Heartworm disease is an extremely serious and potentially fatal condition caused by blood-borne parasites (Dirofilaria immitis) that live in the arteries of the lungs and in the right side of the heart.  Dogs are by far the most effected species and are considered the definitive host, however over 30 species of mammals have been documented with the disease.  Some of these animals include cats, both domestic and wild, ferrets, wolves, coyotes, sea lions, foxes, and in very rare cases, humans.  The disease in dogs has been documented for more than 100 years with the first published finding appearing in 1847 in an issue of “The Western Journal of Medicine and Surgery”.  It wasn’t until the 1920’s that the disease was documented in felines.  Since then the disease has grown to endemic proportions in some areas despite ever increasing awareness and prevention.

The transmission of heartworm disease is dependent upon mosquitoes and an infected host with adult heartworms.  In an infected animal, the adult worms produce offspring, called microfilariae that circulate through the bloodstream.  When the mosquito bites the infected animal the microfilariae are then transferred to the mosquito, where over the course of 14 days they undergo development to a larval stage.  When the mosquito bites another potential host the infected larvae are injected into the skin of the animal through the bite wound.  The larvae will continue to develop to adulthood within the heart and arteries of the host producing more microfilariae and in the process causing considerable damage to the host. In dogs, the larvae will grow to adulthood in four to seven months from the time of infection, while in cats the time frame is seven to eight months.  Typically in cats the life cycle ends at this stage and less than 20% of cats actually produce microfilariae.  Cats typically have fewer adult heartworms, as few as one in some cases, however in dogs the numbers are much higher with as many as 250 or more.


Heartworm disease is widely seen throughout the United States, and in dogs it has been documented in all 50 states.  The conventional wisdom used to be that it was primarily a southern or warm state disease but research has shown this to be false.  All dogs are susceptible to heartworm disease, as well as cats even if they are completely indoors as mosquitoes can come into the home.

In the early stages of the disease a dog may not show any clinical signs of infection; in mild to moderate cases a cough may be detected, as well as exercise intolerance.  Later stages of the disease may cause difficulty breathing, weakness, abnormal heart and lung sounds, and sudden death.  Cats, on the other hand, very rarely shown any outward clinical signs, which makes it more difficult to diagnose.  They may develop life long asthma as a result of exposure.  The more common signs are coughing and shortness of breath, but some cats may seem quite normal then suddenly die due to a reaction in the lungs to the heartworms.  The only sure method of diagnoses, in both cats and dogs, is a blood test.

Treatment of heartworm disease in dogs has progressed greatly over the past several years.  Where once the treatment was almost as risky as the condition itself, now we can successfully treat most cases.  Some statistics say as many as 95% dogs treated for heartworm disease survive it quite well.  Obviously, the more advanced the disease the greater the risk during treatment.  The most common method of treatment in dogs is by injection (complete protocols for Cole Park Veterinary Hospital listed below).

Although it is now possible to effectivley treat heartworm disease in dogs, it is quite intensive and costly, ranging from $600-$900.  It is certainly much easier to prevent, both from a medical standpoint as well as from a monetary one.  Unfortunately, there is no approved drug for treating heartworms in cats.  For cats, we can only treat symptomatically because cats cannot tolerate the drugs used to treat dogs.  Typically, the life span of a heartworm in a cat is around two years, so it is possible for the cat to outlive the heartworms.  During this time it is strongly recommended that the cat be put on approved heartworm prevention.

In North Carolina, it is highly advisable to keep your cats and dogs on heartworm prevention year round.  Heartworm disease is very prevalent in our area.  Unfortunately, we, at Cole Park Veterinary Hospital, see numerous cases of heartworm positive animals every year.  We strongly recommend testing your pet annually and keeping them on monthly heartworm prevention.  Please ask us about the various heartworm preventatives that are available for your cat or dog.

Cole Park Veterinary Hospital Heartworm Treatment Protocols

Heartworm treatment has five steps after the diagnosis has been made:

1.   The first step is a pretreatment examination in which laboratory tests and x-rays are used to evaluate your dog’s condition and ability to undergo heartworm treatment.

2.   The second step involves two days of hospitalization during which your dog receives two intramuscular injections over a 24-hour period.  This treatment is to eliminate the adult worms from the heart and is followed by an extremely important four week enforced rest period at home. 

3.   The third step is to return in one week for a recheck.  The doctor will listen to the heart and lungs to make sure there are no complications from the treatment.   

4.   The fourth step also involves a hospital stay, but only for one day.  During this step the dog receives an oral medication that eliminates the microfilariae from the circulation.  Your dog is kept hospitalized to observe for any reaction to the medication, which is rare.  This step occurs one month from the initial injection. 

5.   The fifth step follows four weeks after step four.  During this visit we will draw a small amount of blood and make sure that step four was successful in removing the microfilariae.  Once this test is negative your dog is then started on monthly heartworm prevention.

Although the treatment is between 90-98% effective, we do not consider your dog completely clear of heartworms until we do the first occult heartworm test and it is negative.  We recommend this test be approximately six months from the initial treatment. 
 

Our Office Hours
Monday, Wednesday & Friday - 7:45am-6:00pm
Tuesday & Thursday - 7:45am-7:00pm
Saturday - 8:45am-2:00pm
Sunday - Closed

For after hours emergencies contact: Veterinary Specialty Hospital  919-233-4911
Pet Emergency Treatment Service (TPETS) 919-489-0615