Development of guidelines for immunological monitoring of gastrointestinal parasites in dairy cattle

Gastrointestinal parasites, such as Ostertagia ostertagi , adversely affect milk production in dairy cattle. These gastrointestinal nematodes are ubiquitous in temperate climates, but clinical signs of infection are rarely seen in adult cattle. An enzyme-linked immunosorbent assay (ELISA) based on a crude Ostertagia antigen has been used to quantify a cow's immunological response to intestinal parasites, and therefore act as a surrogate measurement for parasite load.

Svanova (Uppsala, Sweden) developed a commercial ELISA test (Svanovir ®) available in Europe. This test is designed to be used on milk samples, making the collection process very simple, and bulk-tank (BT) or individual cow milk samples can be used. Results from ELISA tests are normalized, using the controls, and reported as optical density ratios (ODRs), thus permitting the ELISA results to be compared between plates, kits and, to a certain extent, studies and regions.

A large clinical trial, involving over 3,000 cows from nearly 40 herds in 9 provinces, was undertaken to predict the amount of milk loss (kg/cow/day) associated with gastrointestinal parasites and to evaluate the ability of the ELISA to predict the benefit from anthelmintic treatment. Milk samples were collected from cows (>200 days in milk) to measure individual ODRs. Producers applied a treatment (randomly allocated as anthelmintic or placebo) to cows as they calved. Milk production records were acquired from Dairy Herd Improvement (DHI) programmes for individual cows on a monthly basis during the study.

The treatment effect (anthelmintic) on milk loss was expected to depend on the level of parasitism in the cow, where low ODR values from the ELISA test indicated low levels of parasitism. As such, the estimates from the interaction between ODR and treatment on milk production would be able to determine how treatment effect depended on ELISA test results. A fractional polynomial (2-degree) was applied to this interaction since the relationship was nonlinear. To increase statistical power, datasets from two previous smaller, yet similar, Canadian studies were incorporated into the analysis. The large combined dataset was able to predict the amount of individual milk loss (kg/cow/day), based on ELISA test results from individual cows in a herd.

The final objective of the thesis was to utilize data from both individual milk and herd BT samples, from several studies in North America (including this clinical trial) and Europe, to develop guidelines for the use of a commercial ELISA test (Svanovir®) to predict production losses (for the herd) associated with gastrointestinal parasites in dairy cattle.

The guidelines required a series of small analyses in order to transform BT ELISA results into averaged individual milk losses (kg/cow/day). Each step along the series used different datasets from various studies. One of the final products is a nomogram (diagram that solves calculations by drawing simple lines) which was designed to interpret ELISA test results to quantify the estimated economic losses associated with intestinal parasites from two BT milk samples taken within one season. While the nomogram reports values using deterministic methods, stochastic processes were applied to estimate uncertainty around the coefficients and identify influential parameters within the guidelines.