Research Summary - 2
Can we use on-farm tests for diagnosing intramammary infection at dry-off in dairy cows?
| Date/Time: |
9/13/2024
11:45
|
| Author: |
José Denis-Robichaud |
| Clinic: |
Independent researcher |
| City, State, ZIP: |
Amqui, QC G5J2N5 |
J. Denis-Robichaud, DVM, MSc, PhD, Dipl. ACVPM
1
;
N. Barbeau-Grégoire, MSc
2
;
M.-L. Gauthier, DVM, MSc, Dipl. ACVM
3
;
S. Dufour, DVM, PhD
2
;
J.-P. Roy, DVM, MSc, Dipl. ECBHM
2
;
S. Buczinski, DVM, MSc, Dipl. ACVIM
2
;
J. Dubuc, DVM, MSc, DVSc
2
;
1Independent researcher, Amqui, Québec, G5J 2N5, Canada
2Faculté de médecine vétérinaire, Université de Montréal, Saint-Hyacinthe, Québec, J2S 2M2, Canada
3Ministère de l’Agriculture, des Pêcheries et de l’Alimentation du Québec, Saint-Hyacinthe, Québec, J2S 2M2, Canada
Introduction:
Implementing selective
dry cow therapy as an alternative to systematic antimicrobial use at dry-off on
dairy farms can play a role in decreasing antimicrobial use. While on-farm
bacteriological tests have been validated to identifying intramammary infection
(IMI), a cow-side test such as ATP luminometry could be a quick, simple, and
cheap alternative. In this study, we aimed to assess the validity of various
diagnostic tests in identifying intramammary infection (IMI) in dairy cows at
dry-off. We compared the validity of laboratory bacteriological culture, vs. on-farm Petrifilm and Tri-Plate
culture systems, vs. luminometry.
Materials and methods:
To conduct the study, composite milk samples were collected from cows
before dry-off between September 2020 and December 2021. Each sample was tested
with luminometry, Petrifilm (aerobic count plate), Tri-Plate, and laboratory
culture. We categorized IMI using thresholds of ≥ 100 CFU/mL for laboratory
culture, ≥ 50 CFU/mL for Petrifilm, and ≥ 1 CFU for Tri-Plate. For luminometry,
we assessed multiple thresholds of relative light units (RLU; 50 to 300). We
fitted Bayesian latent class models designed for comparing imperfect tests
between them. We estimated the sensitivity and specificity of each test, along
with predictive values and misclassification cost terms (MCT) at varying
false-negative to false-positive ratios, under different prevalence measures
(0.30, 0.50, and 0.70). Results are presented as median estimate with 95% Bayesian
credibility intervals (BCI).
Results:
A total of 333 cows
from a commercial Holstein herd were included in the study. Results indicated
that luminometry showed poor validity across all thresholds tested, with a sensitivity
of 0.51 (95%BCI = 0.43–0.59) and a specificity of 0.38 (95%BCI = 0.26–0.50)
when using a threshold of ≥ 150 RLU. Conversely, laboratory culture, Petrifilm,
and Tri-Plate demonstrated higher sensitivity and specificity values. Indeed, laboratory
culture had a sensitivity of 0.93 (95%BCI = 0.85–0.98) and a specificity of
0.69 (95%BCI = 0.49–0.89), Petrifilm had a sensitivity of 0.91 (95%BCI =
0.80–0.98) and a specificity of 0.71 (95%BCI = 0.51–0.90), while Tri-Plate had a
sensitivity of 0.65 (95%BCI = 0.53–0.82) and a specificity of 0.85 (95%BCI =
0.66–0.97). The bacteriological tests generally had good predictive values,
although the Tri-Plate had lower negative predictive value compared to the laboratory
culture and the Petrifilm. For a prevalence of IMI of 0.30, all bacteriological
tests showed similar MCT. However, at prevalences of 0.50 and 0.70, the Tri-Plate
demonstrated higher MCT in scenarios where the consequences of leaving a cow
with IMI untreated outweighed treating a healthy cow.
Significance:
In conclusion, while luminometry proved ineffective for identifying IMI at dry-off, both Petrifilm and Tri-Plate tests showed performances comparable to laboratory culture, contingent upon IMI prevalence and the relative importance of false-positive vs. false-negative results.
