*****scroll down for an excellent parasitism link from Pfizer
Parasite control, or "deworming" is an integral part of routine health care for the horse. The common intestinal parasites include large and small strongyles, pinworms, roundworms (especially in young horses), stomachworms, bots and tapeworms. The injurious effects of these parasites include competition for nutrients, sucking blood , tissue damage, intestinal obstruction and the elaboration of toxins. It is safe to assume that all horses have a parasite load and in most cases the horse is able to live normally with that load. Problems arise when either the host is stressed in some way or the parasite load is acutely increased or the pathogenicity of the parasites increases. When this occurs the horse may show external signs of parasitism such as weight loss, diarrhea, pot bellied appearance, reduced performance, rough, staring hair coat, colic, etc.
Traditionally horses have been dewormed on a consistent schedule that might vary depending upon the population density, management practices, pasture acreage, etc. It has also been a common practice to rotate the type of dewormer being used to attempt to prevent the occurrance of parasite resistance. Commonly used deworming medications include ivermectin (by far the most commonly used dewormer), fenbendazole (Panacur), pyrantel pamoate (Strongid), moxidectin ( Quest) and praziquantel (specific for tapeworms). Recent research and monitoring has shown that there is definitely current and developing resistance of some parasites to certain medications. This is evidenced both by the inability of a given dewormer to kill a given parasite but also a decrease in the amount of time that a particular parasite might return to pre deworming population levels.
Currently it is recommended to continue to deworm horses on a routine basis and to alternate the product being used. Another method is to monitor the parasite load by fecal examination; either by routine qualitative parasite egg identification (done by the veterinarian) or more specific quantitative fecal egg counts that must be done by a reference laboratory). This allows for a more targeted approach to parasite control.
Management practices such as reducing population densities, pasture rotations, fecal waste removal, feeding off the ground, etc. are also important in a comprehensive parasite control program.
The Pfizer corporation has a useful website on various equine topics.
see link below
One example is parasitism with some pics, technical info, and advice.
