Recent Danish Studies on the Epidemiology of Bovine Parasitic Bronchitis
Epidemiological studies on verminous bronchitis were carried out by weekly observations and sampling of groups of calves and the plots grazed by them. Twenty-one calves were given a light initial infection of Dictyocaulus viviparus in May and turned out to graze on the same day in three groups of seven calves each on plots of different sizes. Moderate patent infections developed in all calves and gave rise to pathogenic pasture larval contaminations after 5 to 6 weeks. Although the magnitude and the horizontal distribution of the pasture contamination varied between the plots, severe outbreaks were observed in all three groups, resulting from prepatent superinfections. The discrepancy between the pasture results and the clinical observations was explained by favourable climatic conditions and large individual variations.
A second experiment was conducted by turning out eight susceptible calves on 1st August on an adjacent plot which had not been grazed and from where a hay crop had been taken. Four of these calves excreted lung-worm larvae four weeks later. This was explained by field-to-field transmission shortly after turning out. A resulting moderate pasture larval contamination was recorded during weeks 5 and 6. It gave rise to an outbreak during weeks 8 to 10. There was a tendency that the calves which did not excrete larvae during the first 6 or 7 weeks, became more affected than the calves which were responsible for the pasture larval contamination. The pasture contamination which was high in autumn, was followed into the next grazing season. The number of infective larvae per kilo of herbage dropped markedly before and after a period with frost and snow and reached an extremely low level in May, whereas the larval contamination of a pasture sample stored during the winter on a hay loft dropped markedly when the herbage dried out in mid-winter.
The infection in the first experiment which, culminated in mid-summer, became undetectable in the pasture samples taken in late autumn, and no larvae were detected in the faeces of eleven calves available during the stabling period. One specimen of D. viviparus was recovered from the lungs of one out of 5 calves slaughtered in May. The remaining six calves turned out into their second grazing season on the same paddock grazed by them the previous year, but no larvae were detected in their faeces or in the pasture. Similarly, no signs of infection were observed in the lungs of two tracer calves or in the faeces of 4 calves which were turned out simultaneously on the same paddock during the second year of observation.
KeywordsInfective Larva Grazing Season Favourable Climatic Condition Faecal Examination Herbage Sample
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