Heartworm Disease in Dogs: Diagnosis,Treatment & Prevention
Heartworm disease is a serious medical condition that often results in severe heart and respiratory illness in affected dogs. It is caused by a helminth parasite Dirofilaria immitis, more commonly known as heartworm. Unlike other internal parasites which mainly invade the gastrointestinal system, heartworms stay inside a dog’s blood vessels and eventually find their way to the animal’s heart where it will lodge itself and grow.
Heartworm infection in dogs causes serious heart and lung problems and can be potentially fatal if not diagnosed and treated early. Proper diagnosis is essential in planning a treatment approach for dogs affected with heartworm disease. This is also one of the few diseases that can be prevented with prophylactic medications.
Heartworm disease in dogs is a blood-borne parasitic nematode (roundworm) known as Dirofilaria immitis transmitted by mosquitoes.
Upwards of 30 species of mosquitoes can act as heartworm transmitters. Mosquitoes ingest immature heartworm larvae, called microfilariae, by feeding on either an infected cat or dog. The microfilariae develop further for 10 to 30 days in the mosquito’s gut and then enter parts of the mosquito’s mouth.
When an infected mosquito bites a dog, it injects larvae into the dog. The larvae then mature over a period of several months, eventually ending up in the right side of the heart and the pulmonary arteries. Once this occurs, they mature into adult heartworms in dogs, and can reproduce about six months from the time of invasion. At approximately eight months after the invasion, heartworm in dogs begin to produce a new crop of microfilariae that will live in the dog’s blood for about one month. By the time this occurs, their lives are in danger even though clinical signs may not be apparent yet.
How is heartworm disease transmitted from one pet to another?
The mosquito plays an essential role in the heartworm life cycle. Adult female heartworms living in an infected dog, fox, coyote, or wolf produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into “infective stage” larvae over a period of 10 to 14 days. Then, when the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited onto the surface of the animal’s skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. Because of the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet.
How Veterinarians Diagnose Heartworm Disease in Dogs
Dogs 7 months and older should be tested for heartworm disease at least annually. If the dog misses a dose of prevention, then she should be tested more frequently. Testing is often done in the hospital at the bedside and requires a small amount of blood.
The most widely used method for diagnosing heartworm disease is antigen-based testing. Antigens are proteins uniquely found on the surface of an organism that are used to detect the presence of that organism in the sample. In this case, the antigens being tested for are produced by the female adult heartworm, and if the test shows positive, then the dog is infected.
Other tests that can be performed include a blood smear or a modified Knott’s test (often a test that is sent out for diagnosis), which are done to check for the presence of circulating microfilariae.
Once diagnosis has been obtained, your veterinarian may recommend more testing, which is used to find out the severity of the infection as well as the amount of risk involved for treatment. Other testing often includes chest radiographs, EKG, blood pressure, cardiac enzyme evaluation (NT-proBNP), echocardiogram, blood work, and urine testing.
Class I dogs are those with the lowest amount of risk for treatment, and Class IV dogs are those often diagnosed with caval syndrome and are at highest risk. This means the worm burden is so great that the worms are blocking blood from exiting the heart. These dogs are dying and require surgical removal of the worms (often done by a specialist) to survive.
Heartworm Symptoms – The Four Stages Of Heartworms In Dogs
Heartworm symptoms in dogs are divided into four stages. It is important to understand that individual stages are not always clearly identifiable and some stages can overlap, but the following information will help educate you about the four major stages, as well as their accompanying heartworm symptoms. The four clinical stages of heartworm begin when your dog has already become infected and the heartworms are present in the dog’s heart:
Stage 1: In dogs, the first stage of heartworm will typically be symptom free. In this stage, the heartworms are present and settling into the heart. However, in stage one the disease has not yet progressed to the point where the heartworms will have produced a new generation of microfilariae and dog’s body will not yet have produced antigens in an amount sufficient for detection.
