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Epidemiological aspects of infectious bovine keratoconjunctivitis in New Zealand : this thesis is presented in partial fulfilment of the requirements of the degree of Master of Veterinary Science at Massey University
Infectious bovine keratoconjunctivitis (IBK) has been recognised in New Zealand only in the past 7 years and there is little information available concerning epidemiological features of the disease under local conditions. There was an urgent need for rational control measures to be formulated and for a preliminary investigation which might indicate particular aspects of the disease deserving more detailed study. The first of two projects was a postal survey involving 400 beef farmers in the Hunterville and Gisborne districts. A questionnaire was sent out to determine management practices which might influence the occurrence of IBK and to obtain an impression of the impact of the disease on the farming industry. The survey results (72% response rate) showed that IBK is widespread in both areas, appears to have been noticed only within recent years and the annual incidence is still rising (1% in 1975 to 28% in 1980 in the Gisborne area). Outbreaks follow a seasonal pattern with most occurring in late summer. All age groups of cattle may be affected, but morbidity rates differ (young stock 18%; adults 10%). A greater susceptibility of the Hereford.breed to the disease was observed only in the Gisborne district. Difficulties with stock management and lower sale prices were two major consequences of IBK reported and concern was expressed over control measures. Only 11% of farmers routinely treated cattle and yet they reported that treatment improved the recovery rate. The other project involved a study of haemolytic strains of Moraxella bovis isolated from cattle eyes and the relationship of infection rates to clinical IBK cases. Three local beef farming properties were chosen because of their previous history of IBK and over a 1 year period identified animals were observed for clinical signs and conjunctival sac samples were collected for culture. Moraxella bovis was identified by its β-haemolysis, colony and organism morphology, and Gram negative staining characteristics, and confirmed biochemically using five tests including alkaline peptonization of litmus milk. Only one farm experienced an outbreak of IBK. The latter property operated a feedlot system, and early results implicated "carrier animals" in the initiation of a new outbreak. The prevalence of M.bovis isolations increased approximately one month before any clinical signs were apparent and infections remained at a higher level throughout the outbreak. A large number of animals which became infecte never developed signs of the disease. (Peak figures in one pen 87% infected and 55% IBK). Seventy-eight percent of those cattle were infected with M.bovis in both eyes at some time, whereas only 23% of the cases of IBK were bilateral. Moraxella bovis infections decreased slowly after the peak, but many cattle remained infected for at least 5 months. The widespread nature of M.bovis infections indicated the desirability for treatment of both eyes of all animals in a group if control of an IBK outbreak is to be attempted, but such treatment is sometimes impracticable. More detailed investigation is needed into the local factors that are responsible for converting a latent infection of M.bovis into IBK.