Fa. Munroe et al., Estimates of within-herd incidence rates of Mycobacterium bovis in Canadian cattle and cervids between 1985 and 1994, PREV VET M, 45(3-4), 2000, pp. 247-256
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.