The idea then, is that within the horse population, there are those horses that don't need to be treated because they are
managing the parasites naturally via their immune system, and are shedding relatively low numbers of eggs, and therefore there's
no medical reason to treat them every time. Those animals need a few treatments at particular times of the year, for particular
reasons, which include other parasites (i.e. bots, tapes, ascarids), but they don't require treatment for the cyathostomes,
except for one to three treatments per year, sources say. The low-egg shedders are constantly supplying a low level of parasite
contamination on the pasture, but it's enough to help dilute out the survivability of the worms. When you treat the high egg
shedders that are shedding the resistant worms, you have a dilution effect.
"We've got to get away from extremely frequent treatments," says Craig Reinemeyer, DVM, PhD, president of East Tennessee Clinical
Research. "We have to determine which drugs are still effective in each and every herd, characterize the susceptibility characteristics
of individual horses, and deworm them in a customized fashion, instead of hitting everybody in the herd with the same thing
at the same time."
Ascarids protruding from a ruptured intestine of a horse.
According to Kaplan, a worm control program should begin when worm transmission to horses changes from negligible to probable,
in late autumn in the hot Southeast and Southwest, and in the early spring in the moderate Northern climates. Veterinarians
should remind owners that there is no point deworming during the summer months in hot climates when transmission of L3 larvae
is low, nor during the winter in the North. Veterinarians also should base parasite control on the differences in susceptibility
between horses. Some horses have very poor natural immunity against internal parasites, and need to be dewormed somewhat frequently,
while there are other horses that have excellent immunity against parasites and can be dewormed less frequently.
For the hot climates, begin in September. At this initial stage of the worm control cycle, all horses should be treated regardless
of FEC, and be so with one of the macrocyclic lactones. This will take care of bots, previously acquired since the spring,
Habronema and Draschia (nematodes responsible for summer sores), sterilize Onchocerca (leg or neck threadworms) females preventing transmission; kill pinworms, kill migrating large strongyles and kill small
strongyles that are in the intestinal lumen.
Inhibited small strongyle larvae in the mucosa of the large intestine.
At this same time an FEC should be done on all horses, and categorize the animals as low (<150 epg, eggs per gram), moderate
(150-500 epg), or high egg shedders (>500 epg). Further worming and decisions are then based on FEC, the need to treat for
tapeworms later in the year (Dec), and the necessity to rotate to other anthelmintics based on the response of the horses
and their parasite load. After December, the primary concern is cyathostomes, but the necessity to worm should be dependent
on the immunity variation between horses.
A key addition to anthelmintic use is pasture management, which helps reduce pasture contamination and thereby reduces re-infection
throughout the horse herd.
"It is a good to rotate the pastures, particularly if the horse owner has a number of animals," Carrillo suggests.
When the grass is getting low and scarce, the animals are taken off the pasture and that field is worked on, including reseeding
and getting the soil back together to renew the pasture. It is good to continue to rotate them as the pastures get warn out.
If a few months pass before a horse gets back to a pasture it's been on before, there will be time for the eggs to hatch and
the larvae to die, and therefore re-infection is potentially a lot less. In general, you will decrease the volume of parasites
that the horse is ingesting from their own fecal contamination of the pasture.