Gastrointestinal nematodes, such as Ostertagia ostertagi and several species of Cooperia,
are ubiquitous in temperate climates and have been shown to have detrimental effects
on production in adult dairy cattle. A published meta-analysis demonstrated that overall,
producers lose approximately 0.35 kg of milk per parasitized cow per day. Enzyme-linked
immunosorbent assays (ELISAs) have the ability to quantify nematode infections in cattle,
and thus, could be used to estimate the amount of milk production loss due to differing
levels of parasitism at the individual cow level.
ELISA results from individual cow milk samples were used to predict milk production
response following a randomized anthelmintic treatment in a large field trial. To increase
statistical power, the data collected from this field trial was pooled with data from two
other published field trials to form an individual patient data meta-analysis (IPDMA).
The ability to predict the effect of anthelmintic treatment on milk production depends on
the level of parasitism quantified by an ELISA measuring milk antibodies against O. ostertagi,
and reported as optical density ratios (ODRs). Therefore, the estimates from the interaction
between ODR and treatment on milk production were used to determine how well the ODR
predicted the response of the treatment. It was anticipated that the relationship between
milk production and ODR was unlikely to be linear, so fractional polynomials were applied
to the continuous ODR values.
The interaction in the field trial showed a trend (p = 0.138) toward a beneficial treatment
effect when the individual ODR values, measured in late lactation and using Svanovir®,
were greater than 0.12. When individual data from two other similar studies were included
in an IPDMA, the interaction terms became statistically significant (p = 0.009) indicating
that there is a beneficial treatment effect when ODR values are slightly elevated. A graph
was used to demonstrate the treatment effect (the estimated difference of kg/cow/day
of milk yield between the treated and placebo cows), with 95% confidence intervals, as
the ODR values increase. It is important to note that the methods of quantifying the ODR
values differed between the three studies in the IPDMA, therefore some caution should be
used when using these final estimated values. However, the shape and magnitude of the
treatment effects, as well as the other fixed model estimates, were very similar between
the field trial and the IPDMA suggesting that any bias would likely be minimal.