Effects of Oral Macrocyclic Lactone Heartworm Preventatives on Retinal Function and Chromatic Pupillary Light Reflex in Healthy Companion Dogs.

Objective: Determine the effect of oral macrocyclic lactone heartworm-preventative medications on retinal function and chromatic pupillary light reflex (cPLR) in healthy dogs.

Animals studied: Cross-sectional retrospective: 60 dogs (n = 33 females) with heartworm medication administration status and electroretinogram (ERG) data available. Prospective clinical study: 25 dogs (n = 10 females) had ERG performed, 18 of which had cPLR performed.

Procedures: Retrospective: ERG amplitudes/peak times were compared between dogs that had or had not received oral heartworm preventatives. Bivariate and multiple variable linear regression models were used to evaluate relationships between ERG testing and heartworm preventive administration status, age, and sex.

Prospective: ERG and cPLR testing were performed at a baseline visit (minimum 14 days since last preventative administration), and a second visit where ERG/cPLR testing was performed 4 h after oral preventative administration. Mixed effects models and Mann-Whitney U statistics were performed.

Results: Retrospective: There was no association between heartworm preventive administration status and ERG amplitudes or peak times (all p-values > 0.12).

Prospective: Heartworm preventative had no effect on light- and dark-adapted ERG amplitudes or peak times (all p-values > 0.56). Similarly, there was no effect on baseline pupil size (p = 0.83), nor on cPLR (p = 0.32).

Conclusion: No significant effects of oral macrocyclic lactones on retinal/cPLR function at preventative doses were identified. While small effects on retinal/cPLR function cannot be completely ruled out, it remains unlikely that these medications cause clinically significant visual deficits at prescribed doses, and proven antiparasitic benefits likely far outweigh small potential ophthalmic risks of administration.