Heartworm positive dogs are hospitalized and given an injection of melarsomine (Rx) (brand name--Immiticide) to kill the adult worms. A second injection of melarsomine is given 24 hours later. This treatment is effective but harsh. Some pets may develop life threatening reactions to the heartworm treatment itself, which is why they must be hospitalized for the heartworm treatment.
Monthly heartworm medications prevent microfilaria from growing into adult worms. Some heartworm treatments also contain additional medications to kill intestinal worms that may potentially inhabit your pet's gut. The American Heartworm Society recommends that pets remain on heartworm protection year round because when the medication isn't given consistently some pets are developing heartworm disease. The Society feels it is easier for most pet caretakers to remember to give a heartworm medicine every month than to give it a few months of the year.