Collie breeds have been shown to be more sensitive to heartworm medications than most other breeds of dogs. This leaves them more susceptible to toxic reactions to these drugs. Research has shown that the reason for this is a genetic defect in the brains of many Collies. This mutation affects the part of the brain that stops certain drugs from building up in their system.
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Genetic Research
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A study involving 40 healthy Collies showed that 35% of the tested Collies were affected by this genetic defect, 42% were carriers of the defect (the mutation is recessive and does not always show through), while the remainder were free of the problem. It is because of this mutation that Collies (and certain other breeds) can suffer from neurologic toxicosis when heartworm medicines such as ivermectin, milbemycin, and moxidectin are administered in high enough doses.
Toxic Reactions to Heartworm Medication
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Early signs of neurologic toxicosis can include salivation, vomiting, dilated pupils and difficulty walking. More serious symptoms can be presented by an inability to stand, unresponsiveness and coma. Almost all toxic reactions in Collies, caused by heartworm medications, have occurred because the canines in question have received unusually high doses.
Ivermectin
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Ivermectin is one of the most popular heartworm medications for dogs. It is the main active ingredient in the Heartgard and Heartgard Plus brand. A typical monthly oral dose of ivermectin in this brand is 6 to 12 micrograms per kilogram of body weight, which is enough to stave off heartworm and other parasites. Ivermectin is available in larger doses, however, because it is also used in cattle for various purposes. As a result of trying to save money many people have tried to use the stronger dose but give smaller portions to their dogs. Unless their calculations are precise (and often they are not), this can lead to the dog receiving an overdose, thus causing a toxic reaction.
Milbemycin
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Like ivermectin, the heartworm medication milbemycin is a once a month oral dosing. Milbemycin is the active ingredient in the Interceptor and the Sentinel brands. This drug is also considered safe for use on all dogs as it has a large safety range (roughly the same as ivermectin). Problems such as neurologic toxicosis do not occur in Collies until the dose reaches about 10 times the normal recommended dose. Most other dogs don't experience any issues until the dose reaches 100 times the recommended dose. This makes Collies more susceptible than other breeds to toxic reactions, but ivermectin and milbemycin are still considered safe and effective heartworm medications due to this large safety range.
Moxidectin
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Moxidectin is an active ingredient in the ProHeart brand of heartworm medication. ProHeart is different than Heartgard, Sentinel and Interceptor in that it places heartworm control solely in the hands of the veterinarian. This is because it is a six month sustained release injection that is only available to veterinarians. This means that Collie owners only have to worry about heartworm.
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