The Effects of Brucella Strain 19 Vaccination Titers in the Tube Agglutination Test: A Case Control Study

on

S. W. Martin and M. R. Hudson* ABSTRACT

reaction serologique des bovins vaccines s'avemoins elevee que celle des bovins non vaccines. Par consequent, les sujets vaccines etaient moins susceptibles d'etre classifies comme positifs que les non vaccines, independamment de la methode d'interpretation des resultats de l'epreuve de l'agglutination lente. Quant on tenait compte de la periode de tolerance consecutive a la vaccination, les sujets vaccines etaient moins susceptibles d'etre classifies comme douteux que les non vaccines. Le nombre de sujets vaccines au sein d'un troupeau ne sembla pas influencer beaucoup les effets de la vaccination ou la prldominance de reacteurs. II sembla par consequent que la vaccination avec la souche 19 conferait une protection minime (en se basant sur la reaction serologique plus faible des sujets vaccines) mais que l'importance de cette protection dependait beaucoup de l'applicabilite de la periode de tolerance cons&cutive a la vaccination, pour classifier les sujets. II faudra etudier plus d fond cet aspect de la vaccination avec la souche 19. ra

The degree of agglutination in the tube agglutination test of the sera of 373 "reactor" cattle was compared to that of 800 "nonreactor" cattle from multiple reactor herds in southwestern Ontario. Vaccinated cattle had lower titers than did nonvaccinated cattle. Thus, vaccinated cattle were less likely to be classified as positive than nonvaccinated cattle irrespective of the method of interpreting the results of the tube test. When the full allowance for vaccination was used, vaccinated cattle were less likely to be classified as suspicious than nonvaccinated cattle. The extent of herd vaccination did not appear to markedly influence the effects of vaccination or the prevalence of reactors. Thus, it appeared that vaccination with strain 19 produced a small protective effect (inferred from the lower titers in vaccinated animals), but that the magnitude of this effect was markedly dependent on the appropriateness of the vaccine allowance for classifying cattle. Further work is needed on this aspect of vaccination with strain 19.

R2SUMA Les auteurs ont utilise l'epreuve de l'agglutination lente pour comparer le degre de reaction serologique entre 373 bovins "reacteurs" et 800 "non reacteurs". Ces animaux appartenaient a des troupeaux du sud-ouest de l'Ontario qui comptaient plusieurs reacteurs. La *Department of Veterinary Microbiology and Immunology, Ontario Veterinary College, University of Guelph, Guelph, Ontario NlG 2W1 (Martin) and Health of Animals Branch, Agriculture Canada, 31 Federal Building, 1323 Fifth Avenue, Prince George, British Columbia V2L 3L6 (Hudson). Submitted January 19, 1977.

Volume 42 - January, 1978

INTRODUCTION The use of strain 19 vaccination for calves was an integral part of the brucellosis eradication programs in Canada and the United States of America for a number of years (1). The general consensus has been that such vaccination was necessary to reduce infection to a level at which eradication by test and slaughter programs was feasible. Abortions, one of the primary methods of spread of Brucella abortus, are much less frequent following vaccination and the incidence and prevalence of infec-

