Page 93 - September 2023

PARASITE RESISTANCE
by Nancy S. Loving, DVM
It is no secret that there are concerns that no new anthelmintic (anti-parasite drugs) products are being developed to deal with
this important facet of equine preventive health care. With this in mind, along with increasing development of parasite resistance, a different approach to parasite control is recommended – selective deworming. As described by the American Association of Equine Practitioners (AAEP) Guidelines
for Parasite Control, this targeted strategy includes “properly timed treatments with effective anthelmintics administered at the appropriate time of the year.”
Selective deworming is based on deworming only as often as necessary to reduce fecal egg counts rather than applying what used to be an historical practice (based on information from the 1970’s) of giving a dewormer to every horse every two months, whether needed it or not. To achieve a targeted treatment goal, selective deworming relies on surveillance through twice annual fecal egg count testing (FEC), which
calculates the number of parasite eggs in an individual horse’s feces. Fecal egg counts must be performed no sooner than 12–16 weeks after a dose of dewormer has been given; the interval is dependent on which anti-parasitic drug is used.
It is known that the majority of internal parasites in any group of animals are concentrated in a minority of the animals – 15–30% shed 80% of the eggs. Fecal egg counts help to identify individual differences in each horse’s immune response and the likelihood of which horses are most responsible for shedding eggs into the environment. They are categorized as low vs. moderate vs. high shedders based
on egg numbers in the feces. In addition, fecal egg counts help to identify which deworming products are no longer effective in parasite control on individual farms.
Enough time has now passed using selective deworming practices to be able to draw conclusions on how well this deworming technique is working. Slowing resistance is dependent on the existence of refugia: Parasites
in refugia are those that are not exposed to an anthelmintic at the time of treatment of others; those worms are still susceptible to deworming products. This includes all stages on pasture (eggs and larvae) as well as parasites residing in animals left untreated.
It is impossible, and actually not preferable, to completely eliminate all internal parasites from horses. So, this can be used to advantage. If lots of parasites exist in refugia, the selection pressure for development of resistance is milder compared to what occurs in small concentrations of refugia. The role of refugia
is to dilute resistant stages of internal parasites – this is accomplished by having worms that are still susceptible to anthelmintics continue to make up the bulk of the internal parasite population. This then delays development of resistance throughout the worm population. Resistance is a genetic adaptation that is passed to succeeding generations of worms; once resistance has developed to a particular anthelmintic, those worm populations are not likely to revert back to susceptibility.
VETERINARY VIEWS
"Selective deworming is based on deworming only as often as necessary to reduce fecal egg counts rather than applying what used to be an historical practice (based on information from the 1970’s) of giving a dewormer to every horse every two months, whether needed it or not."
SPEEDHORSE September 2023 91