Sepsis is a potentially fatal bacterial infection in neonatal foals. In this thesis we have shown that correct antimicrobial treatment has a positive effect on survival in foals with sepsis. In important groups of bacteria that cause sepsis in foals, we observed emergence of resistance to commonly used antimicrobial drugs over time. Furthermore, hospitalized foals are at risk for development of healthcare-associated infections. The antimicrobial susceptibility patterns of the bacteria that cause these infections are unpredictable, making these infections challenging to treat. Strict hygiene protocols are necessary and repeated bacteriological cultures (including susceptibility testing) are advised in hospitalized foals to identify infections at an early stage.
The microorganisms in the gut (the gut microbiota) play an important role in equine health. Location, age, season, horse type and pasture access affect the composition of the gut microbiota in healthy horses. Oral treatment with antimicrobials disturbs the gut microbiota. A gradual recovery was observed within two weeks; however, significant differences could still be detected six months later. Antimicrobial treatment also increased the presence of antimicrobial resistant bacteria in the faeces. Some of these were still excreted in high numbers six months later. Horses might therefore be a potential reservoir of resistant bacteria and could form a risk to animal and human health. Judicious antimicrobial use in veterinary medicine is important to prevent development and spread of antimicrobial resistance.