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U.S. Department of Health and Human Services

Animal & Veterinary

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Focus on Learning: CVM Hosted Lecture on Heartworm and Associated Diseases

by Dr. Melanie McLean, Senior Writer/Editor, Communications

As part of the spring semester’s program of scientific lectures, CVM’s Learning Management Institute hosted John W. McCall, M.S., Ph.D., for a talk about emerging issues in heartworm disease and associated diseases.  Dr. McCall is Professor Emeritus in the Department of Infectious Diseases at the University of Georgia’s College of Veterinary Medicine in Athens, Georgia.  He is also an officer of the American Heartworm Society (AHS).    

Wolbachia and Heartworms

The first part of Dr. McCall’s lecture focused on the association between Wolbachia and the filarial nematode Dirofilaria immitis, better known as heartworm.  Wolbachia are Gram-negative bacteria that live inside the cells of many filarial nematodes, including heartworms. 

Filarial nematodes are parasitic, slender roundworms which typically require two hosts (the intermediate host and the definitive host) to complete the lifecycle.  Filarial nematodes live inside the intermediate host for a short transition period in order to become infective (capable of causing disease).  They reach maturity and reproduce inside the definitive host.  Heartworms use the mosquito as the intermediate host and the dog as the definitive host. 

Filarial nematodes cause various diseases in people and animals, such as canine heartworm disease in dogs.

Wolbachia appear to be necessary for many filarial nematodes, including heartworms, to develop, reproduce, and survive long-term in the definitive host.  When a filarial nematode dies inside the definitive host, Wolbachia are released from the nematode’s cells, exposing the host to the bacteria.  Researchers think that this exposure of the host to the Wolbachia bacteria worsens the severity of the diseases caused by filarial nematodes.

In dogs infected with heartworms, a protein found on the surface of the Wolbachia bacteria, called the Wolbachia surface protein or WSP, may cause the dog’s body to mount a specific immune response against WSP.  This immune response may worsen the dog’s heartworm disease.  Researchers think that WSP may also worsen the lung and kidney inflammation seen in dogs with heartworm disease. 

Heartworm Disease:  Spread, Prevention, and Testing

The second part of Dr. McCall’s lecture focused on the spread of canine heartworm disease and on preventing and testing for the disease.  Heartworm disease continues to spread throughout the United States for several reasons:

  • Relocation of heartworm-positive dogs, especially after Hurricane Katrina in 2005;
  • Natural and man-made environmental changes, leading to more breeding sites for mosquitoes and resulting in an increase in mosquito populations;
  • Introduction of non-native mosquito species, and native mosquito species expanding their territories; and
  • Enlarging pool of unprotected canid populations, for example, pet dogs that are not on heartworm prevention and wild canids, such as coyotes which are very susceptible to heartworms.  The more animals carrying heartworms, the more likely heartworm disease will spread. 

Although there are many FDA-approved products that prevent heartworm disease in dogs, many pet dogs in the United States are not on prevention.  The AHS recommends:

  • Starting puppies on an FDA-approved heartworm prevention product by eight weeks of age;
  • Keeping dogs on year-round heartworm prevention regardless of where the dog lives; and
  • Testing all dogs on prevention every year for heartworms. 

Dr. McCall discussed the accuracy of the various heartworm antigen tests that are on the market.  These blood tests detect specific proteins, called antigens, which are released by adult female heartworms into the dog’s bloodstream.  Antigen tests are highly effective at picking up infections with two or more adult female heartworms that are eight months old and older. 

Dr. McCall also discussed the timing of heartworm tests when a dog is switched from one heartworm prevention product to another or when there is a gap in dosing.  The dog should be tested for heartworms before the new product is started or the old product is resumed (called “Month 0”).  If the result is positive, the dog should be treated for heartworm disease and retested six months after treatment.  If the result is negative, the dog should be started on heartworm prevention and retested four months later.  If the Month 4 result is positive, the dog was infected with heartworms before Month 0 even though the Month 0 test was negative.  The most likely reason for the negative result at Month 0 is that the heartworms were too young to be detected by the antigen test.    

If a veterinarian waits more than four months between the Month 0 test and the retest, it is harder to figure out when the dog became infected with heartworms.  For example, if the dog is not tested until six months after starting the new product or resuming the old product and the result is positive, it is unclear if the dog was infected before or after Month 0.    

Is it Product Failure?

Lack of effectiveness (LOE) of a heartworm prevention product is defined as a dog testing positive for heartworms while on prevention.  But, as Dr. McCall explained, LOE is not always a clear case of product failure.  Possible reasons for LOE unrelated to product failure are: 

  • The dog was under dosed, for example, a fast growing puppy or a dog that gained weight;  
  • The owner gave the product too late or not at all;
  • The dog’s body did not retain the dose, for example, the dog vomited right after being given the dose;
  • The dog’s body did not absorb the drug, for example, a dog with a hyperactive gut due to diarrhea; or
  • The heartworms are resistant to the drug, however, some heartworm experts think that resistance is likely not a big reason for LOE of heartworm prevention products.   



For More Information

Learn more about heartworm disease by visiting: