8) Heart worm disease - symptoms, diagnosis, treatment and prevention Flashcards
Causative agent of heartworm disease
Dirofilaria immitis, transmission by mosquito species (most common: Ades, Anopheles, Culex)
Patent infections in numerous wild and companion animals → most important dogs (most susceptible, 56% develop into adults), also cats (only 6% of larvae develop into adults
Pathogenesis in cats
- Not the primary host, just occasionally affected
- Heart worm-associated respiratory disease (HARD) → no maturation but larvae migrating in lungs
- Initial phase is often misdiagnosed with asthma or allergic bronchitis
dogs - different forms of the worms
depends on number of worms
- live adult worms
o cause direct mechanical trauma,
o immunosuppressive effect
o stimulate immune system to damage vessel intima → proliferative endarteritis and perivascular cuffing with inflammatory cells (high number of eosinophils) = provoke reactive vascular lesions reducing vascular compliance and lumen size within days after adult worms enter the pulmonary artery
- dead adult worms
o even more severe vascular reactions → lung pathology (also in areas not directly in contact with worms)
o worm fragments →thrombi cause embolization + inflammation and finally fibrosis
pathogenesis of heartworm disease in dogs
- HW associated inflammatory mediators cause vasoconstriction → bronchoconstriction (pulmonary hypertension) → leakage from vessels → parenchymal lung inflammation and mild noncardiogenic edema formation
- Pulmonary artery disease → further endothelial damage due to inability of vessels to dilate → can cause ischemia + inflammation → interstitial fibrosis
- Thromboembolism after adulticide treatment
- tricuspid regurgitation due to worms accumulating at the tricuspid valve, pulmonic trunk and right side of the heart → life threatening
➔ long-term infections → chronic lesions and subsequent scarring → Caval syndrome = displace of the adult worms from pulmonary arteries to the right heart → occlusion of the right ventricular outflow tract → pulmonary hypertension and sudden fall in right CO → CHF
➔ chronic pressure causing right side ventricular dilation +/- concentric hypertrophy
➔ right-sided congestive heart failure (→ ascites, chronic hepatic congestion) - active dogs are more affected → activity increases pulmonary arterial pathology and pulmonary artery resistance → higher risk of CHF
➢ liver damage (congestion!)
➢ glomerulonephritis (circulating immune complexes!)
➢ embolization of organs → brain, eyes, limbs etc.
clinical findings in heart worm disease
- moderate:
➢ cough, exercise intolerance, abnormal lung sounds
➢ often don’t show any clinical signs for years → develop gradually - severe:
➢ cough, epistaxis, exercise intolerance, dyna, abnormal heart and lung sounds (caudal lung lobes), enlarged liver (hepatomegaly), syncope, ascites - Caval syndrome:
➢ sudden onset of severe lethargy and weakness, accompanied by hemoglobinuria a hemoglobinuria
➢ signs of right sided CHF
➢ tricuspid cardiac murmur
➢ small dogs are especially prone - overt clinical signs when exercise is increased rapidly → more arterial damage
- weight loss, lethargy, abdominal distension, poor BCS
- radiographic findings: large main pulmonary artery, lobar pulmonary arteritis, truncated and tortuous pulmonary arteries, pulmonary infiltrate, hilar lymphadenopathy, pulmonary parenchymal disease, right heart enlargement → aneurysmal dilation and peripheral occlusion → tortuous proximally dilated vessels
- cat: bronchitis, asthma signs, intermitted vomiting, lethargy, coughing, dyspnea, death of adult worm: acute respiratory distress, shock → may be fatal
diagnosis of heartworm disease
- antigen detection test → best for diagnosis in routine screening of asymptomatic dogs or suspected infection, problem: only detects adult female worms, needs good timing: start preventative treatment with macrolides and then test for antigens 7mo later
- screening for microfilaria → but ~20% of infected dogs are not microfilaremia (figure even higher in dogs with adult worm infection but regular macrolide prophylaxis → just kills microfilariae)
- can do echocardiography but considered unimportant since the antigen and microfilaria tests in combination can already evaluate the worm burden sufficiently, shows → pulmonary arteria dilation, right heart dilation, visible heartworms in pulmonary artery, Caval syndrome (heartworms in right ventricle +/- right atrium
- ECG is usually normal, only ventricular hypertrophy pattern in severe cases of right-sided CHF, enlargement of RV and RA in advanced cases
- X-ray most important to assess severity of infection, can show pulmonary hypertension, right ventricular enlargement, enlarged pulmonary arteries, …evidence of thromboembolism?
- do annual screening with antigen SNAP test (7% of infected dogs negative though!) and microfilaria test recommended annually as well (e.g. modified knots, filter test, direct smear of anti-coagulated blood
- antigen tests in cats often false negative → not sensitive enough for screening, antibody tests positive but can still be positive when there are no alive worms anymore
- cat: historical and physical findings, index of suspicion, thoracic radiograph, echocardiography (more important in cats than in dogs due to difficulties of serological screening), serologic test
treatment day 0 to day 60
Day 0
- test for Ag → begin exercise restriction (the more pronounced the symptoms the more exercise restriction)
- stabilize adequately before therapy
- prednisone 0.5mg/kg BID first week, SID second week, EOD third + fourth week
Day 1 to 28
- administer heartworm preventative (→ kills microfilariae) Doxycycline 10mg/kg BID for 4 weeks
- + prednisone (as prescribed above) + antihistamines if necessary → to reduce risk of anaphylaxis
Day 60
- 1st melarsomine (immiticide) injection 2.5mg/kg IM
- + more prednisone (again as above for 4 weeks)
- Decrease activity level even further
treatment day 60 to day 271
Day 90
- 2nd melarsomine injection
Day 91
- 3rd melarsomine injection
- prednisone (again as above for 4 weeks)
- Continue exercise restriction for 6 to 8 weeks
Day 120
- Test for microfilariae → positive? → treat with microfilaricide and test again after 4 weeks
Day 271
- Ag screening
- before adulticide treatment: assess risk of post adulticide thromboembolism:
- exercise restriction after adulticide treatment is essential (4-6wk after administration)
- pre-adulticide treatment (do this prior to adulticide treatment): Doxycycline 10mg/kg bid for 30 days → kills microfilariae, corticosteroid may also be administered to reduce pulmonary inflammatory lesions, 2mo prior to adulticide: macrolide prophylaxis
- high-risk dogs: stabilize before melarsomine treatment (cage confinement, oxygen, corticosteroids (prednisone), heparin (75-100 U/kg SC tid) → to decrease risk of pulmonary thromboembolism
- dogs with right-sided HF: furosemide (1-2 mg/kg bid) + ACE-inhibitor (enalapril 0.5mg/kg/day), Inodilator (pimobendane 0.25 mg/kg bid) to support myocardial function, Sildenafil (initially 1mg/kg tid) as pulmonary vasodilator
- microfilariae treatment → use macrocyclic lactones: topical product containing imidacloprid and moxidectin
- Surgical extraction of adult worms → indicated for dogs with Caval syndrome
cats treatment of heartworm disease
Cat: no satisfactory treatment → infection often lethal → cats in endemic regions of canine HW disease should receive drug prophylaxis
prevention of heart worm disease
- HW disease can be completely prevented with macrolides (e.g. ivermectin, milbemycin oxime, moxidectin, selamectin): macrocyclic lactones
- Preventative treatment for dogs AND cats in all endemic regions
- Labeled for dogs and cats: selamectin or a combo of imidacloprid/moxidectin
