Common Questions and Answers

I have used this opportunity for reporting on the results of the annual surveys to respond to the more common inquiries I receive on testing for and prevention of HW.

A. In what areas should dogs be tested for heartworm?

The greatest need for testing is in the four heartworm endemic areas in Canada. Those areas are southern Ontario, southern Quebec, southern Manitoba and in the Okanagan Valley in British Columbia. Within those areas there are foci where prevalence of the infection is the highest and where as many dogs as possible should be tested. Outside of those areas, fewer dogs could be tested.

In last year's report, "Heartworm in Atlantic Canada in Dogs in 1995", I provided a map with estimates for various areas in Canada on the probability that HW could be transmitted if infected dogs and suitable mosquitoes were available (Slocombe, et al 1995). This should provide some guidance on the effort and approach needed for testing dogs in nonendemic areas. In summary, there is at least a 90% probability that HW could be transmitted in New Brunswick, Nova Scotia and Prince Edward Island, in most of Ontario and Saskatchewan, in eastern Alberta and in southern Manitoba, Quebec and British Columbia. In Newfoundland the probability of transmission is at most only 60%. The highest prevalence of infection is likely to be found along Canada's southern border because in most of that area there is 100% probability of transmission. HW is unlikely to be established in northern Canada above the 60th parallel.


All practices should test some dogs. In highly endemic areas, all dogs should be tested. In nonendemic areas and especially areas where there is a high potential for HW to occur an aggressive approach should be taken to test (1) new dogs entering the area and (2) dogs intending to travel to endemic areas.

B. What is the best test for screening for heartworm infection?

Antigen tests are more sensitive than microfilariae recovery procedures. However, the antigen that is detected in these tests is produced principally by reproductively active female worms and the amount of antigen appears directly related to the number of those worms. Further, antigen is not detected consistently from female worms until after 8 months of infection. Dogs with few worms, female worms early in the patent period, immature worms, senile worms or only male worms will have extremely low levels or no antigen and tests on these dogs may result in false negatives. In addition, a low percentage of dogs with HW are antigen negative yet microfilaria positive.


ntigen tests are highly specific, but when an antigen test is positive other evidence that the dog may have HW should be sought. The evidence could include data from a repeat of the test or preferably from a different type of antigen test, a microfilarial recovery procedure, clinical signs, radiographic changes, a history of the dog having missed treatments or received inadequate dosage, or that the dog had resided in a highly endemic area and was not on preventive medication.


Dogs seen by a practitioner for the first time should be tested with both a microfilarial recovery test (Knotts or filter) and an antigen test to give the best information on the status of these dogs. For screening dogs that had been on ivermectin and milbemycin the antigen test is the best approach.

C. What time of year should dogs be tested?>

The optimum period for annual routine testing is related to the length of the HW transmission period. The time to start testing can be calculated by adding 6 months, i.e., the length of the prepatent period for D. immitis, to the end of the HW transmission period (Slocombe, et al. 1989). The end of the time for routine testing for dogs on preventive medication would be one month after the "earliest start date" of the HW transmission season. For unprotected dogs in highly endemic areas testing should be completed before the start of the mosquito season. I expect to release information on the heartworm transmission period for Atlantic Canada shortly, but until that time a conservative estimate would be to use the data for Ontario where the transmission period has been documented.


Start Time. The time to start testing would be early April. If testing is done earlier, then a repeat test would be required to ensure that the dog was not infected late in the previous season. These recommendations are for dogs that had not travelled outside of Canada. The time for testing of dogs that had visited endemic areas outside of Canada would be no earlier than 6 months after they had returned.
End Time. Dogs Which Had Not Been on Preventive Medication. If practitioners consider that the infection rate in unprotected dogs is high and dogs are likely to be infected, then such dogs should be tested before the beginning of the mosquito season. Dogs Which Had Been On Heartgard30, Heartgard30 Plus, Interceptor or Sentinel. Dogs should be tested by June 30.

D. When should dogs be placed on preventive medication?

The period for use of medication is related to the length of the HW transmission period (Slocombe, et al. 1989). Medications should start one month after the "earliest start date" of the HW transmission season and end on the first day of the month following the "latest end date" of the transmission season. At this time, a conservative estimate of these times would be to use data as generated for Ontario.


Heartgard30, Heartgard30 Plus, Interceptor, or Sentinel need not be started until July 1 and the last medication can be given on November 1.

E. Should dogs on preventive medication be tested for heartworm?

Periodic retesting is the only way to ensure that dogs are not infected.


After the first season of medication with Heartgard30, Heartgard30 Plus, Interceptor, or Sentinel the dogs should be checked at the start of the next season to certify compliance before prescribing further medication. Thereafter, the frequency of testing of dogs on these medications is at the discretion of the practitioner who must be satisfied about compliance before prescribing the medications. Retesting would certainly be required if there is a suggestion or evidence that the schedule for prophylaxis was incomplete, an uncertain history of exposure to infection, or for dogs that may have grown beyond the range of dose of the medication that had been prescribed. Prescribing the medications without retesting is an "extra label" application and practitioners who take this course of action may wish to obtain from the client a release of liability. Dogs using DEC should be tested every year. For a full discussion on the issue of retesting see Slocombe, 1996.

There is the question of whether the recommended times for annual testing need be followed with dogs tested less frequently than annually. Testing at the recommended times and regardless of the frequency of testing ensures a knowledge of the status of the dog right up to the time of testing. However, adherence to the recommended times becomes increasingly less critical the greater the number of years between tests since the dogs could have had exposure to HW over several seasons. If dogs are tested at other than the recommended times, the owner should be advised on the time period for which that test is valid.

F. Should dogs which had been treated with an adulticide be tested subsequently for effectiveness of the adulticide?

A dog should be tested for HW antigen after use of an adulticide to determine the effectiveness of clearance of adult HW. If the dog is not so tested and is placed on preventive medication for a season there will be difficulty in discerning whether a positive antigen test the following spring is a result of failure of the preventive medication or inadequate clearance of adult worms.


Test a dog 3-4 months after treatment. Antigen from disintegrating worms will not disappear from circulation until 3 months has elapsed after giving the adulticide.

G. What are the concerns when an HW infected dog is not given an adulticide but placed on HW preventive medication?

Adult HW are the primary pathogens in the dog and for most dogs these should be removed with an adulticide. HW disease is a treatable condition and most dogs treated appropriately with an adulticide show considerable physical improvement. However, when there are medically appropriate reasons to not give an adulticide to an infected dog there are several concerns: (1) there is the potential for development or increase in the severity of clinical signs, (2) if the dog is microfilaremic it is a source of infection for other dogs and (3) when the dog is given a preventive medication, which is also microfilaricidal, there is the potential for an allergic reaction to the dying microfilariae. The dog, therefore, should be monitored carefully.


(1) Monitor the dog periodically for development of or increased pathology and clinical signs. (2) When mosquitoes are present confine the dog indoors until the dog is amicrofilaremic. (3) Administer a microfilaricide with adequate monitoring for a reaction to the dying microfilariae. Interceptor, which is also microfilaricidal, is a useful choice. But there is no data, of which I am aware, to support how frequently to use Interceptor in such circumstance. I suggest that the medication be used every 2 weeks for at least 2 months and that the dog be checked.monthly for microfilariae during the period. After the dog is amicrofilaremic, the medication is administered monthly.