The temperate times of the year are the times that parasites are most likely to infect your horse. When the average daily temperature is between 45 and 85 degrees the parasite's eggs are most efficient at becoming infective larvae on the pasture. Once the day's temperature gets above 85 degrees, and especially when it is dry, the parasite larvae on the pasture rapidly die off due to low energy reserves and moisture. The result is a "cleaning" of the pasture. When the day's temperatures stay below 45 degrees the eggs are very slow to develop into infective larvae. However over-wintering eggs and larvae do accumulate until the following spring when the accumulated winter's egg burden on the pasture suddenly blossums as infective larvae in the spring.

No matter which deworming program you choose, testing for efficacy is important. When you test in the deworming cycle effects the validity of the information. A good time to have your deworming program checked is at his yearly vaccination and should be just before a regularly scheduled deworming. Deworming your horse will cause a temporary cessation in egg production by the adult parasites so no matter how wormy your horse is the fecal test will look good just after deworming your horse and up to the prepatency time. What you really want to know is what is happening at the end of a deworming cycle so that you can judge if your pastures are becoming contaminated. A clean test at this time is your best indication what you are doing is working to keep your pastures free of infective parasite larvae, for more see "Testing" below.

Parasite Permissive Horses

An interesting result of close monitoring of fecal tests in herds of horses has shown a large range of susceptibility to parasites in healthy adult horses. Some horses have been found to be very resistant to reinfection and others are very susceptible to reinfection. Most horses fall somewhere between these two extremes. Susceptibility to infection is manifested by a quick return to egg production following deworming and increased egg production between dewormings when compared to the herd as a whole. These horses are important sources of maintaining a higher level of infection in the herd than would normally occur and need to be diagnosed and then singled out for special treatment.

Foals

Dewormers do not work as well in foals as it does in adults. This is partially because foals have a naive immune system that does not help the dewormer suppress parasite population. It has also been conjectured that foals may metabolize the anthelmintic more quickly. For these reasons deworming and monitoring worm burdens in foals should be done more aggressively than in adults. If you obtain a foal that has an uncertain deworming history or appears to be suffering from poor care, best to start slowly with the deworming program. Obstruction due to a massive kill of rounds and inflammatory bowel disease can result from deworming foals with heavy burdens of parasites so best is to start with half doses of dewormer and slowly building to a full dose. The schedules article has specific schedules for foals

Debilitated Horses

Debilitated adults may be heavily parasitized and require extra care when being dewormed. The killing and release of large numbers of parasites may cause a remarkable inflammation of the bowel so it is best is to begin with a half dose of ivermectin and if no problems occur give a full dose 2 weeks later.

Nasogastric tubing to deworm horses is unnecessary if you use good quality parasite treatments and administer them properly. Besides following the instructions carefully, we recommend you follow these instructions to insure the horse gets dosed properly:

• If in doubt about your horse's weight, tapes at the feed store can be had to approximate the horse's weight. These tapes are not terribly accurate so add 15% more to the findings. This is well within a safe dosage for the recommended dewormers.
• It is important that your horse be cooperative when dewormed. Those dewormers are bad tasting their are techniques of positive reinforcement that can train your horse to willingly accept the pasting. If you are having trouble consider positive conditioning to reshape your horses behavior, ...more
• Pastes dewormers work much better on a empty stomach. Though not essential, best is to stall for 6 to 12 hours without food before treatment and then left up to 2 hours before fed or turned out.
• Paste dewormers must be given when the mouth is empty and should be placed as far back in the mouth as possible. The horse should be watched for a few minutes to be sure he does not spit it back out.

The answer is not as clear as you would think. With aggressive use of products and strong anti-parasite management it might be possible to eradicate small strongyles in some herds of horses. The rising incidence of resistant parasites is making this harder all the time. But there is another problem to consider. Naturally-acquired immunity from parasite exposure is an important mechanism that helps horses deal with parasite exposure. To get and keep this immunity, the horses must be exposed to the parasites. Young adult horses without it, when exposed to even moderate to low levels of parasites, develop inflammation of the bowel and some have bouts of colics. In some experiments even death has occured in the naive horses when exposed to contaminated pastures. So even if we have the power to eradicate intestinal parasites, is it best? Only when you can insure that the horse will not be exposed to parasites in the future.