Heartworm Test for Dogs

icon May 9, 2023

Source of Heartworm

Heartworm is primarily spread by mosquitoes. The heartworm lifecycle begins when an infected dog's blood containing microfilariae (baby heartworms) is taken up by a mosquito during a blood meal. The microfilariae develop into infective larvae inside the mosquito, which are then transmitted to other dogs or animals when the mosquito feeds again.

When a mosquito bites a dog, it injects the infective larvae into the dog's bloodstream. Over time, the larvae develop into adult heartworms that can grow up to a foot in length and live in the dog's heart, lungs, and blood vessels. These adult worms can cause serious damage to the dog's cardiovascular system, leading to heart failure, lung disease, and other health complications.

It's important to note that heartworm is not contagious between dogs or other animals. It can only be transmitted by mosquitoes that carry the infective larvae. 

Damage of Heartworm

The damage caused by heartworm disease can be significant. As the worms mature and multiply, they can cause damage to the heart, lungs, and other vital organs. This damage can lead to a range of symptoms, including coughing, difficulty breathing, fatigue, and weight loss.

If left untreated, heartworm disease can progress to a point where it can be life-threatening. The worms can cause irreversible damage to the heart and lungs, which can lead to heart failure, respiratory failure, and even death.

What are the Symptoms of Heartworms in Dogs?

  • Coughing: A persistent cough is one of the most common symptoms of heartworm disease in dogs. The cough is often worse after exercise or excitement.
  • Fatigue: Dogs with heartworm disease may become lethargic and have a reduced energy level.
  • Difficulty breathing: As the heartworms grow and multiply in the heart and lungs, it becomes more difficult for the dog to breathe. This can lead to shortness of breath, rapid breathing, and panting.
  • Reduced appetite and weight loss: Dogs with heartworm disease may lose their appetite and experience weight loss.
  • Swollen abdomen: Dogs with severe heartworm disease may develop a swollen abdomen due to fluid buildup.
  • Collapse or fainting: In some cases, dogs with advanced heartworm disease may collapse or faint due to the strain on their heart and lungs.

The severity of heartworm disease is related to how many heartworms are in the dog, how long the dog has been infected, and how the dog's body reacts to the heartworms. There are four stages of heartworm disease, the further along the stages are, the more severe the disease and the more pronounced the symptoms.

Stage 1

No symptoms or mild symptoms such as occasional coughing.

Stage 2

Mild to moderate symptoms, where the dog may have an occasional cough after exercise or may feel tired.

Stage 3

More severe symptoms such as weakness, persistent cough, or even refusal to exercise, breathing difficulties and heart failure. For stages 2 and 3 of heartworm disease, changes in the heart and lungs can usually be seen on a chest x-ray.

Stage 4

Also known as Kavalier's syndrome. The dog's body is so bursting with heartworm that the blood flow back to the heart is blocked by a large number of heartworms. Kavalier syndrome can be life-threatening and surgery to clear the heartworms is the only treatment option. However, surgery is risky and even surgery can kill a dog. Of course, not all dogs with heartworm disease will develop Kaval syndrome. But failure to detect or treat heartworm disease in time can also damage a dog's heart, lungs, liver, and kidneys, and even lead to death.

Heartworm Test for Dogs

When a dog is infected with heartworm, there are few (though of course there can be) early signs of disease, so it is important to test for the presence of the parasite by a heartworm test. Only a small blood sample needs to be taken from the pet and can be tested for the presence of heartworm proteins to determine if infection is present. If the test is positive, further testing may be required. 
For example

  • ECG: to detect the heart's electrical activity.
  • X-rays: to detect enlargement of the heart.
  • Ultrasound of the heart: to detect the heart complex, the number of heartworms, and the degree of damage.

However, a negative blood test result may also indicate that the dog is infected with heartworms, as shown below for a false negative blood result

If the dog is in the early stages of infection or is only carrying male heartworms

  • Younger dogs may carry fewer heartworms and have a negative blood test. The Knott test is used to detect heartworms (microfilariae) by centrifuging a blood sample and using a high concentration of the deposited fraction.
  • The infection has been present for less than 5 months
  • As it takes several months for heartworm microfilariae to develop in the body and be detected by the test, it is very important to have two blood tests on the dog if the dog is not on regular preventive treatment. Each blood collection should be performed 6-7 months apart in order to confirm heartworm infection.

Secondary diagnosis:

X-rays and taking an ultrasound of the heart can be used to identify suspected cases of heartworm infection and as an aid to identify the extent of infection.

What Happens During a Heartworm Test

During a heartworm test, a small amount of blood is drawn from the dog's vein and sent to a laboratory for analysis. The test typically takes about 10-15 minutes, and results are usually available within a few days.

If the test is positive, your veterinarian may recommend additional testing, such as X-rays or ultrasound, to determine the extent of the infestation. Treatment options will depend on the stage of the infection and the overall health of the dog.

