Researchers at the University of Vienna, lead by Christine Aurich, have found that the non-infectious diarrhea in foals may not require any specific treatment.
The scientists, whose findings are published in the current issue of the journal Veterinary Microbiology, said many young animals developed diarrhea shortly after birth.
A major switch in intestinal bacteria within the first two weeks of life appears to be directly responsible for diarrhea in many foals.
Their findings show that the intestinal flora of foals undergoes a major switch within the first two weeks of life. The change appears to be directly responsible for diarrhea.
Many methods have been suggested to avoid the problem, including supplementing the mothers' diets with beta-carotene (ß-carotene), which is known to be helpful in preventing diarrhea in young calves.
However, Juliane Kuhl, a member of Aurich's research team, has shown that the supplement has no real effect on the incidence of diarrhea in foals.
Kuhl, Aurich and their collaborators set about finding out the cause of the diarrhea and how it could be treated.
They examined the bacteria in the feces of foals and their mothers, as well as measuring the levels of antibodies (gamma-globulins) in the animals' blood.
Foals are born with low concentrations of antibodies - effectively without functional immune systems. However, gamma-globulins (or y-globulins) are transferred to the foal with the mothers' milk for a few hours after birth.
As foals can synthesise the antibodies in sufficient amounts only after they are about four weeks of age, adequate milk intake immediately after birth is essential - which argues strongly against the practice of limiting the time foals are allowed to suckle their mothers, another method that has been proposed to help prevent diarrhea.
If foals take up sufficient amounts of their mothers' first milk (colostrum), there is no period in the horses' early development where the levels of gamma-globulins are dramatically reduced.
And as Kuhl was able to show that foals with low gamma-globulin levels did not develop diarrhea more often than those with much higher levels, the incidence of diarrhea cannot be related to a weakened immune system.
The bacteria in the feces give a good indication of the animals' intestinal flora, which is difficult to monitor directly. The scientists found little change over time in the nature of bacteria in the mothers' faeces, although they did observe dramatic differences in the bacteria in the foals' faeces.
Foals are born with very low amounts of bacteria in their intestines but are colonized by E. coli within the first day of their lives.
In contrast, the number of foals with Enterococcus remains low until about 10 days following birth, after which these bacteria can be detected in the majority of animals.
Other bacteria, such as Streptococcus and Staphylococcus, arrive even later, between two and four weeks after birth, by which time the foals' intestinal flora is essentially indistinguishable from that of their mothers.
Interestingly, the researchers found that the changes in the bacterial flora closely parallel the development of diarrhea.
Kuhl is careful to note that "we have not yet shown that diarrhea results directly from the switch in intestinal bacteria, although our data make it seem very likely that this is the case".
The implications are that foals are predisposed to developing diarrhea at a very young age.
As the condition clears up fairly quickly without the need for antibiotic treatment, food withdrawal or food supplements such as beta-carotene, breeders should simply accept that many of their animals will suffer from the condition.
Kuhl is happy to concede that "our result might not be exactly what horse-breeders or the feed industry hoped to hear".
"Not all foals develop diarrhea but the vast majority of them do - and do not suffer any long-term consequences from it."
The research work in the study was carried out at the University of Veterinary Medicine, Vienna and the Graf Lehndorff Institute for Equine Science, Neustadt (Dosse), Germany.
The researchers were Aurich, Kuhl, Nora Winterhoff, Manuela Wulf, Florian Schweigert, Ilse Schwendenwein, Rupert Bruckmaier, Jörg Aurich and Peter Kutzer.