Aspects of Immunisation of Calves Against Dictyocaulus viviparus

The studies reported in this thesis were undertaken to explore ways in which an improved vaccine could be developed against the disease caused by the cattle lungworm Dictyocaulus viviparus. The current vaccine is a live, non-sterile, oral preparation of x-irradiated larvae, although more recently a gamma-irradiated oral vaccine has been marketed in the Netherlands and the U.K. Each is administered orally in two doses 28 days apart. It was thought that it would be advantageous from the viewpoint of future licensing requirements, if live vaccine could be rendered microbiologically sterile and that certain commercial benefits could be obtained from packaging the vaccine in plastic rather than glass bottles. The first experimental chapter, Chapter Three, deals with the problems of producing a sterile suspension of larvae without harming their viability or infectivity. A suitable means of achieving this was found to be a one hour treatment with a 0.0024% w/v solution of the monosodium salt of 5,5'-Dichloro-2,2'-dihydroxydiphenylmethane (dichlorophen). Chapter Four is concerned with the packaging of vaccine in plastic bottles. The main problem in this respect was that in earlier studies a large proportion of the larvae were found to adhere to the surface of the bottle and in consequence were not delivered to the calf at dosing. However, various newer plastics were examined in the present work and polyethylene tetraphthalate (P. E. T. ) was found to give results comparable to glass. As a stepping-stone towards the use of a killed antigen extract as a vaccine, it was thought possible that, in a two dose vaccine, the initial dose could remain as the current oral preparation and be followed up with an injection of killed larval extract 28 days later. This regime was investigated in Chapter Five and, in the method attempted, gave poor protection against subsequent challenge. Now that it was possible to render the irradiated larvae sterile, it was considered feasible to investigate the immunogenicity of parenterally administered irradiated larvae. The experiments on this aspect are presented in Chapter Six. It was found that subcutaneous administration of two doses of irradiated larvae 4 weeks apart gave 95% protection against experimental challenge. In the concluding experimental chapter, Chapter Seven, the use of sterile, injectable vaccine was further studied and the process of sterilising the larval suspension simplified and refined. In addition, the required evidence of safety, efficacy and sterility was acquired in preparation for an application to be made for a Ministry of Agriculture certificate to permit field testing of this new formulation of bovine lungworm vaccine