When to Test Your Dog for Heartworm

All dogs should be tested for heartworm infection once a year and this can usually be scheduled as part of a regular check-up for preventative care. The following are guidelines for testing and scheduling:

  • Puppies under 7 months of age can be started on heartworm prevention without testing (dogs need at least 6 months after infection to test positive), but should be tested 6 months after their initial vet visit, then 6 months after that, and annually thereafter to ensure they are heartworm free.
  • Adult dogs over 7 months of age and dogs that have not been on prophylactic medication previously need to be tested before starting heartworm prevention. They also need to be tested after 6 months and 12 months, and annually thereafter.
  • Owners should consult their veterinarian and restart monthly prophylaxis immediately, then re-test the affected dog after 6 months. The reason for re-testing is that the heartworm age must be around 7 months before a diagnosis can be made.

Annual testing is necessary, even if the dog is on heartworm prevention all year round, to ensure that the prevention program is effective. Heartworm medication is very effective but the dog is still at risk of infection. If you miss a monthly dose or administer the medication too late, you can leave the dog unprotected. Even if the owner gives the medication as recommended, the dog may spit up or vomit the heartworm medication or rub off the topical medication. Heartworm prevention is of course very effective, but not 100% effective. Without testing, it is impossible to know if a dog needs treatment.

How Can I Prevent Heartworm Disease in My Dog?

  • Use Preventive Medication

There are several safe and effective medications available to prevent heartworm disease in dogs. These medications are typically given once a month in the form of a chewable tablet or topical application. Some medications also protect against other parasites such as fleas and ticks.

Topical Application

--Abamectin B1 to kill or remove fleas, flea eggs, ticks, lice, and mosquitoes on the skin

Puainta® Topical Deworming Drops to Kill Fleas & Ticks in Dogs/Cats

How to use

Choose dosage based on weight

Size Weight Dosage
Small under 5kg 0.5ml(1 tube)
Medium 5-10kg 0.5-1ml(1-2 tubes)
Big 10-20kg about 1-2ml(2-4 tubes)
Large 20-30kg about 2-3ml(4-6 tubes)

A Tablet

--The main drugs used to treat heartworm disease are praziquantel, arsenious acid and avermectin. Avermectin is the first choice for heartworm disease, arsenious acid is a derivative of avermectin, arsenious acid has low toxicity and few side effects.

Puainta® 血吸虫/并殖吸虫病防御片

  • Test Your Dog Annually

Even if your dog is on preventive medication, it is still important to have them tested for heartworms annually. This can catch any infections early on before they cause serious damage.

  • Minimize Exposure to Mosquitoes

Since heartworms are spread through mosquito bites, it is important to take steps to minimize your dog's exposure to mosquitoes. This includes keeping them indoors during peak mosquito activity, using mosquito repellents, and eliminating standing water around your home.

  • Keep Your Dog Healthy

A healthy immune system can help your dog fight off infections, including heartworms. Make sure your dog gets regular exercise, a balanced diet, and routine veterinary care.

How Often Do Dogs Need Heartworm Medicine?

Most heartworm preventive medications are given once a month, either in the form of a chewable tablet or a topical application. Some medications also protect against other parasites such as fleas and ticks. It is important to follow the manufacturer's instructions for the specific medication you are using, as well as your veterinarian's advice.

In addition to monthly preventive medication, it is recommended that dogs be tested for heartworms annually, even if they are on preventive medication. This can help catch any infections early on before they cause serious damage.

Can I Give My Dog Heartworm Medicine Without Testing?

It is not recommended to give your dog heartworm preventive medication without first testing them for heartworm disease. This is because giving medication to a dog that already has heartworm disease can be dangerous and potentially harmful.

If a dog with an active heartworm infection is given preventive medication, it can cause a severe and potentially fatal reaction as the heartworms die off. This is known as a "dead worm" reaction and can cause symptoms such as vomiting, diarrhea, lethargy, and loss of appetite.

In addition, giving heartworm preventive medication to a dog with an active heartworm infection can also delay diagnosis and treatment. This can allow the infection to progress and cause more serious damage to the dog's health.

What to Feed a Dog with Heartworms?

If your dog has been diagnosed with heartworm, it's important to follow your veterinarian's recommended treatment plan. In terms of diet, there isn't a specific food that can cure heartworms, but a healthy and balanced diet can help support your dog's overall health during treatment.

Your dog's diet should include high-quality protein from sources such as meat, fish, and eggs, along with healthy fats from sources such as fish oil or flaxseed oil. A diet rich in fruits and vegetables can also provide antioxidants and other nutrients that can support your dog's immune system.

It's important to avoid giving your dog foods that are high in salt or artificial preservatives, as these can be harmful to a dog with heartworms. Additionally, it's important to limit your dog's activity during treatment and to provide plenty of fresh water to help keep your dog hydrated.

Can You See Heartworms in Dog Poop?

No, heartworms cannot typically be seen in dog poop. Adult heartworms live in the heart and lungs of dogs and can cause significant damage to these organs. Microscopic baby heartworms called microfilariae circulate in the bloodstream and are usually not visible to the naked eye.

If your dog has been diagnosed with heartworms, your veterinarian will likely perform blood tests to confirm the diagnosis and assess the severity of the infection. In some cases, your veterinarian may also use imaging tests such as X-rays or ultrasounds to evaluate the extent of the damage caused by the heartworms.



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