Fig. 3.

Window of infection analyses for three heartworm-positive patients. The window of infection (WOI) is the period of time in which the current infection is most likely to have occurred and includes the period of time from 9 months prior to the last negative heartworm test through the date 6 months prior to the positive heartworm test. The finding of purchase gaps 45 days or greater within the window of infection argue against product failure and, instead, indicate compliance failure. In these diagrams, a white circle represents a single heartworm preventive dose, an asterisk indicates the purchase of additional preventive medication, and a positive or negative symbol denotes the time and result of heartworm testing for the patient in question. These dogs (a, b, c) appeared to have consistent heartworm prevention coverage during initial scrutiny of patient records, but gaps >45 days were apparent in the WOI analyses. These gaps are symbolized by red coloration of the horizontal timeline. It is of interest that all three dogs showed good compliance after the first purchase of medication but were infected prior to having received heartworm prophylaxis