What is heartworm disease?

Heartworm disease (HWD) is a serious and fatal disease caused by the parasitic roundworm Dirofilaria immitis. It is passed from one dog or cat to another through mosquito bites. Although it is a dangerous disease, it can be prevented.

In all cases, the vector is always a female mosquito. It introduces the microfilariae, or parasite larvae, into the circulatory system of its chosen host. In tropical countries such as ours where mosquitoes are quite prevalent, heartworm disease is a common occurrence. Several species of mosquitoes commonly found in the Philippines such as Aedes, Culex, and Anopheles, are all carriers of the parasite. It is for this reason that I recommend dogs and cats who are three to four months old to undergo heartworm testing for early detection and prevention of the disease.

Diagnosis and screening

Antigen (AG) tests veterinarians can administer preventive treatment or prophylaxis for dogs. These tests are highly specific and sensitive, and are reliable in detecting heartworm infections (HWI).

For cats, AG testing alone will not suffice as it is harder to detect adult heartworms in cats simply because they are less likely to become infected. Although they are not ideal hosts, cats are not exempted from infection; and if left untreated, their immune system could suffer. Thus, additional tests called antibody (AB) tests must also be administered to further increase the detection of adult heartworm in cats. These tests can detect heartworms as early as two months after infection.

It’s often heartbreaking for us veterinarians to tell our clients that their companion animals have HWI or HWD. While it is difficult to break the unfortunate news, it is important that we communicate the treatment process and risks involved in order for clients can manage their expectations; we do our best to answer any questions that clients may have as well.

Although adulticidal treatment for HWI or HWD is straightforward (whether through medication alone or surgery), the outcome still varies depending on the individual dog’s response to the treatment. If the dog tests negative for microfilaria, prophylaxis ranging from topical, oral, to injectables will be administered.

Ways to avoid heartworm disease

Prevention must be the cornerstone of reducing – if not eliminating – the risk of heartworm infection. As a veterinarian, I insist on this for the sake of your companion animals’ wellbeing.

1. Periodic testing on a semi-annual basis
This is recommended for outdoor dogs living in tropical countries. It is also recommended even for dogs and cats who are currently on prophylactic medications, as this could ascertain whether the current prophylaxis is effective.

2. Restrict exposure to mosquitos
Move your companion animals indoors. Make sure there are no standing water around your home to prevent mosquitoes from breeding.

3. Follow your veterinarian’s orders
Reinfection is likely if there is poor compliance to the prescribed prophylaxis, even after being cleared in test findings in post-adulticidal therapy. Visit your veterinarian for regular testing and screening.

Tricky treatment

There is currently no FDA approved treatment for cats; and medications that are available for dogs are not safe for them. However, continuous monitoring and preventive health maintenance are key to ensure that your cat remains heartworm-free.

If you cat tests positive for HWI or HWD, a long-term management plan supported by your veterinarian will keep your cat’s health stable.

Things to watch out for

Below are the different manifestations according to severity

Class I: Few or no clinical signs
In their 1992 paper, Genchi and colleagues said that “the development of the parasite in cats takes longer compared to dogs.” Moreover, “Many cats tolerate the infection without any noticeable clinical signs or with signs manifested only transiently and sometimes sudden death may arise without warning,” as stated by McCall and others in their 2008 research.

Class II: Few or moderate clinical signs

Class III: Severe clinical signs
Congestive heart failure as manifested by abdominal distention, ascites (abnormal buildup of fluid in the abdomen), hepatosplenomegaly (enlargement of liver and spleen) induced by pulmonary hypertension.

Class IV: Caval Syndrome
Carries the greatest risk of complications when treated. At this point, the disease will be very hard to treat and may require major post-adulticidal treatment. It is declared as a life-threatening presentation of heartworm disease.

Surgical removal of the heartworms from the right atrium and possibly, the right ventricle is necessary. However, even with successful surgical removal of heartworms, mortality rate remains at the 30-40% range. Organ failure and disseminated intravascular coagulation (blood clots form all over the body causing multiple organ failure and consumption of platelets leading to bleeding), can happen after or even before the heartworm extraction.

Adverse Reactions

Dead or dying larvae (microfilariae) may be found in the veins, arteries, and chambers of your companion animal’s heart as a result of treatment. If this clotted material finds its way to the lungs, complications may arise. The blockage could trigger a series of cardio-respiratory problems, such as rapid heart rate, severe coughing, alkalosis, and inflammation of other organs, leading to heart enlargement, which may be fatal.

Moreover, the bacteria released by dead microfilariae called Wolbachia pipientis may elicit an immune response.

Unfortunately, not all treatments result to the elimination of the parasite. Even in well-supervised treatments, unintended consequences occur. If this is the case, how do we deal with heartworm disease? I will have to insist at this point that regular consultations be done with your veterinarian.

Prevention is better than cure

A typical clinical cursory examination may reveal nothing for asymptomatic patients. Oftentimes, infections and diseases are detected in routine screenings. However, the most commonly observed Class I complaint that warrants heartworm screening is coughing. Other complaints are exercise intolerance, weight loss or loss of body condition, and syncope or collapsing episodes.

One such patient I encountered, a Labrador Retriever (whom I suspect got infected outdoors), had a distended abdomen and rasping cough. The dog had significant loss of muscle mass, looked malnourished, and had labored breathing.

Unfortunately, the dog was not able to undergo recommended testing, routine screening, nor prophylaxis. After my evaluation, I advised his human companions not to push through with treatment as there was a high risk of post-adulticidal therapy becoming unsuccessful. Sadly, the dog passed away after a week’s confinement.

An animal’s prognosis or likelihood of recovery after adulticidal therapy depends critically in the staging of the disease.

In the current Philippine setting, a veterinarians’ advice is based largely on sufficient clinical data as to how the disease is manifested, the consequences post-treatment, as well as treatment protocols.veterinarian.

Zoonotic potential

Zoonosis refers to disease that can be spread by animals to humans. In their paper entitled “What is happening outside North America regarding human dirofilariasis” published in 2005, Simón and colleagues stated that “the presence of D. immitis in dogs constitutes a risk for the human population.” They further stated that “the human host is the causative agent of the pulmonary dirofilariasis and in many cases produces benign pulmonary nodules which can initially be misidentified as malignant tumors.”

Lest I cause a panic among dog owners, documented cases of zoonosis for HWD are rare With that being said, I advise pet parents to follow the prophylaxis schedule prescribed by your veterinarian religiously to ensure that your companion animals are protected against infection. In this way, you are not only protecting your pet’s health, but yours and the rest of the general population’s, too.

This appeared in Animal Scene magazine’s December 2019 issue.

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