Heartworm chemoprophylaxis requires authorization by a licensed
veterinarian having a valid relationship with the client and patient. To
establish this relationship, heartworm prevention should be discussed
with the client. If records of past treatment and testing do not exist,
it may be necessary to test the patient before dispensing or prescribing
chemoprophylaxis. Options for effective chemoprophylaxis include
several drugs administered either in oral, topical, or parenteral
formulations at a monthly or 6-month interval.
Canine heartworm infection is preventable despite the inherently high
susceptibility of dogs. Because all dogs living in heartworm-endemic
areas are at risk, chemoprophylaxis is a high priority. Puppies should
be started on chemoprophylaxis as early as possible, no later than 8
weeks of age. Puppies started on a heartworm preventive after 8 weeks of
age should be tested 6 months after the initial dose, then annually
thereafter. Before initiating a preventive regime on dogs 7 months of
age or older, antigen testing should be performed and in appropriate
instances, also tested for microfilariae (see PRIMARY DIAGNOSTIC
SCREENING). This will avoid unnecessary delay in detecting subclinical
infections and potential confusion concerning effectiveness of the
prevention program if a pre-existing infection becomes evident after
beginning chemoprophylaxis (e.g., chemoprophylaxis initiated during the
pre-patent period).
Evidence strongly suggests that by reducing the reservoir population
through increasing the number of dogs receiving chemoprophylaxis, a
disproportionately large decrease in the prevalence of infection among
unprotected dogs may occur relative to the percentage of additional dogs
receiving chemoprophylaxis. This collateral protection spreads the
umbrella of chemoprophylaxis most effectively in communities where
heartworm prevalence and dog population density are both relatively low.
Even though continuous, year-round transmission may not occur
throughout the country, year-round use of broad-spectrum
chemoprophylaxis products with endoparasitic and/or ectoparasitic
activity during this extended period should enhance compliance and may
assist in preventing pathogenic and/or zoonotic parasitic infections.
