margalida. 2016. Potential threat to Eurasian griffon vultures in Spain from veterinary use of the drug diclofenac. Journal of Applied Ecology 53: 993-1003.

Ogada, D.L.,  et al. 2016. Another continental vulture crisis: Africa’s vultures collapsing toward extinction. Conservation Letters 9(2): 89-97.

Markandya, A., T. Taylor, A. Longo, M.N. Murty, S. Murty, and K. Dhavala K. 2008. Counting the cost of vulture decline – an appraisal of the human health and other benefits of vultures in India. Ecological Economics 67:194-204.

Prakash, V, et al. 2012. The population decline of Gyps vultures in India and Nepal has slowed since veterinary use of Diclofenac was banned.  PLoS ONE 7(11): e49118. doi:10.1371/journal.pone.0049118

Photo/figure credits from the Tweetstorm can be found on the figures, or at these locations:

(4, 6, 8, and 15) BirdLife South Africa has a bunch of great fact cards that are worth sharing. You can check out the rest here.

(7) Thanks, Disney, for our childhoods.

(10) Photo credit to Corrinne

(12) Figure credit to Steven Vanek

(13) Find this and other excellent illustrations here

(16) From here

(18) The LA times has a great series of condor release photos here

(19) Thanks to Ginger at NCEAS for being our photographer!

Parasites and de-extinction

[We’re still taking a break from the “how to become a successful parasite ecologist” post series. More on that in a few weeks!]

Sometime during my undergraduate education, I was required to prepare for and participate in a class debate exercise regarding whether we should bring animals like the woolly mammoth back from extinction. In the years since, I haven’t kept up with that literature at all, so I was quite surprised to read this opening line in a recent paper: “De-extinction is rapidly transitioning from scientific aspiration to inevitability.” Wow!

But that wasn’t even the most exciting part of the paper. Wood et al. (2017) went on to point out that to successfully ‘resurrect’ extinct species, we would need to ensure that the appropriate abiotic and biotic environments exist to sustain those resurrected species. You know what that means, don’t you? Parasites. If we’re going to resurrect extinct species, we need to give them parasites.

Here’s a quote from the paper. I hope it makes you ponder things… I certainly did.

“Would it be possible to genetically manufacture a parasite fauna and microbiota to suit the resurrected species, perhaps using palaeoecological data as a guide? Or would a mixture of extant parasites and microbiota, from species with a similar ecological niche, be sufficient? What implications would there be of a failure to adequately reconstruct these obligate microbiotic communities for the resurrected species and the ecosystem within which it is to be embedded?”

Reference:

Wood, J. R., Perry, G. L. W. and Wilmshurst, J. M. 2017. Using palaeoecology to determine baseline ecological requirements and interaction networks for de-extinction candidate species. Funct Ecol, 31: 1012–1020. doi:10.1111/1365-2435.12773

 

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.

(Not) North Pole Parasites: Mistletoe

I meant to continue blogging about North Pole Parasites today, moving on to talk about Trichinella in polar bears, but this article by Tommy Leung reminded me that mistletoe is really the ultimate Christmas parasite. So instead, I wrote you guys a poem. Happy Holidays!

EDIT: Oh, and for cool footage, check out this video!

Everything you thought you knew about keystone sea stars is wrong

…ok, maybe not everything. I’m just getting into the clickbait title fad. But you probably are incorrectly citing Paine (1966), so you’ll be glad you clicked!

Bob Paine, a giant in ecology, recently passed away. He left behind an incredible legacy of ideas and students, and one insanely famous paper: “Food web complexity and species diversity.” If you’re an ecologist, you know that in the experiment described in that paper, Paine removed sea stars from the rocky intertidal and then recorded what happened. In particular, when sea stars were removed, he found that mussels (a favorite delicacy of sea stars) took over more of the primary substrate, crowding out other space-holding species and thus reducing the total number of space-holding species.

According to a recent paper by Lafferty and Suchanek (2016)(PDF link), ecologists usually cite Paine (1966) when they say something like, “predators increase biodiversity by fostering co-existence among competitors.” Most papers never specify which components of biodiversity actually increase when sea stars are present (i.e., primary space-holders). But it turns out that it is important to be specific, because in the rocky intertidal, sea stars actually greatly reduce biodiversity! By eating mussels, sea stars reduce the surface area of an important 3D habitat full of epibionts and parasites and tiny free-living organisms that live among the mussel shells. So, when you cite Paine (1966), you should specify that sea stars increase primary space-holder biodiversity, but reduce total community diversity.

As a side note, my favorite quote from this paper was: “Whelks are like little wolves in slow motion.” Go read it!

Those purple things are mussels. You get the cartoons you pay for on this blog. 😛

Reference:

Lafferty, K.D., and T.H. Suchanek. 2016. Revisiting Paine’s 1966 Sea Star Removal Experiment, the Most-Cited Empirical Article in the American Naturalist. The American Naturalist.