Studies on newcastle disease and some associated local virus strains in Kenya.
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Newcastle disease is an infectious and highly contagious disease affecting mainly chickens and turkeys. Various other birds as well as in may be affected by the virus. In recent years, the disease has caused severe economic losses in Kenya. Outbreaks of the disease have occurred even in areas where, in the past, vaccination had been practiced regularly. Though much work has been done in other countries, very little work has been carried out in Kenya regarding the biological and biophysical nature of the virus .and also the disease. It was therefore necessary to study the occurrence of the disease in Kenya and the behaviour of some associated locally isolated virus strains. Data on the outbreaks of the disease and on positive diagnoses was collected from the annual reports of the Kenya Veterinary Department with the kind permission of Dr. Chema, Deputy Director of Livestock Development (Research), and used to study the epidemiology and ecology 0 the disease. Thirty two virus isolates were recovered from clinical materials submitted for laboratory diagnosis from different parts of Kenya. Their biophysical and biological characteristics were determined, namely:- (xiii) 1. 'l'he ability of the virus to haemagglutinate fowl, equine, bovine, ovine, caprine and canine erythrocytes. 2. Inactivation of the haemar;agglutination activity and infectivity by heat, ultraviolet light and disinfectants. 3. Serological relationships by cross haemaggIu'ti.na.Lton-d.nhLb.i, tion tests. 4~ Residual and eluted virus using 2 strains at room temperature. 5. Times to elution at 400. 6. Heplication and cytopathogenicity of the virus in various cell cultures. '7I • Viability of the virus in various ma t er-i.a'l s at room temperature (water, feed , litter J soiI and feo.thers )• . The disease was found to be widely distributed throughout the country. Analysis of positive diagnoses of the disease for the period 1967 - 1980 showed a trend with three peaks in a year coinciding with the cold and we t periods in the year. The highest peak occurred in the June July period. The trend of positive diagnoses and number of outbreaks declined progressively with the use of the killed vaccine that was first used in 1958 -- 1959. By 1965, the Leve I (xiv) of the disease had reached a low value after vhich there was a dramatic rise. The level of the disease declinBd again in 1972 - 1973 with the introduction of the F strain vaccine. All the isolates haemagglutinated fowl, bovine, ovine and canine erythrocytes. Most of them haemagglutinated caprine erythrocyte"'. Five isolates haemaggutilna t.8:u1 equl.ne ery t_hrocy tes au-!- both 4°C 37°C, 10 haemagglutinated the same cells at 37°C and and not at 4°C while 17 isolates failed to haemagglutinate them at all. Twenty isolates had heat stable haemagglutinins anc1 12 had heat labile haemagglutinins. Some strains were heat labile for haemaggl1.l.tination activity and infectivit.y wh i.Le others »iex:e stable in both parameters. Ultraviolet light treatDent increased the titre fourfold within 4 hours of exposure for some isolate s wh i Le for others the titre remained unchanged. Thirteen isolates were classified as fast elutors and 19 as slow eLut ors, Graphic(~l presentation of residual and eluted virus against 'ti.me showed characteristic hyperbolic curves. Cross haemagglutination-inhibition tests among 6 isolates and 2 vaccine strains and their antisera sh9wed high titTes for each virus and its homologous antiserum but highly var-Lab Le reactions with heterologous antisera. All isolates tested produced cytopathic effects in cells of chicken embryo, sheep lung, sheep kidney and calf kidney cells characterised by rounding up of cells and complete degeneration within 96 hours. Eosinophilic cytoplasmic inclusion bodies were also observed. Syncytia formation was observed only in cells of the chicken embryo. The virus multiplied to higher titres in chicken embryo lung cells, chicken embryo intestinal cells and chicken embryo heart cells than in chicken embryo fibroblastso The virus was destroyed in 10 minutes by 1% and 0.1% lysol, 1% sodium hypochlorite, 70% ethyl alcohol and 1:400, 1:600, 1:2000 and 1:6000 dilutions of biocid-30. The virus could not.be destroyed completely by 4% formalin in 10 minutes. The virus survived for 7 days in water and feathers, 10 days in soil and 14 days in mash and litter at room temperature. The virus survived for 21 days in water containing 2% (w/v) bovine serum albumin (BSA), 28 days in water containing 10% BSA· and 49 days in water containing 20% BSA. These findings indicate clearly that there are marked variations among the Newcastle disease virus isolates present in Kenya. This may explain some of the field observations related to the Newcastle disease.