Nutritional influences on the expression of genotypic resistance to gastrointestinal nematode infection in sheep

This paper reviews experiments investigating the responses of resistant and susceptible genotypes of sheep to gastrointestinal nematode infection under differential nutrition. Using faecal egg count as a measure of host resistance, differences between genotypes appeared to be greatest under conditions of low nutrient availability and under such conditions both resistant and susceptible genotypes generally responded to supplemental protein by reducing faecal egg count. However, when nutritional conditions were moderate to good, responses to additional protein tended to be observed only in susceptible genotypes. In general, host genetic resistance was associated with reliable reductions in faecal egg count of moderate to large magnitude, while nutritional intervention was less reliable at reducing faecal egg count, and induced reductions of smaller magnitude. The situation was very different when examining host resilience to infection, as determined by sheep productivity in the face of infection. Increased host resistance was rarely associated with improved growth or production during the period of infection. In contrast, nutritional supplementation reliably increased host productivity irrespective of infection status and prevailing nutritional conditions. A general model of the relationship between nutrient availability and host resistance and resilience in resistant and susceptible genotypes is postulated. Taken together, the studies reviewed in this paper suggest that selection for host resistance and strategic nutritional intervention have complementary roles in the integrated control of gastrointestinal nematode infection in sheep. The former will contribute primarily to the epidemiology of the disease by reliably reducing faecal egg output over a wide range of conditions, reducing host challenge and the number of treatment interventions required. The latter will reliably boost host resilience to infection, with lesser effects on resistance, and the economic rationale for use will require accounting for the full spectrum of production and disease response

Absolute quantification using real-time polymerase chain reaction of Marek's disease virus serotype 2 in field dust samples, feather tips and spleens

Methods for Taqman quantitative real-time PCR (qPCR) assays to detect the three serotypes of Marek's disease virus (MDV) are available, and absolute quantification has been developed for MDV serotype 1 and serotype 3. The development of a method for absolute quantification of Marek's disease virus serotype 2 (MDV2) is described in this paper. Using plasmid DNA, the lower detection limit of the MDV2 assay was determined to be 10 copies. Three independent assay runs showed highly reproducible Ct values and calculated copy numbers, with mean intra- and inter-assay coefficients of variation of less than 3% for Ct and less than 21.5% for calculated copy number. Absolute quantification of MDV2 was performed successfully on dust samples collected from poultry farms across Australia, material from infectious spleens and feather tips from chickens vaccinated with an attenuated strain of MDV2. Thus, it is now possible to use qPCR assays for absolute quantification of all three serotypes of MDV in a sample

Absolute quantitation of Marek's disease virus and Herpesvirus of turkeys in chicken lymphocyte, feather tip and dust samples using real-time PCR

Methods for Taqman quantitative real-time PCR (qPCR) assays to detect the three serotypes of Marek's disease virus (MDV) are available, and absolute quantification has been developed for MDV serotype 1 and serotype 3. The development of a method for absolute quantification of Marek's disease virus serotype 2 (MDV2) is described in this paper. Using plasmid DNA, the lower detection limit of the MDV2 assay was determined to be 10 copies. Three independent assay runs showed highly reproducible Ct values and calculated copy numbers, with mean intra- and inter-assay coefficients of variation of less than 3% for Ct and less than 21.5% for calculated copy number. Absolute quantification of MDV2 was performed successfully on dust samples collected from poultry farms across Australia, material from infectious spleens and feather tips from chickens vaccinated with an attenuated strain of MDV2. Thus, it is now possible to use qPCR assays for absolute quantification of all three serotypes of MDV in a sample

Evaluation of growth rates and resistance to nematodes of Deccani and Bannur lambs and their crosses with Garole

Sheep rearing is a traditional occupation of about 85 000 shepherd families on the Deccan plateau in theMaharashtra State of India. They rear Deccani (D) sheep which usually bear only single lambs. Prolificacy is animportant trait for the efficiency of meat producing sheep. It was decided to evaluate and utilize Indian sheepgenetic resources with a view to improving the efficiency of sheep production on the Deccan plateau. Acrossbreeding experiment was conducted over 4 years, using rams of the D, Bannur (B) and Garole (G) breeds andD and B ewes with the aim of developing recommendations for the appropriate breed combination of a likelycomposite. It was found that crossing with G reduces live weight and growth rates significantly compared with Dbut lambs sired by G rams were more resistant to naturally acquired gastro-intestinal nematode infections and toartificial challenge withHaemonchus contortusthan those sired by D or B rams. The G breed, being from ahumid environment is, however, not adapted to the semi-arid Deccan plateau. The higher productivity (in terms ofweight of lamb weaned) of twin-bearing ewes compared with those bearing singles was evident even in extensiverearing conditions. The finding of increased resistance to gastro-intestinal nematodes in the G breed, which alsocarries a major gene for prolificacy, highlights the potential for inclusion of G in a composite breed. Negative effectson growth and survival from inclusion of the Garole need to be carefully managed

Immunosuppressive effects of Marek's disease virus (MDV) and herpesvirus of turkeys (HVT) in broiler chickens and the protective effect of HVT vaccination against MDV challenge

Much of the impact of Marek's disease in broiler chickens is considered to be due to immunosuppression induced by Marek's disease virus (MDV). The present study evaluates the effects of an Australian isolate of pathogenic MDV (strain MPF 57) and a non-pathogenic vaccinal strain of herpesvirus of turkeys (HVT) (strain FC 126) on the immune system of commercial broiler chickens for 35 days following challenge at days 0 or 3 of age. It also investigates the extent of protection provided by HVT vaccine against MDV-induced immunosuppression. Immune system variables, including relative lymphoid organ weight, blood lymphocyte phenotype (CD45+/CD3+, putatively T, and CD45+/LC+, putatively B) and antibody production following vaccination against infectious bronchitis (IB) at hatch, were used to assess the immune status of chickens. Immunosuppression was also assessed by susceptibility to secondary challenge with pathogenic Escherichia coli on day 29 post-MDV challenge. MDV infection reduced the weight of the thymus and bursa of Fabricius, the numbers of circulating T lymphocytes and B lymphocytes, and IB antibody titre. The timing of these effects varied. MDV infection greatly increased susceptibility to E. coli infection. HVT alone caused mild depletion of T and B lymphocytes but no effect on immune organ weight or IB titre. Vaccination with HVT provided good protection against most of the immunosuppressive effects of MDV but not against MDV-induced growth impairment and reduced responsiveness to IB vaccination, suggesting that recent Australian strains of MDV may be evolving in virulence to overcome the protective effects of HVT