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The objective in this study was to identify risk factors for flock …

Home » Biology Articles » Agriculture » Animal Production » A farm-level study of risk factors associated with the colonization of broiler flocks with Campylobacter spp. in Iceland, 2001 – 2004 » Results

- A farm-level study of risk factors associated with the colonization of broiler flocks with Campylobacter spp. in Iceland, 2001 – 2004

Descriptive summary

Data were available for 792 flocks on 33 farms, and of these, 217 (27.4%) tested positive for Campylobacter. The median number of flocks per farm was 14 (mean 24, range 1 to 146), and the median number of positive flocks per farm was 3 (mean 7, range 0 to 55). The proportion of positive flocks per farm ranged from 0 to 75%, with a median and mean of 25%. Three farms did not have any positive flocks; these were primarily smaller farms that each raised a total of 1 to 9 flocks during the four summer seasons of the study. Other domestic livestock on farms included cattle only (1 farm), pigs only (1 farm), sheep only (2 farms), and sheep plus cattle and/or horses (5 farms).

Of the 792 flocks raised during the four summer seasons, the total production was 5,828,772 broilers. This figure was slightly less (5,659,534 broilers) for the 28 farms (758 flocks) included in the multivariable analyses. The median age at slaughter of flocks raised during the summer seasons was 37 days (mean 37, range 31 to 100). The age distribution for flocks included in the multivariable analyses was similar although the maximum age was 63 days. The number of houses per farm ranged from 1 to 15 (median 2, mean 2.5). Individual flocks ranged in size from 604 to 21,772 broilers (median 6,275, mean 7,366). A large proportion of flocks (72%) were slaughtered in one catch lot. For flocks with more than one catch lot, the mean catch lot size was 5,065 broilers (range 330 to 14,867). Each catch lot was sampled at slaughter. Of the 217 positive flocks, 14 flocks were slaughtered in three catch lots with four samples per catch lot for a total of 12 samples per flock, 46 flocks were slaughtered in two catch lots with four samples per catch lot for a total of eight samples per flock, and the remaining 157 flocks were slaughtered in one catch lot with four samples per flock. On the basis of catch lot sampling, out of 291 catch lots, 266 were positive in all samples (91.4%), 2 were positive in three samples (0.7%), 6 were positive in two samples (2.1%), and 17 were positive in one sample (5.8%). On a flock basis, 14 of the 217 positive flocks were positive in only one pooled sample, likely indicating early stages of flock colonization.

The characteristics of farms excluded from the analyses due to missing data for one or more variables are shown in Table 3. There were no obvious patterns among the excluded farms other than none of the farms raised other livestock and all had either one or two houses. The proportion of positive flocks on these farms ranged from 25% to 75%.

Table 3. Characteristics of farms excluded from the farm-level analyses due to missing data

Multivariable analysis

The variables "manure spread on fields in summer season" and "manure spread on fields in winter season" were strongly and positively correlated with each other (τb = 0.86). Of the 28 farms included in the multivariable analysis, there were 9 farms that spread manure in both summer and winter, 17 farms that did not spread manure in either season, and 2 farms that spread manure in the summer but not in the winter.

Coefficients and p-values for the variables in each model are presented in Table 4. For categorical variables, exponentiation of the coefficient represents the increase (positive coefficient) or decrease (negative coefficient) in the risk of Campylobacter when the factor was present on the farm compared to when it was not present on the farm. For example, using the coefficient of 0.92 from the manual backward selection model using "manure spread in summer", the risk of a flock being colonized with Campylobacter was 2.5 times higher (e.g. e0.92 = 2.5) on farms that spread manure on fields in the summer season compared to farms that did not spread manure in the summer. Exponentiation of the coefficient for the continuous variables represents the increase in the risk of Campylobacter as the median flock size increased by 1,000 birds, and the increase in risk for each additional house on the farm (see discussion). The p-values in Table 4 represent the probability that the increase or decrease in risk was due to chance alone. For example, the p-value of 0.025 for manure spreading in the summer indicates that there was a 2.5% probability that the observed increased risk of Campylobacter colonization was due to chance.

Table 4. Six logistic models for farm-level factors associated with Campylobacter in broilers in Iceland (n = 28a)

For each variable in Table 4, a range of coefficients and p-values are presented. The values differ depending on the model selection method. The presence of data in the table is an indication that the variable was associated with Campylobacter colonization in the respective model, whereas the absence of data indicates that the variable was not associated with flock colonization (i.e. the variable was either removed (backward-type models) or it was not eligible for addition (forward-type methods)). The variables are listed in descending order, such that factors identified as being associated with Campylobacter in all models are the top of the table. For example, increasing median flock size was identified as a strong risk factor in all six models, whereas an all-in-all-out policy was not a significant predictor in any of the models. Factors that were significantly associated with colonization regardless of modelling approach could be considered to have a greater relative importance in the epidemiology of Campylobacter on broiler farms in Iceland.

In general, the factors associated with an increased risk of Campylobacter were increasing median flock size, spreading manure on the farm in the winter, and increasing the number of broiler houses on the farm. Protective factors included the use of official or official treated water on the farm compared to the use of non-official untreated water, storing manure on the farm at any time of year, and the presence of other domestic livestock on the farm.

In the automated forward selection and forward stepwise models, one farm had a large residual (standardized Pearson's residual = 3.2) relative to the residuals of the other farms. The characteristics of this farm were: non-official water, one house, an all-in-all-out system, manure was spread and stored at all times of the year, absence of other livestock and poultry, and a mean flock size of 4,579 birds. Although this farm had a much higher proportion (7/15) of positive flocks than predicted (2.5/15), it did not have undue influence on the models.

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