A Worming Rotation That Still Works: Resistance in Desert Yards

For thirty years the standard advice was to worm every horse on the yard every eight weeks. It was simple, it was easy to administer, and it has produced widespread anthelmintic resistance in small redworm across every region where it was practised.

There are only three classes of equine anthelmintic in general use. Resistance is documented in all three. No new class is coming.

What went wrong

Worming every animal on a fixed interval means that at any given moment, essentially every parasite on the property is being exposed to the drug. The only worms that survive are the resistant ones, and they repopulate the pasture unopposed.

The concept that fixes this is refugia — maintaining a population of parasites that has not been exposed to the drug, to dilute the resistant survivors. You do that by not treating every animal every time.

The 80/20 rule

Parasite burdens are not evenly distributed. In most groups, roughly 20% of animals carry roughly 80% of the worm burden. Those animals need treating. Most of the rest do not, most of the time.

Identifying them requires a faecal egg count, which is inexpensive, quick, and the single highest-value change most yards can make.

Building the programme

1. Faecal egg count everything. Establish who the high shedders are. Repeat two to three times a year.

2. Treat on the count, not the calendar. A common threshold is 200 eggs per gram, though your veterinarian may set it differently for your circumstances.

3. Run a faecal egg count reduction test. Count before treatment and again fourteen days after. An effective drug should reduce the count by well over 90%. If it does not, you have resistance to that class on your property and you need to know that before the next season, not after it.

4. Treat all youngstock. Animals under three carry higher burdens and have less immunity. They come off the count-based programme.

5. Cover tapeworm and encysted stages strategically. Tapeworm needs praziquantel or a double dose of pyrantel; encysted small redworm needs moxidectin or a five-day fenbendazole course. Neither shows reliably on a standard egg count.

6. Weigh the animal. Under-dosing is a direct driver of resistance, and eyeballing weight consistently under-estimates it. Use a weigh tape at minimum.

Desert-specific factors

Hot, dry conditions kill free-living larvae on pasture quickly — which is genuine good news and reduces transmission pressure substantially compared with temperate climates.

The offsetting factors:

• Irrigated paddocks create wet microclimates where larvae survive well
• High stocking density in pens concentrates contamination
• Shared water and feed points concentrate it further
• Communal housing, common in camel management, means the group is the epidemiological unit rather than the individual

Dung removal matters more than any drug. Twice-weekly collection from pens and paddocks removes the eggs before they become infective larvae, and it is the intervention with no resistance risk attached.

Camels

Camels share several parasites with horses and have some of their own, and dosing rates are not always equivalent. Work with a veterinarian familiar with camel parasitology rather than extrapolating from equine data.

The goal is not zero worms. It never was. The goal is keeping burdens below the level that causes disease, while preserving the drugs so they still work in ten years.