Is There A Correlation Between Gulf Hurricanes And Heartworm Cases In Dogs?

At first glance, I know that you’re wondering….say what?  I thought that the problem with heartworm disease was that a larval-carrying mosquito bit a vulnerable dog (not on a preventive), and that was the way the process started.  What does wind and water have to do with this?  Aside from maybe a bit of heat and humidity and a transfer vector, what else does dirofilaria immitus need to thrive?

At the recent Triennial Symposium of the American Heartworm Society in New Orleans, Dr. Clarke Atkins, DVM, DACVIM (SAIM, Cardiology), addressed this seemingly off-beat topic.  After communicating with inland veterinarians in places like Shreveport, LA and Oxford, MS, Dr. Atkins found that positive diagnoses of heartworms and adulticide treatments became more prevalent in the year after hurricanes affected the gulf.  The core question of course is why?

According to Dr. Atkins, as we as coast people know, when there is a shake-up of our structures, like homes, fences and neighborhoods from damages, pets roam free.  Atkins called it post-storm “chaos”.  Dogs went free, and in the case of Katrina, many were rescued and relocated to inland shelters.  From that point, many were vaccinated, heartworm tested, dewormed and adopted out.  It is estimated that 250,000 dogs from the greater New Orleans area and up to 100,000 from South Mississippi were processed in this manner.  Of course, many dogs were found to be heartworm positive-so we’ll call this pre-existing disease.

Then, Dr. Atkins cites the point that refugia-native animal populations- contribute to the problems well.  In the Mississippi delta, other canine and canine-type hosts (wolves, coyotes, wild dogs, etc.) also enhance the spread of the disease.  Once again, just as with unprotected domestic dogs, these hosts are just as dangerous as any to propagating more potential for the spread of heartworm disease.

Then there’s water.  This is not to blame our coastal flooding but waters to our north.  The increased rain from the hurricanes contributes to not only standing water problems, but even significant rises in lake levels.  After the storm-laden hurricane season of 2005, Lake Sardis near Oxford even rose to record levels after Katrina, Rita and other named storms from that year.  Do mosquitoes love standing water?  Oh yeah.  There’s your connection.

Dr. Atkins had a chart that showed the lag effect of positive heartworm cases diagnosed after Hurricanes Elena (1985), Georges (1998), Lili (2002), Ivan (2004), Katrina (2005), Gustav (2008), and Isaac (2012).  The same chart showed the quiescent periods of lower heartworm diagnostic rates in the times when we had no hurricanes in the previous year.  Although there was a question as to whether the “delta” should be singled out as a local hotbed for heartworms, I personally think that the three main reasons Atkins cited stand on their merits whether we focus on that specific area of Mississippi or use broader parameters.

So what about the people who adopted and had these dogs treated?  Should pet owners in Massachusetts or Pennsylvania be concerned that the risk of heartworm disease goes up in their region once these dogs are transferred up?  Dr. Atkins says yes, because the presence of a heartworm positive dog greatly increases the potential for the disease, especially if the area is vulnerable (mosquitoes present and unprotected dogs).  That statement really re-framed my thinking, as for years I have always held that without the bite of a vector-host (mosquito), direct communicability of heartworms from dog-to-dog was never believed to be a problem.  My thinking has indeed expanded on this subject.

I guess this old dog needed to learn a new trick.

Have a great week with your pets!

Dr. Chris Duke

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