Signs of a Heartworm Medication Overdose
Heartworm preventatives belong to a family of drugs called the avermectins. As Parasitipedia explains, in overdose situations, these drugs interfere with nerve transmission and cause the central nervous system to respond abnormally to stimulation. Typical symptoms include:
If your dog displays any of the above symptoms, especially after an overdose of heartworm medication, contact the vet immediately. While there is no antidote to toxicity, the vet may be able to give medications to minimize further absorption of the active ingredient and to reduce the effect of the toxicity.
Several breeds have a hereditary sensitivity to ivermectin, but the following breeds are the most commonly affected:
A mutation in the MDR1 gene is responsible for this genetic susceptibility. This mutated gene may also make the dog more susceptible to a variety of other drugs. Individual dogs of the above-mentioned breeds do not all possess the mutant gene. Testing is the only technique to determine whether a dog possesses the mutant MDR1 gene. To perform the test, cells from inside the dogs cheek are scraped and sent to a lab for genetic analysis. If youre interested in getting your dog tested, talk to your veterinarian.
Your vet is always the best source of information pertaining to your specific circumstances and the information below is provided for general interest only. Follow all veterinary advice and do not attempt to treat your dog without guidance from your veterinarian if you suspect an overdose has occurred.
The greatest risk with expired medication is that it has lost some of its effectiveness. Therefore, if the dog eats a recommended dose of an expired med, contact your vet to inquire when you should give the next dose of in-date medication. The answer will vary depending on the conditions in which the medication was stored and how out of date it was.
Because the medication will have some residual action if the dog has an overdose, contact the vet as you would if an overdose of an in-date medication had been given, and proceed accordingly. Using out-of-date medication means there is a risk the dog was not protected and may have picked up heartworm in the interim. You must discuss this with your vet because treating with a preventative if the dog has adult heartworms could make the dog very sick.
Collies, in particular, are a special case. Some Collies have a genetic mutation which makes their brain more vulnerable to the effects of ivermectin medications. Always contact your vet immediately if your collie has an overdose. Plumbs Veterinary Drug Formulary suggests signs of toxicity are most likely to occur with doses in excess of 1 milligram per kilogram of body weight (equivalent to 1,000 micrograms per kg of body weight). Thus, a 10-kg (roughly 22 pound) dog may show toxicity when exposed to 10 mg (or 10,000 micrograms) of ivermectin.
A single dose of Heartgard Plus for a dog weighing less than 25 kg contains 68 micrograms. Thus, the same 10-kg dog would need to eat around 147 tablets for an overdose to occur. However, some dogs are more sensitive to the active ingredients than others. Never be complacent and always seek veterinary advice if the dog may have had an overdose.
What’s In Heartgard, Anyway?
Heartgard contains two active ingredients: Ivermectin and pyrantel pamoate.
Ivermectin is a chemical compound that is originally derived from fermentation processes of organisms that reside in the earth and soil.
First discovered on a Japanese golf course, it is now used in heartworm medications such as Heartgard and Iverhart to kill heartworms at the larval stage when they haven’t yet matured and migrated into the arteries of the lungs.
Not only is ivermectin used in tiny doses to kill heartworms in a dog’s body, it is also used in much higher concentrations (think 50-60x!) in other medications to combat tough parasites such as demodectic and sarcoptic mange.
Pyrantel pamoate is used as a dewormer in Heartgard, making it a complete protective solution by targeting stomach parasites such as hookworm and roundworm.
It works by paralyzing and killing worms in the stomach and intestines, and renders them unable to attach themselves to intestinal walls- resulting in expulsion in the feces.
It can take a few hours of digestion before the active ingredients ivermectin and pyrantel pamoate provide relief against heartworms.
Though rare, some dogs may experience mild side effects even with proper dosage of heartworm medication (1.1% of administered doses according to studies). These side effects can include vomiting, diarrhea, lethargy, and in serious cases seizures and allergic reactions.
Dogs may also experience a loss of appetite and a higher frequency of diarrhea and vomiting for the next 24 hours as parasites are eliminated from the body.
Some dogs are also naturally predisposed to experiencing reactions after ingesting heartworm medication- most commonly herding breeds like Collies and Heelers. Even still, adverse symptoms are rare and most frequently occur only when an overdose has taken place.
Can I Give My Dog A Higher Dose Of Heartgard?
Heartgard is a very safe product, and even if you give your dog a higher dose it most likely won’t cause any health issues.
With that said, there is really no need to give your dog a higher dose of Heartgard.
It won’t make the medication any more effective than if you give it in proper amounts, and it is always best to follow medication dosage guidelines in case your dog does have extra sensitivity to the drug.
If it’s a situation where you have truly forgotten whether you have given your dog Heartgard this month (see below section), then the truth is that it may be safer to give it a potential extra dose of Heartgard than to miss a dose.
This will ensure that your dog remains protected from heartworm infection, while the risk of medicinal side effects is kept small.
However, consult your vet first before giving the extra dose, and if your dog does display symptoms of diarrhea, vomiting or lethargy, take it to the animal hospital immediately for an examination.
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