Heartworms in Dogs, Causes, Transmission, Symptoms, Diagnosis, Treatment.

Heartworms in Dogs

Heartworms in dogs are caused by the parasite Dirofilaria immitis. The parasite manifests in the dog’s heart, blood vessel, and lung arteries. It matures over time to a full adult with little or no symptoms. Here is more on the causes, transmission, and symptoms. Learn more about the life cycle, diagnosis, and effective treatment measures.

Causes and Transmission of Heartworms Disease

Dirofilaria immitis also known as dirofilariasis can only be transferred from the host to host by a mosquito. This means that the more mosquitoes there are in your area, the higher the chances of your dog getting infected. Dogs are the natural and definitive hosts of heartworms. Thus the parasite can grow into an adult, mate and reproduce while still in the dog. Other hosts include the cat, coyotes, or foxes, etc.

Note that: the mode of transmission of Dirofilaria immitis is only through an infected mosquito; a dog or any other animal host animal can’t affect or deliver the worms to another animal directly.

Heartworms cause tissue damage
Heartworms cause tissue damage

What happens is that the female heartworm mates with a male heartworm they produce Microfilariae (Young heartworms) in larvae form.  The Microfilariae flow into the bloodstreams and live in the smaller blood vessel of the dog. Here is where the mosquito comes in. You see the Microfilariae cannot develop to adults without going through the development cycle in mosquitoes, so it stays dormant awaiting mosquito ingestion.

After a mosquito bites an infected dog, it ingests the larvae. It takes 10 to 30 days for them to develop into infective larvae. The mosquito then bites another host or the same host transmitting the infective larvae into the dog’s subcutaneous tissue. Larvae flow into the bloodstream. Here the larvae now can mature into an adult in the dog’s heart, large blood vessels, and lung arteries. It takes 5-7 months from the point of infection till maturity.

How the Heartworm affects the Dog’s Body

The worm blocks blood from entering the heart. It also congests lung arteries and large blood vessels. This causes damage to the lung vessels and tissues. Congestion brings about congestive heart failure, malfunction of body organs due to lack of oxygen supply. If left untreated it leads to death.

The microfilariae in cats don’t flow in the bloodstream. That is why the dog is the most definitive host pet.

Lifecycle of Heartworms

Stage 1

First, the mosquito bites a dog with heartworm disease, and as a result of feeding, it ingests the microfilariae from the dog where the microfilariae develop into infective larvae within the mosquito. The stage of development takes about 10 to 30 days then the infective larvae to the moves to the mosquito’s salivary gland.

Lifecycle of Heartworms in Dogs
The lifecycle of Heartworms in Dogs

The development period depends on the temperature. It’s more rapid in temperatures between 75°F -90°F, less active in 60°F -74°F temperatures and cannot develop in temperatures under 57°F.

Stage 2

The infective larvae are then deposited into the subcutaneous tissue of the dog. It matures and then molts to the next stage. It takes 1 -2 weeks to develop then molt.

Stage 3

After stage two it moves to the dog’s muscles and blood vessels. Here it grows for three weeks to 7 weeks before it molts to an immature adult.

Stage 4

The immature adults are then carried through the bloodstream to the right side of the heart. Here they develop into full-size adults where they increase in length and width. Fully mature female worms are 8 -14 inches long and males are 5-8 inches long.

After full maturity the female mates with a mature male producing microfilariae which go into the bloodstream starting the process all over again.

The average number of worms (worm burden) in dogs is 16 worms; however, the number can range from 1- 300 worms. A full adult worm can live up to 7 years in a dog.

Common places with high recorded heartworm disease in dogs

There are familiar places where heartworm disease is more prevalent. Swampy areas or places with temperature ranging from75°F to 90°F are most affected. Here are some states in the USA with high heartworm disease rate than others.

  • Mississippi
  • New Jersey
  • Louisiana
  • South Carolina
  • Alabama
  • Texas
  • Arkansas

Outside the USA

  • Korea
  • Japan
  • Warmer parts of Canada
  • Australia
  • Middle East
  • Southern Europe

Symptoms of Heartworms in Dogs

The severity of heartworms is directly proportional to their numbers in the host, which can result in early onset of symptoms. The lung arteries and heart are the major affected parts by heartworms. Their symptoms are mild and can sometimes go unnoticed.

Here are the clinical signs in dogs associated with heartworm disease:

Mild disease

In this stage, the dog only shows coughing signs.

Moderate disease

Here the symptoms increase with a harder cough, abnormal lung sounds, fatigue after mild activity and reluctant to exercise.

Severe case of the disease

  • Dry cough
  • Fatigue after mild activity
  • The dog is unwilling to play
  • Difficulty breathing
  • Enlarged liver (hepatomegaly)
  • Abnormal lung sounds
  • Irregular heart beats
  • The dog’s coat loses luster
  • Collapse -Brought on due to lack of oxygen due to blockage of the blood
  • Bulging chest
  • Weight loss or anorexia


You’re required to take your dog for tests and prevention treatment off heartworms disease even if you don’t see any clinical signs. The diagnosis is inclusive of clinical signs and several diagnostic procedures such as Radiology, Serology or Ultrasonography, etc.

Here are the diagnostic procedures are done by veterinarian depending on how your dog presents;

1. Serology

Serology is a blood test. It detects specific antigens produced by female adult heartworms that later enter into the dog’s blood system. The test results depend on the amount of antigens produced by the worms to ensure accurate results mostly in clinic test.

