The Effect of Season upon the Life Cycle of Haemonchus contortus in Experimentally Infected Lambs
This project was developed to study the life cycle of the most pathogenic gastrointestinal nematode (GIN) affecting small ruminants, Haemonchus contortus. The main goal of the study was to test the effects that the environment and the age of the adult worm have on the resulting egg’s ability to hatch in vitro and the larvae’s ability to undergo exsheathment both artificially in vitro as well as in vivo within the rumen. A secondary study objective was to compare the results from the two exsheathment assays in order to see how well they correlate. This study was designed to run in a series of cycles according to the start of each season (cycles denoted as: Fall 1, Winter, Spring, Summer and Fall 2). Each cycle began with the infection of two genetically related donor lambs with H. contortus larvae (L3). Larvae and eggs were harvested from the donor lambs for 4–6 months (increasing worm ages) for each cycle and were subjected to in vitro egg hatch and in vitro and in vivo exsheathment assays, respectively. Eggs were incubated in a well plate at 26 °C and after 24 hours of incubation, hatchability of eggs was assessed under the microscope. For the in vitro exsheathment assay, larvae were bubbled with CO2 for 15 minutes and incubated at 37 °C for 18 hours and were then observed under the microscope to determine percent live exsheathment, as well as viability. For the in vivo assay, larvae were added to a containment capsule and suspended in the rumen of four fistulated ewes for 8 hours, recovered and examined under the microscope to determine percent live exsheathment, as well as viability. Data collected from the two exsheathment assays were statistically compared to one another according to seasonal effects and age of worm effects. It was found that neither season nor age of worm had any effect on egg hatchability. There was a season*worm age effect on both in vitro and in vivo exsheathment and viability across multiple seasons and worm ages tested. Increased variability in in vitro exsheathment rates was detected during the Fall 1, Summer and Fall 2 cycles, with lower variability during the Winter and Spring cycles. Upon comparing the results of the two assays, it was determined that the in vitro assay yielded higher viability rates, but lower and more variable exsheathment rates when compared to the in vivo assay. The results of this study indicate that both season and age of worm have an impact on exsheathment of H. contortus larvae both in vitro and in vivo, especially during the Fall and Summer seasons. The findings of this study also indicate that the in vitro and in vivo assay yield variable results according to different seasons and worm ages. This study has shed light on the role that both season and age of worm play in the exsheathment stage of the H. contortus life cycle, indicating that these factors need to be studied in more depth in future research. Additionally, more work should be done to compare different in vitro and in vivo exsheathment assays in order to refine methodology for how H. contortus is studied in anthelmintic research.
"The Effect of Season upon the Life Cycle of Haemonchus contortus in Experimentally Infected Lambs"
Dissertations and Master's Theses (Campus Access).