Estimates of within-herd incidence rates of Mycobacterium bovis in Canadian cattle and cervids between 1985 and 1994

We analysed the individual-animal data from six of the nine outbreaks of tu berculosis in Canadian cattle and cervids from 1985 to 1994. A "positive/re actor" animal was one which had either a positive culture or a positive or suspicious reaction on a mid-cervical, comparative cervical, or gross or hi stopathological test for tuberculosis. Individual-animal data were collecte d only for herds which had one or more positive/reactor animals. Data were collected from the outbreak records in the Regional or District offices of Agriculture and Agri-food Canada's Animal and Plant Health Directorate. The within-herd spread of Mycobacterium bovis was studied by determining the m ost-likely date at which die herd was first exposed to M. bovis and the num ber of reactions which had developed by the time the herd was investigated. The animal-time units at risk in the herd were probably overestimated, res ulting in conservative estimates of the within-herd incidence rates. Negati ve-binomial regression was used to investigate factors which might have inf luenced the within-herd spread of tuberculosis, Increasing age appeared to be a risk factor for bring a positive/ reactor animal. When compared to ani mals 0-12 months old, animals 13-24 months old had an incidence I ate ratio (IRR) of 7.6, while animals >24 months old had an IRR of 10.4 (p=0.009). A ctual and predicted incidence rates for tuberculosis in mature( >24 months old) animals were calculated, Actual and predicted incidence rates were sim ilar for cervids, within an outbreak. There was more variability between ac tual and predicted rates in the dairy and beef animals. In the one outbreak (Ontario) where there were positive/reactor cervid, dairy and beef herds, the actual incidence rate for cervids (IR-9.3 cases per 100 animal-years) w as almost twice that of dairy cattle (IR=5.0) and three times that of beef cattle (IR=3.1). (C) 2000 Elsevier Science B.V. All rights reserved.