Browsing by Author "Nel, Cornelius Loftus"

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    The comparison of immunological responses of Brahman, Nguni and Angus cattle infested with Rhipicephalus microplus and Rhipicephalus decoloratus
    (Stellenbosch : Stellenbosch University, 2017-03) Nel, Cornelius Loftus; Dzama, Kennedy; Marufu, Munyaradzi Christopher; Jonsson, Nicholas N.; Dube, B.; Stellenbosch University. Faculty of AgriSciences. Dept. of Animal Sciences.
    ENGLISH ABSTRACT: Ticks and tick borne diseases pose major threats to modern South African cattle production. Rhipicephalus microplus and R. decoloratus are important tick species currently spread throughout most of Southern Africa. Current control methods are not considered sustainable because of various economic, social and environmental concerns. Host resistance to ticks is a characteristic of cattle and is dependent on breed type. An understanding of these resistance mechanisms is necessary if host resistance is to be exploited as an alternative control method. Host reaction to infestation is specific to the tick species. It was hypothesized that tick resistance is a product of co-evolution between host and parasite and a breed will thus show superior resistance to tick species that it has a historical relationship with. The parasite-host pair with African origin, R. decoloratus and the Bos taurus africanus Nguni, was thus chosen along with the pair of Asiatic origin, R. microplus and the Bos indicus Brahman. A European breed, the Bos taurus Angus was also included due to their known susceptibility to Rhipicephalus ticks. Following the collection of control samples on all animals (n=36), one half (n=6) of breed group (n=12) was artificially infested with roughly 100 unfed larvae of R. microplus while the other half was similarly infested with R. decoloratus. Approximately 12 hours’ post infestation, multiple blood samples were drawn and skin biopsy samples were collected from visible parasitized sites of all animals. The remaining ticks were allowed to mature and tick counts were performed on day 18 post infestation. The blood samples were used for comprehensive haematology and serum biochemistry profiles while the skin biopsy sites were sectioned for cell counts and histopathological scoring of tissue using hematoxylin and eosin staining. There was no significant interaction between breed and tick species for counts, haematology, biochemistry or cutaneous cell counts and breed and tick species was used as fixed effects for assessment. Regarding day 18 tick counts, the Brahman breed displayed lower (p<0.01) tick counts compared to both the Nguni and Angus breeds. Rhipicephalus microplus displayed a higher success rate (p<0.05) compared to R. decoloratus across all breeds. At the 12-hour time point, the Brahman breed displayed a lower (p<0.05) level of mean cell volume (40.94 fl). The Nguni breed displayed a lower (p<0.05) level of platelets (311.59 x 109/dl). No haematological differences were observed for tick species. The Angus breed displayed a lower (p<0.05) absolute level of circulating neutrophils (3.65 x 109/l) and a higher (p<0.05) level of lymphocytes (9.69 x 109/l) compared to the Nguni, but not Brahman breed. The Nguni displayed a higher (p<0.05) absolute level of eosinophils (0.43 x 109/l) compared to the Brahman, but not Angus breed. Regarding serum biochemistry, the Brahman breed displayed higher (p<0.05) albumin levels (28.85 g/l) compared to both breeds and higher (p<0.05) alanine transferase (59.70 U/l) levels compared to the Angus breed. The Angus breed displayed higher (p<0.05) levels of blood urea nitrogen (5.21 mmol/l) compared to the Brahman breed. The Brahman breed displayed lower levels of fibrinogen (1.77 g/l) than the Nguni and Angus breeds. Animals infested with R. microplus displayed a higher (p<0.05) serum globulin level (43.37 g/l) than those infested with R. decoloratus. Overall, alanine transferase (-0.36, p<0.05), alkaline phosphatase (-0.36, p<0.05) and fibrinogen (0.39, p<0.05) showed weak, but significant correlations to day 18 tick counts. No differences within breed and tick species groups were observed within the number of cellular infiltrates or histopathology scores. Within all treatment groups, the level recorded for cutaneous infiltrates or histopathology scores post infestation was higher (p<0.05) than the control values. It was concluded that a specific evolutionary relationship is not necessarily the primary contributor to the manifestation of the resistant phenotype and a high level of cross resistance is possible. R. microplus has a superior parasitic aggression which will have an influence on its displacement of R. decoloratus. Immunological parameters are important when assessing tick-host relationships, but the influence on the host includes a wider range of factors. The 12-hour interval is promising for further investigations, but higher intensities of infestation are recommended to increase the reliability of assessments.
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    Studies on genetic responses and genomic characterisation in South African and Australian sheep
    (Stellenbosch : Stellenbosch University, 2022-04) Nel, Cornelius Loftus; Cloete, Schalk W. P.; Dzama, Kennedy; Swan, A. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Animal Sciences.
    ENGLISH ABSTRACT: The current scope of small stock breeding in South African (SA) Merinos is constrained. In contrast to Australia (AUS) and New Zealand, no traits indicative of animal resilience or fitness are currently being recorded. Furthermore, the use of genomic selection (GS) could be a valuable tool to widen the scope of breeding objectives, but has not been formally explored on local populations. SA hosts multiple Merino resource flocks that are well recorded for a series of difficult to measure fitness traits, which can be exploited to the benefit of evaluating similar traits in commercial Merinos. The Elsenburg Merino flock is a resource flock that has been divergently selected for reproductive success on number of lambs weaned (NLW) that separates the H-Line (positive selection) from the L-Line (negative selection). The first aim of this study was to elaborate genetic parameters and realized genetic trends obtained in the Elsenburg flock as a demonstration of responses to genetic selection on a lowly heritable, sex-limited trait recorded late in an animal’s lifetime. Apart from a focus on production and reproduction traits, the genetics and factors influencing lamb survival received special attention. A second aim of this study was to use marker data to better define the population genetic architecture of important SA and AUS sheep breeds, with a focus on the across country compatibility of SA and AUS ovine genetic resources. A third aim of this study was to assess the benefits of genomic information in genetic prediction of SA Merinos. Genetic trends in the Elsenburg lines showed that long term divergent selection for NLW did not have severely detrimental effects on the genetic change in other production traits, with a possible exception of clean fleece weight and staple strength. Despite the low heritability, favorable genetic trends (~1% of the overall mean) per year were reported for NLW in the H-Line. Genetic change was more moderate, but also worthwhile for component traits, namely conception rate and litter size. Survival to weaning, as a trait of the lamb, also showed a rate of gain close to 1% in the early years of the experiment, which contradicted the premise that lamb survival is not amenable to genetic selection. As measured by rectal temperature, H-Line lambs also performed better during cold stress conditions, and the fitness of H-Line lambs was strongly linked to increased cold stress adaption. Population genetic parameters such as linkage disequilibrium and effective population size reiterated the fact that the genetic diversity of sheep can be high both across and within breeds, especially for Merinos. According to across country imputation, there is opportunity to combine SA and AUS Merino databases. This was supported by parameters of divergence (FST), principal component analysis and relatedness, but a narrow spectrum approach of specific populations is most likely to deliver the best results. The outcomes of GS were promising when a single-step GBLUP was used to predict genetic merit in unvalidated candidates for a series of production and reproduction traits. However, results varied across flocks and traits, and more research is needed to optimize these results. The study provided a foundation for further research on these and related topics.