Baker Institute |  Animal Health

An Overview of Canine Heartworm Infections

Canine heartworms, scientific name Dirofilara immitis and members of the nematode family, were discovered more than a century ago and were once only a problem in southern climates. Today however they are recognized as a major global parasite affecting dogs, wolves, coyotes, foxes, and occasionally cats and ferrets. Because it is such a devastating disease, veterinarians actively encourage all patients to treat and prevent heartworm using a wide variety of drugs and medications. Thankfully, modern drugs are highly effective, but it is vital that every owner remain current with preventative medication for his or her dog.

Why and how might my dog become infected?

The heartworm requires two separate hosts in order to complete its six- or seven-month life cycle – a mosquito and a mammal such as a dog or a cat. Adult worms in already-infected animals live in the chambers of the heart and the pulmonary arteries. The adults reproduce and large numbers of microfilariae are discharged into the bloodstream. A mosquito ingests microfilariae when feeding on an already-infected mammal and then serves as an intermediate host for the larval stage of the worm. The larvae develop in the gut of the mosquito for one to four weeks and then migrate to the mouthparts of the mosquito and wait to be injected into a new host. As an infected mosquito pierces the skin of a potential host, the matured larvae enter the body, molt, and move to the chambers of the heart over the course of several months. Once the larvae reach the heart and/or pulmonary arteries, they mature into adult worms and may remain alive for up to three years. Heartworms may reach 12 inches in length, and a single, heavily-infested dog may harbor up to 250 parasites.

How is a heartworm infection diagnosed?

The first sign of a heartworm infection is generally a soft cough that can worsen with exercise but is often mild enough to be easily dismissed. As the number of adult worms increases however the cough will worsen and the dog will have more and more difficulty with physical activity. Infected animals tire easily, lose weight, and become weak and listless. The once-mild cough may worsen yet again and begin to produce blood. If heartworms are left untreated even walking will become impossible for the dog and death will eventually occur as a result of congestive heart failure.

Vena caval syndrome may occur when, left to their own devices, a mass of worms leaves the heart and becomes lodged in the posterior vena cava. This leads to total loss of appetite, collapse, and almost certain death.

If suspicion of a heartworm infection has been aroused, ultimate diagnosis is generally achieved via blood tests conducted in a diagnostic laboratory. The ELISA test is capable of detecting antigen even if microfilariae are not yet present in the bloodstream. If the disease has progressed to such a stage where microfilariae have been produced and are in the blood, a Knott’s test and filtering procedures may be used to isolate the larvae. Radiographs may also be useful as they can visually detect the presence of adult worms in the heart.

What are the treatment options for dogs with heartworm infections?

All but the most severe cases of heartworm can usually be successfully treated. The goal of treatment is to kill all living worms and all living microfilariae. Generally, those dogs with only mild symptoms and a lighter worm load will have the easiest road to recovery. The greater the level of infection (i.e. the greater the number of worms), the more complications might arise during treatment procedures. Before any course of treatment is begun, the individual dog must be examined to determine if he is strong enough to withstand the trauma of therapy.

Once the health of the dog has been established, the first step is to kill all adult worms using an arsenic compound. The drug is administered in two doses a day for two days, followed by a period of inactivity to allow the dog’s body a chance to absorb the dead worms. Three to four weeks after administration of the initial arsenic compound, the second step is to kill the microfilariae that are still living in the dog’s bloodstream. This requires a daily dosage of medication for seven days. At the end of the week, blood tests are done to see if microfilariae can be detected. If a cure has not been achieved, the dosage is increased until no traces of the larvae remain. Follow-up studies are recommended up to a year after initial treatment to ensure that all worms and larvae have in fact been killed.

Surgical removal of worms has been indicated in some extremely advanced cases of heartworm infection, but this is rare and carries inherent dangers. If your dog is diagnosed as having heartworms, your veterinarian is the best source of information regarding treatment and follow-up therapy.

What steps can I take to minimize the risk of infection for my dog?

Modern veterinary medicine has blessed us with a vast array of drugs to prevent heartworm infection in our dogs. The products come in many different forms, including injections, daily or monthly tablets, and monthly topical medications. Which form of prevention is right for your dog will depend upon where you live and your individual needs. Your veterinarian will be more than happy to provide you with the information you need to decide upon the proper course of prevention. What is most important is that preventative measures are taken, as heartworms can be devastating to your dog’s health.

Additional resources on the web:

American Heartworm Society
The definitive source for information on heartworm infection. Provides excellent descriptions of heartworm disease for a wide variety of audiences, from children to veterinarians.

For information on specific heartworm preventatives, visit the manufacturers' websites:

Proheart®6 from Fort Dodge Animal Health
Filaribits Plus® Chewable Tablets from Pfizer Animal Health.
Revolution® from Pfizer Animal Health.
Heartgard® and Heartgard® Plus from Merial.