Changes in the Battle Against Heartworm

Every year you bring your dog to the vet, we do a heartworm test.  It’s habit and routine and most people are aware of heartworms, what they are, and what we do to prevent them.  Most people have never had a dog with heartworm, and may not remember the days of giving a daily preventive.  Over the last 20 years, preventives have changed and we only have to do a preventive monthly, or even every 6 months.  Since it is not as common as it once was, some people aren’t aware of heartworms and how serious it can be.  I want to take a little time to get a little more in depth on this devastating parasite and why we veterinarians feel it is so critical to prevent these parasites.

Heartworm is a worm that lives in the right side of the heart.  How it gets there is a mystery to most people.  Dogs are not the only animal that can get and carry heartworms.  Several wildlife species can also carry them.  An infected host (dog, wildlife) will have larva called microfilaria in their bloodstream.  When this host is bit by a mosquito, these larva are taken in by the mosquito.  Once ingested, they go through a molt phase and become a more mature larval form and hang out in the mosquitoes salivary glands.  When this mosquito takes a blood meal now, it will inject saliva into the victim, also injecting this larval form of heartworms.  If the new host is an appropriate one (like your dog) the larva will go though a maturation process and end up as adult heartworms in the right side of the heart.  If there are male and female worms, they will product microfilaria, so mosquitoes who bite your dog will now transmit heartworms from your dog as well.  Over time, if there are enough worms, they will take up most of the space in the right side of the heart and decrease the bloodflow.  They will also cause the right side of the heart to become enlarged.  Over time, your dog will become lethargic, often cough, have exercise intolerance, and end up in heart failure.  If you don’t mind a little graphic video, watch this showing surgical removal of heartworms in a dog with a severe heartworm burden. Davey’s Gift

If we catch the infection soon enough, we can treat them for having heartworms, but it is not an easy process.  The medication used is administered in 3 injections, spaced out over several months, the whole process taking about 6 months.  During heartworm treatment, dogs need to be strictly cage rested.  This means no walks, no playtime, nothing that will cause the heart rate to go up and dying worms to embolize to the lungs.

All dogs are at risk, no matter their lifestyle.  It doesn’t matter if your dog is an outside dog or a pampered pup who’s feet  never touch the grass.  Mosquitoes live inside as well as out and one of the more common dogs I’ve diagnosed over the years is the small and toy breed dog who’s owners didn’t think was at risk.

For decades we have not changed our prevention protocols.  Medications have changed a bit and how frequently we need to administer them has decreased, but we have always focused on treating the heartworm larva after the dog is infected.  Heartworm preventives work by eliminating the larva form, so if your dog was bit in the last month, when you give the dose today, it will kill those circulating larva that were contracted last month.  It is a retroactive medication and prevents the adult heartworm from developing, but not heartworms in general.  From 2013 to 2015 there has been a 166% increase in reported positive heartworm cases in the US.  It has been diagnosed in all 50 states and several foreign countries.  We are also seeing a strain of heartworm in the southern US that is resistant to our current preventives.

We have targeted other vector born diseases using a bi-modal or multi-modal approach for years.  Many are familiar with the permethrin empregnated bednets to block malaria transmission, lymphatic filariasis, and river blindness, all transmitted by mosquitoes and black flies.  We use topical sprays when we go outside to prevent mosquito bites and the different diseases they may carry, but we haven’t done much to prevent the bite on our 4 legged friends.  Current recommendations by the American Heartworm Society now include use of a repellent topical to prevent the mosquito bite, and environmental measures to cut down on mosquito populations.  Environmental prevention includes vectra mosquitopreventing standing water, treatment of the environment for mosquito larvae, and keeping pets indoors during peak feeding times.  A new research project on prevention of mosquito bites stopping the transmission of heartworms was recently performed using Vectra 3D and heartworm positive dogs.  I won’t bore you with the details, but in this study, Vectra 3D was 100% effective in blocking the transmission of heartworm larva to mosquitoes.  Mosquitoes basically refused to feed on dogs treated with Vectra 3D.  It’s not often we get 100% in anything, so this was a pretty astounding result.  For more information visit Fight Heartworm Now

There are lots of choices out there for preventives and they are not all created equally.  If you want to do everything to prevent your pet from getting heartworms, the combination of a heartworm prevention AND a repellent are your best bet.  We can help you determine the best combination for you and your pet based on your pet’s lifestyle and health history.  The cost of prevention is a fraction of the cost of treating a positive pet.  We are here to help you keep your pet happy, healthy, and live a long life!  If you have any questions, please let us know!


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