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Guidelines for the Diagnosis, Prevention and Management of Heartworm (Dirofilaria immitis) Infection in Dogs


This is an excerpt taken from a full article which can be found on the American Heartworm Society site.

Prepared and approved by the Executive Board of the American Heartworm Society (Officers: Dr. Charles Thomas Nelson, President; Dr. Donald W. Doiron, Past President; Dr. John W. McCall, Vice President; Dr. Sheldon B. Rubin, Secretary-Treasurer; Dr. Lynn F. Buzhardt, Dr. Wallace Graham, and Dr. Susan L. Longhofer, Board Members; Dr. Jorge Guerrero, Symposium Chair; Dr. Carol Robertson-Plouch, Symposium Co-Chair; and Dr. Allan Paul, Editor)


These recommendations are based on the latest information presented at the 2004 triennial symposium of the American Heartworm Society. Revisions to the last recommendations, published in 2003, are based on new research and additional clinical experience, particularly in the areas of heartworm chemoprophylaxis, and serologic testing/retesting. This document focuses primarily on procedures and largely omits discussion of the better known pathophysiologic mechanisms and clinical features of heartworm disease in dogs. Guidelines for the diagnosis, treatment and prevention of heartworm infection in cats are contained in a separate document.


As lack of effectiveness has been reported for all macrocyclic lactones, annual retesting is an integral part of ensuring that prophylaxis is achieved and maintained. Where heartworm transmission has a local seasonal cycle, scheduling for retesting should take into consideration the seven-month predetection period used for primary screening.

Evaluation of Product Efficacy Following Noncompliance and Changing Products

Evaluation of the efficacy of a heartworm preventive product administered to unprotected dogs over six months of age or to dogs in which there is a known or suspected breach in dosing compliance for three months or longer should be done with caution. Such a dog should be tested for antigen prior to starting (or resuming) preventive therapy and if antigen-negative, should be tested again four and nine months later. Considering that an antigen test may be positive as early as five months after infection and most dogs are positive by nine months (and assuming that noncompliance during the nine-month period was not an issue), a positive test result before dosing is started (or resumed) and/or at four months indicates that the dog was infected prior to initiation of dosing. If the dog is positive only at nine months, it is equally possible that the dog was infected prior to the first dose or that the product failed. If the dog is positive only after nine months, product failure is most likely the reason for infection.

When changing chemoprophylaxis products and noncompliance is not an issue, special consideration needs to be taken for assessing heartworm infection status and evaluating efficacy of the products. To most effectively evaluate the efficacy of the original and new products, the dog should be tested for antigen prior to changing products, three months (for monthly products and DEC) or four months (6-month-injectable products) after changing products and again five months later (i.e., eight to nine months after changing products). If the dog is positive prior to changing products and/or at three to four months, the original product was ineffective. A positive test only at eight to nine months after changing products could be due to failure of either the original or the new product. Thereafter, a positive test result is most likely due to lack of full efficacy of the new product, if all previous tests were negative.

Practical considerations will probably result in testing at the time of the late start/resumption of chemoprophylaxis in a non-compliant case or product change in a compliant case, six months later (which gives only equivocal results as to which product has been ineffective) and then at the next annual examination. However, for medicalegal reasons and for manufacturer efficacy guarantees, additional testing at three to four months and eight to nine months, rather than at six months, is strongly recommended.

Summary of Retesting Results

Original Product Test at change Test timing after change (months) Positive results Confirmatory testing (months) Positive results Subsequent testing (months) Positive results (if earlier negative)
DEC Yes 3 Failure of original 8 Failure of either 8 Failure of new
Macrocyclic lactone (oral or topical) Yes 3 Failure of original 8 Failure of either 8 Failure of new
Moxidectin SR Yes 4 Failure of original 9 Failure of either 9 Failure of new
Unprotected dog or non-compliance >3 months At (re-) initiation 4 Infected prior to initiation 9 Infected prior or failure of original 9 Failure of new

Monthly Ivermectin, Milbemycin Oxime, Moxidectin and Selamectin

Macrocyclic lactone chemoprophylaxis will clear microfilariae from the blood of dogs with patent infections by exerting a direct or indirect microfilaricidal effect, depending on the specific product used, and retarding repopulation by gradually suppressing embryogenesis. With uninterrupted dosing, elimination of microfilariae is usually complete within six to 12 months of oral dosing with monthly macrocyclic lactones or one month following moxidectin SR injection. Once the adults are sterilized, clearance is generally permanent unless the dog is re-infected. In the event a pre-existing prepatent infection matures after starting macrocyclic lactone chemoprophylaxis, microfilariae are unlikely to be found, or appear only transiently in small numbers. Since macrocyclic lactone chemoprophylaxis may negate microfilaria testing and microfilariae do not contribute to heartworm antigenemia, antigen testing is the most reliable method of retesting.

To verify that a chemoprophylaxis program has been successfully started, retesting approximately seven months following the end of the first transmission season is advised. A negative antigen test at this time generally ensures that a prepatent infection did not precede initiation of chemoprophylaxis, and that an adequate dose was administered to dogs started on chemoprophylaxis before attaining their mature weight.

Annual retesting will not fulfill the objective of early detection if performed indiscriminately within the calendar year without regard for the seven-month predetection period (see Test Timing for Optimal Results). For example, testing in early January will not detect an infection occurring in late July. If the next annual retest is performed the following January, the effective testing interval is 18 months.

Moxidectin SR Injections

Since administration of this form of chemoprophylaxis is completely under the control of a veterinarian, the medical record should leave no doubt about the timing and frequency of treatment. A retest should be performed after completion of the first cycle of protection to ensure that a prepatent infection was not present. As with all chemoprophylaxis products, periodic testing will ensure there have been no efficacy breaks

Daily DEC

The chance is greater that brief interruptions in DEC administration will cause breaks in heartworm protection. In the event a microfilaremia should occur, these dogs are at serious risk of developing potentially fatal reactions following resumption of DEC chemoprophylaxis. Since antigen testing may miss an occasional microfilaremic dog, a microfilaria test must be run before resuming seasonal prophylaxis with this drug. Even if DEC is given daily throughout the year without interruption, it is still prudent to retest annually for microfilariae. Antigen testing is recommended highly for its greater sensitivity but is not a substitute for microfilaria testing when DEC is used for chemoprophylaxis.

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