The doomsday scenario of parasites resistant to all deworming drugs has not materialized, but we are facing some challenges.  This will give you current information on resistance, your treatment options and future directions.

Note: I have deliberately not used any brand names. There are several brands associated with the same active ingredient in dewormers and it's a good idea to get in the habit of actually looking at the active ingredient rather than the brand name to be sure you are getting what you need.

Ascarids (Parascaris, roundworms).  This is a major health threat for young horses. Healthy adults are rarely infected but aged horses may lose their ability to resist.  These are large worms and can even obstruct the intestine in foals.  Their immature forms migrate through the liver and lungs. They are a common cause of "snots" in young horses.

Ascarids in many areas are resistant to ivermectin, moxidectin and in some instances also pyrantel pamoate. They are still sensitive to the benzimadazoles (e.g. oxibendazole, fenbendazole) and piperazine.

Small Strongyles (Cyathostomin sp.).  These parasites of the large intestine are the major parasite of adult horses.  The immature forms can go into a dormant cyst in the wall of the intestine and cause considerable damage when they emerge.

The small Strongyles are resistant to all drugs except the macrocyclic lactones ivermectin, abamectin and moxidectin.  Even these drugs don't work like they did in the past. Fecal egg counts are reduced for approximately 30 days in contrast to the several months previously obtained with these drugs.

Pinworms (Oxyuris).  Pinworms occupy the most distal portions of the intestinal tract and emerge from the anus to lay eggs around the anal area. This produces intense irritation and itching.

There are multiple reports, both anecdotal and formal research, of macrocyclic lactones failing to remove pinworms. However, there are other reports showing the expected effectiveness. It is unclear at this time whether there is emerging resistance or if some issues may be related to inaccurate dosing. Resistance of pinworms to other dewormer drug classes has not been reported.

There are no known resistance problems with tapeworms, Strongyloides westerii (a problem in young foals), lungworms or large Strongyles (Strongylus species). However, large Strongyles are relatively rare since the introduction of the macrocyclic lactones and may actually share the resistance profile of the small Strongyles.

So far these problems are troubling but surmountable. However, there are no new drugs on the horizon to come to the rescue if issues worsen. There is another solution though, and it's ready to break onto the scene.  Duddingtonia flagrans.

I first wrote about the potential for predatory fungi to assist in parasite control about 20 years ago. D. flagrans is a fungus naturally found in the environment which feeds on the infective larval stages of multiple parasites including large and small Strongyles, roundworms, threadworms (Strongyloides) and pinworms. D. flagrans traps and consumes the larvae before they can infect the horse.  They are fed to the horse in an inactive spore state and remain inactive until passed in the horse's manure.

For detailed safety information see this document from the EPA:

D. flagrans has been approved for use in horses in the USA, New Zealand and Australia. It is set to hit the market in New Zealand and Australia very shortly. Although already approved by the EPA, USA use will require approval by the individual states before it can be marketed.

Smart control of intestinal parasites requires a knowledge of the species threatening your horse, how various parasites respond to each dewormer class and environmental control to limit exposure.  D. flagrans will make the job of limiting exposure much easier.