Posted on October 18, 2016











(Brevardaidtoanimals.com, 2016)

Heartworm disease is a serious and potentially fatal disease in dogs and cats in Singapore, but rarely humans. As its name suggests, it is caused by worms that can grow a foot-long and predominantly resides in the heart, lungs and associated blood vessels of affected animals. These parasites cause acute lung disease, heart failure and damage to the other organs of the body.

Dogs are natural hosts for heartworms, essentially meaning it spends its entire life in the dog, maturing, mating and procreating. If left untreated, the population of worms in the dog can increase to as many as several hundreds. The disease has potential to cause severe damage to the heart, lungs and arteries of the dog, effectively affecting its health and quality of life even after the parasites are gone. For this reason, prevention by annual injection (Proheart SR-12) or oral medication (Heartgaard/Interceptor tablets) is the best option available. In the event that the dog had been infected with heartworms, treatment should be administered as early upon detection as possible.

Heartworm disease in cats differs from that in dogs. Cats are atypical hosts for heartworms and most worms do not survive to maturity in cats. Cats with adult heartworms usually just have one to three worms. This however, means that the disease often goes undetected or undiagnosed in cats, leading to heartworm associated respiratory disease. Furthermore, there is no heartworm medication for cats so prevention is the only means of protecting cats from heartworm disease.

Method of transmittance

When mature heartworms living in a dog procreates, they produce microscopic baby worms called microfilaria that circulates in the bloodstream. When a mosquito feeds on the blood of the dog, the microfilaria gets taken in and develops into the “infective stage” larvae over 10-14 days. Following which, the larvae would then get deposited on the surface of another animal’s skin and enter the blood stream through the mosquito’s bite wound. Once entered a new host, the larvae takes 6 months to mature into adults. Once matured, these parasites can live up to 5-7 years in dogs and 2-3 years in cats.

Signs & Symptoms


In the initial stages of the disease, many dogs show little or no symptoms at all. As the infection progresses, symptoms would be more likely to develop. Dogs with high activity level heavily infected with heartworms or those with other health problems often has more obvious clinical signs surface.

Symptoms may include mild persistent cough, lethargy, refusing to exercise or do any vigorous activity, inappetance and weight loss. Dogs may also appear to have a swollen or distended belly due to the accumulation of fluid in the abdomen. Those who are hosts to large numbers of heartworms can develop sudden obstructions of blood flow within the heart, resulting in a life-threatening form of cardiovascular collapse. This is called caval syndrome, characterized by a sudden onset of laboured breathing, pale gums, and dark bloody or coffee-coloured urine. Without prompt surgical removal of the obstruction, most dogs die.


Signs of heartworm disease in cats can appear very subtly or very dramatically. These signs include coughing, wheezing, occasional vomiting, inappetance or weight loss. In certain cases, infected cats may experience difficulty walking, fainting, seizures or accumulation of fluid in the abdomen. However, the first sign in some cases is the sudden collapse or death of the cat.

Is my pet at risk for heartworm disease?

Many factors are considered when it comes to a disease which can be transmitted by mosquitoes. Mosquitoes can be bred in typically any household or housing estates with stagnant water and they can have blood meals from one dog to another. For this reason, heartworm disease can be combatted by prevention measures, such as getting rid of stagnant water, to a certain extent. However, weather factors also play a part in the spread of mosquitoes. Strong winds can easily blow mosquitoes throughout the country, effectively spreading the disease. Also, due to the fact that mosquitoes can survive in literally anywhere, both indoor and free-roaming pets can be affected. All in all, the best prevention method from heartworm disease is to start your pet on monthly or annual heartworm preventions.

Why should my pet be tested for heartworms?

Heartworm disease is a severe and progressive disease. Early detection and treatment increases the chances of recovery. Usually if your pet is not on heartworm prevention or is late for routine prevention, a heartworm test would be done to detect the presence of heartworm proteins, which would in turn indicate the presence of heartworms. At My Family Vet, a rapid canine heartworm test kit is used and results would be achieved within a few short minutes, effectively speeding up the decision on commence of heartworm prevention or treatment. This test kit only involves a small blood sample which would be drawn from your pet.

Puppies under 6 months of age are safe to start heartworm prevention without taking a heartworm test because it takes at least 6 months for a heartworm to mature and test positive. Dogs over 6 months require testing before commencing heartworm preventives. Heartworm medications are highly effective but dogs can still be infected if you are late or miss the one routine dose. Heartworm tests are also effective in ensuring that the prevention programme is working properly.

Heartworm disease in cats is much tougher to detect in cats because they are less likely to have matured heartworms. Likewise in Singapore, cases of cats with mature heartworms are rarely seen. My Family Vet clinic does have test kits for cats but we would only carry out the test if deemed appropriate, relevant to exposure and risk.

What happens if my dog is heartworm positive?

