Susceptibility of Lawsonia to Antibiotics
Lawsonia intracellularis is an important cause of disease in weanling foals, causing a disease called proliferative enteropathy. Antibiotics, mainly erythromycin,are usually used as part of the treatment for this condition. However, little is known about the true antibiotic susceptibility of this bacterium. The problem is that Lawsonia does not grow in culture plates in a lab like most other bacteria with which we deal - it can only grow in cells, which makes it very difficult to test for antibiotic resistance. Antibiotic therapy for this disease is therefore chosen based on basic knowledge about the bacterium and anecdotal information about how animals respond to treatment. Obviously, this is not the ideal situation.
A little more information is now available on this topic. In a recent edition of Veterinary Microbiology, researchers from the University of Minnesota published a study of the activity of different antibiotics on L. intracellularis. The study, by Suphot Wattanaphansak and colleagues, only involved 10 isolates of the bacterium, all of which were from pigs. (Lawsonia infection is an even bigger problem in pigs than it is in horses). They only tested with a few drugs that are used in horses, and they used an unvalidated (but reasonable) test, so it's very important to be careful (and conservative) when extrapolating the results to Lawsonia in horses. However, they did have some potentially equine-relevant results. One important finding was that there was variation in susceptibility to some drugs between isolates, meaning some strains were susceptible and some were resistant to certain drugs. Although this isn't really surprising, it emphasizes the fact that the same treatment may not be suitable for all cases - antibiotic resistance can be present, and if it is it's critical to identify and address the issue. This particular study showed that tetracycline, a drug sometimes used in horses, had "intermediate activity" against Lawsonia... not great, but not horrible. Unfortunately, they didn't test erythromcyin or chloramphenicol, which are two drugs more commonly used to treat Lawsonia in foals.
This study doesn't tell us much more about how to treat horses with this infection, but it does emphasize the need to test equine strains against drugs used in horses to see if we are doing things right, whether resistance to important equine drugs is present, and to determine if there are better and faster ways to detect resistance.
More information about Lawsonia intracellularis can be found on the equIDblog Resources page and in our archives.
