DOI: 10.5061/dryad.9ghx3ffsr
title: Etiology, persistence, and risk factors of subclinical mastitis in a meat-producing sheep flock

description: Subclinical mastitis, the asymptomatic inflammation of the udder often
 caused by bacterial infection, is an important economic and welfare
 concern in meat-producing flocks. We conducted a longitudinal
 observational study to: (1) characterize the etiology, prevalence,
 incidence, and persistence of intramammary infections during the first 8
 weeks of lactation, and (2) to investigate the potential risk factors of
 lactation stage, parity, and the number of lambs reared. Duplicate milk
 samples (n=592) were collected aseptically from each udder half of 37
 clinically healthy ewes (9 Hampshire, 22 Polypay, and 6 Targhee) weekly
 for their first 8 weeks of lactation. A sample was considered a true
 infection if at least 100 CFU/mL of a bacterial species were isolated. Out
 of a total of 560 non-contaminated duplicate udder half samples, 186
 (33.2%) were infected. The prevalence of subclinical mastitis in the first
 week of lactation was 26.0% at the udder half level and increased over
 lactation to 47.1% by 8 weeks. Over the course of lactation, 34 of 37
 (91.9%) ewes had an intramammary infection in at least one udder half. The
 most common bacteria isolated were non-aureus Staphylococci (72.1%),
 Staphylococcus aureus (7.5%), and Mannheimia haemolytica (5.9%). The
 weekly incidence rate of new infections was 18.0 ± 10.2% (mean ± SD).
 Infections lasted 2.1 ± 2.1 weeks, with 9.0% present the full 8 weeks of
 observation. Spontaneous cures occurred in 36.0% of infection bouts but
 varied from 0 to 100% depending on the pathogen. The probability of
 subclinical mastitis infection increased as lactation progressed for ewes
 rearing single or multiple lambs and for ewes in their first or second
 parity, but not for ewes in later parities. In conclusion, intramammary
 infections in this flock were prevalent across lactation, parities, and
 litter sizes, and varied in their duration and spontaneous cure rate.
 Improved knowledge of the etiological agents and risk factors involved in
 subclinical mastitis is important in developing effective control measures
 in meat-producing flocks.
description: # Etiology, persistence, and risk factors of subclinical mastitis in a
 meat-producing sheep flock
 [https://doi.org/10.5061/dryad.9ghx3ffsr](https://doi.org/10.5061/dryad.9ghx3ffsr) ## Description of the data and file structure These materials provide the data and R script used in this longitudinal observational study of subclinical mastitis in sheep and are referenced in the related article by the same name. ### Files and variables #### File: Peckler\_etal\_JAS\_SheepMastitisData\_241125.xlsx **Description:** Excel file containing the bacteriological results from milk samples collected in duplicate from each udder half of 39 ewes weekly over the first 8 weeks of lactation. The second sheet contains descriptions of each column variable. Blank cells indicate NA values. ##### Variables | Variable | Description | | :------------------------------- | :---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | EweID | Unique ID for each ewe (n=39) | | DOB | Ewe's date of birth | | Breed | Breed of the ewe (Targhee, Polypay, or Hampshire) | | Cohort | Each ewe belonged to a cohort that lambed in Spring or Fall 2021 | | Parity | The ewe's total number of lactations (including the current one). Ranges from 1 to 9.  | | ParityGroup | Ewes were categorized into first, second, third, or &gt;third parity | | LambingDate | Date the ewe gave birth | | LambsBorn | The number of lambs the ewe birthed | | LambsReared | The number of lambs the ewe reared | | MultipleLambs | A binary variable indicating whether the ewe reared a single lamb (0) or multiple lambs (1) | | UdderHalf | Left (L) or right (R) udder half | | DateSampled | Date that the udder half was milk sampled | | DaysSinceLambing | The number of days since the ewe lambed when the milk samples were taken. Ranges from 2 to 53 days. | | WeeksSinceLambing | The number of weeks since the ewe lambed when the milk samples wer taken. Ranges from 1 to 8 weeks. | | Bacteria1\_ID\_Sample1\* | Identification of the first bacterial species (Bacteria1) identified in the culture of the first milk sample taken (Sample1). Milk samples were taken in duplicate for each udder half. | | Bacteria1\_Level\_Sample1\* | Level classifying the number of colony-forming units for the first bacterial species (Bacteria1) identified on the culture of the first milk sample taken (Sample1). | | Bacteria2\_ID\_Sample1\* | Identification of the second bacterial species (Bacteria2) identified in the culture of the first milk sample taken (Sample1). Milk samples were taken in duplicate for each udder half. | | Bacteria2\_Level\_Sample1\* | Level classifying the number of colony-forming units for the second bacterial species (Bacteria2) identified in the culture of the first milk sample taken (Sample1).  | | Bacteria3\_ID\_Sample1\* | Identification of the third bacterial species (Bacteria3) identified in the culture of the first milk sample taken (Sample1). Milk samples were taken in duplicate for each udder half. | | Bacteria3\_Level\_Sample1\* | Level classifying the number of colony-forming units for the third bacterial species (Bacteria3) identified in the culture of the first milk sample taken (Sample1). | | Bacteria1\_ID\_Sample2\* | Identification of the first bacterial species (Bacteria1) identified in the culture of the second milk sample taken (Sample2). Milk samples were taken in duplicate for each udder half. | | Bacteria1\_Level\_Sample2\* | Level classifying the number of colony-forming units for the first bacterial species (Bacteria1) identified on the culture of the second milk sample taken (Sample2). | | Bacteria2\_ID\_Sample2\* | Identification of the second bacterial species (Bacteria2) identified in the culture of the second milk sample taken (Sample2). Milk samples were taken in duplicate for each udder half. | | Bacteria2\_Level\_Sample2\* | Level classifying the number of colony-forming units for the second bacterial species (Bacteria2) identified in the culture of the second milk sample taken (Sample2). | | Sample1Diagnosis\* | Diagnosis of Sample1, using criteria described in the related article. | | Sample2Diagnosis\* | Diagnosis of Sample2, using criteria described in the related article. | | Diagnosis\* | Final diagnosis of udder half based on diagnoses from Samples 1 and 2, using criteria described in the related article. | | BacteriaPresent\* | Binary variable indicating whether the udder half was considered infected (1) or not (0). | | Persistence\* | Binary variable indicating whether the same infection may have been present the previous week (1) or is a new infection (0). | | Bout\* | Unique ID for each infection bout that lasted at least two weeks (i.e., considered persistent) | | BoutDuration\* | The number of weeks each infection bout lasted. | | Cure\* | Binary variable indicating whether the infection spontaneously cured (1) or not (0). | | | | | \*Blank cells indicate NA values | | ## Code/software #### File: Peckler\_etal\_JAS\_SheepMastitisDataAnalysis\_241125.nb.html **Description:** R script to run the descriptive and statistical analyses on the data in the Excel file and generate the figures in the related article. The nb.html file, which includes annotated code and output, can be opened in any web browser or in RStudio (e.g., using File -&gt; Open File).