Parasite Perspectives: Thinking Outside the Box

Examining the age-old problem of parasites in a new way might lead to innovative solutions for equine parasite infections.

Scientists tend to think in certain traditional ways when they approach their chosen topic. It is often difficult to change or do things differently, and those that do are often considered heretics in the field. Author Gregory Maguire, who wrote the book Wicked, described his retelling of classic fairy tales by looking at them from a different proverbial camera angle. These new "shots" would bring different characters and happenings in the story into focus and provide an alternate perspective. Creative thinking is all about taking a subject you know and looking at it in a new way.

Scientists have been studying parasites for centuries. Many research hours have been spent developing new treatments, finding out their effects on the host, and even examining the life cycles. Some parasitologists are now approaching the subject of parasites from some interesting alternate angles.

An Ecological Perspective to Parasites

Researchers are examining parasites through a wide-angle lens, using an ecological perspective and dialoguing about how parasites should fit into the "food web" theory.

Discover magazine ran an article by Carl Zimmer recounting the fact that every living thing in nature has at least one parasite living with it or in it, and the higher level species often have many more. In fact, Michael Sukhdeo, MSc, PhD, professor in the Department of Ecology, Evolution, and Natural Resources at Rutgers University, tells us, "We estimate that 60-70% of all animal species on Earth are parasites. Here parasites are defined as any animal species that lives all or most of its life on or within a single animal host (but excluding bacteria and viruses)."

This means in the grand scheme of things, parasites are an important part of the ecology of our world.

An ecosystem is a community of organisms in which several food chains having a mutual or reciprocal relation form what's termed a food web. Food chains are defined by as a group of organisms interrelated, in that each member of the group feeds upon the one below it and is eaten by the organism above it in the chain.

As scientists begin the attempt to integrate parasites into existing food webs, it becomes clear these parasites radically affect the traditional perception of food webs. Most studies find parasites represent about 75% of all of the connections in a given ecology.

Parasites have the ability to regulate the populations of their hosts by causing illness in, and death of, the hosts.

Horse owners should also be concerned about ecotoxicity (toxins in the environment). Modern deworming drugs kill a wide variety of parasites, and drug residues pass with the feces in surprisingly high levels, says Sukhdeo. The drug residue kills beneficial bugs on the pasture (i.e., dung beetles).

How else do parasites affect the ecosystem? Predators that are most abundant in a food web are the most susceptible to parasite infection and become more vulnerable as parasites evolve to adapt to these hosts. Rarer host species are least likely to be affected by parasites, as they are "less of a sure bet" when parasites are choosing a host, notes Sukhdeo.

Kevin D. Lafferty, PhD, is a research ecologist for the U.S. Geological Survey's Western Ecological Research Center. In his article, "Parasites dominate food web links," he writes. "Parasites strongly affect food web structure. Indeed, they disproportionately dominate food web links. Most food webs have ignored parasites because parasites are hidden and are perceived to have negligible biomass, yet some parasites have population dynamic impacts that are hugely disproportionate to their small size."

This perspective represents a paradigm shift of massive proportions. Scientists still have not decided on the best models to use to incorporate parasites into food web models, and the shift certainly raises additional questions.

The Parasite's World

Shifting from a macro view of parasites as they relate to the total ecology, we move to the micro view, where scientists are exploring the world from the perspective of the parasite itself. Konrad Lorenz, Niko Tinbergen, and Karl von Frisch were awarded the Nobel Prize in 1973 for their work on the idea of species-specific perceptual worlds in animals. It is now one of the core assumptions in modern animal research.

Only recently have scientists begun to apply this approach to the world of parasites.

From the parasite's perspective, the world is extremely limited in scope, but very focused on one thing--survival. Parasites seem to care very little about what happens around them unless it directly affects their life cycles.

Parasites are remarkably adaptive over time and can even cause changes in host behavior to suit their survival needs. For example, a common human infection, toxoplasmosis (why pregnant women should not clean a cat litter box), changes the behavior of infected mice so they become unafraid of--and even attracted to--cats, the definitive host in the parasite's life cycle.

Sukhdeo has studied parasite behavior for some time and explains, "A critical idea is that all animals possess complex patterns of innate behavior which can be released by extremely specific signals from the environment. Parasites live in ecologically predictable worlds, particularly once inside the host."

It seems that for many parasites, complex behavior patterns can be set off by a very small number of environmental signals. The parasite has one job--to get into the host and find its way to the place inside where it can thrive. If it is unable to complete this mission, it is destined to die. So, it expends tremendous amounts of energy trying to accomplish this one simple mission in life.

