Heartworm Prevention and Treatment
Pensacola, Florida

1. What are they?
a. Heartworms are a blood parasite. Adult heartworms live in the heart. If a male and female are present, they will mate to produce baby heartworms, called microfilariae. These microfilariae invade the bloodstream. Microfilariae are what we normally check for when we do a heartworm check during a hospital visit.

2. Where do they come from?
a. Heartworms are carried by mosquitoes. Mosquitoes are found in the South all year round. The heartworm begins its development inside the mosquito (intermediate host) and completes its development into an adult in the dog. When a mosquito bites a dog, it injects the developing heartworm into the dog’s skin, where it migrates along the blood vessels to the heart and completes its development into an adult. This entire process takes about six to seven months to complete.

3. How can I prevent them?
a. If your dog is put on preventative before 5 months of age and continues without fail, then the dog will not get heartworms. If your dog is not on preventative and is over 6 months of age, it must be tested (a small blood sample is obtained) and checked for microfilariae. If the test is negative for microfilariae (none seen), then you can safely put the dog on preventative. If the blood sample is strongly positive for microfilariae, then you should not put the dog on preventative because it may cause illness or death.

4. Can you get rid of the worms if my dog has them?
a. Yes, for most heartworm-positive dogs, there is an inexpensive, safe treatment performed by Scenic Hills Veterinary Hospital. The course is a minimum of four treatments, at two weeks apart. Once the microfilariae are cleared, the dog can be safely started on heartworm preventative.

5. Can cats or people get them?
a. Healthy people and cats are naturally immune. However, there have been cases where immune-suppressed individuals have gotten heartworms.

6. Preventatives?
a. Heartgard® is the most effective and only preventative that we recommend. Heartgard® will also help prevent intestinal parasites (whipworms, roundworms and hookworms) and is cost-effective. Although other heartworm preventatives are available, none are as effective, safe and economical as the one described above. Because other preventatives are not as effective, safe or economical, we do not recommend them.

Policy regarding the dispensing of heartworm preventatives

The following policy is provided to help answer questions regarding the dispensing of a heartworm preventative for your pet.

Legal requirements 
All heartworm preventatives are labeled prescription drugs, requiring veterinarians to follow federal and state regulations for prescribing and dispensing them. These regulations require a valid veterinarian-client-patient relationship, which necessitates a patient record and recent physical exam.*

Medical guidelines
Heartworm blood tests are performed on all dogs over 6 months of age. The test results are useful in minimizing the risk of adverse reactions and in assessing the patient’s infection status. If heartworm embryos (microfilariae) are present in the blood in large numbers, heartworm preventatives can cause a potentially life-threatening reaction at the start of treatment, or when treatment is resumed. A short lapse in administering monthly preventatives (less than one month) does not present an undue risk in resuming their use. Heartworm preventatives are very effective, but each year, we detect infection in dogs that have been receiving preventative therapy. These infections are usually due to known or suspected non-compliance in administering the preventative, or to the incomplete consumption of the preventative.

At Scenic Hills Veterinary Hospital, our primary concern is to dispense heartworm prevention medication in a manner that ensures your pet’s safety and protection while also complying with state and federal laws. To achieve that goal, we have adopted the following guidelines for pets receiving heartworm preventatives:

  1. A current patient record must be on file.
  2. The pet must be examined at least every 18 months.
  3. A visiting client must bring their pet to the hospital for a heartworm test and establish a veterinarian-client-patient relationship.
  4. A current heartworm test is required before preventative therapy can be initiated. This test may need to be repeated before renewing the prescription. To be refilled, our records must show that the amount purchased is appropriate for dosage compliance.
  5. Any deviation from these guidelines must occur within the context of a veterinarian-client-patient relationship. A staff veterinarian will be happy to discuss this policy with you.

*Florida Statutes 472.214(1) (y) 1 and 474.2165, Florida Department of Professional Regulation. Board of Veterinary Medicine. January 1993.

Canine heartworm treatment

Heartworms live in the heart. Until a large number of heartworms collect, there are no symptoms. When symptoms develop it is too late to treat safely. Those dogs that survive treatment seldom recover completely. Dead heartworms are even more damaging than live heartworms. A large number of worms dying at once can cause the death of the patient. Dogs generally tolerate one live heartworm per pound of body weight without symptoms, but dead worms are about 10 times as dangerous. A 50-pound dog would die if even 10 worms died suddenly. Most cases of heartworms are detected quite early, but there are not tests that will accurately determine whether a dog has five or 50 heartworms.

