Heartworm Infection Symptoms and Prevention Medication Comparison for Dogs

Heartworm infection is a preventable disease—regular medication can effectively protect your dog. However, once infected, treatment is not only expensive and painful but may also cause permanent heart and lung damage. Preventive medications are highly safe with minimal side effects, making this an essential aspect of daily care that every owner should take seriously.

## What Is Heartworm?

Heartworm is a parasite transmitted through mosquito bites. When a mosquito carrying heartworm larvae bites a dog, the larvae enter the bloodstream and develop over approximately six months, eventually growing into adult worms in the heart and pulmonary arteries.

Adult worms can grow up to 30 cm long and live for 5-7 years. As the number of worms increases, they severely obstruct blood flow, causing damage to the heart and lungs.

## What Are the Symptoms of Infection?

Heartworm infection shows almost no symptoms in the early stages, which is why many owners easily overlook it. By the time symptoms appear, the infection has usually progressed significantly.

**Early Symptoms:**
* Decreased activity level
* Mild dry cough
* Easy fatigue after exercise

**Mid to Late-Stage Symptoms:**
* Persistent coughing, coughing up blood
* Decreased appetite, weight loss
* Ascites (enlarged abdomen)
* Lethargy
* Difficulty breathing

**Severe Cases:**
* Blood in urine, anemia
* Heart failure
* Sudden death

## Common Preventive Medication Comparison

Most heartworm preventive medications on the market belong to the "macrocyclic lactone" class, which effectively kills heartworm larvae (L3 and L4 stages). Long-term use also has some effect on adult worms.

**Common Products:**

| Product Name | Active Ingredient | Form | Features |
|-------------|------------------|------|----------|
| Heartgard (Interceptor) | Mibemycin oxime | Oral tablet | Specifically prevents heartworm, lower dosage |
| NexGard Spectra | Milbemycin + Afoxolaner | Oral tablet | Treats internal and external parasites simultaneously |
| Heartgard Plus | Ivermectin | Oral tablet | Lower price, caution needed for herding breeds |
| Revolution | Selamectin | Topical | External application, also prevents fleas |

**Regarding Herding Breeds:**

Some herding dogs (Collies, Border Collies, etc.) carry the MDR1 gene mutation and are more sensitive to Ivermectin (the active ingredient in Heartgard Plus). If your dog is a herding breed, it's recommended to choose Interceptor or NexGard Spectra, which use Mibemycin/Milbemycin—relatively safer ingredients.

## Important Considerations Before Starting Medication

**Always test for heartworm first!**

If your dog already has adult heartworms, giving preventive medication directly may trigger a severe allergic reaction or even sudden death. This is because the preventive medication kills large numbers of microfilariae, and the dead worms can cause vascular embolism.

The following situations require a blood test first:
* Adult dogs using preventive medication for the first time
* Dogs that have missed medication for more than two months
* Uncertainty about whether previous doses were given on time

## How to Use Preventive Medication

1. It's recommended to start medication from 6-8 weeks of age
2. Administer monthly on schedule without interruption
3. Prevention should continue year-round (mosquitoes are present year-round in Taiwan)
4. Heartworm larvae develop to a stage that's difficult to kill within about 30-40 days in the body, making timely medication crucial

**Side Effects of Preventive Medication:**

Preventive medications are highly safe, with side effects being extremely rare. In clinical trials, NexGard Spectra showed no significant problems even at 5 times the maximum dose. However, if your dog experiences vomiting, diarrhea, or lethargy after taking medication, consult a veterinarian.

## Prevention Is Better Than Cure

Heartworm treatment requires arsenic-based drugs to kill adult worms, during which the dog must strictly limit activity for several weeks to prevent dead worms from causing embolism. Treatment costs are high, and even with successful treatment, heart and lung function may already be damaged.

In comparison, a monthly preventive pill is simple and safe. It's the most worthwhile health investment.

If your dog has never been tested for heartworm or hasn't had medication in a long time, please take them to a veterinary clinic for testing as soon as possible. Only after confirming they're clear should you begin a prevention plan.

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**References:**
* [Canine Heartworm Diagnosis, Treatment, and Prevention Recommendations - Doupi Animal Hospital](https://peace-vet.com/canine-heartworm-disease/)
* [NexGard Spectra | Understanding Heartworm](https://nexgard.com.tw/heartworm)
* [Complete Guide to Heartworm Symptoms and Prevention Drug Side Effects - Zoetis](https://www.pet-care.com.tw/DoctorArticleDetail.aspx?id=32f3fda7-fae9-4602-8463-c0ce9873d103)
* [What Are Dog Heartworm Symptoms? - DogCatStar](https://www.dogcatstar.com/blog/heart-worm/)
* [Heartworm Is a Dog and Cat Killer! - CommonHealth Magazine](https://www.commonhealth.com.tw/article/90383)

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