Page 94 - September 2023

VETERINARY VIEWS
"For some time, some countries in Europe, like Denmark, Austria, and Germany for example, have been on a prescription-based deworming protocol with diligent surveillance by veterinarians."
Parasite resistance cannot be concluded on any given farm without proper fecal egg count testing.
WHAT THE DATA SHOWS
Generalizations can be made about the frequency of resistance to specific classes
of dewormers, such as the benzimidazoles, pyrantel, and macrocyclic lactones (M/L that include ivermectin and moxidectin). The AAEP guidelines emphasize, however, that the occurrence of resistance is very variable and large differences are found between individual farms. Resistance cannot be concluded on any given farm without proper fecal egg count testing. The previously used, but ineffective, rotational strategy every two months is no longer as important as ensuring that all anthelmintics used are still effective.
For some time, some countries in Europe, like Denmark, Austria, and Germany for example, have been on a prescription-based deworming protocol with diligent surveillance by veterinarians. Due to the intense involvement of veterinarians in parasite control, anthelmintic treatment intensity is much less, and the degree of parasite surveillance is higher in some European countries. Despite that, European and American studies document widespread internal parasite resistance. The findings are important when considering which anthelmintics still retain efficacy.
Overall, strongyle parasites are widely resistant to benzimidazoles, somewhat resistant to pyrantel salts, and there is evidence of emerging resistance to macrocyclic lactones (M/L), such as ivermectin and moxidectin.
Double resistance (benzimidazoles and pyrantel) has been documented on several farms in the USA and Europe.
There may be farms where benzimadazole efficacy is still high against strongyles, but it is important to test this through FECRT (fecal
egg count reduction testing). FECRT tests fecal egg counts prior to deworming and then again two weeks after deworming to assess efficacy
of the drug product used. Even after more
than 20 years without using benzimidazoles, resistance is still present in the strongyle population although it remains an excellent drug against ascarids.
Ascarids (Parascaris equorum) display almost the opposite pattern to strongyles – they show high levels of M/L resistance and a few reports of pyrantel resistance. At this point, resistance to pyrantel is most commonly seen in small strongyles (cyathostomins), but there are reports of resistance in ascarids as well.
It is suggested that If horse owners stop using M/L products exclusively and use them less often, it may be possible to mitigate the progress of parasite resistance.
ADDITIONAL CONSIDERATIONS
Kentucky and Copenhagen) is in development. To date, there are no signs of drug resistance by bloodworms so any anthelmintic drug should work. However, only some of the currently available anthelmintics are capable of treating migrating stages of bloodworms as they pass through lung and liver.
It is a general consensus that the selective approach to deworming is useful in adult horses; however, it is not recommended for foals and young horses. Fecal egg counts
are not reliable as monitoring tools in foals,
so ascarid worm burdens could accumulate thereby putting foals at risk for life-threatening, small intestinal impaction.
Traditionally, foals have been dewormed every two months, with or without drug rotation, but almost certainly without benefit of any diagnostics. It is still recommended to treat foals 4-5 times within their first year. It isn’t necessary to avoid the use of M/L altogether in young horses; just have some basis of efficacy when using them and don’t use these products exclusively in the first year of life. Yearlings and two-year olds are considered “high shedders”
so are dewormed 3-4 times per year with an appropriate dose of an effective product.
SEASONAL STRATEGIES
Timing of treatment is important – the objective is to control shedding of eggs into
the environment. An owner’s approach varies relative to geographic location. In Northern temperate climates (roughly north of the Ohio River), strongyle eggs hatch and develop into infective larvae in spring, summer, and into late autumn. However, infective larvae survive well through the winter months on pasture. The objective of parasite control is to limit environmental contamination and subsequent infectivity – this is best done by stopping egg production during those seasons when parasite eggs can turn into future adults. In Northern temperate zones, the annual strongyle transmission is April through October – that’s when chemical interventions should take place.
92 SPEEDHORSE September 2023
One big concern about the selective
deworming protocol is the possible resurgence
of Strongylus vulgaris (bloodworms) due to
a less intensive deworming frequency. One
study suggests a relationship between
selective therapy and recurrence
of bloodworms. This
parasite is definitely
still present and so
it is important
to implement
surveillance horses. and control
measures. A
blood test for
detection of
bloodworms
(University of
In Southern temperate regions, the opposite
It is a general consensus that the selective approach to deworming is useful in adult horses; however, it is not recommended for foals and young