Studies on the population dynamics of Teladorsagia circumcincta

Richardson, Katherine (2000) Studies on the population dynamics of Teladorsagia circumcincta. PhD thesis, University of Glasgow.

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An initial study was conducted to investigate the recovery rates of Teladorsagia circumcincta infective larvae from small herbage samples, as most methods have been developed for isolating infective larvae from large, field samples. A direct count technique (DCT) was developed, enabling high levels (on average 80- 90%) of infective larvae seeded to be recovered. This technique was used to investigate the influence of temperature, relative humidity and light intensity on larval migration, using growth chambers. The effect of two different sward types was also investigated, using a young, growing sward and a mature, tillered sward. The results suggest that initial migration onto the sward may be very rapid and independent of external stimuli. The majority of the larvae were recovered from the bottom third of the sward. A greater number of larvae were recovered from the mature sward than from the young sward, which may be due to the tillering of the sward, so creating pockets where moisture could be trapped and so altering the micro-environment. The optimum conditions for migration of T. circumcincta vary according to the time after seeding that the pots were harvested. Lambs fistulated at the oesophagus were used to investigate larval intake using trays of rye grass seeded with known numbers of infective larvae and to investigate whether grazing behaviour influenced larval intake. The influence of sward height was also investigated, using a 3cm and a 6cm sward. The bite depth was shown to differ on different height swards but this did not have a significant effect on the number of larvae ingested. The periparturient ewe is thought to be the major source of contamination for the young growing lamb. To investigate the period of greatest susceptibility to T. circumcincta, three groups of ewes were given a single dose of 10,000 infective larvae either 7, 28 or 56 days post partum (DPP). Their faecal egg count and blood parameters were measured over a period of 56 days post infection (PI). A group of worm naive and barren ewes were also included to act as positive and negative controls. The groups found to be most susceptible were the ewes given the larvae 7 and 28 DPP, and the ewes given the larvae 56 DPP were the most refractory to infection, showing a similar level of infection to that of the barren ewes. One theory as to why young, growing animals to do not mount an effective immune response is that there is competition for available nutrients, especially protein, between growth, repair of the damaged intestinal tract due to parasite infection and the immune response. If sufficient protein was available in the diet of these animals, it is hypothesised that this competition would be eliminated. To test this theory, two experiments were conducted, one using lambs of approximately 3 1/2 months of age at the start of the study and one using lambs of approximately 5 1/2 months of age. The lambs were divided into three groups and fed either a basal ration (MP), high protein ration (HP) or the basal ration supplemented with methionine, an essential sulphur amino acid, protected from degradation in the rumen. There was no evidence of either increased resilience, the ability of an animal to perform despite parasite challenge, or increased resistance, the ability to suppress establishment of parasites, resulting from either the increased protein ration or the supplementation of methionine. The use of computer models offers a very cost effective method to examine management strategies and investigate the development of anthelmintic resistance. In order to simulate what level of reduction in faecal egg count would need to be achieved in the ewe by the addition of dietary protein, to have a significant impact on the infection of the lambs, an anthelmintic bolus of varying efficacy was administered, then a ten year simulation run. A model developed by Drs. Barnes and Dobson was used, with weather data supplied by the Rothamstead Institute used to parameterise the model for UK conditions. The management strategy modelled was that used by one of the Institute's farms, using 120 ewes and their twin lambs. The results obtained show a very similar pattern to experimental data generated over a three year period by Barrett (1987), but of a much smaller magnitude. Nevertheless, the predictor models are an extremely useful and versatile tool.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Parasitology
Colleges/Schools: College of Medical Veterinary and Life Sciences
Supervisor's Name: Jackson, Dr. Frank, Coop, Dr. Bob and Duncan, Professor Jimmy
Date of Award: 2000
Depositing User: Enlighten Team
Unique ID: glathesis:2000-72144
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 17 May 2019 12:48
Last Modified: 05 Apr 2022 11:30

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