Ostertagia ostertagi exposure in dairy cows in the alpine space: implications for region-specific assessments

Pasture-based management of dairy cattle in temperate regions leads to infections with gastrointestinal nematodes, with Ostertagia ostertagi recognised as the most pathogenic species of these. However, little is known about O. ostertagi exposure in alpine regions. Here, we aimed to explore the O. ostertagi seropositivity in a large sample of dairy cows from two Austrian federal states that differ substantially in herd size, productivity and use of (alpine) pastures, and to assess associations between parasite exposure, milk yield, and farm management factors.

Bulk tank milk (BTM) samples from 1241 dairy farms in the federal states of Upper Austria (n = 742) and Tyrol (n = 499), of which the dairy herds comprised a total of 25,985 adult cows, were analysed for anti-O. ostertagi immunoglobulin G at the end of the grazing season 2018. Anti-Ostertagia antibody levels in milk [measured as the optical density (OD) ratio (ODR)] from herds with known grazing were evaluated for associations with production and management data via linear models to (1) evaluate the effect of farm management practices on herd exposure to O. ostertagi, and (2) assess the association of O. ostertagi exposure and management practices with milk production.

Of all the investigated herds, 65.3% were positive for O. ostertagi infections (ODR ≥ 0.5), with a mean ODR of 0.59 (25th-75th percentile 0.42-0.75). Herds in Tyrol had higher mean BTM-ODR values (0.73; 25th-75th percentile 0.61-0.87) than those in Upper Austria (0.50; 0.38-0.63); however, alpine grazing was not associated with higher ODR values in either Austrian state. In Upper Austria, organic farming was significantly associated with higher O. ostertagi exposure, whereas in Tyrol, larger herd sizes were linked to increased exposure. Lower milk yield was significantly associated with decreasing cow herd size and organic farming in both federal states, but not with increasing O. ostertagi ODR.

Marked differences in the exposure of Austrian dairy cattle to O. ostertagi are likely explained by contrasting grazing management between the investigated states and by specific characteristics of each region. No association between increased parasite exposure and milk yield was observed, highlighting the need for region-specific investigations. Small-scale and/or alpine cattle farming with predominantly dual-purpose breeds requires a tailored risk assessment to support sustainable parasite control strategies.