Heartworm Disease in Dogs and Cats (NAVLE Notes)
Heartworm disease, caused by Dirofilaria immitis, is a serious parasitic infection that affects the cardiovascular and pulmonary systems. Clinical presentation and treatment differ between dogs and cats.
Classic Case
- Canine Heartworm Disease:
- Mild: No clinical signs or a mild cough.
- Moderate: Mild signs with exercise intolerance.
- Severe: Moderate signs plus dyspnea, abdominal distension.
- Caval Syndrome: Acute onset of lethargy or weakness, hemoglobinuria (coffee-colored urine).
- Feline Heartworm Disease:
- Signs: Vomiting, intermittent cough, increased respiratory rate, sudden death.
Diagnosis
- Etiology: Dirofilaria immitis
- Canine Diagnostics:
- Heartworm Antigen SNAP Test: Detects protein from female worms, with earliest detection at 5 months post-infection. False negatives can occur due to antigen-antibody complex formation, immature females, light infection, or male-only infection.
- Microfilaria Testing: Recommended concurrently with antigen testing, as 7% of infected dogs may test antigen-negative and microfilaria-positive. Options include Modified Knott’s, filter test, or direct blood smear.
- Thoracic Radiography: May show enlarged, tortuous pulmonary arteries, right heart enlargement, and pulmonary parenchymal disease.
- Ultrasonography: Can visualize worms in pulmonary arteries.
- Feline Diagnostics:
- Radiography: Suggestive changes only seen in ~50% of cats, resembling bronchitis or asthma.
- Antigen Testing: False negatives can occur with male-only infections or low worm burden.
- Antibody Testing: Positive test only confirms prior infection.
- Ultrasonography: May show worms in pulmonary arteries.
Treatment
- Canine Treatment:
- Doxycycline: Administered daily for 30 days before adulticide therapy to reduce Wolbachia bacteria, essential for heartworm survival.
- Macrocyclic Lactones (Heartworm Preventatives): Start two months before adulticide therapy to eliminate susceptible larvae and microfilaria (not adult worms). Pre-treat with diphenhydramine and corticosteroids if microfilaria-positive to prevent anaphylaxis.
- Adulticide Therapy:
- Melarsomine Dihydrochloride: Three-dose protocol recommended; first dose, then two doses one month later (24 hours apart), killing 98% of adult worms.
- Exercise Restriction: Strict exercise restriction during and for 6-8 weeks post-treatment.
- Corticosteroids: Used to manage pulmonary thromboembolism and started with doxycycline and macrocytic lactone in symptomatic or microfilaria-positive dogs.
- Surgical Extraction: For caval syndrome, typically requiring referral.
- Feline Treatment:
- Melarsomine: Not recommended for cats.
- Prednisolone: Used to manage lung inflammation.
Key Points
- Prognosis:
- Canine: Prognosis varies:
- Mild-moderate cases have a good prognosis.
- Severe cases are fair to guarded.
- Caval syndrome cases have a poor to grave prognosis.
- Feline: Prognosis is generally guarded to fair.
NAVLE-Style Practice Questions on Heartworm Disease
Question 1
A 4-year-old mixed-breed dog presents with a mild cough and exercise intolerance. Thoracic radiographs show right heart enlargement and tortuous pulmonary arteries. What is the most likely diagnosis?
- A) Mitral valve disease
- B) Dilated cardiomyopathy
- C) Heartworm disease
- D) Chronic bronchitis
Correct Answer: C
Explanation: The signs, breed, age, and radiographic findings suggest heartworm disease, which commonly presents with pulmonary and right heart changes in symptomatic cases.
Question 2
A dog diagnosed with heartworm disease is started on doxycycline and macrocyclic lactones. What is the purpose of this pre-treatment before starting adulticide therapy?
- A) To kill adult worms before they reproduce
- B) To prevent new infections and eliminate Wolbachia
- C) To provide immediate improvement in pulmonary symptoms
- D) To prevent thromboembolism
Correct Answer: B
Explanation: Doxycycline reduces Wolbachia, an essential bacteria for heartworm survival, while macrocyclic lactones prevent new infections and eliminate susceptible larvae and microfilaria. This pre-treatment improves adulticide efficacy.
Question 3
A 7-year-old Labrador with heartworm disease suddenly develops hemoglobinuria and weakness. What is the most likely explanation?
- A) Pulmonary edema
- B) Caval syndrome
- C) Bacterial infection
- D) Heart failure
Correct Answer: B
Explanation: Hemoglobinuria and acute weakness in a dog with heartworm disease are hallmark signs of caval syndrome, where heartworms obstruct blood flow in the heart, leading to hemolysis and severe systemic effects.
For study notes visit cracknavle.com