I am a little hesitant to tell people outside of my research field what I study, which is surprising if you know me - I love talking about my research. It’s fascinating (and I’m not at all biased). I would like to think my enthusiasm for my research project is contagious; however, my fervor is often met with varying degrees of disgust. Why?
Because I study parasites. And not just any parasites; I study the roundworms, tapeworms, and flukes of rats and mice. Not only is there a stigma against parasites, but also against the rodents I collect. I will spare you the explanation of why rodents are awesome and mostly misunderstood, but I will be delving a bit more into the study of parasites.
These groups of parasites, the roundworms, tapeworms, and flukes, are collectively known as helminths. In most cases, these helminths reside in the body cavity, organs, and various tissues of their hosts. While their creepy crawly nature may repulse some people, they are remarkably important to individual hosts, populations, communities, and to the ecosystem as a whole, yet they are often ignored in many fields of study in the biological sciences. There is a growing body of literature attesting to their importance even outside of human and veterinary medicine (see citations below for a few examples).
Before scientists can determine the role of parasites in host evolution, community ecology, trophic ecology, and so on, we must first know more about parasite diversity and abundance. Parasites are largely understudied, and species new to science are still being discovered and described. Parasitological surveys of hosts have merit for documenting the diversity and abundance of parasitic species in hosts.
I approached Dr. Anna Phillips at the American Society of Parasitologists’ annual meeting in July 2015 about joining one of her research projects. Anna, a research zoologist and one of the curators of the invertebrate collections, is collaborating with researchers in Spain to investigate the parasite fauna of rodents and tree shrews from Borneo. Few studies have examined the parasites of these species from these localities, so this investigation is important to determine the diversity and abundance of the parasites of these hosts.
We have a few goals for this research. First, we will document the helminth diversity and abundance in these hosts. Second, we will describe species new to science. This will allow for these helminth species to be included in future ecological and evolutionary studies and considered in conservation management plans. Third, these helminths will be DNA barcoded – certain genes will be sequenced and input into an open access database for future use by other scientists (for example, for identification or phylogenetic studies).
This past May, I spent three weeks going through some of the rodent specimens looking for parasites. Unfortunately, I was only able to get through a small portion of the rodent samples – fortunately, this was due to the high numbers of parasites found in each specimen. Morphological and molecular identification for these first samples is ongoing; we hope to have the rest of the parasites collected and identified by next year. After identification, the parasite specimens will be deposited into the National Parasite Collection for future scientific use.
With such a large number of helminths found in the intestines of each rodent, you may wonder if the rodents and tree shrews were alive when they were collected. Yes, they were alive! So were they sick?
That is a more interesting question. Parasites can affect their hosts in a variety of ways – they can influence host reproduction, behavior, health, and life span. The effects and their magnitude differ between parasite species and host species, and are influenced by the number of parasite species and the number of individual parasites infecting the host.
Back to the original question – what about the rodents and tree shrews? I did not collect these specific rodents and tree shrews so I don’t know if they were lethargic, skinny, weak, acting strangely, in breeding condition, etc. We do have some clues about the parasites’ effects on their host’s health though. I examined the organs where the parasites were located, and in most cases, I only found slight physical damage, often to the intestinal villi. There were some specimens with more serious damage: intussusceptions, where a portion of the intestine inverts itself, and multiple perforations of the intestinal and stomach wall were present.
Intussusceptions in humans and other animals can cause abdominal pain, vomiting, and diarrhea. While this condition has been linked to infection with parasites, there are other causes too. There is no way for us to definitively say that these intussusceptions were results of parasitic infection. If they were linked, we would be able to conclude that these parasites are having a negative impact on their hosts’ health.
We can conclude that the perforations of the organ walls were caused by the parasites. I found a number of nematodes half buried in the walls of the stomach and the large intestine. I did not find any of the worms passing completely through the organ wall; rather, the walls were thickened where the parasites had burrowed in. The hosts likely suffered abdominal pain when the worms originally burrowed into the organ walls, and may have also experienced blood in the feces or diarrhea.
With parasitological studies such as this one, we are limited to making observations and finding correlations between a parasite’s presence and possible effects on the host. Experimental and manipulative studies are required to definitively link cause and effect, but observational studies are still very important for determining the biodiversity and abundance of parasites, as well as driving the formation of hypotheses linking parasites and their effects on the host.
In the next few months, we will continue to sort through the rodent and tree shrew specimens, collecting, sorting, and counting the helminths found. Helminth specimens will be sequenced for DNA barcoding, prepared for museum installation, and described as a new species if they are species new to science. We look forward to the exciting discoveries and results from this work!
By Whitney Preisser, a fourth year doctoral student in the Department of Wildlife and Fisheries Sciences at Texas A&M University. Her research investigates the latitudinal diversity gradient of parasitic helminths in North America. She can be reached at wpreisse *at* tamu.edu.
- Altizer, S, CL Nunn, and P Lindenfors. 2007. Do threatened hosts have fewer parasites? A comparative study in primates. Journal of Animal Ecology. 76:304-314.
- Fenton, A and MA Brockhurst. 2008. The role of specialist parasites in structuring host communities. Ecology Research. 23:795-804.
- Palm, HW, S Kleinertz, and S Rückert. 2011. Parasite diversity as an indicator of environmental change? An example from tropical grouper (Epinephelus fuscoguttatus) mariculture in Indonesia. Parasitology. 138:1793-1803.
- Ricklefs, RE. 2015. Intrinsic dynamics of the regional community. Ecology Letters. 18:497-503.
- Wood, CL, JE Byers, KL Cottingham, I Altman, MJ Donahue, and AMH Blakeslee. 2007. Parasites alter community structure. Proceedings of the National Academy of Sciences of the United States of America. 104:9335-9339.
- Dougherty, ER, CJ Carlson, VM Bueno, KR Burgio, CA Cizauskas, DP Seidel, NC Harris. 2016. Paradigms for parasite conservation. Conservation Biology. 4:724-733.