19 Aug 2026
Scholars found peak Lawsonia intracellularis exposure occurred between six and nine months of age and in the autumn-winter period.
Chris Simpson
Job Title
Image: Piaffe Photography / Adobe Stock
Researchers investigating the prevalence of the bacteria causing equine proliferative enteropathy have identified the peak risk period in foals.
Vets from Rossdales Equine Hospital and Diagnostic Centre found seasonal and age-related peaks in seropositivity for Lawsonia intracellularis, which can inform vaccination strategies.
The scholars conducted a first of its kind study exploring seroprevalence and faecal shedding of L intracellularis in UK foals during their first year of life.
They examined 47 foals from 6 Thoroughbred stud farms – 37 of which completed the study – at birth and at three-month intervals throughout the 2014 breeding season, a year prior to widespread vaccinations being implemented.
At birth, 36 mares and 30 foals (77% and 64%, respectively) were seropositive, and there was a positive linear relationship between mare and foal serum titres and between foal serum titres and immunoglobulin G levels.
Only 22% of foals that completed the study (8 out of 37) did not seroconvert while more than three-quarters did, indicating exposure to the bacterium following loss of maternal antibodies.
At least one foal from each farm seroconverted, which the authors suggested indicates exposure to L intracellularis is widespread, although they acknowledged that all the farms were located within the same county.
Peak timing for seroconversion occurred between six and nine months of age and between September and January.
The authors noted that such epidemiological information is critical to the development of practical and effective screening and vaccination programmes, but that locality-specific measures are also required.
Current recommendations are to administer the vaccine 30 days prior to anticipated disease. All foals remained asymptomatic throughout the study period and only two faecal samples out of 159 tested were positive. The paper noted this would suggest healthy foals do not significantly contribute to the environmental pathogen load, but also underscores the sporadic nature of clinical disease and complicates epidemiological surveillance.
The authors called for widespread screening of the most susceptible populations at the times of year associated with maximal disease and for further, more up-to-date UK-based studies, using larger, more diverse equine populations spanning longer timeframes with smaller sampling intervals.
Rossdales research associate and co-author Vicky Colgate said: “This study has helped us identify when foals on UK stud farms are most likely to encounter Lawsonia intracellularis for the first time.
“The results suggest that exposure most commonly occurs between six and nine months of age, after maternal immunity has declined, which provides valuable information when considering vaccination strategies.
“By understanding this risk period more clearly, breeders and veterinarians can make more informed decisions about protecting youngstock from this potentially serious disease.”
Co-author and RCVS-recognised equine internal medicine specialist Caroline Ribonnet added: “As the first UK study to investigate the seroprevalence and timing of Lawsonia intracellularis exposure in foals, these findings provide an important foundation for future research.
“The work improves our understanding of how widely the bacterium is present on stud farms and offers evidence that can help guide both disease surveillance and preventive health care strategies in the years ahead.”