The most common type of serologic test is Enzyme-Linked Immunosorbent Assay (ELIZA). Overall test accuracy ranges from 75 to 98%. What may make the test ineffective is when there are only male worms present and also if all the female worms are immature.

2. Electrocardiogram.

This type of test uses a machine that detects and traces electric current produces by the dog’s heart.  The tests try to identify if there is an abnormality in the dog’s heartbeat. It also checks whether there is a change in the size of the dog’s heart (larger).

This test is not dependable as there are other diseases that cause a change in the size of your dog’s heart. Cardiomyopathy is one of them.

3. Blood test for microfilariae

Microfilariae test in the dog’s blood can be accomplished by either use of Knott’s test or test using a microscope. If any microfilariae are identified then, it’s concluded that the dogs have heartworms disease. The test is quick but and has an accuracy level of 80 %.

The setback comes in when there is a blood parasite that looks the same as microfilariae. The body might be fighting back to destroy microfilariae hence there absence in the body.

4. Use of ultrasound

This technique shows the heart chamber size and looks for heartworms presence in the main pulmonary artery and also the ventricles.

5. Radiology

Radiographs (X-rays) of the heart and lungs help to determine abnormalities. Some of the visible defects are swelling of the heart or lungs, enlargement of the artery from the heart to the lung (pulmonary arteries) and inflammation in the lungs or heart.

The decision on what type of test to take depends on what the vet thinks and sees as a most appropriate step.

Clinical observation, medical history and travel history will help the vet assess and treat your dog better.

Treatment of Heartworms in Dogs

The earlier heartworms disease is diagnosed, the better the chances of effective treatment. It will leave your dog in a better condition with minimal complications. The entire treatment procedure is comprised of FDA (Food and Drug Administration) approved medication.

Some of the medications that have been used to treat heartworms in dogs are toxic. Here is how to cure heartworms in dogs;

Use of Antibiotics and Steroids

The Vet will likely give antibiotics to your dog. It is a prevention procedure is done to kill a bacterium known as Wolbachia which is released by the heartworms when they die. Wolbachia cause inflammation in the lungs and kidney which worsens the heartworms disease. It also hinders the treatment process.

The antibiotics and steroids are used before actual treatment begins. Pre-treating dogs with doxycycline are necessary so as to destroy Wolbachia and prevent malaise that occurs during treatment.

Adulticide Therapy

There are only two types of drugs approved by the FDA in the treatment of heartworms in dogs. After completion of the first course of drug (steroids and antibiotics) the vet uses one of these drugs which are organic arsenical compounds, to kill the worms. Your dog will be hospitalized for this treatment which will include a series of injections that take at least 65 days. The drugs are Thiacetarsamide sodium (Caparsolate) and Immiticide (Melarsomine).

Treatment to kill of Microfilariae

Approximately one month (1 to 6 weeks) after completing Adulticide Therapy, the vet gives the dogs medication that kills of microfilariae. It’s necessary to kill off any more risk of your dog having heartworms disease.

Precautions You Should Take

During and several weeks after treatment your dog may experience thromboembolic complications. Thus your dog’s activity levels should be minimal or limited. The thromboembolism is caused by Wolbachia or dead worms (before they are absorbed in the body). This may cause obstruction in the arteries. That is why the dog shouldn’t exercise- it increases blood flow causing capillaries rupture because of high pressure caused by congestion.

The cost for treatment usually falls under the range of $300 to $400.

Factors Considered in Treating Heartworms in Dogs

There are factors that can increase ill health in your dog; these factors also influence the initiative your vet will take in tackling your dog’s health problems.

1. The amount of liver, heart and other body organs damage

An X-ray will help determine if your dog’s organs have hard mild, moderate or severe damages. The length of time and size of heartworms in the dog’s body contribute a lot on the degree of injury the dog’s liver, heart and kidney incurred.

2. The dog’s size

Smaller dogs are at a higher risk than larger dogs this is because of their arteries sizes and body organs size. The diameter of the pulmonary arteries in dogs is greater than in small dogs.

3. The period since infection

The longer the time since your dog has been infected by heartworms the higher the risks of circulatory damage and vice versa. Recently infected dogs and also young dogs have little damage because the worms haven’t increased in number and haven’t grown.

4. Number of heartworms in your dog

The number of parasites in your dog’s body means that more arteries are blocked. There is also congestion in the liver arteries and heart. The amount mostly is affected by mosquito population and duration of infection.

5. The sex of adult heartworm

The male adult heartworm is smaller than the female. Thus the female worm causes more damage because of its size.

The good thing is that treatment of heartworms is usually successful, so there is no need for surgery which becomes necessary when the condition is severe. The drugs work quicker on male heartworms than females.

How to Prevent Heartworms in Dogs

There are three steps can be done to prevent your dog from getting heartworms disease. Regular checkup

A blood test can be done to see the blood body count. An x-ray can also be done to see the size of the heart and liver

Reduce mosquito exposure

Mosquito free environment and making your environment less hospitable to mosquitos helps a lot in prevention.

Routine preventive measures

The prevention cost is way less than treatment; it’s $35 to $80 which depends on the size and age of your dog.  There are four approved FDA drugs used in prevention measure. They include:

  • Ivermectin- Minimum age your dog should be to given the preventive is six to eight weeks.
  • Milbemycin oxime- Minimum age your dog should be to given the preventive is four weeks.
  • Selamectin- Minimum age your dog should be to given the preventive is six weeks.
  • Moxidectin- Minimum age your dog should be to given the preventive is eight weeks to six months.

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