Risk Factors Influencing Parasite Counts Between- and Within-Litters in Columbian Ground squirrels (Urocitellus columbianus)
Date
2019-03-26
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
The number of parasites carried by a host varies from host to host due to individualities that influence the risk of host exposure to parasites, subsequent invasion of the host and colonization by parasites, and the degree of parasite proliferation within the host. Factors that contribute to differences in risk include variation in host and parasite characteristics as well as in the surrounding environment. Features of the host that may impact parasite count include risk factors such as age, body size, genetic resistances, hormone levels and avoidance behaviors, whereas risk factors derived from the parasite comprises their capacity to find and colonize a new host, and mode of transmission (whether through physical contact, ingestion, inhalation, etc.). Environmental risk factors arise from fluctuations in climate, including microclimate, and the distributions of both host and parasite across the landscape - whether they are in tightly-clumped groupings or diffusely spread. Previous literature has primarily focused on factors that impact the risk of carrying parasites in adult hosts - with the notable exception of clutch size and parasite load. To my knowledge, no study as of yet has placed its central focus on comprehensively exploring risk factors in juvenile hosts with limited previous exposure to parasites. Columbian ground squirrels (Urocitellus columbianus) are used in this study to investigate risk factors to parasitism in juvenile hosts to fleas, lice, and Eimeria, a single-celled intestinal parasite. Columbian ground squirrels offer an ideal study system because offspring are born in environmentally and socially-isolated natal burrows which only the mother enters, making her the sole source of parental care and parasites to her offspring. Two-thirds of litters have multiple paternity, enabling investigation into the role of the father’s genetics. The risk factors tested were separated in two groupings: 1) those that varied between-litters, and 2) those that varied within-litters. Between-litter risk factors included: mother’s condition upon emergence from hibernation, mother’s age, mother’s parasite count, the number of times the natal burrow was changed, litter size, and host density around the natal burrow. Within-litter risk factors included weight, sex, and paternity. Offspring flea counts increased with local host density within 30 m of the nest burrow, and were higher in male offspring, and in offspring with lower body weight. Offspring louse counts increased with host density within 20 m of the natal burrow, and were higher in female offspring and lighter offspring. Eimeria oocyst counts in the feces were higher in offspring that moved natal burrows more frequently. Additional risk factors that had adequate influence to be included in the final models were: for flea count, maternal condition at emergence, for louse count, maternal louse count at weaning and maternal emergence condition, and for Eimeria oocyst count, litter size, mother’s age, and offspring weight. For both fleas and lice, risk increased with host density around the natal burrow; however the effective distance was different: 30 m for fleas and 20 m for lice. While fleas can jump from host-to-host in addition to living off host for short periods of time, lice require direct host-to-host contact which will shorten the effective distance. On the other hand, I propose one of the predominate risk factors influencing high Eimeria oocyst counts is maternal stress. Although stress was not directly measured in this study, offspring that carried more Eimeria had mothers that were older, who had produced larger litters, who had moved natal burrows more often, and who had other closely nesting adult females. All of these could possibly contribute to chronic stress which is known to weaken the immune system, and could additionally lower the ability of the mother to care for her litter and lead to increased risk of Eimeria infection. This study shows that factors which increased risk to parasite infestation in offspring were due to the interaction between the biology of the host and the parasite, as seen with parasite transmission mode for fleas and lice and its relationship to host density, and stress-related immune weakness and Eimeria oocyst count. The re-location of litters to new natal burrows, considered a method of parasite reduction, did not reduce offspring parasite counts. Instead, it appears that parasites infested the natal burrows when the mother prepared them in the weeks after she mated (i.e. during gestation). This made post-mating maternal parasite counts one of the most relevant maternal counts. The other maternal parasite count that consistently had relatively high correlation to offspring parasite count at weaning was maternal count at the same time, reflecting temporally recent transfers of parasites between mothers and their offspring. Sex, weight and yearly environmental conditions were also revealed to be relatively strong risk factors over others such as litter size, number of natal burrow changes, paternity, maternal age, maternal condition, and maternal parasite counts. Investigating offspring in their role as a host gives unique insights into the interactions between hosts and their parasites that are difficult to quantify in adult hosts. Effects stemming from offspring’s mother were found to have stronger effects than those of paternity or litter. Previously well-studied factors, such as litter size, were not found to be strong explanatory factors in this study; instead other factors that were associated with the specific biology of the host and parasite were more relevant, such as host density for fleas and lice, and for Eimeria counts, factors that could induce maternal stress such as neighbouring females, large litters and old age. The more holistic approach in this study, verses a study on the relationship between only a few variables, hopes to provide a clearer impression of the relative weight of risk each factor in determining host parasite count.
Description
Keywords
Columbian ground squirrel, Urocitellus columbianus, parasite load, offspring, litter
Citation
Hammer, T. L. (2019). Risk factors influencing parasite counts between-and within-litters in Columbian ground squirrels (Urocitellus columbianus) (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.