Stage 2: Stage two of heartworms in dogs is accompanied by anywhere from no symptoms to moderate symptoms including intolerance for exercise and a more lingering cough. The heartworms have been present long enough in the body for antibody production and probable microfilariae production. During this phase, heartworm disease may be detected with blood tests.
Stage 3: By stage three of heartworms in dogs, the symptoms of the disease will be very noticeable and have a big impact on your dog’s health. Dogs will cough and experience fatigue after exercise, may be reluctant to exercise at all, and can have trouble breathing. During this stage, dogs may also cough up blood. By stage three, the disease is quite evident on x-rays. The worms in the heart and large vessels will be obvious on x rays.
Stage 4: Dogs in stage four of heartworm disease have very visible heartworm disease symptoms. These symptoms are accompanied by long-term implications for the dog’s health. These dogs are very ill. The symptoms are similar to Stage 3 but more severe. Dogs will be reluctant to exercise, tired after exercising, and will exhibit a cough. They will probably experience trouble breathing as well. Testing may reveal the impact of the disease in the form of abnormal sounds within the dog’s heart and lungs and an enlarged liver. Even with treatment, this stage of the disease carries a high risk of long term debilitation and possible death.
The severity of heartworms in dogs is directly dependent upon:
- The number of worms present in a dog’s body
- The duration of the incubation
- The response of the infected do, in fighting off the infestation
As heartworm disease progresses through each stage, treatment methods become increasingly invasive. This is a big reason why early detection plays a major role in the options and ability for your dog to recover. Remain aware of any changes in your dog’s behavior. Keep an eye out for any changes in behavior that align with the symptoms of heartworm .
Other heartworm symptoms include:
- Anemia
- Fainting Spells
- Right Sided Chronic Heart Failure
- High Blood Pressure
- Rapid Heart Beat
It is important to understand that the symptoms listed above are indicative of advanced stage heartworm disease. Unlike medications that are used to prevent heartworm in dogs, the medications that are used to kill an advanced stage heartworm infection carry a higher rate of potential side effects, can be painful for the dog and are costly to the owner. In addition, the treatment will require considerable downtime for your dog’s normal exercise routine while the dog recovers from the infection.
Treatment of Heartworm Disease in Dogs
Once your dog is diagnosed, your veterinarian will most likely explain to you next steps including treatment options, more diagnostics, and time frame for follow-up visits.
First, your dog should have his activities restricted as exercise can increase the potential for the heartworms to dislodge and cause clots elsewhere in the body. Additionally, if your dog has circulating microfilariae in his bloodstream, mosquitoes, after ingesting a blood meal from your dog, can then transmit the parasite to others, so limited exposure to the outside is recommended.
Certain medications may be prescribed, such as:
- Steroids: to decrease inflammation created by the worm itself
- Antibiotics: doxycycline is used to kill Wolbachia, a symbiote organism that lives within the heartworm. Without the symbiote, the host heartworm becomes easier to kill and secondary inflammation is minimized.
- Specific kind of heartworm preventive: to prevent younger worms from developing into adults and to rid the bloodstream of any circulating microfilariae
An injection containing the arsenic-based compound melarsomine will be given to your dog 60 days, 90 days, and 91 days after diagnosis by the veterinarian. This is a medication designed to kill the adult heartworms and is usually administered in the lower back deep into the muscle. As it is painful, pain medications will most likely be sent home at those visits as well.
Prevention of Heartworm Disease in Dogs
The best way to treat your dog is to do your best to prevent the disease in the first place with year-round heartworm prevention. The good news is that there are multiple types and forms of heartworm prevention on the market, and they are all affordable.
There are tablets, topicals, and even injectable versions that can provide anywhere from 1 month to 12 months of protection. There are even products that are combined with flea and tick control to give your dog a more comprehensive preventive profile.
All products are designed to kill the L3 and/or L4 heartworm larvae, and some will clear the blood system from circulating microfilariae.
