Heartworm medication side effects

Question
We started giving Bailey ( our 4yr old male Irish Setter) the  
Heartfard Plus medication for dogs 51-100lbs (he is about  
85lbs). He received his first monthly dose in Aug. Early  
September he suffered a violent seizure. 6 weeks later he  
suffered another and for the last two weeks he has had one  
a week.

The only corresponding element that we can figure is the  
administering of this medicine. We are very concerned as  
he has been very happy and healthy until we started this  
medication. He is AKC registered, and has championship  
bloodlines.  

Are there any serious side effects of this medication and  
are there alternative treatments for heartworms?  

Thanks for your help.  

Answer
You should have blood work checked to rule out underlying medical problems such as liver disease, underactive thyroid, electrolyte imbalances, low blood sugar, low calcium levels and elevated white blood counts as well as screen for Lyme Disease which could all contribute to seizures.  

Epilepsy will usually start in young to middle aged dogs and most likely is coincendental as far as the Heartgard as the medication because it is only in his system for 48 hrs.  

I would suggest you contact Merial in regards to the medication side effects and take him to the vet for blood tests.  At my practice we use Interceptor, which is a different form of heartworm medication, but again unlikely to cause seizures.

If the seizures are that violent and weekly, then he needs to be placed on anti-seizure medication.  But blood testing must be done first to rule out other causes.  I would recommend Potassium Bromide versus Phenobarb since he is young.