Heartworm condition in dogs can be successfully treated. Firstly, your dog’s physical activity level has to be reduced as soon as the diagnosis is confirmed. Physical exertion increases the rate of damage to the heart by the worms so the more severe the symptoms, the less activity your dog should have. Before the treatment begins, the disease has to be stabilized with appropriate therapy, which would be advised by your veterinarian. In serious cases, the stabilization process can take several months. Upon successful stabilization of the disease, your veterinarian will then commence heartworm treatment. After the course of treatment has been completed, another heartworm test would be done to confirm the elimination of all the heartworms. To prevent another infection, it is best to start your dog on routine preventions.

What happens if my cat is heartworm positive?

Cats can be infected with heartworms as well. However, unlike dogs, cats are not ideal hosts for heartworms and infections usually resolve on their own although it can leave them with lasting respiratory damage. Heartworms in the circulatory system also affect the immune system causing coughing, wheezing and difficulty breathing. Unfortunately, worms can also migrate to other parts of the body, such as the brain, eye and spinal cord. Severe complications such as blood clots in the lungs and lung inflammation can also develop following the death of worms in the body.

Unlike dogs, cats can fall severely ill when their body plays hosts to just 1 or 2 worms. Though there are no available treatments for heartworm disease in cats, the disease can often be resolved with good veterinary care.

Immiticide – For Heartworm Positive Dogs


  • To avoid over dosage, a dog must be weighed carefully to ensure an exact calculation of the injection volume. As an owner, your dog trusts you the most and therefore, it is important that you help calm your dog while the nurse is weighing it, so as to get an accurate measurement.
  • During the injection, the dog must be in a relaxed state and be restrained properly. If injected partially subcutaneously in the fats or muscles, an accumulation of watery fluid in the tissues may develop. As an owner, you can also play a part in calming your dog down for the injection.


  • Occasional temporary pains might occur during the injections. A short period of appetite loss may also occur for 1-2 days.
  • Some animals may become agitated with or without tremor and excess salivation for a brief period after injection. However, do not worry as the period of the effect is usually very brief.
  • Following the first week of treatment, you may observe signs of fever, anorexia and depression in your dog that may indicate a respiratory reaction to the presence of dead worms.
  • Obstruction of blood vessels caused by dislodged dead worms may also be observed 7 to 20 days after treatment. Signs are inclusive of fatigue, depression, anorexia, rapid and laboured breathing. Specific treatment should be administered and the animal monitored properly.


  • Heartworm treatment drugs cannot be used on pregnant dogs
  • In the case of a severe heartworm infection, the initial treatment may have to be surgical, followed by drug therapy 1-2 months later depending on the condition of your dog.

Description of clinical classes

Class 1

No disease/ Subclinical heartworm disease

Clinical Examination: All examinations are normal and the dog usually displays no symptoms of the disease.

Thoracic Spine Radiography: No specific wounds, or only slight densities around blood vessels, damage limited to the liver.

Prognosis: Good

Treatment: Direct immiticide and ensure your dog has enough rest


Class 2

Moderate heartworm disease

History: Sometimes decreased tolerance to usual exercise, athletic performance and occasional coughing.

Clinical examination: Few symptoms

  • Alteration of general conditions
  • Loss of condition
  • Associated signs (anemia, benign liver or kidney involvement)
  • Increased pulse (right chest)

Electrocardiogram: Wall of right muscle chamber in the heart thickening may be visible

Thoracic Spine Radiography:

  • Typical wounds
  • Heart: Right chamber enlargement due to dilation and thickening of muscle wall
  • Pulmonary arteries: Distinct irregularities of lobar arteries, enlargement of main pulmonary artery
  • Pulmonary tissue: Densities on alveoli and area around blood vessels


  • Anaemia: Hematocrit between 20% and 30%
  • Proteinura: 2+

Prognosis: Good to fair

Treatment: Direct immiticide, necessary rest and treatment depending on symptoms displayed.


Class 3

Severe heartworm disease


  • Exercise intolerance and limited exertion
  • Anorexia and weight loss
  • Persistent cough and breathing difficulty
  • Coughing of blood
  • Temporary loss of consciousness caused by fall in blood pressure

Clinical Examination: Severe symptoms

  • Poor general condition
  • Breathing difficulty, rapid breathing, increased respiratory sounds
  • Marked anaemia and pale mucous membranes (cheeks, lips, mouth)
  • Jugular distension and deteriorating jugular pulse
  • Tricuspid valve (in right side of the heart) insufficiency, pulmonary insufficiency
  • Liver and kidney insufficiency
  • Coughing of blood due to fusion of artery and bronchial tube in very severe cases


  • Thickening of right ventricular muscle wall
  • Typical wounds

Prognosis: Guarded


  • All necessary symptomatic treatment
  • STRICT rest
  • Immiticide 1-2 weeks later; 1 single injection


Class 4

Very severe heartworm disease

Result in Caval Syndrome

Prognosis: Very guarded


  • Surgical removal of worms
  • All necessary symptomatic treatment
  • STRICT rest
  • Immiticide 1-2 weeks later; 1 single injection