If your dog tests positive for heartworm disease, it is important to discuss the specific type of preventative needed while treating it, as there are only a few that should be given to minimize secondary complications.
You should speak with your veterinarian to decide 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 to cause heartworm disease.
Heartworm prevention is given in the form of a monthly chewable or a long acting injection. The chewables are readily accepted by most dogs as a treat. It is very important that the chewable be given once a month on the same day of the month to make sure that your dog is adequately protected. Side effects of the chewable medication are rare. However, as with any medication, please call if you notice any changes in your dog’s health or behavior. The long acting injection is a great option for busy families and provides protection for a minimum of 6 months. Both types of prevention also provide broad-spectrum intestinal parasite control for both pet and family safety. New products are launched all the time and our veterinarians will continue to review available options for your pets and will advise you of our current best recommendations at your next visit.
Heartworm Disease in Cats
Mosquitos can also infect cats with heartworm. However, cats are not natural hosts for heartworm. Therefore, cats are less prone to heartworm than dogs
Heartworm disease in cats is a bit different than in dogs. Heartworms in cats do not live as long. The average lifespan is only 2 to 4 years. They do not grow as long, and fewer of them mature into adults. Usually, a cat has only one or two worms. However, because of a cat’s small body size, a cat with only a few worms is still considered to be heavily infected.
In cats, it takes 7 to 8 months for infective larvae to mature into adult heartworms and produce larvae. This is about one month longer than in dogs. The presence of larvae in a cat’s bloodstream is uncommon. Only 20 percent of cats with heartworm disease have larvae in the bloodstream, compared to 80 to 90 percent of dogs with heartworm disease. Also, the presence of larvae in the bloodstream is inconsistent and short-lived in cats.
What are the Symptoms of Heartworm Disease in Cats?
- Difficulty or labored breathing
- Vomiting
- Decreased activity
- Loss of appetite
- Weight loss
- Fluid in the lungs.
Not all cats with heartworm disease show symptoms. However, cats that show symptoms usually show signs of breathing difficulties due to the lung damage caused by the heartworms.
Cats typically show symptoms of heartworm disease at two different points. One is when the immature heartworms arrive in the arteries of the heart and lungs. The other is when the adult heartworms die. Cats with heartworm disease rarely show signs of heart failure. However, some infected cats die suddenly from heartworm disease without ever showing signs of being sick. And in some cases, cats have been able to rid themselves of heartworms without ever having any symptoms.
It is harder to detect heartworm infections in cats than in dogs. Veterinarians generally use two types of blood tests in combination to check a cat for heartworms. However, negative test results do not rule out heartworm infection. Positive test results may or may not mean that there is an active heartworm infection. A veterinarian uses the results of both blood tests, along with the cat’s symptoms and the results of other tests such as x-rays and an ultrasound of the heart, to determine if a cat has heartworm disease.
Infection in Cats
After an infected mosquito bites a cat, the immature heartworms arrive in the heart and lung arteries in about 3 to 4 months. Many of these immature heartworms die, causing a strong inflammatory response in the cat’s lungs. We call this response heartworm associated respiratory disease (HARD). It refers to breathing difficulties, such as trouble breathing, increased breathing rate, and cough. It may be difficult to distinguish HARD from feline asthma or feline bronchitis.
When the adult heartworms die, they release toxins into the cat’s bloodstream which causes lung damage, leading to respiratory problems or sudden death. Even the death of one worm can be fatal for a cat.
There is no FDA-approved drug to treat heartworm disease in cats, although symptoms may be managed with medications. If a veterinarian can detect heartworms by ultrasound, surgical removal of adult heartworms may be an option. But surgery is risky. And if the heartworms are not removed intact, there can be potentially serious complications, such as shock and death.
COMPILED & EDITED BY-DR UDAY KUMAR,PET CONSULTANT,HAJIPUR
REFERENCE-ON REQUEST
https://www.pashudhanpraharee.com/heart-worm-disease-in-felines/
