There is no reason why dogs should still be contracting heartworm disease!

In our day and age, there is no reason why any dog should be contracting heartworm disease.  For decades, heartworm disease has been among the most safely and effectively preventable diseases in dogs, yet we continue to see cases with regularity.

The domestic dog is a definitive host of the heartworm parasite.  Once a dog is infected with heartworm larvae, they will mature to adult worms in 6 months and begin replicating.  Left untreated, the worms will continue to colonize the heart as pictured above.  The end stage of heartworm disease is congestive heart failure, but many dogs succumb to the disease before that from complications such as thromboembolism (clots that form and cut off blood supply to vital organs) or vasculitis (inflammation of blood vessels).

Below in bold are false rationales that I commonly hear from dog owners for choosing not to treat their dogs with heartworm preventative medication.  I refute each of these false rationales in italics.

Heartworm prevention medications have chemicals that will harm my dog.

The safety of heartworm preventative medication administration in dogs has been long established since the 1990s.  There are a number of options for heartworm prevention that fall into a general category of compounds called macrocyclic lactones.  These compounds work by paralyzing the neuromuscular system of invertebrate animals – such as worm parasites – causing them to detach and be expelled from the dog.  Macrocyclic lactones do not cause any harm to the neuromuscular apparatus of vertebrate host – in this case the dog – especially at the tiny doses necessary to kill the microscopic infectious heartworm larvae that lead to heartworm. 

My dog is not at risk because he is never around other dogs

Heartworm disease is not spread from one dog to another.  It is spread by an infected mosquito feeding on a dog and inoculating him with microscopic infectious larvae.  Mosquitoes do not needto feed on another dog in order to become a carrier.  Feral cats, bobcats, and wild canids such as coyotes and foxes that mosquitoes readily feed on are all potential carriers of heartworm.  The bottom line is, wherever there are mosquitoes, your dog is at risk for heartworm disease.

If my dog gets heartworm, I will just put him on medication then

With conventional testing, heartworm is not detectable until 6 months after the initial point of infection when the larvae have matured to adult worms.  Adult worms cannot not by killed by macrocyclic lactones, so by this point, preventative medications are ineffective.

Heartworm is treatable, right? Why not just let him be and treat him if he gets heartworm?

Treatment for heartworm is a very involved and expensive process that is also painful for the dog.  The protocol begins with a series of chest x-rays and blood work to rule out any red flags that may negatively affect treatment or affect prognosis.  This is followed by a one-month course of an antibiotic called doxycycline, after which the dog comes in for his first treatment.

Treatment consists of an arsenic-based compound called melarsomine, administered by injection deep into the lumbar muscle. The dog is hospitalized for the day and monitored for adverse effects.

The dog is then discharged to the care of the owner, treated with a steroid, and must be kept confined and calm over the next month.  The dog then returns for the final two injections, each administered in the same manner 24 hours apart.

The injections cause significant discomfort at the injection sites for several days.  Complications could arise from the body’s reaction to the dead worms, thromboembolism, vasculitis, or toxicity from the melarsomine.

With all of these points considered, it simply makes no sense to opt out of treating your dog with a monthly heartworm preventative that has long established safety data and will spare him the morbidity of the disease, the risk of treatment, and the owner the expense and emotional toll of treatment.

Dr. Roger Welton is a practicing veterinarian and highly regarded media personality through a number of topics and platforms. He is the author of his top selling memoir The Man In The White Coat: A Veterinarian’s Tail Of Love. In addition to this writing this blog, Dr. Welton also hosts the popular Space Coast Pet Podcast.  He is the CEO and chief attending veterinarian of Premier Veterinary Care in Viera, FL.