Lesson 19 of 20
In Progress

Feline Heartworm Disease

  1. General Remarks:
    • fewer adult worms are present (1 to 5)
    • clinical signs are usually more severe, usually due to heartworm death. The relative space occupied by the worms in the pulmonary arteries is larger as cats have smaller pulmonary arteries.
    • Occult Heartworm Disease (amicrofilaremia) is more common – 80% of infected cats. Occult infection occurs due to host immune-mediated clearance of microfilariae and suppression of microfilariae production.
    • where microfilaria are present they are usually of low numbers and of short life span. Mircofilaremia seldom persists beyond 228 days post infection (circulating microfilariae appear about 195 days post infection). Thus microfilaremia is short lived.
    • prepatent period is approx 7 months
    • the mechanisms of pulmonary pathology in the cat are similar and more severe than in dogs. Embolization of pulmonary arteries due to early heartworm death and host reactions are more intense than in the dog
    • aberrant migration is more common in cats
    • life span of adult worms is much shorter in the cat – 2-3 years.
    • pathology is related to:
      • Arrival of immature heartworms in the pulmonary arteries and their death
        1. Occurs about 3-4 months post infection
        2. Incites an inflammatory arteritis
        3. The clinical picture looks like allergic airway disease (asthma) – called Heartworm Associated Respiratory Disease (HARD)
      • Death of adult heartworms
        1. Live heartworms suppress the immune response
          • This permits many cats to tolerate their infection without ill effects
        2. Once mature worms begin to die – the degenerating worms incite pulmonary inflammation and thromboembolism
          • Single worm infections and their death can cause even cause severe reactions
      • Caval infections are rare in cats – due to small worm burden?
    • Whereas the antigen test to diagnose heartworm is the best test (gold standard) in dogs; because unisex infections consisting of only male worms or symptomatic immature infections are more common in cats, none of the available antigen tests can be relied upon to rule out heartworm disease in cats.
  2. Signalment:
    • cats of all ages usually 3-6 years; no breed predilection
    • 75% are males
  3. History:
    • many cats tolerate their infection without noticeable clinical signs
    • cats may die acutely, exhibit chronic signs or be asymptomatic
    • lethargy, anorexia, and weight loss
    • vomiting intermittently (50%)
    • coughing/dyspnea (50%)
    • sudden death
    • ascites, exercise intolerance, and signs of right heart failure are rare
  4. Physical Examination:
    • harsh lung sounds
    • fine crackles on auscultation of pulmonary field
    • CNS signs may occur due to aberrant filarial migration
  5. Radiology:
    • the best diagnostic tool
    • enlargement, tortuosity, and pruning of the pulmonary arteries especially the caudal pulmonary arteries
    • alveolar disease with patchy coalescent areas or consolidated lung lobes may be noted
    • pleural effusion (30%)
    • arterial changes are not as prominent in the cat as the dog
    • main pulmonary artery enlargement is not detected
    • pulmonary infiltrate with eosinophils (30%)
  6. Electrocardiography:
    • right heart enlargement (25%)
  7. Blood Work:
    • Hematology
      • eosinophilia – 33% incidence, usually resolves in 7 months
      • basophilia – not common, usually resolves
        hyperglobulinemia – usually present
    • Concentration techniques
      • most cats have no circulating microfilaria
      • same techniques as for dogs
    • Immunodiagnostics
      • IFA and ELISA are useful; sensitivity is about 50% with the ELISA tests
        • The antigen tests are highly effective at detecting even single adult female worm infections, however not good to detect unisex male or immature worm infections.
        • 50% – 70% of cats have at least one adult female worm
      • 30% of cats are IFA positive (rare for false positive)
        1. Antibody tests have the advantage of detecting both male and female worms, and larvae (antibody can be detected as early as 2 months post infection.
        2. Antibody tests do not indicate continued infection – only that exposure occurred.
        3. Different antibody tests vary in their sensitivity to each stage of larval development.
        4. Most cats that are antibody positive have only been transiently infected to the L4 larval stage.
      • Since both L5 larvae and adult worms are capable of causing clinical disease in the cat, both antigen and antibody tests are useful and should both be employed
  8. Goals of Therapy:
    1. Supportive therapy
      • severe exercise restriction
      • diuretics
      • corticosteroids: prednisone (1-2 mg/kg SID-TID) for cases of severe lung disease due to thromboembolism
    2. Adulticide therapy
      • a treatment of last resort for cats in stable condition but with clinical signs not controlled by corticosteroids
      • thiacetarsamide (0.22 ml/kg BID x 2 days)
      • melarsomine is toxic to cats at doses as low as 3.5 mg/kg
      • ivermectin at 24 ug/kg monthly administered for 2 years will reduce worm burden by 65%
      • to date there is no evidence that any adulticide therapy increases survival in cats
    3. Surgery
      • ideal to remove worms and not destroy/damage them in situ
    Comment: Massive thromboembolism occurs with killing adult worms; cats should be hospitalized and observed for 2 weeks post treatment.
    1. Microfilaricide therapy
      • usually not required
      • levamisole is effective
    2. Preventative
      • usually not required in Canada
      • monthly prophylaxis is safe and effective in areas where heartworm is endemic in dogs
        • oral (ivermectin or milbemycine oxime) or topical (moxidectin or selamectin)
        • these agents can be given even in cats that are antibody or antigen positive since miicrofilaremia is uncommon
  9. Prognosis:
    • most cats are successfully treated with restriction and corticosteroids
    • if right heart failure is present, they are difficult to manage
    • detectable antigenemia will disappear by 4-5 months of spontaneous or adulticide induce elimination