Salmonella enteriticus, Neorickettsia risticii (Potomac Horse Fever), Clostridium difficile and Clostridium perfringens are most commonly associated with infectious diarrhea in adults. Foals can have a variety of
agents including viral causes and bacterial such as Lawsonia intracellularis. However, it is estimated that less than half of the cases of enterocolitis, an etiologic agent is ever identified in acute
diarrhea of the horse. Other causes include parasisitism with larval cyathostomiasis a suspected culprit in adults and several
verninous parasites in foals. Non-infectious causes are likley under appreciated and under recognized and these primarily
include dietary changes (composition or quantity), toxicity (e.g., heavy metals, phenylbutazone or cantharadin toxicosis),
sand and antibiotic-associated diarrhea. In all the of the non-infectious causes, horses can become febrile and toxic due
to the severe secondary effects associated with massive changes in flora and secondary endotoxemia due to changes in composition
of gram negative flora. Nonetheless, any febrile, diarrheic horse should be treated as infectious and biosecurity precautions
Like humans, initial infection is likely foodborne in the adult, but the role of maternal-to-foal transmission during neonatal
life has been largely uninvestigated. Furthermore, the role that foal carriage plays in environmental maintenance and cycling
is largely unappreciated. Salmonellosis is considered by some to be the most common cause of infectious enterocolitis in
adult horses, although a recent report from California suggested that Clostridium difficile might be more common in that state. The reported prevalence of infection with Salmonella has been variable, ranging from
less than 1% (NAHMS) to 70% with higher prevalence of Salmonella associated with 1) the presence of clinically affected horses;
2) method of detection); and 3) climates with warmer months of the year (summer and autumn) associated with outbreaks in Florida
and Fall and winter in North Dakato. According to Cohen (1997, 1998) the risk factors for equine enteric salmonellosis include
1) transportation, 2) change in diet, 3) antimicrobial treatment, 4) surgery, 5) common nasogastric tubes, 6) wet and dark
conditions, and 7) other gastrointestinal disorders (e.g., impaction colic). Foals are at risk for bacteremia/sepsis caused
by Salmonella. Mares are commonly considered to be the source of infection, although are usually without clinical signs of
disease (Madigan). It has been proposed that the underlying mechanisms for known risk factors is a change in the intestinal
microflora. In the United States, Salmonella enterica serovar Newport is the serovar most implipaced in large hospital outbreaks.
This serovar is considered by CDC to be under epdemic spread in animals and humans.
Comments on Laboratory Findings/Ancillary Testing
Diarrhea is the most commonly observed clinical sign. Not all horses with salmonellosis enterocolitis will develop diarrhea
nor will they be febrile. Presence of fecal leukocytes may help to attribute any colitis to an invasive pathogen. For fecal
culture, collect several grams of feces. Rectal swabs are insensitive. Transport using suitable transport media (e.g., Ames
aerobic culture media) and keep cold. Double wrap all samples and wipe off outside of sample with 10% bleach before sending.
Samples can be transported in selenite broth if processed within 24 hours of collection. Five samples for culture and three
fecal samples for PCR must be submitted before a horse or foal can be declared negative. For fecal PCR, this technique appears
to be highly sensitive and a positive result likely carries greater importance in a foal than adult horse with diarrhea.
All foals should have a blood culture performed. This is particularly useful in foals less than 1 month of age, as young foals
with intestinal Salmonellosis are frequently bacteremic.