Cat Wolf is a veterinary technician who works in a small animal clinic with the occasional exotic patient.
What Is Heartworm?
Like many pet owners, you probably have heard of fleas, ticks, mites, and other nasty intestinal parasites that can afflict your fuzzy (or scaley or feathered) friends. One parasite in particular, however, is absolutely terrifying. It's as deadly as rabies, but with a very quiet means of infiltration. What could this terrible species be?
They're called heartworms, or Dirofilaria immitis, a species of roundworm that infects the cardiovascular system of its primary host. Infection is spread by mosquitoes—its secondary host—which means it's far more prevalent in places that are warm, wet, and otherwise tropical. However, that doesn't mean that living in a desert will protect your pet!
Heartworms cause (creatively) "heartworm disease," which is difficult to detect during the first six months of infection. After being bitten by a carrier mosquito, the unlucky host carries the microscopic larval worms in its bloodstream until they reach the lungs and heart. (Rarely, heartworms can become "lost" and end up somewhere outside of the usual destinations, such as the eye or a different major artery elsewhere, causing unusual and complex symptoms such as blindness.)
As heartworms mature in the heart, they cause more obvious problems, including (but not limited to) coughing, fatigue, exercise intolerance, and other symptoms generally associated with heart complications. As cases advance without treatment, the tissue of the lungs may begin to break down and cause the afflicted animal to cough up blood. The patient may lose weight and become weak or may develop fainting spells. Eventually, a significant infection will cause congestive heart failure, and the patient will die.
How Do I Know if My Pet Is Infected?
Heartworm is easy to detect. There's a simple test for both dogs and cats that only requires three drops of the patient's blood and about ten minutes. It's a simple "snap" test, which means it's a small plastic indicator on which there are small dots that will react with various proteins, antigens, and other markers for various infections. (The one we use in my clinic for dogs also tests for Ehrlichia, Anaplasma, and Lyme diseases, all tick-borne diseases.) Based on what dots turn blue at the end of the test, your veterinarian or veterinary technician will be able to determine whether or not your pet is positive for heartworm infection.
Three drops of blood are mixed with four drops of conjugate, then poured into the "well" of the plastic unit. The mixture then runs up the "window," covering the indicator dots. Once it covers the window completely, the technician "snaps" the test downward, breaking the fluid-filled compartment housed on the underside of the plastic unit and causing the fluid to push the blood mixture back toward the well. Any blood containing the appropriate antigens then slowly turns blue on the corresponding dot. Don't panic if you see one blue dot! Most tests have a "control" indicator to prove that the test is working correctly.
What if My Pet Is Infected?
When it comes to heartworm, treatment is much more difficult than prevention. If your pet tests positive for heartworm, be prepared for a long and difficult journey.
Veterinary Heartworm Treatment in Dogs
For dogs, there is a treatment available that takes nearly a year to complete. First, your veterinarian will likely prescribe an antibiotic and steroids, as a particular species of bacteria often carried by heartworms has shown some association with the inflammation brought about by heartworm disease. After that, there will be at least three injections into your dog's lower back muscles (which are about as unpleasantly painful as you'd imagine they would be), each no more or less than 30 days apart. After the injections are administered, the dog must be kept under close hospital monitoring to ensure that there is no adverse reaction to the injections.
After that ordeal, the patient must then be kept on strict kennel rest for no less than six months to ensure that they don't develop complications from the dead worms potentially lodging in important blood vessels. Then, they will need to be retested for heartworm disease, which, if they still have larvae in their system, they very well could still have. If so, they will require more injections and more kennel rest.
All of this treatment costs thousands of dollars, and it's not guaranteed to work. Sometimes, the infection is so severe it cannot be treated in any other way than by surgically extracting the adult heartworms, which costs even more and has a fairly high risk of complications. It is, however, a better alternative than not treating the animal at all.
Veterinary Heartworm Treatment in Cats
Cats are a different story. Heartworm disease affects cats differently, as they are not the target species. Cats have a remarkable inflammatory response that actually tends to kill off most heartworm larvae. This means that instead of 25-40 adult heartworms, they are more likely to develop two or three.
Cats are also more likely to have the larvae become "lost" and end up in non-target organs and areas of the cat's body. Due to the nature of the inflammatory response, symptoms vary from coughing, difficulty breathing, and fatigue to vomiting, anorexia, blindness, and convulsions. Cats are far more likely than dogs to suddenly die from heartworm disease without warning.
Given all this, and given that cats are more apt to throw false-negatives on anything other than an antigen-test, the American Heartworm Society recommends trying to wait out the 2 to 3-year lifespan of the heartworms. This, of course, is provided the cat does not appear sick, in which case you would have had to diagnose the problem asymptomatically.
