What is the step-by-step treatment for heartworm (dirofilaria immitis) infection in dogs?

Step-by-Step Treatment of Heartworm in Dogs

The treatment of heartworm disease in dogs follows a standardized protocol using melarsomine dihydrochloride as the adulticide, combined with macrocyclic lactone preventives and doxycycline, with the specific regimen determined by disease severity classification. 1

Pre-Treatment Assessment and Stabilization

Before initiating adulticide therapy, classify the disease severity and stabilize the patient 1:

• Class 1 (Mild): Asymptomatic to mild disease with no radiographic abnormalities or anemia. May have general loss of condition, fatigue on exercise, or occasional cough 1
• Class 2 (Moderate): Radiographic signs present (right ventricular enlargement, pulmonary artery enlargement, perivascular densities), anemia (PCV 20-30%), or mild proteinuria (2+). Stabilize prior to treatment if necessary 1
• Class 3 (Severe): Cardiac cachexia, constant fatigue, persistent cough, dyspnea, right heart failure signs (ascites, jugular pulse), severe radiographic changes, significant anemia (PCV <20%), or proteinuria (>2+). Must be stabilized prior to treatment 1

Treatment Protocol Steps

Step 1: Initiate Macrocyclic Lactone Preventive (Day 1)

Begin heartworm preventive immediately at diagnosis 2:

• Moxidectin (topical or injectable) is highly effective and well-tolerated, even in microfilaremic dogs 3, 4
• Ivermectin has the most potent safety-net and adulticidal activity against younger worms 5
• Milbemycin oxime has demonstrated efficacy against 3-month-old heartworms with repeated treatments 2

For microfilaremic dogs: Pretreat with prednisolone and clopidogrel for 1 week before first moxidectin application. Give dexamethasone and chloropyramine on day 1 to prevent adverse reactions from destroyed microfilariae 3

Step 2: Doxycycline Administration (Days 1-28)

Administer doxycycline 10 mg/kg BID for 28-30 days to target Wolbachia bacteria 3, 4:

• Give with probiotics to reduce gastrointestinal side effects 3
• If gastrointestinal side effects occur (6% of cases), lower initial dose to 5 mg/kg BID and provide symptomatic therapy 3
• This targets the bacterial endosymbiont and enhances overall treatment efficacy 4

Step 3: Continue Monthly Preventive

Continue macrocyclic lactone monthly on days 30,60, and 90 3:

• This eliminates susceptible larvae and provides continuous protection 3
• Monthly treatment is needed to produce full drug effects 5
• No "susceptibility gap" exists when combining continued macrocyclic lactone therapy with the three-dose melarsomine protocol 2

Step 4: Melarsomine Adulticide Therapy

The dosing regimen depends on disease classification 1:

For Class 1 and 2 Dogs:

• Day 60: First melarsomine injection (2.5 mg/kg IM) 3
• Day 90: Second melarsomine injection (2.5 mg/kg IM) 3
• Day 91: Third melarsomine injection (2.5 mg/kg IM) 3

For Class 3 Dogs:

Use the alternate dosing regimen per FDA label 1

Administration technique:

• Determine injection depth by ultrasound examination of lumbar muscles to avoid local complications 3
• Give butorphanol and dexamethasone immediately before each melarsomine injection 3
• Starting on day 60, administer dalteparin for 10 days to decrease pulmonary thromboembolism risk 3

Step 5: Exercise Restriction

Strict exercise restriction is critical throughout treatment and for at least 6-8 weeks after the final melarsomine injection 5:

• Less active dogs are at lower risk of severe thromboembolism and death 5
• Working dogs may be at higher risk and require more intensive monitoring 5
• Worsening radiographic/echocardiographic findings have been documented in dogs given prolonged treatment without adequate exercise restriction 5

Step 6: Monitoring and Follow-Up

Day 120 (4 months post-treatment): Clinical recovery expected 3

• All microfilariae should be eliminated 3, 4
• Dogs treated with moxidectin/doxycycline become microfilariae-negative within 1 month 4

6-9 months post-treatment: Antigen testing 3, 4

• Dogs treated with melarsomine typically become antigen-negative by 5 months 4
• Dogs treated with moxidectin/doxycycline combination start becoming antigen-negative at 4 months, with most negative by 9 months 4

12,18, and 24 months: Radiographs and echocardiography 4

• Monitor for worsening pulmonary patterns or pulmonary hypertension 4

Expected Complications and Management

Local reactions at injection site 3:

• No local side effects: 30% of dogs 3
• Mild local side effects: 66% of dogs 3
• Severe local swelling: 4% of dogs 3

Systemic post-therapeutic complications (rare) 3:

• Transient anorexia and diarrhea: 2% 3
• Coughing and mild dyspnea: 2% 3

Critical Pitfalls to Avoid

Do not delay treatment upon diagnosis—starting both melarsomine and macrocyclic lactones immediately provides maximal heartworm clearance and prevents disease progression 2

Do not use this protocol in asymptomatic working dogs or dogs with clinical/radiographic evidence of heartworm disease without extreme caution, as worsening can occur despite treatment 5

Ensure adequate exercise restriction—failure to restrict activity increases thromboembolism risk significantly 5

Concomitant Therapy

Melarsomine can be safely administered with anti-inflammatories, antibiotics, insecticides, and heartworm prophylactics with no adverse drug interactions 1