Parasitism and Stress

Research in the Davis lab is broadly focused on animal “ecophysiology”, with study subjects ranging from birds, to butterflies, to beetles. REU students can make many contributions to these ongoing projects, with a combination of field work and laboratory experiments.  A recent thematic area involves asking, how can animals cope with the daily stressors in their lives while being parasitized? A useful study subject for these experiments is a common forest-dwelling beetle (horned passalus, pictured), which is naturally-parasitized by a seemingly benign nematode (pictured). This parasite appears to cause little outward harm to its host, but during times of duress or heightened activity, there is in fact an observable cost to being parasitized.

In the summer of 2020, an REU student will conduct an experiment that tests the behavioral and physiological reactions of parasitized and unparasitized beetles to a mild, non-lethal stressor, to further understand how parasites impact their hosts. The details of this project will depend in part on the interests of the student. The ideal applicant for these projects is someone who is OK with traipsing through chigger-infested forests, is able to work with (handle) bugs, and who is not squeamish about icky dissections.

Mentor: Andy Davis
Type of project: Empirical, Lab- or field-based

Effects of parasites and predators on heart rates of Daphnia laevis using an innovative electronic stethoscope

Lutchie M. Carrasquillo, a student at the University of Puerto Rico at Arecibo, worked with Christian Hurd and Dr. Andy Davis to study the effects of parasitism on Daphnia heart rate using a new methodology.

Abstract: Daphnia are a model organism often used in investigations of chemical toxicity, and for biology classes. Measuring changes in heart rate is a commonly-used approach to assess responses to toxins. However, these assessments are usually done manually, which is time consuming and tedious. We developed a novel apparatus for monitoring changes in Daphia heart rates in real-time, without harming the animals. We used this approach to investigate how heart rate changes in response to naturally-occurring parasites (epibionts) and a natural Daphnia predator (glassworms). Our results showed Daphnia heart rates were not greatly affected by these, but we did discover an unusual diurnal effect, where the heart rate response differed between the morning trials and the evening trials.


Nematode parasite reduces the fight or flight reaction in its host

Felicia Ebot-Ojong, a junior from the University of Georgia, worked with Dr. Andy Davis to study the effect of nematode parasites on the fight-or-flight reaction of infected beetles.

Abstract:  Parasites cause a range of unfavorable effects on host fitness, including reducing available energy. This could be detrimental in situations where the host needs to escape a harmful situation (in a “fight-or-flight” situation), though this is rarely studied in parasitology research. The bess beetle (Odontotaenius disjunctus) inhabits forests in the eastern United States and is host to a naturally-occurring nematode parasite (Chondronema passali), which can be extremely abundant within hosts. The goal of this project was to evaluate the how nematode infection affects the fight-or-flight responses of the beetles under simulated predator attack. A total of 150 beetles were collected locally and stored individually in plastic containers for 3 weeks. Each week, we assessed and video-recorded behavioral reactions to a series of “pokes and prods”, then scored level of the reactions in the videos. We assessed the number of vocalizations during attack, plus levels of physical resistance using a scoring system (Fig. 2). Beetles were later dissected and their level of nematode infection and gender was recorded. We found that beetles with nematode infections tended to have reduced fight-or-flight reactions to the predator attack compared to unparasitized individuals. The level of nematode infection had no effect on the amount of vocalization made by each individual during an attack. These results show that nematode infection has an energetic cost during situations where the host needs to respond quickly.

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A protozoan parasite weakens the stress reaction of monarch butterflies

Jovani Raya, a student from Abaraham Baldwin Agricultural College, worked with Dr. Sonia Altizer and Dr. Andy Davis to study the effect of infection on the stress reaction of monarch butterflies.

Abstract:  The protozoan parasite Ophryocystis elektroscirrha (OE) affects the adult mortality, longevity, body size, and flight ability of monarch butterflies (Danaus plexippus). However, very little research on how the parasite influences the stress response in monarchs has been conducted. We examined the effects of parasite infection and larval rearing densities on the monarch stress response. Monarch larvae were inoculated with parasite spores and reared in low (2 larva) or high (10 larva) densities. When the monarch larvae reached pupation, we assessed their stress reactions. To produce the stress response in the pupae, physical disturbance was applied for 20 seconds. After the disturbance, the pupa was placed on a device that detects movement within animal tissue and can record the movements of the heartbeat. This recording allowed us to count the number of beats per minute. The result showed that infection was a significant predictor of the magnitude and duration of pupa HR; infected monarchs had lower reactions. Lower reactions would negatively affect how well infected monarchs could cope with daily stressors, especially during the arduous fall migration.

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Can Internal Parasites Affect Wound-healing Rates in Insects?

Lexi Calderon, a student from the University of Redlands, worked with Dr. Andy Davis and members of his lab to study parasites affect wound-healing in bess beetles.

Abstract: By definition, parasites depend on the resources of their host to survive. This relationship can result in a decrease of energy and fitness for the host. The parasitic nematode Chondronema passali resides in the hemocoel cavity of the bess beetle, Odontotaenius disjunctus. Although this parasite is non-lethal, a single beetle can harbor thousands of nematodes. Previous research has demonstrated this parasite affects the stress reaction of beetles, but very little research has investigated the effect parasites have on the host’s ability to heal a wound. Wound healing can be thought of as an indicator of the effectiveness of the immune system and by studying healing we can infer the effect this parasite has on the fitness of its host. We conducted a series of experiments where beetles were wounded with a dremel rotary drill and observed every hour for 12 hours after initial wounding. Each hour beetles were given a value from 1-5 to measure their status in the wound healing process, and values were summed to generate a ‘wound healing score’ for each beetle. Beetles were killed and dissected following the experiment to define gender and parasite abundance. Out of 188 beetles, 83% were infected with C.passali. Wound healing scores were not significantly predicted by parasite status. Beetle weight was a predictor of wound healing scores where heavier beetles had higher scores. Oxygen consumption was also measured in a subset of beetles after wounding, and we found parasitized beetles tended to have higher respiration (10% higher) than non-infected beetles.

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Do Parasite Infections Effect Fighting Ability in Beetles

David Vasquez, a student from Virginia Tech, worked with Andy Davis in the Odum School of Ecology to examine the effect that parasite infections have on the fighting ability of beetles.

David Vasquez1, Andy Davis2

1Virginia Polytechnic Institute and State University,

2Odum School of Ecology, University of Georgia

Parasites, by definition, subsist off their host’s resources, which can drain energy. This can have negative consequences for the host, especially during energy-intensive activities. Fighting is common in most animals that are territorial, or that are protective of young. Few studies have examined the effect of parasites on fighting capacity in animals. The bess beetle (Odontotaenius disjunctus) is a saprolytic insect common in forests within the eastern United States, and it is susceptible to a naturally-occurring nematode parasite (Chondronema passali). We examined the effect of infections on the outcome of staged fights in this beetle. Beetles were selected based on weight (so that each pair contained similarly-sized individuals), then placed in a small wooded arena to observe fighting behavior. A video camera recorded 3 minutes of fighting. Afterwards, beetles were killed and dissected to determine gender and parasite status. From the videos, an external observer recorded the number of bouts, wins and losses for each pair, and the overall winner. A total of 78 beetles were used in the experiment; 40% were infected with C. passali. 31 infected beetles were the overall winner in 52% of their matches, while uninfected beetles won 48% of fights (this was not significantly different X2 test). However, when fights were grouped by infection severity, we found that beetles with the highest infection score (thousands of nematodes) won 71% of their battles, while the least-infected beetles only won 25% of the time. This is counter-intuitive to the idea that infection has a negative energetic effect on host fitness.

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