If the cat appears to be suffering from effects of the inflammatory response their body has mounted against the heartworms, your veterinarian may suggest steroids to help alleviate distress. Otherwise, the only other real option to treat an active heartworm infection in cats is to administer the same medication used for dogs . which is notoriously toxic to cats and can cause lethal damage on its own.
So, What Can I Do?
Luckily, heartworm prevention is a far simpler matter than heartworm treatment. If you stay ahead of heartworm, you'll save a lot of money and effort.
Heartworm Prevention for Dogs
For dogs, there are once-a-month tablets they can consume that will release medication into their bloodstream and interfere with the nervous systems of any developing larvae. For those of you with picky pups, there is an injectable option available that lasts for six months. This is especially useful if you're anything like me and have trouble remembering when the last time it was that you gave your pet their heartworm prevention pill.
Heartworm Prevention in Cats
For cats, there are several forms of chewable tablets, pills, and topical applications that you administer monthly, but as of yet, there are no injection-based preventions available. Just because your cat is indoor-only does not mean they are not at risk! Even indoor kitties should be put on a careful regimen of heartworm-prevention meds. Mosquitoes can get inside with one swing of your door, and all it takes is one carrying the disease to ruin your cat's health.
Prevention Is Key
Prevention is a must! Heartworm disease is nasty, expensive, painful, and a serious risk to your pet's health and welfare. If you have ferrets, they can also be infected, so be sure to talk to your veterinarian about prevention options for them. Oh, and in case you're wondering . no, this disease does not affect humans. Our bodies are inhospitable to the larvae!
Questions & Answers
Question: Are rats susceptible to heartworms?
Answer: Thankfully, there is no evidence that rodents, lagomorphs, or mustelids are at risk for contracting heartworm disease.
BirdPerson on June 15, 2020:
Are Birds prone to get heartworms? pls tell me as I have never read about this before
Dirofilaria immitis, also known as heartworm or dog heartworm, is a parasitic roundworm that is a type of filarial worm, a small thread-like worm, that causes dirofilariasis. It is spread from host to host through the bites of mosquitoes. There are four genera of mosquitoes that transmit dirofilariasis, Aedes, Culex, Anopheles, and Mansonia.  The definitive host is the dog, but it can also infect cats, wolves, coyotes, jackals, foxes, ferrets, bears, seals, sea lions and, under rare circumstances, humans. 
Dirofilaria immitis is commonly called "heartworm". Adult heartworms often reside in the pulmonary arterial system (lung arteries) as well as the heart, and a major health effect in the infected animal host is a manifestation of damage to its lung vessels and tissues.  In cases involving advanced worm infestation, adult heartworms may migrate to the right heart and the pulmonary artery. Heartworm infection may result in serious complications for the infected host if left untreated, eventually leading to death, most often as a result of secondary congestive heart failure.
Heartworm Disease - The Basics: What Do You Need to Know?
Heartworm disease is caused by the parasite Dirofilaria immitis. This is a worm that lives in the heart, lungs, and surrounding vasculature. It is a serious disease that primarily affects the heart and lungs but can also affect the liver, kidneys, central nervous system, and if left untreated, can cause death.
Heartworms are transmitted by mosquitoes: they take a blood meal from an infected animal and transmit the microfilariae (larval stage/baby worms) into another animal with subsequent blood meals. These microfilariae will then make their way to the heart where they grow into adult worms, causing heartworm disease. Mosquitoes are required for the parasite’s life cycle which means that a dog cannot re-infect itself.
Both dogs and cats can get heartworm disease from mosquitoes! A cat is an atypical host, and unfortunately many times goes undiagnosed. In some cats, 1-3 adult worms can be devastating and create respiratory issues, and one of the main risk factors for cats developing feline asthma is heartworm! The treatment that we use for dogs cannot be used in cats, so prevention is key for kitties.
What are the symptoms?
Some dogs are asymptomatic, meaning that they act normally. There are no changes in their breathing, exercise levels, or appetites. With chronic infections or heavy worm-burdens, owners can notice coughing, exercise intolerance (unable to go on a walk without stopping and/or coughing), decreased appetite, sleeping more, and even weight loss.
Clinical signs in cats can be very subtle to very dramatic. These symptoms can include coughing, asthma-type symptoms, vomiting, weight loss, lack of appetite, and fluid build up in the abdomen.
There are 4 grades to heartworm disease:
Grade I: Asymptomatic dog, tests positive on the annual test that is recommended by veterinarians. Chest x-rays, blood work and urine testing is normal.
Grade II: Asymptomatic or mild symptoms in dogs. Chest x-rays will show some abnormalities or the pet may have mild changes on blood work and urine testing.
Grade III: Symptomatic dogs, chest x-rays show obvious changes and blood work and urine testing is very consistent with chronic inflammation and parasitic infection.