Research to identify these environmental signals and how they help parasites navigate has been a focus of Sukhdeo's lab for some time. Inside a specific host, which provides a closed and predictable environment, parasites respond to different chemical triggers, which help them navigate to where they want to be.

The liver fluke, for example, uses bile as one of its triggers. Once a liver fluke encounters bile, it begins to wiggle like crazy, breaks out of its cyst, and progresses toward the liver.

Other parasites respond to different triggers.

Small strongyles, for example, respond to a common digestive enzyme called pepsin. The trigger allows them to enter a different locomotor stage so they can effectively reach the best places to feed and thrive. In this case, the prime locations would be the large intestine, colon, and cecum. If small strongyles didn't encounter pepsin, they would not know to make the next move to reach their goal.

In studying this Sukhdeo began to wonder what would happen if you were to expose the parasites to their triggers at different times. Sure enough, he could trigger a change in behavior by introducing the trigger compound artificially at different places and stages in the host's system.

Parasites in Public

So, what would happen if scientists were to alter triggers and the environment before the parasite enters the host? Trials were set up to study this.

The team at Rutgers looked at applying pepsin to small strongyle larvae in the pasture. Sukhdeo recalls, "We were simply spraying it (pepsin) onto the field using a commercial sprayer."

The trigger succeeded in killing the larvae in the field because they were tricked to believe they were inside the host. They struggled until they used up their remaining energy and died. Amazing, right? This treatment might eliminate the ongoing cycle of small strongyle infection in horses. Traditional deworming cannot change the fact that as soon as you eliminate the internal parasites, horses graze and pick up more of them. Here, researchers discovered a solution. It even turned out to be economically viable.

"Our tests were done on subsets of large fields, and based on our application rates, we estimated the treatment costs at $10-$15 per acre using crude pepsin preparations," Sukhdeo said.

So why are we not all spraying our fields? There was no funding to take these tests further and allow scientists to test pepsin safely and develop it into a commercial product. Thus, research stalled.

Finally, in yet another change of perspective, Sukhdeo and his team took a look at how the environment outside of the host affects the navigation of parasites. In one study they examined strongyles that seemed to "dry up" and lie dormant in the field for periods of time. They found that parasites in this state can be very hard to kill in the environment because they can enter an anhydrobiotic state (a state of apparent suspended animation) when completely desiccated (fully dried up). The data from this study suggest that "anhydrobiosis in larvae promotes survival at freezing temperatures, decreases metabolic activity, and prolongs survival under natural field conditions."

It explains why long dry spells do not kill off the parasite population in a field. The anhydrobiotic state is a natural survival mechanism of the larvae.

Another study looked at how worms navigated around fields to be eaten by grazing horses. Horses are considered to be naturally coprophobic, which means they don't really like to eat near where they defecate. As part of their life cycle small strongyle eggs are eliminated in the feces and enter the early larval stages. Small strongyle larvae, once they reach the later stages of their life cycle, are known to be mobile, but usually only for short distances. So, how do they travel the sometimes great distances to the prime feeding areas? Sukhdeo and his team set up a study to find out.

"We did a series of ground elevation studies, which were conducted in horse fields populated with naturally infected horses," he says. "Almost all of the parasites recovered were small strongyles. We determined that above-ground flows of rainwater were consistently responsible for transporting the strongyle larvae over the large distances in the pasture. The strongyle larvae then used their own locomotive action to climb up the blades of grass to be ingested by the horses."

In summary, the parasites effectively used the natural rain cycles and water runoff patterns to be hitchhikers on the road to their desired destination.

It is interesting to note that horses in another study with a higher pasture population density overcame their natural coprophobic tendency and both defecated and grazed throughout the field. Parasites in that study were, thus, not required to migrate for ingestion.

Take-Home Message

Parasites, it seems, are a much bigger part of our world than we have given them credit for. While taking up little biomass, they have a huge impact on the ecosystems we all live in. Scientists are just beginning to realize the extent of that impact as they attempt to integrate parasites into the complex views and models of our ecosystems.

With the rapid development and progress in the realm of environmental science, parasites might play a new role in our world perception. Changing the way a problem is approached (changing the proverbial camera angle) can often lead to amazing insights. It will no doubt lead to a variety of innovations in the future.

As parasitologists wrestle with challenges such as anthelmintic resistance, perhaps changing the direction of approach to the problem will one day lead to innovative solutions, such as the pepsin treatment. That is, if funding promotes forward progress.

The more we know about parasites, the better we can learn to adapt our methods to keep our horses and ourselves healthier. Kudos to the scientists who continue to think "outside the box."

About the Author

Liza Holland

Liza Holland is a freelance writer and voice talent based in Lexington, Ky.

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