Lufenuron is used worldwide for control of fleas on cats and dogs. This agent disrupts the synthesis of chitin, a structural component of the exoskeleton of arthropods. Chitin is also a component of the outer cell wall of fungi. A retrospective study suggested that lufenuron treatment was associated with recovery in a large number of dogs and cats with superficial fungal infections. This experimental trial was designed to determine the effects of lufenuron treatment in cats on the establishment and course of Microsporum canis infection in infected cats. Cats were between 14 and 16 weeks of age and weighed between 1 and 1.3 kg at the start of the study. One group of eight cats received the commercially available oral lufenuron suspension in accordance with label instructions. Four doses were administered to each cat in canned cat food 28 to 30 days apart before challenge exposure and additional dosages were given once every 28 to 30 days for the rest of the study. Another group of eight cats received the commercially available injectable lufenuron suspension in accordance with label instructions. The first injection was given 4 months before the challenge exposure and a second injection was given 6 months later. A group of eight cats received only the suspension vehicle for the oral preparation at the same dosing schedule as the cats in the oral treatment group.

The cats were challenged by exposure to experimentally infected cats. All treated and control cats developed dermatophytosis. Infection developed more slowly in the cats treated with lufenuron than in the control cats. The cats receiving oral lufenuron were the slowest to develop lesions. Once established, infection resolved in approximately the same time for all groups.

There are several possibilities why the results of this study varied from field reports of efficacy. Since the lufenuron was administered before the challenge exposure, it is possible that the organisms may have been able to adapt when they encountered the drug in the skin. If the organism is already established when the drug is given, the transient retardation of growth may give the animal's already-stimulated immune system a chance to overcome the infection. Another possibility is genetic differences in chitin synthesis among fungi, and a third is that the historically given dose may be lower than necessary to be effective. Recent anecdotal reports stress that clinicians use a variety of doses in field cases. An effective dosage has not been established. It is also possible that the successful treatment mentioned in some anecdotal reports was the result of self-cure. This study was supported by Novartis Animal Health.

Effects of lufenuron treatment in cats on the establishment and course of Microsporum canis infection following exposure to infected cats. DeBoer DJ, Moriello KA, Blum JL, Volk LM. JAVMA 222:1216-1220, 2003.