Grade IV: Severely symptomatic dogs, chest x-rays show enlarged and abnormal vessels they may have fluid build-up in the abdomen and are in right-sided congestive heart failure. These pets have a guarded prognosis (and in some cases treatment may need to involve surgical extraction of the worms from the heart, through the jugular vein)
Why is annual testing recommended if my pet is on regular prevention?
Heartworm disease can be devastating. The earlier the detection, the better chances for survival. Since many dogs are asymptomatic at time of diagnosis, the only way it is found is through an annual test, which requires only a small amount of blood
All pets over the age of 7 months old should be tested for heartworm disease on an annual basis, but we start giving the heartworm preventative medication as young as 8 weeks of age.
How is heartworm disease treated?
If your dog has been found to have heartworm disease and all the testing indicates that it is safe to then go ahead with treatment, it is done with a medication called Immiticide (an arsenic derivative!). The American Heartworm Society recommends giving three injections: one injection on day one and the other two injections one month later, 24 hours apart. Post-injection care includes strict exercise restriction for 30 days (so, for a traditional treatment - that means TWO MONTHS of STRICT restrictions), keep them on all prescribed medications (often steroids to reduce inflammation in the lungs, sedatives as needed and pain medications for injection-site discomfort) for the heartworm disease, and monthly heartworm prevention.
There is no approved treatment for cats.
What is the best way to prevent this disease?
Keeping dogs and cats on monthly prescription preventatives, year round (even in the cold months), is the best way to prevent this disease. The two main ways to administer this are topical or oral medications. Both are only available as prescriptions through a veterinarian.
This is definitely a disease where prevention is a lot better (and cheaper) than treatment!
The life cycle and intricacies of treatment are a lot more complicated that the basic information we’ve provided here. If you’re interested in learning more - ask your veterinarian! At Clarendon Animal Care we work with a number of local rescue groups and manage heartworm positive dogs frequently - we’re always happy to answer any questions you may have about this disease - detection, prevention, management, and general biology/life cycle.
How do I prevent my pet from getting heartworm?
There are many heartworm preventatives available. If your dog has not had heartworm medication for more than six months, a blood test is necessary before you can commence preventative treatment, just to make sure they don’t already have heartworm. Prevention should begin at six to eight weeks of age. Medications come in either tablet form or as top-spot application, and many brands help protect your pet against intestinal worms and other parasites like fleas as well. The most convenient and effective heartworm prevention is a yearly injection administered by your veterinarian. This can be given from 12 weeks of age, then boosted at 6 months.
Tablets or top-spot applications are the best products available to prevent heartworm infection in cats. You can purchase high-quality heartworm products that also cover intestinal worms, with some also controlling fleas. We know your cat can be difficult to medicate, so regular visits to the vet where a professional can administer them for you is a good idea.
Remember that prevention is always the best medicine. Keep your pet happy and healthy all year round by sticking to a good preventative treatment plan. They’ll thank you for it.
What Are the Symptoms of Heartworm in Cats?
No region is exempt from the concern of feline heartworm disease – in fact, heartworm diagnosis occurs in every state. This is why knowing the symptoms of this disease is relevant information to familiarize yourself with.
Unfortunately, a cat can have adult heartworms, but that doesn’t mean they will show symptoms. Some lucky felines are able to spontaneously rid themselves of these parasites and heartworm larvae from their pulmonary arteries without ever having shown symptoms.
On the opposite side of the spectrum, others may have heartworm infections that cause unexpected, sudden death – again, without the display of any symptoms.
Signs of heartworm disease can appear nonspecific and even mimic one of the many other feline diseases. Some of these obscure, nonspecific symptoms include loss of appetite, decreased activity, vomiting, and weight loss. Heart failure symptoms are less common, but there are rare cases of it.
Symptoms of heartworm in cats can vary depending on the stage of the worms themselves. An immature worm parasite might resemble feline asthma and include these signs:
- Difficulty breathing
- Intermittent coughing
- Vague malaise
- HARD (Heartworm Associated Respiratory Disease)
- Weight loss
More mature, adult heartworms may present themselves with these signs:
- Intermittent vomiting
- Difficulty breathing
- Fluid in the belly
- Acute respiratory distress
Immature heartworm larvae can arrive in the heart and lung pulmonary arteries within 3-4 months following a bite from an infected mosquito. A lot of these immature parasites die, which causes the cat’s lungs to have a strong inflammatory response. This is known as HARD, or heartworm associated respiratory disease, due to the respiratory symptoms (such as coughing, increased respiratory rate, and trouble breathing) being the most obvious. However, feline bronchitis and feline asthma present quite similarly to HARD, making them difficult to distinguish.
If adult heartworms die, they attack the blood vessels. Toxins are released into the bloodstream that causes damage to the lungs, which can lead to dangerous respiratory problems and even death. Even just one worm dying can be fatal.