Young foals are susceptible to respiratory disease of the lower airway, especially Rhodococcus equi pneumonia prior to 6 months of age. Though not the most common foal pneumonia, the illness caused by Rhodococcus equi continues to be a major cause of loss of life and is a particularly insidious in foals.
The organism, a gram-positive coccobacillus, commonly causes infection in hot, dry environments via the inhalation of dust
from manure-contaminated soil. Organic acids normally found in horse manure promote the growth of the bacteria, which also
is enhanced by summer heat. The dry conditions promote aerosolization of the bacteria. The pneumonia progresses fairly rapidly
as the infected foal gradually loses condition and develops a chronic cough, wheezing, fever and increased respiratory rate.
Untreated foals develop progressive crackling sounds noted upon thoracic auscultation within the entire lung, accompanied
by harsh inspiratory sounds. Foals also can develop an intestinal infection due to swallowed infected sputum. An infected
foal frequently appears quite well until just before death, which is why the disease is often missed. The mortality rate can
be as high as 60 percent to 70 percent in untreated foals. Even those foals that recover sometimes are limited in their performance
later in life. To be successful, treatment must be started in the early stages of the condition.
Some farms endemic, others not
It is curious that on some farms the disease is endemic but not on others. John Prescott, DVM, Department of Pathobiology
at the Ontario Veterinary College, says it is primarily a matter of numbers and concentration of foals. Others factors include
the sandiness and acidity, and possibly other properties of the soil, the amount of dust generated, and the length and extent
of heat in the summer. Established horse farms, those that have raised horses for several years, are more likely to show greater
incidence of disease. Contamination does not come from adult horses that are immune and do not shed bacteria in their manure,
though foals younger than 3 months readily shed R. equi from their intestinal tract, contaminating the soil.
Adult horses can shed the organism in their feces, says Stephen A. Hines, DVM, PhD, Dipl. ACVP, professor in the Department
of Veterinary Microbiology & Pathology at Washington State University's College of Veterinary Medicine. It's just that they
don't tend to share near as high of numbers as even asymptomatic foals.
"If you have a horse on your place, you've almost certainly got R. equi there — even though it may not be causing a big problem," Hines says. "Heck, if you've got a farm of any kind, I'm confident
you can find R. equi in the soil and manure."
Virulence of the organism also plays a role in differentiating farms of heavy incidence of disease from those with few cases
because farms with endemic disease have larger proportions of virulent R. equi
According to Keith Chaffin, DVM, MS, Dipl. ACVIM, Texas A&M University, there have been several studies that have looked at
farm-level risk factors for R. equi. Chaffin and colleagues did a study at 64 farms with and without R. equi pneumonia. They found several farm-level risk factors, including large acreage and a large population of mares and foals.
The higher the density of foals, the higher the risk of disease.
Not a disease of neglect, management factors were not found to be important.
"The farms that had R. equi pneumonia were more likely to be well-managed," Chaffin says. "Those that would do all the things that we as veterinarians
tell them to do — test for failure of passive transfer of immunity, attend foaling, vaccinate and deworm are at high risk."
The density of foals is the biggest factor, Hines agrees.
"If there are a lot of foals, there is a lot of Rhodococcus," he says. "The foals are the high shedders. You get a cycle with
high environmental load and high infectious doses. The thing that is correlated best is the number of horses per acre."