Bacteria, viruses, and protozoa can all cause diarrhoea in foals, and probably of equal concern, can cause diarrhoea which is readily transmissible to other in-contact foals, resulting in outbreaks on farms and studs. Parasitic diarrhoea was dealt with in the section on non-infectious diarrhoeas, although it could be argued that it should be categorised as an infection caused by worms.
Bacterial diarrhoea is seen in foals of all ages but is a major cause of death in neonatal foals (a foal in its first week of life). Diarrhoea is a predominant clinical sign associated with septicaemia in young foals. This is a systemic disease associated with pathogens and/or their toxins in the foal’s bloodstream and, if seen, needs to be addressed urgently.
Salmonella, E. coli and Clostridium are three of the main bacterial culprits that are responsible for severe diarrhoea, usually in neonates, but can occur in foals of all ages. The bacteria infect the foal via ingestion through the mouth, with the mare often being the source of contamination. These bacteria either live in the environment or are part of the mares’ usual gut flora, which in times of high stress multiply and are passed out in the faeces. Some mares carry salmonella within their gut without showing any clinical signs (“carriers”). These carrier mares, when foaling down, excrete salmonella bacteria in their faeces, which runs down the perineum onto the vulval lips, then down between the legs and onto the udder. When the foal seeks the udder, it will lick/suckle all the areas adjacent to the udder and therefore can ingest large numbers of pathogenic bacteria that have been excreted by the mare, resulting in septicaemia and diarrhoea.
Hence the importance of good management and attention to detail when foaling mares down at home, as good hygiene and management procedures can be essential in reducing the transfer of bacteria to foals. Other bacteria associated with diarrhoea in foals include Bacteroides, Campylobacter, and Leptospira spp.
Foals that fail to ingest enough colostrum are particularly susceptible to infections, allowing other less common bacteria to proliferate in their system and cause diarrhoea. Measuring the foal’s IgG level 12 hours after birth and treating susceptible foals with plasma can help prevent this type of diarrhoea occurring.
Older foals can also succumb to bacterial diarrhoeas; this can be a standalone condition or seen in conjunction with other clinical signs such as weight loss and coughing.