Relationship between gastrointestinal (GI) nematode infection and immune response in dairy cows

Intensive pasture grazing systems commonly seen in New Zealand dairy farms provide favourable conditions for the development of gastrointestinal (GI) parasitism. The objectives of this study were to seek associations between levels of serum or milk antibody to gastrointestinal nematode parasites and faecal egg counts (FEC), milk yield, infective L3 larvae ingested from pasture and worm burden. This study was performed on the Lincoln University Research dairy Farm (LURDF) from October, 2011 to June, 2012. In Experiment A, a group of lactating dairy cows grazing pasture at a high stocking rate (HSE, n=31) and fed supplements were compared with a low stocking rate group (LSE, n=30) without supplements, both of which grazed a combination of new and established (old) pastures. Monthly measurements were performed for faecal egg counts (FEC), pasture L3 larvae level, daily milk yield and anti-Ostertagia antibody level (OD) in pooled milk or serum. In Experiment B, 60 lactating dairy cows were treated with anthelmintic and randomly allocated to one of three groups i.e. GI, GII & GIII. In GI (n=20), cows were not artificially infected (controls). In GII (n=20), cows were artificially infected twice with infective larvae (oral dose of 50×103 L3/cow) which was twice truncated 10 days after infection with an anthelmintic treatment. In GIII (n=20), cows were artificially infected once with L3 (oral dose of 50×103 L3/cow) without truncation. Individual faecal egg counts and anti-Ostertagia antibody levels (OD) in pooled serum were measured. At the end of the trials, 17 cull cows (6-Exp. A; 11-Exp. B) were slaughtered and worm burdens were quantified. A significant positive relationship was observed between daily milk yield and serum OD (but not milk OD) in both the HSE (r=0.909; p=0.002) and the LSE (r=0.908; p=0.002) groups in Experiment A. This is likely to be due to a low level of infective larval ingestion leading to a decline in immunity over the lactation period rather than a direct link. The mean worm burden was not found to be significantly different (p= 0.492) between the HSE and LSE groups. The correlation between anti-Ostertagia antibody levels (OD) in serum and worm burdens in the slaughtered animals was negative (r= -0.805, p=0.054). There were no significant differences in the milk OD (p=0.126) or serum OD (p=0.500) between the HSE and LSE groups. There was no significant difference (p=0.26) between the HSE and LSE groups with regard to FEC, but there was a trend (p=0.059) for a greater percentage of younger (cluster of 24 years) cows to have positive FEC. There was a positive correlation (r=0.868, p=0.056) between FEC and pasture L3 levels in the HSE group. Pasture L3 levels showed a significant difference in the old versus new pasture (p=0.009) and a trend for higher levels (p=0.092) in the HSE versus the LSE groups. In Exp. B, there was a positive relationship between the percentage of cows with positive FEC and anti- Ostertagia antibody levels (OD) in serum (r=0.559, p=0.030). The mean worm burden did not differ significantly (p=0.370) among the treatment groups (GI, GII & GIII). Anti-Ostertagia antibody levels (OD) in serum did not differ between groups, and worm burdens in the slaughtered animals did not correlate with antibody titre in serum (r= -0.331, p=0.320). In summary, there is evidence that levels of parasite antibody in the serum and percentage of positive FEC may be related. The difference in stocking rate appears to have no obvious impact on gastrointestinal worm infections in adult dairy cows. However, the low numbers of animals in this study prevent any association with worm burden and hence there is a need for this research to be repeated in a larger study for validation.