Work to understand the genetic structure of Rhodococcus equi may lead to new measures to control the disease in foals.
Speaking at the Thoroughbred Racing and Breeding Seminar, Professor Jose A Vazquez-Boland from the Microbial Pathogenesis Unit at Edinburgh University reviewed the progress made in understanding the biology, ecology and virulence mechansisms of Rhodococcus equi through studying the organism’s genome.
R equi typically affects foals up to six months old , and causes chronic broncho-pneumonia with abscesses in the lungs. Other forms of the disease occur including infection of the intestine and lymph nodes.
Rhodococcus equi does not only infect horses. It is an opportunistic human pathogen associated with immune suppression diseases. Up to 4% of suspected bovine tuberculosis cases at the abattoir are, in fact, due to R equi.
The organism is found in soil especially in presence of horse faeces, which provides the substrate on which it feeds. It is well equipped to resist desiccation. Foals are infected by inhaling dust contaminated with Rhodococcus equi.
Treatment is difficult. The organism is resistant to many common antibiotics. It lives within the macrophages, making it difficult for antibiotics to reach it.
A long course of suitable antibiotics, - such as erythromycin and rifampin - is required to effect a cure.
Understanding the complex mechanisms used by the bacteria to cause disease may reveal the microbe’s “Achilles heel” that could be targeted by vaccines and drugs.
So what have the scientists learnt about this organism from studying its genetic make-up?
They identified a number of features that indicate that it is ideally suited to its environment. It has a unique metabolism, different from most other bacteria, as it feeds not on sugars but on short chain fatty acids - such as lactic acid and acetic acid (found within the intestine.) So it doesn’t compete with the bacteria in the horse’s gut, but feeds off their by products.
Rhodococcus equi has genes that confer resistance to many antibiotics. Why does it have so many resistance genes? These antibiotic resistance factors are not acquired by transfer from other bacteria - but they are an "innate" feature of the organism, acquired early on during evolution to resist natural antibiotics produced by soil organisms.
“Among the virulence factors we have identified, we have found one in particular that we think might be a good target for a vaccine.” Disease-causing strains of the bacteria have long processes or “pili”, which they use to attach to the host cells.
“We identified the genes responsible (“Rp1 adhesin”) and raised antibodies against them.” These antiRp1 antibodies prevent the bacteria attaching to the cells and so prevent infection getting established. So this would be a good candidate for a vaccine, currently under investigation.
The foal’s immune system is not well developed at the time it is exposed to infection. So there is probably little chance of producing a vaccine that can be used in the foal to prevent invasive diseases based on cellular immunity. But by vaccinating the mare, it may be possible to protect the foal with passive immunity through the colostrum, particularly if targeting the early stages of infection (attachment to host cells).
Rhodococcus equi genomics: looking into the soul of a horse pathogen.
Professor Jose A Vazquez-Boland. (firstname.lastname@example.org).
Thoroughbred Racing and Breeding Seminar Handbook 2009 p12.