Strategic parasite control in horses is about balance in a relationship.
A parasite depends upon its host, the horse, for its survival, but gives little in return.
It would be a bad investment for a parasite to upset the delicate balance with its host, as living accommodations would then become unavailable.
The balance shifts in favour of the parasite when time and space in the environment give it an advantage normally not available.
During 60 million years of evolution, horses have been in perpetual movement, covering large tracts of land.
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Domestication has changed the relationship between horses and their environment. The relationship between the parasite and its host changed as well.
Part of the parasite’s life cycle relies on the chance moment that a horse will consume its infective larval form in the environment and perpetuate the circle.
Any factors that enhance this chance moment expose the horse to the burden of parasites.
Parasites use feces as a clever vehicle of operation.
Control strategies aimed at disrupting this part of the life cycle limit the opportunity for chance moments that link the horse and the parasite.
Increased numbers of horses in smaller areas can dramatically contaminate a horse’s environment.
Intensive grazing is likely to mean marked fouling of pastures with high pasture larval infection.
The amount of time the larval form requires in its environment to become infective is influenced by weather.
Ideal conditions
Infective larva have little tolerance to sunlight and drying conditions. They flourish in warm, damp weather.
By observing “lawns” and “roughs” in a horse’s pasture, we get a glimpse of evolutionary wisdom.
These lawns and roughs are a win-win situation for both parties.
Horses defecate in longer grasses or roughs but they prefer to graze in the lawns.
This reduces the number of incoming infective parasites that come in contact with the horse.
The roughs help the parasite’s development because the longer grasses shield them from harsh environment exposure.
Effective control strategies provide sufficient space so roughs do not become lawns.
Harrowing pastures during hot, dry weather, alternate grazing with cattle or sheep, prolonged pasture destocking and, when practical, pasture sweeping all interrupt the parasite-horse cycle.
Pasture sweeping or vacuuming has the added advantage of eliminating the roughs and increasing the grazing areas by up to 100 percent.
Parasite egg output in the feces varies with the seasons, peaking in spring and summer.
Control practices during these times substantially reduce parasites in pastures.
Monitoring fecal egg output over the seasons is useful. A general trend is that during the cooler to coldest months of the year, fecal egg output decreases considerably.
If you wait a few months for spring, egg output resumes, often in prolific amounts.
Strategies that focus on these months of prolific egg laying reduce the burden of parasites in the environment.
By gaining an understanding of time and space in the relationship between the horse, its parasites and their environment, we can use deworming tools wisely.
Factors such as stocking rates, pasture geography, pasture quality, past parasite control measures, pasture management and manure management shift the balance in favour of the horse and the environment.
