Horses with pituitary pars intermedia dysfunction, more commonly known as PPID or Cushing’s disease, are more likely to have higher fecal egg counts than healthy horses, according to a recent article in the Journal of the American Veterinary Association.
Although it’s widely believed that older horses have less immune resistance to parasites in general, it hasn’t been adequately studied. This study found that age alone doesn’t affect the reappearance of parasite eggs.
The study used 29 healthy horses (aged 4 to 35) and 13 horses with PPID (aged 13 to 33). The PPID horses were chosen based on clinical signs of the disease. Control horses were chosen based on a lack of these signs. Fecal egg counts were performed using a modified Wisconsin floation method at two-week intervals before and after treatment with ivermectin.
The study found that horses with PPID had higher fecal egg counts before and at eight, 10 and 12 weeks after ivermectin treatment, compared to site-matched healthy horses. Resistance to ivermectin was not found.
Bottom Line: If your horse is showing clinical signs of Cushing’s disease, a more aggressive parasite prevention program may be needed. We’re not convinced that other older horses can be monitored less closely and advise you to continue to monitor them for worms.