Canine filariasis, also known as heartworm disease, is an infection caused by parasites transmitted by mosquitoes. It is essential to start administering preventive medications from around April, when mosquitoes start to appear, to prevent the onset of the disease. Although it may be challenging to remember to administer the medication every month, it is a 100% preventable disease, so it is important to understand the significance of prevention. Below, I will explain in detail how filariasis is transmitted and how to prevent it.
About Canine Filariasis
Filariasis is transmitted by mosquitoes and parasitizes the heart and pulmonary arteries. In its early stages, it usually shows no symptoms. However, even if infected and asymptomatic, if adult worms produce a large number of microfilariae that circulate in the blood, the infected animal can become a source of infection. As the disease progresses, it can cause various symptoms such as:
- Coughing
- Rapid breathing
- Fatigue and reluctance to move (exercise intolerance)
- Anemia
- Abdominal swelling and weight gain
In severe cases, it can lead to heart failure or respiratory failure, potentially resulting in death. Therefore, it is crucial to prevent the disease through the use of deworming medications, which can prevent the onset 100%.
How Filariasis Infects Dogs
When a mosquito bites an animal infected with filariasis, microfilariae (L1) enter the mosquito’s body. Inside the mosquito, the microfilariae undergo two molts to become L1→L2→L3, which are infectious larvae. These L3 larvae enter the animal’s body through the mosquito’s bite.
Once inside the animal, the larvae (L3) molt twice more under the skin (in fat or muscle), becoming L3→L4→L5, and move into the bloodstream in about 3-4 months. Upon reaching the heart and pulmonary arteries, L5 larvae mature into adult worms, mate, and produce microfilariae. It takes about 6-7 months from the time the larvae enter the animal’s body for microfilariae to be produced. When mosquitoes bite an infected animal, they ingest the microfilariae, which grow and develop in the mosquito’s body, perpetuating the cycle.
Feline Heartworm Disease
Heartworm disease is primarily seen in dogs, but cats can also be infected, showing different clinical signs and manifestations compared to dogs.
Most heartworms entering a cat’s body die before maturing due to the cat’s immune response. Therefore, it is rare for heartworms to mature inside a cat. However, if heartworms reach the heart or pulmonary arteries and die there, they can obstruct blood vessels or cause acute inflammation, leading to symptoms such as coughing, difficulty breathing, vomiting, and weight loss. In rare cases, heartworms can also cause sudden death.
By the time symptoms appear, the heartworms are often already dead, and unlike in dogs, there are no specific diagnostic tests for heartworm infection in cats. Therefore, detection often relies on imaging techniques such as CT scans. Treatment for lung and vascular lesions resulting from heartworm disease is often less effective and time-consuming. Like in dogs, heartworm disease is fully preventable, so taking appropriate preventive measures can protect your cat’s health.
Preventing Canine Filariasis
Mosquitoes are most active when temperatures are between 20°C and 30°C. They start appearing around April and can remain active even in cooler periods of October and November. For the larvae to develop into L3 in the mosquito, an average temperature of at least 14°C is required. In Tokyo, the average temperature drops below 14°C only after late December, so we recommend the prevention season for canine filariasis from April to December.
1. Testing
Before administering deworming medication, it is necessary to confirm that the animal is not already infected with filariasis. Administering deworming medication to an already infected animal can cause a severe allergic reaction or shock due to the sudden death of many microfilariae. Therefore, always use deworming medication after confirming a negative blood test.
2. Administration
Tablet Type
Requires monthly administration. It can be given with food or treats, or placed directly in the back of the mouth. It is safe for dogs with food allergies. However, it can be challenging to administer to dogs that are sensitive to taste and spit out the medication.
Chewable Type
Requires monthly administration, or every three months depending on the formulation. These are chewable tablets, which most dogs readily accept, but caution is needed for dogs with food allergies.
Spot-on Type
Requires monthly administration. The liquid is applied to the skin on the back of the neck. This type is suitable for dogs that refuse tablets or treats and those with food allergies. The medication is absorbed through the skin, and bathing is possible after a certain period. However, it may be difficult to apply to dogs with sensitive skin or those that move a lot.
Injection Type
Provides year-round prevention with a single injection. This method ensures prevention without the worry of forgetting to administer the medication. However, additional medication for other parasites like fleas and ticks is required. Rarely, dogs may have allergic reactions such as fever, facial swelling, or anaphylactic shock.
Other Precautions
If you forget to administer the medication
Contact us to determine the duration the medication was missed. Stopping the medication out of fear of re-administering without testing increases the risk of infection. We will advise on how to proceed.
If the dog vomits after taking the medication
Inform the veterinarian how long after administration the vomiting occurred. We will determine if additional dosing is necessary.
If the dog is bothered by the spot-on medication application site
The dog may show discomfort until the medication dries. Rarely, transient hypersensitivity reactions (such as itching, redness, dermatitis, or hair loss) may occur. If symptoms persist or worsen, consult the veterinarian immediately as the medication may not be suitable.
Is annual testing necessary?
While it is not mandatory if the medication is administered year-round, there is a chance the dog may have missed a dose or vomited it up unnoticed. It is recommended to test before starting a new prevention season, even if there is leftover medication from the previous year.
