Heartworm Disease in Dogs (NAVLE Notes)
Heartworm disease, caused by Dirofilaria immitis, is a serious and potentially fatal condition affecting the heart and lungs. Here’s an overview of the clinical stages, diagnostic methods, treatment, and prevention.
Classic Case
- Stages of Severity:
- Class I: Minimal clinical signs.
- Class II: Cough present.
- Class III:
- Cough, hemoptysis, weight loss, lethargy, exercise intolerance
- Ascites secondary to heart failure, radiographic changes
- Class IV (Caval Syndrome):
- Signs of shock: Weakness, pale mucous membranes, slow capillary refill time (CRT)
- Dark urine, tachypnea, dyspnea, distended jugular veins
- +/- Ascites
Diagnosis
- Etiology: Dirofilaria immitis
- Antigen Testing:
- Sensitive and specific, but detects only adult female worms.
- Microfilaria Testing:
- Detects about 80% of infections.
- CBC and Chemistry:
- Eosinophilia, basophilia, hyperglobulinemia, hypoalbuminemia.
- Urinalysis:
- Proteinuria, bilirubinuria, hemoglobinuria.
- Thoracic Radiography:
- Enlargement of main pulmonary artery, tortuous caudal lobar pulmonary arteries, pulmonary infiltrate, hilar lymphadenopathy, right ventricular enlargement.
- Echocardiography:
- Confirms caval syndrome by visualizing heartworm masses in the right atrium and ventricle, obstructing the tricuspid valve.
Treatment
- Pretreatment:
- 30 days of doxycycline to eliminate Wolbachia pipiens, a symbiotic bacterium in heartworms.
- Monthly macrolide preventatives (ivermectin, milbemycin oxime, moxidectin, selamectin).
- Adulticide Therapy (3-dose protocol with melarsomine):
- STRICT exercise restriction is essential to avoid complications.
- +/- Corticosteroids to reduce inflammation.
- Caval Syndrome:
- Surgical removal of heartworms using long alligator forceps via the jugular vein.
Prevention
- Year-Round Preventatives:
- Monthly macrolide preventatives, starting at 6-8 weeks of age, are nearly 100% effective.
Key Points
- Mosquitoes (Aedes, Anopheles, Culex) are intermediate hosts.
- Life Cycle:
- Mosquitoes ingest microfilariae, which mature into infective larvae (L3) and are transmitted during feeding. L4 larvae develop in tissues and migrate to the heart and pulmonary arteries within 2-4 months.
- Prognosis: Guarded to poor, particularly for dogs with caval syndrome, where mortality is high (30-40%).
NAVLE-Style Practice Questions on Heartworm Disease
Question 1
A 5-year-old Golden Retriever presents with a persistent cough, lethargy, and exercise intolerance. Physical exam reveals ascites and weight loss. Thoracic radiographs show enlargement of the main pulmonary artery and caudal lobar pulmonary arteries. What is the most likely diagnosis?
- A) Pulmonary hypertension
- B) Canine influenza
- C) Class III heartworm disease
- D) Class I heartworm disease
Correct Answer: C
Explanation: The presence of cough, lethargy, weight loss, exercise intolerance, ascites, and radiographic changes is consistent with Class III heartworm disease, which involves significant cardiopulmonary symptoms.
Question 2
A dog diagnosed with Class IV heartworm disease is admitted to the clinic with dark urine, distended jugular veins, and tachypnea. What is the most appropriate intervention?
- A) Administer melarsomine immediately
- B) Perform surgical removal of heartworms via jugular vein
- C) Begin monthly ivermectin treatments
- D) Initiate corticosteroid therapy
Correct Answer: B
Explanation: Class IV heartworm disease (caval syndrome) requires immediate surgical removal of heartworms to relieve obstruction in the right atrium and ventricle. Administering melarsomine would not be appropriate due to the acute obstruction.
Question 3
A 4-year-old Labrador has been diagnosed with heartworm disease and started on monthly ivermectin. What other pre-treatment should be administered before adulticide therapy?
- A) Furosemide to reduce pulmonary congestion
- B) Doxycycline to eliminate Wolbachia bacteria
- C) Melarsomine immediately
- D) Praziquantel for concurrent parasites
Correct Answer: B
Explanation: Doxycycline is administered to reduce Wolbachia bacteria, which has a symbiotic relationship with heartworms, improving the effectiveness and safety of adulticide therapy.
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