I'd think that if thousands of dogs had died after being given Trifexis, it would be in the scientific literature. I also agree with whoever it was that cautioned against taking everything you read on the internet as gospel. As Mark points out, there are other ingredients that can cause reactions. If you live in the south, you are in essence playing with fire if you don't use HW preventives. Thank you Mark for adding some well-reasoned comments to this discussion. If your dog is one of the few that has adverse reactions to one monthly heartworm preventative (one manufacturer's drug formulation), work with your vet to find another (different drug formulation) to which your dog does not have adverse reactions. Studies continue as to the source(s) of the loss of effectiveness.Īs someone else pointed out, you can treat with monthly preventative to kill of microfilariae or you can gamble your dog won't get bitten by an infected mosquito and then have to treat with a much more dangerous drug and the risks associated with killed heartworms being pumped through your dog's heart. This may be related to the percentage of mosquitos that are infected with microfilariae, the level of infection of microfilariae in infected mosquitos (dose of microfilariae being given to dogs), or the genetics of the microfilariae (or a combination). There is evidence that in some regions the monthly heartworm preventatives are not 100% effective. This phenomenon occurs with vaccines which are also complex formulations with many ingredients. The dog could have an underlying condition that predisposes it to react to one of the other ingredients. Some dogs can and do react to these other ingredients just because a dog reacts to one of these drug formulations (given within the safe working range) does not automatically mean the dog reacted to the active drug. Heartworm preventatives are not just the active drug they all contain other ingredients (a drug formulation). This gives the safe working range for each drug the wider the working range the safer the drug. ![]() ![]() The key number to compare is the difference between the minimum effective dose and the dose at which adverse reactions occur or the multiple of the minimum effective dose at which adverse reactions occur. Each drug has a unique minimum effective dose (microgram/lb body weight). Just because one drug has a lower minimum effective dose (microgram/lb body weight) does not mean it will be less likely to cause an adverse reaction than another. The duration of effectiveness of the monthly preventatives is not related to the drug but to the lifecycle of the microfilariae maturing into heartworms.Īll heartworm preventatives are in the same class of drug they all work the same way. Heartworm preventatives prevent a heartworm infection by killing off microfilariae before they can mature into heartworms dogs still have a microfilariae infection. I have a few thoughts to add to this discussion.
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