The use of negative binomial modelling in a longitudinal study of gastrointestinal parasite burdens in Canadian dairy cows

Abstract

The epidemiology of bovine gastrointestinal nematodes was investigated through a 1-year (October 1999 to September 2000) longitudinal study in 38 Canadian dairy herds from 4 different provinces (Prince Edward Island, Quebec, Ontario, Saskatchewan). For each herd, fecal egg counts from 8 randomly selected animals were performed on a monthly or quarterly basis. Larval cultures were performed once, to determine the species breakdown of the parasites. All producers were interviewed regarding herd management practices. The observed fecal egg counts were low in this study, with a range from 0 to 419 nematode eggs per 5 g of feces. The mean count was 9.8 and the median was 1. Standard transformations failed to normalize the data, which followed an over-dispersed Poisson distribution. A zero inflated negative binomial model was applied to assess factors that would influence the fecal egg counts. Identified associations were: egg counts were lowest in the winter and highest in the late spring; first-lactation cattle had higher counts than older cows; if manure was spread mechanically on pastures used by lactating cattle the egg counts were higher; and if manure was spread on heifer-pastures, the adult cows had lower counts. In herds where pasture use was more extensive, the cattle had higher fecal egg counts. The difference in pasture exposure was found to be a main contributor to an observed difference in fecal egg counts among herds in the 4 provinces.

Figure 1. Distribution of fecal egg counts (FEC) as ep5g, square root (ep5g) and ln(ep5g + 1). Data from 1946 samples collected form 38 Canadian dairy herds between October 1999 and September 2000.

Figure 2. Variation in fecal egg counts (FEC) from cows in 4 different regions of Canada, from October 1999 to September 2000. Data from 1946 samples collected from 38 herds.

Figure 3. The variation in fecal egg counts (FEC) for cows from primiparous and multiparous cows in Canada, from October 1999 to September 2000. There were 1946 samples from 38 dairy herds.

Figure 4. The proportion O. ostertagi of the total number of larva, from cultures taken in the fall of 1999. Presented by age group and province.