The effect of the indurative lymphocytic mastitis caused by infection with maedi-visna virus was quantified by comparing the pre-weaning growth of lambs from infected and uninfected ewes under the same conditions. A total of 73 infected, but clinically healthy, ewes and 75 ewes from a maedi-visna virus-free source were purchased to form a new flock; they were all three years old. The ewes were mated and the flock was managed as a normal field flock. Serum samples were taken at regular intervals and tested for antibodies to maedi-visna virus. The lambs were weighed at birth and at 14, 30, 50 and, finally, 80 days old, when they were weaned. The ewes were slaughtered, their udders were examined histologically, and the lesions were assessed by counting the typical lymphocytic follicles. Sixty-six per cent of the ewes that were seropositive at slaughter appeared to have follicles. A statistically significant association was found between the number of follicles in the udder and the reduction in the growth rate of the lambs. Lambs from ewes with the mean number of follicles weighed 1.7 kg less at weaning.
Two independent nationwide surveys of randomly selected sheep farmers were conducted: in one a questionnaire was mailed to farmers and in the other the farmers were interviewed. The questionnaire was designed to obtain data on the occurrence of scrapie in the national sheep flock of the Netherlands. Farmers owning 30 or more ewes were selected from a database at the Ministry of Agriculture, Nature Management and Fisheries, resulting in 10,000 addresses. Two lists of names were compiled at random. In the postal survey 700 questionnaires were distributed, and 137 farmers were interviewed directly. The response rate in the postal survey was 37 per cent and in the interview survey 100 per cent. The results from the two surveys did not differ significantly. Six per cent of the farmers indicated that they had at some time observed signs of scrapie in their flock. For the total sheep population in the Netherlands, the prevalence of flocks with sheep showing signs of scrapie was between 3.8 per cent and 8.4 per cent (with 95 per cent certainty). In 1989 and 1990 the incidence rate within infected flocks was 1.27 cases/100 ewes/year, and the incidence rate for the total sample population was 0.1 case/100 ewes/year.
Summary The results of the first four years of the Dutch national voluntary maedi-visna control program, which was launched on January 1, 1982, are presented. At the end of the observation period, 1711 breeding flocks representing 70% of the registered breeding flocks participated. The program is based on accreditation of flocks that have passed two successive serological tests with an interval of six months between and post-accreditation tests every 12 months. The flocks have to conform to a set of specific regulations. Sheep sold from such flocks receive a certificate stating that their origin was accredited. A total of 1212 flocks gained accredition. Of these flocks, 29.5% were free from infection from the beginning, 35.6% were created by total replacement of the original stock, 19.6% employed repeated testing and culling of positives, and 15.3% were created by artificial rearing of colostrum-deprived lambs. A total of 22 flocks lost accreditation due to detection of 36 seropositives. These positives and their progeny were culled, and all flocks regained accreditation after passing the two negative flock tests required. In a total of seven flocks, an unusual course of events during the pre-accreditation stage was observed; this was mainly attributed to late seroconversions and uncommon degrees of horizontal transmission. The results indicate that this certification yields a substantial guarantee of freedom from maedi-visna virus and they indirectly show that the basic design of the program has been adequate.
Sheep in a flock in which 88 per cent of the ewes had antibodies to maedi-visna virus were clinically examined for udder induration during lactation and after drying off. On both occasions about half of the ewes had indurated udders. Histological examination revealed lymphocytic mastitis associated with maedi-visna virus infection, in the udders of six of 25 hoggs (24 per cent), 21 of 39 shearlings (53.8 per cent) and 42 of 67 ewes (62.7 per cent). Distinct lung lesions were found in 8 per cent of the hoggs, 12.5 per cent of the shearlings and 10 per cent of the ewes. The results of a clinical examination of dry udders were correlated with the histological findings.
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