We’ve talked about the effects of seasons on parasite development and the ability of heat to kill larvae.
But parasites didn’t become as common as they are without being able to play hardball.
Climate can certainly give them a hard time, but temperatures and moisture levels in all seasons vary widely from region to region.
To this end, instead of talking about the seasons, let’s look at specific temperatures and how they affect the life cycles of equine parasites.
Strongyles eggs, for example, can occasionally be killed by a hard frost, but the majority will hold on. They are able to hatch at temperatures as low as 7deg Celsius. However, at those kinds of temperatures development into an infective larval stage could take several weeks. At optimal conditions during warmer months, an egg deposited in dung can develop to its infective larval stage in as little as five days in temperatures around 26 or 27deg Celsius.
These infective third-stage larvae are much tougher, and are sealed in a protective membrane, which means they can survive only on stored energy. They are well able to withstand frost and icy temperatures. Summer temperatures increase their metabolism and they can exhaust their stored energy in a matter of two or three weeks.
In winter temperatures, they are using up little or none of their energy stores. They can ride out the winter chill for months or until ingested by a horse, where the warm internal temperatures spark them into action.
Horses do not like grazing near their dung – a feature not uncommon in other species. To this end, a strongyle larvae ready to infect a horse will not find dung to be prime real estate.
Its chances will be much better if it can get into the pasture proper, and to this end it relies on rain, birds pulling apart the manure piles and any other mechanical means of distribution, such as a passing horse clipping a pile with its hoof or a horse owner out with harrows.
Infective larvae tend to prefer the vegetative layer close to the soil, so it’s in your interests to prevent horses grazing grass too short. Leaving the grass longer has added benefits: it will come away faster and will prove more drought-resistant if conditions are beginning to dry out.
Ascarid eggs are even tougher and can survive in a pasture environment for up to 10 years, with development again triggered by warmth.
Tapeworm eggs, as we learned earlier, rely on the orabatid mite as a vector. Frost can kill tapeworm eggs, but there will be plenty of infected mites to see the species through the colder months.
Moisture is another necessary element in the development of parasite larvae in pasture. Several factors come into play here. Generally, the moisture in the horse manure will linger long enough to allow parasites to complete their pasture development, perhaps topped up with the occasional shower.
However, hot and dry conditions can result in a moisture deficit which will slow, or even halt, development.
For most people in temperate and sub tropical areas, spring and autumn will generally provide great conditions for parasite development in pasture. Summers will be somewhat more variable, depending upon how hot they get and how much moisture is available.
» Next: Doing a fecal egg count
First published on Horsetalk.co.nz in February, 2009