Lesson 18, Topic 5
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Therapy may not always be 100% effective, however clinical signs and respiratory function can be markedly improved even though all worms may not be killed.

Goals of therapy:

  1. Manage heart failure if present
    1. Cautious use of diuretics
    2. Low Na diet
    3. Strict exercise restriction
    4. +/- arterial vasodilators
      • i. Sildenafil is a phosphodiesterase V inhibitor that may be particularly useful in treating pulmonary artery hypertension
      • ii. Amlodipine or hydralazine are alternative vasodilators
    5. +/- positive inotropes particularly pimobendan due to its inodilating properties
    6. After 7-14 days of exercise restriction and heart failure therapy, start adulticide therapy
  2. Adulticide therapy
    1. Melarsomine (Immiticide)
      1. The adulticide of choice
      2. Administered by deep IM injection in the lumbar muscles (L3-L5). Strict adherence must be paid to the manufacturer’s instructions.
      3. The standard protocol can be administered for dogs at low risk of thromboembolic complications
        1. 2.5 mg/kg IM q 24 hr for 2 injections
        2. A second series of two injections 24 hours apart may be repeated in 4 months to address worms that were immature during the first series (and thus not susceptible to the adulticide).
      4. An alternate dosing protocol is recommended for dogs with more advanced disease, thus at greater risk of embolic disease
        1. A single injection of 2.5 mg/kg IM initially
        2. In 4-6 weeks follow with the two injection protocol given 24 hours apart
        3. The lower initial dose is expected to result in fewer worms killed to reduce the embolic load on the lungs
        4. This protocol is the treatment of choice by the American Heartworm Society, regardless of disease severity
      5. Diminishing anti-inflammatory doses of prednisone can be used to reduce the risk of pulmonary arteritis and embolization
      6. It is critical to enforce strict exercise restriction for a period of at least one month after each series of treatments
      7. Releasing (antigen test) should be performed 6 months post-adulticide treatment
      8. Ivermectin
        1. Continuous monthly administration of prophylactic doses of ivermectin is effective against young adult heartworms
        2. The adulticidal effect takes 1-2 years, however. Thus the infection persists and causes disease during this period of time. As such, ivermectin is not a substitute for conventional adulticide therapy and should only be considered for those who decline conventional adulticide therapy.
        3. Exercise restriction must be enforced for the duration (1-2 years) of this treatment.
        4. Periodic monitoring (Q 4-6 months) must be performed.
      9. Microfilaricide therapy
        1. Should be initiated as soon as a diagnosis of heartworm disease is made, thus this is often before adulticide therapy is initiated
        2. The macrocyclic lactones are the safest and most effective microfilaricidal agents
        1. Ivermectin, milbemycin oxime, moxidectin, selamectin
        2. Administer chemoprophylactic doses
        3. Complete elimination of microfilariae occurs within 6 months of uninterrupted treatment
        4. Milbemycin oxime is the most potent microfilaricide at the recommended dose and produces the most rapid clearance rate
          • Associated with the rapid death of a large number of microfilariae (especially in cases of large infection) some dogs may experience shock-like signs of tachycardia, weak pulses, pale mucus membranes and dyspnea (2% incidence). Volume expansion and administration of corticosteroids may be beneficial. Other reactions (4% incidence) include lethargy, anorexia, and vomiting. These reactions are mild, last <48 hours and usually require no treatment. Fatalities are rare.
        5. Prophylaxis:
          1. Heartworm is a preventable disease and chemoprophylaxis should be a priority for all dogs in endemic areas.
          2. Puppies as young as 8 weeks of age should be placed on prophylaxis
          3. Reduces the reservoir population
          4. For all dogs over 6 months of age heartworm status should be determined prior to starting chemoprophylaxis.
          5. Macrocyclic lactones are the drugs of choice (ivermectin, milbemycin oxime, moxidectin, selamectin)
            1. Kill microfilariae, 3rd and 4th stage larvae, and young adult heartworms in some cases
            2. Administered monthly
            3. Very safe
            4. Very effective
            5. Provide retroactive efficacy of at least 1 month
            6. Short term lapses in administration should not affect protection
            7. Begin prophylaxis within one month of the anticipated start of transmission and the last dose should be given within one month after transmission ceases
            8. Can begin medication in asymptomatic or mildly symptomatic infected dogs (see above)
            9. Dogs with moderate or severe infection may experience complications with the killing of many microfilariae simultaneously (see above), therefore initiation of treatment in hospital is recommended
            10. Collie dogs (autosomal recessive inheritance) and other p glycoprotein deficient dogs are unusually sensitive to high doses of ivermectin (doses in excess of 16 times the minimum effective prophylactic dose). Toxicosis has been reported with overdosage of other macrocyclic lactones.
          6. Diethylcarbamazine (DEC) is another chemoprophylactic choice however it requires daily dosing thus is rarely used (especially now in the face of availability of the monthly macrocyclic lactones)
            1. Efficacy is entirely dependent on daily use
            2. Therapy should begin 2 weeks before the onset of mosquito season and continue until 2 months after the end of mosquito season (first frost)
            3. Dogs must be shown to be amicrofilaremic before starting DEC as severe and potentially life-threatening reactions may occur when given to microfilaremic dogs
              • Reactions usually occur within 30-60 minutes and involve diarrhea, vomiting, depression, lethargy, incoordination, tachycardia, bradycardia, dyspnea, and peripheral circulatory shock