What Is Heartworm Disease in Dogs?

Dirofilariaimmitis is the organism that causes heartworm disease not only indogs, but also in cats, ferrets, and other mammals. It is a large worm,reaching up to a foot or more in length, and as it completes its life cycle,which takes about six to seven months, it ends up in the heart and pulmonaryvessels, where it can live for several more years. As the heart becomes cloggedwith worms, there is less blood it can push out to the rest of the body, andheart failure can result.

Symptoms of Heartworm Disease in Dogs

The severity of the infection will berelated to the symptoms present, and symptoms of heartworm disease are relatedto the organs affected: the heart and lungs. Symptoms ofteninclude:  

·       Cough  

·       Lethargy  

·       Exercise intolerance  

·       Collapse  

·       Sudden death   

Some dogs may show weight loss, difficultybreathing, and even excessivepanting. Left untreated, dogs may go on to experience right-sided heart failure andascites (buildup of fluid in the abdomen).

Causes of Heartworm Disease in Dogs

Mosquitoes serve as the primary vector(carrier) for transmission of heartworm disease; transmission cannot occur fromone dog to another. As mosquitoes bite and take a blood meal from an infectedhost, they ingest circulating microfilariae, or young immature heartworms.Inside the mosquito, the microfilariae undergo three stages of larvaldevelopment (called L1, L2, and L3).  

When the same mosquito bites a dog, the L3is deposited onto the dog’s skin, and it then migrates into the dog’s body anddevelops into L4. Then as an L5, it migrates throughout the tissues andbloodstream, winding up in the heart, where it takes up residence as an adult.This entire process usually takes about four months to complete.

A few months later, around 7 months of age,the adult females become sexually mature, mate, and produce microfilariae. Thecommercially prepared tests to diagnose heartworm disease in the veterinaryhospital detect antigens (proteins uniquely found on the surface of an organismthat are used to detect the presence of that organism in the sample) producedby the female adult heartworm; that’s why testing usually starts around 7months of age.

How Veterinarians Diagnose Heartworm Disease in Dogs

Dogs 7 months and older should be testedfor heartworm disease at least annually. If the dog misses a dose ofprevention, then she should be tested more frequently. Testing is often done inthe hospital at the bedside and requires a small amount of blood.  

The most widely used method for diagnosingheartworm disease is antigen-based testing. Antigens are proteins uniquelyfound on the surface of an organism that are used to detect the presence ofthat organism in the sample. In this case, the antigens being tested for areproduced by the female adult heartworm, and if the test shows positive, thenthe dog is infected.  

Other tests that can be performed include ablood smear or a modified Knott’s test (often a test that is sent out fordiagnosis), which are done to check for the presence of circulatingmicrofilariae.  

Once diagnosis has been obtained, yourveterinarian may recommend more testing, which is used to find out the severityof the infection as well as the amount of risk involved for treatment. Othertesting often includes chest radiographs, EKG, blood pressure, cardiac enzymeevaluation (NT-proBNP), echocardiogram, blood work, and urinetesting.  

Class I dogs are those with the lowestamount of risk for treatment, and Class IV dogs are those often diagnosed withcaval syndrome and are at highest risk. This means the worm burden is so greatthat the worms are blocking blood from exiting the heart. These dogs are dyingand require surgical removal of the worms (often done by a specialist) tosurvive.

Treatment of Heartworm Disease in Dogs

Once your dog is diagnosed, yourveterinarian will most likely explain to you next steps including treatmentoptions, more diagnostics, and time frame for follow-upvisits.  

First, your dog should have his activitiesrestricted as exercise can increase the potential for the heartworms todislodge and cause clots elsewhere in the body. Additionally, if your dog hascirculating microfilariae in his bloodstream, mosquitoes, after ingesting ablood meal from your dog, can then transmit the parasite to others, so limitedexposure to the outside is recommended.  

Certain medications may be prescribed, suchas:

·       Steroids: to decrease inflammation created by the wormitself

·       Antibiotics: doxycycline is usedto kill Wolbachia, a symbiote organism that lives within the heartworm. Withoutthe symbiote, the host heartworm becomes easier to kill and secondaryinflammation is minimized.

·       Specific kind of heartworm preventive: to prevent younger wormsfrom developing into adults and to rid the bloodstream of any circulatingmicrofilariae   

An injection containing the arsenic-basedcompound melarsomine will be given to your dog 60 days, 90 days, and 91 daysafter diagnosis by the veterinarian. This is a medication designed to kill theadult heartworms and is usually administered in the lower back deep into themuscle. As it is painful, pain medications will most likely be sent home atthose visits as well.     

Prevention of Heartworm Disease in Dogs

The best way to treat your dog is to doyour best to prevent the disease in the first place with year-round heartwormprevention. The good news is that there are multiple types and forms ofheartworm prevention on the market, and they are all affordable.  

There are tablets, topicals, and eveninjectable versions that can provide anywhere from 1 month to 12 months ofprotection. There are even products that are combined with flea and tickcontrol to give your dog a more comprehensive preventive profile.  

All products are designed to kill the L3and/or L4 heartworm larvae, and some will clear the blood system fromcirculating microfilariae.  

If your dog tests positive for heartwormdisease, it is important to discuss the specific type of preventative neededwhile treating it, as there are only a few that should be given to minimizesecondary complications.  

You should speak with your veterinarian todecide the best type of prevention for your dog’s lifestyle and your budget.Limiting your dog’s exposure to mosquitoes will also help, but in some places,limiting exposure is nearly impossible. It only takes one infected mosquito tocause heartworm disease.  

Recovery and Management of Heartworm Disease in Dogs

Treatment forheartworm disease is not risk free. Dogs that undergo heartworm treatment asdiscussed above can suffer from anaphylaxis (shock), emboli (clots), and suddendeath, not to mention the possibility of abscess (pocket of pus) formation atthe site of melarsomine injection and the emotional distress from months ofexercise restriction.  

Dogs can also suffer from long-term healthrisks from the damage caused by the worms to their heart and lungs. Scarringand inflammation (swelling) generated by the worms makes it difficult for bloodto be pumped through the heart and lungs, and right-sided heart failure candevelop, even with successful treatment.

The degree of severity will affect theprognosis, and the sooner the disease is caught and treated, the greaterlikelihood there is for a good outcome. Unfortunately, dogs that suffer fromheartworm disease do not get immunity and are at risk for becoming infectedagain in the future. That is why year-round prevention is critical for yourdog’s health.

Always chat with your veterinarian firstbefore starting a new medication for your dog. They will help you determine thebest medication based on your dog's health.

Heartworm Disease in Dogs FAQs

Can heartworms affect humans?

Heartworm disease has primarily been knownto cause health-related issues in dogs, cats, and ferrets. Yet heartwormdisease can affect multiple mammals, and according to the CDC, that includeshumans. Fortunately, your dog is not contagious: the disease in humans isacquired from the mosquito itself and is not that common.

Can heartworms in dogs be cured?

Yes. If caught early and treatedappropriately, your dog may go on to have a good quality life. Unfortunately, some dogs may experience undesirable consequences either fromtreatment or from the disease itself and may end up with lifelong complications.

What are the first signs of heartworms in dogs?

Some dogs may not show any signs,especially if they live a more sedentary lifestyle. Others, however, may showexercise intolerance and coughing.

What is the survival rate for dogs with heartworms?

Dogs presenting with Class I heartwormdisease, and even Class II, have a much better prognosis and survivability thanthose in Class III and IV. Class IV dogs require surgery for lifesavingtreatment and will die without it.