Heartworm Prevention
Holly Frisby, DVM, MS
Veterinary Services Department, Drs. Foster & Smith, Inc.

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The best program for prevention of heartworm infection includes using preventatives, performing routine heartworm testing, and reducing exposure to mosquitoes.

Heartworm preventatives

Medications used to prevent heartworm infections are called preventatives. The first thing to remember is that preventatives are NOT used to kill the adult worms. Special drugs called adulticides must be used to kill the adults. These drugs will be discussed in the treatment sections. Some preventatives can cause severe problems if given to animals with adult heartworms. Follow the recommendations of your veterinarian and the manufacturer of your preventative in regard to testing prior to giving the preventative.

A number of heartworm preventatives for dogs are on the market. Some are given daily, others, monthly. Recently, an injectable preventative is available that is given once every 6 months. Some heartworm preventatives, or drugs that are combined with them, will control other parasites.

Monthly Preventatives

When determining when to give monthly preventatives, remember they kill larvae the animal was exposed to in the preceding thirty days. For instance a monthly preventative given on July 1 will kill larvae the animal acquired from June 1 to July 1. The monthly preventatives do NOT work forward in time. A monthly preventative given July 1 will NOT have an effect on larvae the animal acquires later in July.

In some areas of the country where mosquitoes are present year round, preventatives must also be given year round. In colder climates, preventatives are started 2-3 weeks after the first mosquitoes appear and are given until after the first hard frost (when the mosquitoes are killed or hibernate).

Daily Preventatives

The daily preventative called diethylcarbamazine kills larvae the dog is exposed to the day the preventative is given. It does NOT work backward. So a major disadvantage of the daily preventative is that if even 2-3 consecutive doses are missed, the larvae introduced into the dog during that time will not be killed, even if the preventative is resumed. Also, severe reactions can result if diethylcarbamazine is given to a dog with heartworm infection. In addition, diethylcarbamazine can occasionally cause severe, sometimes fatal, liver disease in dogs.

A daily or monthly preventative should be given to all dogs. Remember that mosquitoes can get indoors, so even though your dog may not go outside, the dog is still susceptible.

Testing

When and how often dogs should be tested is a matter of debate. It somewhat depends upon how common heartworm disease is where the animal lives, what heartworm preventative the dog is taking, and how long the mosquito season lasts. In areas where mosquitoes are seasonal, the timing of the test is an important consideration.

Testing dogs on a monthly preventative, given seasonally

Remember that the time interval between when the dog is infected with microfilaria and the development of the microfilaria into adult worms is about 6 months. If we perform an antigen test less than 6 months after the last possible bite from an infected mosquito, immature heartworms may be present but the antigen test will not detect them since it only detects adult worms. Therefore it is advised that for dogs on seasonal preventative, any testing should be performed no earlier than 6.5 months after the last possible date of transmission (the last date preventative medication was given).

Testing dogs on a monthly preventative, given year-round

For a dog that receives a monthly heartworm preventative year round, the American Heartworm Society suggests that the interval between tests could be 2-3 years. This is only if there were no missed or late doses of preventative. If a dose was missed or late, the dog should be tested on an annual basis. It is also recommended that a test be performed after the first year the dog has been on the preventative. Heartworm disease is very common in South Florida and mosquito season is practically year-round. Therefore, the Humane Society of Greater Miami requires that all dogs, regardless of whether they are on monthly preventative or not, be tested annually.

Testing dogs on a daily preventative

Dogs receiving diethylcarbamazine on a seasonal basis should definitely be tested before the preventative is resumed in the spring or summer. Dogs receiving this daily preventative all year round must be tested annually because of the severe reactions that are possible if diethylcarbamazine is given to a dog with adult heartworms.

In areas where heartworm preventatives are given seasonally, a puppy that was not alive during the previous mosquito season would not have to be tested. For instance, in northern Wisconsin, puppies born in November do not need to be tested prior to starting them on heartworm preventative the next spring/summer. Remember, any puppy less that 6.5 months old is probably going to have a negative antigen test since any larvae the pup was exposed to would not have had time to mature and be detected by the antigen test.

 

© 2000 Drs. Foster and Smith, Inc.
Reprinted as a courtesy and with permission from
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