There are three options if your dog has heartworms:

  1. No treatment: If you elect no treatment, and your dog gets no more heartworm larva from mosquito bites, your dog could still live a normal life and could actually free itself of heartworms over a period of several years. Without treatment, it is more likely that your dog will continue to get more heartworms until it finally develops symptoms and irreversible damage has been done.
  2. Extreme treatments have been used in the past when alternatives were more limited. These medications contained arsenic and are poisonous to both heartworms and patients. Immiticide is now the only drug of this type on the market. The problems with this drug are twofold. The drug itself can kill or severely weaken your dog. In cases where heartworms are killed, the added stress of even a few dead worms can kill your dog. In cases where many heartworms are killed at once, the dog will probably die of pulmonary emboli. The death rate using immiticide in dogs with minimal numbers of heartworms is still one or two percent. The death rate of dogs when this drug is used with moderate heartworm infections approaches 20 percent. Using immiticide, the death rate of dogs with severe heartworm infections is about 100 percent. Most dogs treated this way are antigen positive for many months and even years.
  3. Mild treatment is what we recommend for nearly all dogs with heartworms. This treatment uses Ivermectin as the active ingredient, and other drugs to decrease the reaction to dying microfilaria. Ivermectin is extremely non-toxic. The dosage is begun at a low level and increases at each subsequent treatment to help prevent the death of a large number of heartworms at one time. When the treatment is completed, a blood test for microfilaria will assure that heartworm prevention can be safely started. Heartgard® is the prevention recommended for dogs that have been treated, because it contains Ivermectin. Ivermectin also kills the adult heartworms over a period of several months.

Experience in this hospital, with treatment of over 6,000 cases of heartworms, has shown that the mild treatment is the treatment of choice. Data presented at the 1995, 1998 and 2001 Heartworm Symposiums shows that repeated doses of Ivermectin rid a dog of adult heartworms over a period of about 18 months. This low rate of heartworm death is tolerated without stress or symptoms. Although you should watch for any abnormality with your pet, the incidence of problems is only about one percent. The only problem with this treatment occurs when several heartworms die at once. This seldom happens, because the medication used is so mild. If it does happen, it generally can be treated because the dog is not weakened by a drug containing arsenic. The death rate in mild to moderately affected dogs with this type of treatment is less than one in a thousand. This treatment takes four to five visits, spaced at two-week intervals, but the interval can be from 10 days to three weeks. As with arsenic treatment, antigen tests can be positive for over one year. Remember that non-stop prevention should be continued for at least three years, even if you move to an area that may not normally use year-round preventative.

Any heartworm treatment is a waste of your time and money unless you are prepared to make sure that an effective schedule of heartworm prevention will be continued. Heartgard® is the recommended prevention after any heartworm treatment. Heartgard®, at the preventative dosage, acts on adult heartworms to help clear the infection. It is perfectly safe to use on dogs that are antigen-positive provided the microfilaria have been cleared. After any heartworm treatment, the antigen heartworm tests have a greater incidence of false positive results since the test is for antigens that are released from dead heartworms as well as live heartworms. In some cases, emboli of dead worms can take years to completely dissolve.

Heartworm preventatives are prescription products, because there are associated hazards if used incorrectly; when used correctly, they are very safe. All preventatives have warnings about usage on dogs that have adult heartworms. This warning actually concerns use in dogs with very high levels of microfilaria. The presence of adult heartworms by themselves does not cause problems with preventatives. Adult heartworms produce baby heartworms called microfilaria, which, in high numbers, can cause reactions when preventatives are begun. If preventatives are stopped for more than a couple months, microfilaria counts can increase to a hazardous level. A microfilaria heartworm check is needed to ensure safe resumption of the medication. Scenic Hill Veterinary Hospital keeps complete records of each patient’s heartworm preventative usage to ensure safety and effectiveness. This record-keeping is required by Florida law.

Prevention prescription note
We perform free heartworm and fecal checks with most visits in order to make heartworm prevention use more affordable and safe. We sell the safest and most effective preventative, at a minimum markup, which includes any prescription fee. We do not prescribe combination products unless a recurrent intestinal worm problem exists because drug resistance and allergies can occur. At the time of a paid exam, a prescription for purchase of Heartgard® elsewhere will be written for no charge and a qualified refill within a year will be written for a reasonable, legal prescription fee. However, if a client wishes to purchase Heartgard® elsewhere, we will no longer be able to provide any services to any pet belonging to said client in the future.