Gastrointestinal parasites in lactating dairy cattle: Immunological monitoring and its relationship with production

Gastrointestinal parasitism in cattle, caused mainly by Ostertagia ostertagi and several species of Cooperia, is an important cause of economic losses worldwide. The most detrimental effects of gastrointestinal nematodes (GIN) are caused by subclinical parasitism. The effect of GIN on milk production was evaluated in a meta-analysis of 75 clinical trials. After controlling for publication bias and/or small study effect, an estimate of 0.35 kg/cow/day was obtained, suggesting that GIN do affect milk production. Historically, the level of GIN has been estimated by using the fecal egg counts (FEC) technique, but this method performs poorly in adult animals. Consequently, other diagnostic techniques have been suggested. One of them is an ELISA using a saline extract of a crude adult O. ostertagi antigen. This ELISA showed a good repeatability within plates and between batches of antigen. The ELISA results were not affected by the use of preservatives or by freezing of the milk samples. ELISA optical density ratios (ODR) and total IgG levels were moderately correlated, with both increasing toward the end of the lactation. After controlling for age, season, herd and SCC, an increase in milk production of 10 kg/day was associated with a reduction of 0.04 in ODR. These findings suggested that ODR values were not greatly influenced by production factors but that they might be adjusted for the level of milk production in order to compare ODR values from cows at different stages of lactation. A bulk tank milk survey of all the diary farms in PEI was carried out during the Fall of 2000. Exposure of cows to pasture and whole herd anthelmintic treatment were associated with ODR levels. An increase in ODR levels from the 25th to the 75th percentile was associated with a drop in milk production of 1.2 kg/cow/day. These results indicated that this ELISA is a potentially useful diagnostic technique to measure parasite exposure in adult dairy cows and that GIN have an important impact on milk production.

As part of the evaluation of the ELISA, a longitudinal study was performed where milk, serum and fecal samples were collected from 38 farms. The ODR values increased with cow age and tended to decrease during the housing period and start increasing in the spring before the cows went out to pasture. (Abstract shortened by UMI.).