What is heartworm and why does my vet recommend testing and preventatives for it?

April is heartworm prevention month. Heartworm disease is a potentially fatal disease in pets that is transmitted by the bite of mosquitoes and leads to heart failure, significant lung disease, or damage to other organs.  These worms can grow to be a foot long and reside mainly in the animal’s heart and lungs.  Heartworms cannot be seen in the feces as they are found in the circulatory system – this requires a different test than the yearly stool sample!

Transmission of heartworm occurs through mosquitoes. A mosquito bites an infected dog, wolf, coyote, or fox and ingests the immature worms circulating in the blood.  The mosquito then can transmit heartworm through its bite.  Once the dog is infected, if another mosquito bites it, the transmission will occur again.

According to the Companion Animal Parasite Council there were 1,336 cases of heartworm just in Pennsylvania dogs in 2015.  Each of these has the potential to be bitten again by a mosquito and continue to spread the infection.  Adding to that – numerous dogs each month are being transported into Pennsylvania for adoption.  Since Katrina, transportation of southern pets for adoption is how the Pennsylvania heartworm incidence has grown.

We all get bitten my mosquitoes – and yes, they do come in the house, so all dogs are at risk.  Thankfully, in Pennsylvania, cats seem to have a lower risk.  Mosquito bites cannot be prevented but progression of the heartworm disease can. The medicine in your monthly heartworm preventative kills the immature stages of heartworm.  It is important not to miss a dose because if any of those immature worms reach adulthood then your pet’s treatment options change drastically.  This is why we recommend year-round administration of heartworm prevention.  As an added bonus, heartworm preventatives also protect your pet against several kinds of gastrointestinal parasites.

Why test for heartworm yearly?  The earlier in the disease process that heartworm is caught, the better chance your dog has of surviving.  If you are giving prevention monthly, your dog has minimal chance of becoming positive. However, if you happened to be late giving a dose, missed more than one month, or maybe your dog had an upset stomach one of the days it was given – they may not be fully protected. We want to know if it is safe to continue preventatives or if we need to change gears and treat your pet.

If a positive heartworm test is confirmed, treatment itself comes with its own risks to your pet and can be quite costly.  Treatment medications work to clear the infection and to decrease the potential of severe adverse drug reactions.  At the center of treatment is a series of injections of melarsamine (Immiticide), which is an arsenic-based compound.  Sounds scary, but that is how serious heartworm is – it takes a very strong drug to kill. During treatment the dog must be restricted from all activity to reduce the risk of damage from the dying worms moving in the bloodstream.  The option of treating heartworm disease with preventatives is not recommended because waiting up to 7 years for those adult worms to die can cause a lot of internal damage.  The American Heartworm Society only recommends use of melarsamine as the treatment.  This treatment can cost well over $1000 if all goes well with no adverse reactions.  A year of heartworm prevention only costs between $70 and $125 depending on the brand of prevention and size of your dog.

The option is yours – you can spend between $6 and $11 a month to protect your dog from a horrible disease, or you can take your chances that worms set up residence in your pet’s heart.  If those worms do set in, the treatment can be dangerous and quite costly.   Please remember to protect those hearts that love you so much!

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