A cohort study of factors associated with the incidence rate of keratoconjunctivitis in dairy heifers farmed under Mediterranean climatic conditions.

Abstract

Bovine keratoconjunctivitis (BK) is a common ocular disease in cattle, often linked to symbiotic bacteria with pathogenic potential, such as Moraxella bovis. Although treatable, BK impacts productivity, animal welfare, and antimicrobial stewardship in food-producing systems. This study estimated the incidence rate of BK among dairy heifers and evaluated animal- and herd-level risk factors, including the field efficacy of a commercial M. bovis vaccine. A year-long prospective cohort study was conducted in 636 dairy heifers across nine farms in South Africa's Mediterranean climatic zone. Heifers were monitored monthly for general health and BK signs. Conjunctival swabs were collected to detect M. bovis, M. bovoculi, and Mesomycoplasma bovoculi. Risk factor data were obtained via structured interviews, environmental monitoring, and weather records. BK incidence density rate (IDR), vaccine efficacy, and risk factor associations were assessed using Poisson models. The BK IDR was 25.1 cases per 100 eye-years (95% CI: 20.7-30.4), peaking in summer. No microbial agents were significantly associated with BK IDR, underscoring its multifactorial nature. Incidence rates were similar between vaccinated and unvaccinated heifers, supporting evidence that current vaccines offer inconsistent protection. Significant animal-level risk factors included younger age, poor body condition, and peri-orbital dermatophytosis. Heifers in drylot enclosures with consistent nutrition showed lower BK incidence. Seasonal increases in solar radiation, lachryphagous fly abundance, and pyrethroid pesticide use were linked to higher BK IDR. Findings support a paradigm shift in BK prevention, emphasizing nutritional resilience, welfare-based heifer management, integrated pest control, and responsible pesticide use.