Heartworm Cases on the Rise - Is Your Hunting Dog Protected?
By J. Paul Jackson - Retriever Editor

It appears that heartworm disease is on the rise, and no matter where you live or hunt, your dog may be at risk.   In this issue we will take a look at this growing problem, and what you can do to protect your dog from this deadly parasite.

Heartworm disease is a very dangerous and often fatal condition characterized by the presence of adult heartworms (a roundworm known as dirofilaria immitus) living in the arteries of the lungs and the right side of the heart of dogs, cats and other mammals.  These adult heartworms cause disease by clogging the arteries and reducing blood supply.  They also can interfere with heart-valve function and increase the workload on the heart.  Left untreated in canines, heartworm disease makes dogs extremely sick and is often fatal.

Early symptoms of heartworm infection include a dry cough and shortness of breath, a loss of stamina and abnormal heart and lung sounds.  In later stages the dog will experience weight loss, congestive heart failure, swelling of the legs and abdomen due to fluid accumulation and eventually death.

Heartworm larvae can be passed from one dog to another only by the bite of a mosquito.  In fact, without mosquitoes, adult heartworms cannot exist.  In order to reach adulthood, heartworm larvae known as microfilariae must first develop for 10 to14 days inside the body of a mosquito.  This means that adult heartworms residing inside a dog cannot produce more adult heartworms within that animal without the help of a mosquito.

Once bitten by a larvae-carrying mosquito, the microfilariae continue to develop inside the dog resulting in adult heartworms.  Estimates vary on how long it takes from when a dog is bitten by an infected mosquito until a blood test can accurately detect the antigens produced by adult female worms.  The AHS says seven months, but an article published by the Center for Veterinary Medicine says “about six months.” 

The American Heartworm Society reports that heartworm cases have been reported in all 50 states.  A survey of 12,000 veterinary clinics conducted by AHS and Merial, maker of Hartgard, found that more than 250,000 cases of heartworm were reported in 2004.

Most cases occurred along the rivers and wetlands of the Mississippi Flyway and along the coastal areas of the southern and eastern portions of the country.  In other words, duck hunting country is heartworm heaven.

While most cases of heartworm can be successfully treated, it is a very rigorous and costly ordeal.  The treatment therapy involves giving the dog an arsenic compound (yes, I did say arsenic) to kill the adult worms followed by a heavy dose of preventative to kill the larvae.  To reduce the chance of death due to complications the dog must remain inactive for four to six weeks following the initiation of the treatment therapy.

To make matters worse, treatment is not cheap.  The price of treating an infected dog can range from $350 to $900. 

“An ounce of prevention” is a much simpler matter.  There are a number of monthly preventatives available by prescription from your local veterinarian.  The two most widely used are Heartgard (Ivermectin) manufactured by Merial, and Interceptor (milbemycin) produced by Novartis.  These preventatives cost about $6 per month and are given orally in the form of a meat chunk or chewable tablet.   

Preventative medicines should be given every month at the same time of the month.  Most importantly, they should be given year round.  According to Dr. Jimmy Crisp of the Dyer County (Tennessee) Animal Hospital, “some people think that there is no risk of heartworm transmission during the winter, but that is strictly not the case.  In most regions of the country there is a possibility of transmission throughout the year.  We have been instructing our clients to keep their pets protected 12 months a year”. 

This fact is particularly true in the southern regions of the country.  I have found mosquitoes alive and well in my kennel facility as late as December, and anyone who has spent much time in an Arkansas rice field pit can tell you that they are there on all but the coldest days.

While heartworm is often the result of improper medication administration, a branch of the Food and Drug Administration reports that may not always be the case.  According an article by Dr. Martine Hartogensis in the November/December issue of the FDA Veterinarian, the Center for Veterinary Medicine (CVM) has received, “an increasing number of reports of lack of effectiveness in products designed to prevent heartworm disease in dogs”. 

Dr. Hartongensis writes that the CVM database included 5,794 reports of lack of effectiveness of preventatives.  “It could be that owners are not properly administering the drugs, or some dogs might not swallow the pills or could later vomit and lose the drug,” wrote Dr. Hartongensis. 

“However, the database also includes 1,301 reports CVM specialists have analyzed and determined to be definitely related to product failure.  These cases were well documented concerning administration of the product according to the label, proper purchase history, and negative heartworm antigen tests prior to initiation of the drug and at least seven months after beginning prevention, followed by a positive antigen test”.

Heartworm preventative medications historically have been safe and effective.  In fact, FDA approval of heartworm prevention products requires 100 percent efficacy in the pre-approval clinical trials.

“However,” wrote Dr. Hartogensis, “the real world is not exactly like clinical trials. Real world conditions such as patient variability, geographic considerations and owner compliance may be contributing to the effectiveness problems with these products.

“In 2005, CVM’s Division of Surveillance asked the sponsors of all marketed heartworm preventatives to refrain from claiming 100 percent effectiveness in promotion and advertising materials…”

Representatives of both Merial and Novartis assured me that they do not believe the drugs are failing.  Carrie Simmons, field sales representative for Merial, told me that her company “believes [our] preventatives to be 100 percent effective in the prevention of heartworm disease in dogs when administered as labeled”. 

She also told me that she had no knowledge of failures for her product.  When asked if she thought heartworm disease was on the rise, Simmons said that she did not.  She did say, however, that “thanks to the antigen test, more cases are being detected and reported”.

According to Novartis spokesman Joseph Burkett, “there is no evidence that our drugs are failing.  Those (1,300) cases merely reflect that the product was administered correctly, but what if the dog vomits it up before it is absorbed?  In such a case the dog would still be susceptible to heartworms, but the product would not truly have failed”.

So why the increased number of heartworm cases?  I contacted numerous veterinarians in my area concerning this matter. Some felt that the increase in reported cases could be attributed in part to more frequent screening by pet owners.  Others felt that the parasite may be gaining a resistance to the preventative medicines. 

Says AHS, “...animals on heartworm prevention occasionally test positive for heartworms. This apparent lack of efficacy is usually due to owner compliance failure, travel or relocation of the animal to an area of active heartworm transmission, or unknown (or misdiagnosed) prior infection.  Annual testing gives owners peace of mind in knowing that their pet is free of heartworms.”

A number of these vets also told me that a research study was being conducted by Dr. Byron Blackburn of the Auburn University College of Veterinary Medicine.  In a phone interview, Dr. Blackburn confirmed that he was looking into the subject.  He said his research was ongoing, and that it would be a year or more before any solid scientific answers would be available.

Regardless of which of these factors are in play, the number one cause of infection remains non-compliance on the part of dog owners. 

To make sure that this is not the case in your home, Dr. Crisp recommends that you have your dog tested at least once a year, that you keep him on preventative year round and that you give the preventative the same day each month. 

He further states that you should always be sure that your dog is receiving the correct dosage.  More than one hapless owner has had their 70-pound dog test positive and then discovered that they were still giving the 25-pound dosage of preventive prescribed for the dog as a pup.

If the preventative medicines truly are 100 percent effective, this approach should be enough.  But just to be on the safe side, some kennels are taking a more aggressive stance toward prevention.  I know of two kennels that are giving the preventative twice monthly.  Others are giving two drugs at different intervals each month.

There are other options as well. As always, hunters should consult their veterinarians to determine which course of action is best for their dog.

And remember—for any plan to work, it must be implemented.