35

tion in vaccinated cattle are much lower parison animals (controls) for the reactor than in nonvaccinated cattle (10, 16, 20, animals (cases). Data were obtained from 22, 25, 32, 34, 36). All of these factors the first herd test at which the above criappear to have aided the initial success of teria for number and type of reactors were satisfied. the vaccination programs. Laboratory personnel designate cattle as As the prevalence of infection decreases, the value of vaccination also apparently de- either negative, suspicious or positive after creases and the liabilities from vaccination considering the reaction to the tube test, become more important. These liabilities the reaction to the plate agglutination test, include the tendency of the vaccine to mask the vaccination status of the animal and infection by preventing abortions, and the the history of the herd with respect to bruoccurrence of false reactors because of the cellosis. For purposes of comparison, the shedding of strain 19 in the milk and/or as cattle were reclassified, by the authors, a result of persistent vaccine titers (20, 24, using only the individual recorded tube test 29). Thus, Canadian officials deemphasized reactions, taking into account the vaccine calfhood vaccination in the late 1960's. Sub- status of the animal. Another reclassificasequently, there has been a drastic reduc- tion was performed, by the authors, using tion in the number of calves vaccinated with only the recorded tube test reactions, ignoring the vaccine status of the animal. strain 19 (17). The primary statistical method for evaluRecent increases in the prevalence of brucellosis have stimulated renewed interest ating the significance and the strength of in the role of vaccination in the eradication any association between vaccination and program. Thus, this study was designed to reactor status, was the odds ratio (13). assess the effect of strain 19 vaccination on An odds ratio of one implies no association, tube agglutination test (tube test) titers i.e. no effect of vaccination on titers. An under the conditions of presumed field ex- odds ratio of less than one implies a protecposure that exist in southwestern Ontario. tive effect of the vaccine, i.e. vaccinated cattle have lower titers than nonvaccinated cattle. The significance of departures of the odds ratio from one was measured by the Chi-square statistic which compares the natural logarithm of the odds ratio to its standard error. The odds ratio was chosen MATERIALS AND METHODS because, in addition to the evaluation of the effects of sampling error (the Chisquare test of significance), the magnitude of the odds ratio indicates the strength of the association between vaccination and The data were obtained from records of reactor (13). Where possible, the the Animal Pathology Division Laboratory results ofstatus were analyzed by other of the Health of Animals Branch of Agri- the same method studies facilitate to comparisons culture Canada in Guelph, Ontario. Six counties were randomly selected from with the results of this study. a list of counties, in southwestern Ontario, having a high prevalence of brucellosis during 1975-76. All farms within these six counties, with two or more reactors, at least one of which being positive, were identified RESULTS and the data obtained for the variables shown in Table I. (A reactor was defined as an animal whose tube test results were interpreted as suspicious or positive and The data collected described 400 reactor whose titer in the tube test was 550). Two animals (one with unknown vaccination negative cattle, those listed closest to the status) and 800 control cattle in 78 herds. "reactor" animal on Form 61-D', were also There were 26 cattle among the 399 reactor selected. The negative cattle served as com- cattle of known vaccination status which had 2+ reactions at the 1/50 dilution and no reactions at higher dilutions in the tube IA listing of cattle in the order sampled on the farm. test. Two (7.6%) of these 26 were vaccinHealth of Animals Branch, Agriculture Canada.

36

Can. J. comp. Med.

TABLE I. Variables Used for Study of Effects of Vaccination in Cattle in Ontario. (Data collected from Health of Animals Branch, Agriculture Canada)

Variable Type of herd Number of animals tested in the herd Number of animals vaccinated in the herd Percentage of animals vaccinated Vaccination status of animal Tube agglutination test (tube test) reaction (extent of agglutination at 1/50, 1/100 and 1/1200 dilutions) Interpretation

Beef =

=

Code/Comment 0; Dairy = 1; Mixed 2; Not recorded =

9

Actual number Actual number Actual percentage Nonvacc = 0; Vacc = 1; Not recorded = 9 - = 0; + = 1; ++ = 2; +++ = 3; ++++ = 4 Negative = 0; Suspicious = 1; Positive = 2

TABLE II. The Titer of Vaccinated and Unvaccinated Cattle and the Proportion of Vaccinated Cattle at each Titer. (Data from Herds with Multiple Reactors in Southwestern Ontario during 1975)

Tube Agglutination Titer 100 200

The effects of Brucella strain 19 vaccination on titers in the tube agglutination test: a case control study.

The Effects of Brucella Strain 19 Vaccination Titers in the Tube Agglutination Test: A Case Control Study on S. W. Martin and M. R. Hudson* ABSTRACT...
1MB Sizes 0 Downloads 0 Views