[Currently important swine diseases].
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Swine Vesicular Disease virus ingested by the adult fly Calliphora persists several days in the digestive tract of the insect and is eliminated in feces. The virus ingested by the insect at larval stage has been recovered from the digestive tract and feces of adult flies. Thus, the dissemination of the virus, even in a limited fashion, seems to be possible and attention is chiefly centered on the second process.
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The Swine Vesicular Disease virus yields a heterogenic plaque population consisting of large round plaques of 8-10 mm, small uniform plaques with slightly indented contours, measuring 1-3 mm, and single plaques of transient form and size. The reisolates of the large (Lpf) plaques give a population that is similar to the initial virus, while the cloning of the small (Mpf) plaques leads to a homogenic population of such plaques. In vivo, the virus of the large plaques manifests enhanced virulence for swine. On the other hand, the selected small-plaque variant is apathogenic, which makes it possible to produce a live avirulent vaccine.
The authors took a survey of foot-and-mouth disease samples of myocardium and tonsil from swine which was died without clinicals signs of foot-and-mouth disease, with isolation of virus, type O, A and C. It was observed and accentuated relation between the incidence of hog cholera, pneumonia and atipic foot-and-mouth disease, especially from suckling pigs.
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Pigs exposed to swine vesicular disease virus developed vesicular lesions by postinoculation day 2. Lesions first appeared on the coronary band and then on the dewclaw, tongue, snout, lips, and bulbs of the heels. The onset of viremia coincided with febrile response and the appearance of vesicles. Virus was isolated from the nasal discharge, esophageal-pharyngeal fluid, and feces as early as postinoculation day 1. Greater amounts of virus were isolated from samples collected during the first week of infection, and lesser amounts from samples collected during the second week. The appearance and the distribution of specific fluorescence in various tissues indicated that during the development of swine vesicular disease virus infection, the epithelial tissues were initially involved, followed by a generalized infection of lymph tissues, and subsequently, a primary viremia. Seroconversion was detectable as early as postinoculation day 4. A mild nonsuppurative meningoencephalomyelitis throughout the CNS was observed in both inoculated and contact-exposed pigs. The olfactory bulbs were most severely and were frequently affected, particularly in contact pigs. The most severe brain lesions were found in pigs 3 to 4 days after the onset of viremia; contact pigs showed more severe brain lesions than inoculated pigs. Microscopic changes were also found in the coronary band, snout, tongue, and heart.
Pigs inoculated intravenously with swine vesicular disease virus (UKG strain), those inoculated with coxsackievirus B5, and other pigs exposed by pen contact to the same viruses developed diffuse encephalomyelitis. Perivascular cuffing, with lymphocytes and formation of neuroglia cell foci, were most prominent in telencephalon, diencephalon, and mesencephalon. Encephalitis was of mild to severe intensity. Severity of lesions was more extensive and severe in the pigs exposed to swine vesicular disease virus. Pen contact exposure to either of the 2 viruses caused a more severe central nervous system reaction than did intravenous inoculation. The type and the distribution of lesions produced by the 2 viruses indicate that they may be related.
Two day old piglets were inoculated intravenously with 1 ml of swine vesicular disease virus UK-G 27-72 isolate. Using infectivity tests, immunofluorescent staining and gross and histopathological examination, pathogenesis of the infection was studied in tissue specimens collected daily from one through seven days postinoculation. Swine vesicular disease virus had a strong affinity for the epithelia of the tongue, snout, coronary band and lips, the myocardium and the lymphoid elements of the tonsil and the brain stem. The virus had the greatest affinity for the epithelium of the tongue. However, there was no evidence that the tongue was the initial replication site for swine vesicular disease virus. Prickle cells in the stratum spinosum appear to be the primary targets for the virus. The necrotic foci in the stratum spinosum appeared first, followed the next day by reticular degeneration and multilocular intraepidermal vesicular formation. In the digestive tract and most of the other visceral organs the short duration and sudden drop of the virus titres and the negative fluorescence and pathological findings suggest that these are not important sites for the replication of swine vesicular disease virus in this experiment. The virus was recovered from most of the central nervous tissue specimens. Although the piglets had significant central nervous system lesions, signs of impaired central nervous system function were not detected. However, subtle nervous signs could have been obscured by difficulties in locomotion resulting from severe lesions of the feet.
Pig sera were assayed for antibodies to swine vesicular disease virus by (a) the radial immuno-diffusion technique combined with autoradiography and (by serum neutralisation tests. The former was more sensitive and was used for initial screening of sera while the latter was used to obtain estimates of titres of positive sera. In a survey of 1759 sera collected at slaughterhouses there were 14 significant titres from a total of seven premises situated in localities where the disease had been known to occur, and it was concluded that this did not indicate wither widespread undetected disease or the occurence of inapparent infection in the pig population.
Purified suspensions of Coxsackie B5 virus and swine vesicular disease virus (SVDV) were prepared by harvesting and purifying cell pack virus. Crossed immunoelectrophoresis was carried out with purified N and H antigen fractions (full and empty particles). Relative migration velocity (RMV) was calculated for the N antigen fraction of 3 SVD viruses (UKG72, HK71 and Italy 66) and 2 Coxsackie B5 viruses (Faulkner and 8068). The prototype strain of Coxsackie B5 virus (Faulkner) had a low RMV almost identical to that of the first isolated strain of SVDV, Italy 66. A recent isolate of Coxsackie B5 virus (8068, isolated in UK in 1973) had a relatively high RMV very close to that of the UKG72 strain of SVDV isolated in the UK in 1972. Also the Hong Kong strain of SVDV (HK71) had a high RMV value. These observations are considered in relation to the emergence of swine vesicular disease.
Swine Vesicular Disease virus (SVDV) did not survive drying at high relative humidities (r.h.) but there was little virus loss at low r.h. Purified virus dried in films was inactivated by formaldehyde fumigation only at high r.h. Inactivation was also influenced by the suspending medium from which the virus was dried. Purified virus resuspended in distilled water and then dried, was rapidly killed, but that in tissue culture fluid survived.