A statistical approach for evaluating the effectiveness of heartworm preventive drugs: what does 100% efficacy really mean?

Table 1 Efficacy data for various trials reported in Snyder et al. [10] and [11] using the MP3 strain of Dirofilaria immitis and treatment using milbemycin oxime (0.5–0.75 mg/kg) and spinosad (30–45 mg/kg)

Treatment regimen^a,b	Number of worms recovered in treated dogs	Geometric mean	Model-based mean	Model-based 95% CI
30 days^a	1	99.8	99.8	99.2, 100
30 days^b	5	98.99	98.51	97.36, 99.44
45 days^b	5	98.87	98.52	97.37, 99.45
30 and 60 days^b	1	99.63	99.51	98.59, 100
15 and 45 days^b	0	100	100	N/A
30, 60 and 90 days^b	0	100	100	N/A

One, two or three doses of drug were administered either 30, 45, 30/60, 15/45, or 30/60/90 days post inoculation with infective L3. Model based mean and 95% confidence intervals (CI) were calculated using the parametric bootstrap model described in the text
^aData from Snyder et al., [10]
^bData from Snyder et al., [11]