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 that 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 instituted.
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.
Comments on Treatment
Antimicrobials are controversial in adult Salmonellosis but are frequently recommended for all juvenile horses with enterocolitis.
Antimicrobials that have been used frequently to treat salmonella infections in foals include chloramphenicol, ceftiofur,
and enrofloxacin. Recent evidence does demonstrate the transfer of antimicoribial resistance genes between E. coli and Salmonella
in horse strains, however, cattle and poultry are still the major carriers of multiresistantce genes in Salmonella. Nonetheless
equine salmonella isolates can carry their own unique resistance genes and a specific mutation has been identified that leads
to fluoroquinolone resistance. The S. enterica serovar Newport this is undergoing expansion across the U.S. has been characterized
on the basis of its resistance. Thus, based on recent work with animal and human strains, antimicrobieals should be avoided
in all cases of animal salmaonellosis except where evidence of neonatal infection in foals indicates systemic spread (osteomyelitis,
lung, liver) and the treatment is therefore life saving.