One of the hallmarks of pituitary pars intermedia dysfunction (PPID), aka Cushing's Disease, is a weakening of the immune response. This includes waning immunity to intestinal parasites.
Even ascarids, or Parascarus equorum, which are typically found only in foals, can become a problem for horses with PPID when their adult immunity to them fails.
In 2024, Horner, et al., reported on strongyle egg reappearance rates in horses with PPID versus controls. The egg reappearance rate is how quickly eggs are found in the manure sample after the horse is dewormed.
A fecal egg count of over 200 eggs per gram is usually considered to indicate deworming is needed. A third of the PPID horses had reached that number by day 42 after deworming and half were there by day 56. Total average egg counts were also considerably higher in the PPID group. This was consistent with results of an earlier study by McFarlane, et al.
This means that if you have gone to once or twice a year deworming, as many people have, your PPID horse may be having long periods of parasite damage. The density also builds up in their environment.
All of that said, there are some PPID horses that do not have high parasite burdens. Their risk depends both on their level of exposure and how advanced their disease is. PPID is slowly progressive even in treated horses.
What to do?
- Assume your horse is at high risk.
- Once a year, check fecal egg counts 45 and 90 days after deworming to see if the horse needs more frequent treatments.
- If you find that actionable egg counts are present at shorter intervals than 6 months/180 days, deworm the horse at that interval all year long. It is suspected some strongyles may slow or stop their egg-laying over winter but they are still there and are still maturing. Don't skip a treatment.
- Use only ivermectin or moxidectin, also with praziquantel once a year for tapeworms. (If the horse has uncontrolled insulin levels use 30 days of Strongid C instead.) There is widespread resistance to other dewormers. The one exception to this is if ascarids are found. Consult your vet for advice on best dewormers for ascarids in your area.
Finally, do not use mail-in fecal services. Eggs will hatch if the sample is not handled correctly. Collect freshly passed manure with a gloved hand. Turn the glove inside out as you take it off and put the sample in a plastic or glass container with a lid and refrigerate. Take it to your vet's office on ice and if it is going to an outside lab request that it be sent on ice.
PPID horses have enough problems to deal with. Parasites shouldn't be one of them.
