Science and the Breeder: Ringworm and Lufenuron

Carol W. Johnson, DVM PhD

Microsporum canis, the most common cause of ringworm, is a parasitic fungus that is highly adapted to cats. Once introduced into a cattery, ringworm can rapidly spread and can infect most or all of the cats in a cattery before the breeder is aware that there even is a problem. In an infected cattery, the lesions often appear to resolve in adult cats after several months and kittens often will no longer show lesions as they approach maturity. In most cases, however, these cats still harbor low levels of fungus and serve to infect the kittens that are born into a cattery. Ringworm can also infect humans. While ringworm infections tend to be self-limiting in most adults, children and immunosuppressed individuals may develop severe infections that may take weeks or months to eradicate. One elderly woman told me of suffering for months after her children surprised her with a purebred kitten to provide companionship during her post-kidney transplant convalescence. The kitten carried ringworm and the poor woman developed huge skin sores and lost her hair. She became quite ill from the antifungal treatment, then almost lost her transplanted kidney during the effort to get rid of the fungal infection. Children, also, can get very severe lesions and ringworm infections of the head are considered to be fairly serious by pediatricians.

Some of the biggest impediments to all catteries becoming ringworm-free has been the expense of treatment and the limited safety of the less expensive drugs. Griseofulvin (Fulvicin) is inexpensive and moderately effective against ringworm, but causes severe neutropenia and immunosuppression that can be fatal in up to 10% of the treated cats. Itraconazole (Sporonox) is much more effective and safer, but can cost up to $200 to treat a cat. Those of us who claim a ringworm-free cattery have often had to spend thousands of dollars to maintain that status and treatment of a large cattery is often beyond the economic ability of some breeders. However, an exciting recent article [Ben-Ziony Y, Arzi B. Use of lufenuron for treating fungal infections of dogs and cats: 297 cases (1997-1999) JAVMA 217(10) (Nov 15) 2000] describes what may prove to be an economically viable treatment and prevention for ringworm.

A veterinarian in Israel noted that dogs and cats treated with lufenuron for flea prevention did not appear to develop ringworm, even though they were not being treated for ringworm. Lufenuron (sold in the US by Novartis under the brand name PROGRAM) is a chitin synthetase inhibitor used for flea control. Chitin is a structural molecule in the exoskeleton of insects and their eggs. After administration, lufenuron sequesters in the fat and is slowly released into blood where it is ingested by a female flea. The drug then interferes with the production of chitin in the eggs, which leads to the eggs drying out after they are laid. Chitin is not found in mammalian tissues, but is a structural component of the fungal cell wall. Thus, this drug does not appear to affect mammalian enzymes and as a result has had an excellent safety record when used according to its product insert.

The Israeli clinic systematically tested lufenuron in ringworm-infected cats and dogs. Over the 2-year period, they treated 201 cats and followed 23 on a daily basis. Most cats treated with lufenuron doses ranging from 51.2 to 266 mg/kg (23.1 to 120.9 mg/lb) cultured negative for ringworm in 8.3 days and began growing hair in 12 days. Four cats either cultured positive for ringworm or developed lesions again but responded well after a second treatment. None of the cats showed signs of toxicity.

I had some concerns regarding the safety of this treatment because the article used between 5 and 20 times the recommended dose used to treat fleas, so I called Novartis’ Customer Service line and spoke with a veterinarian. He reported that Novartis had no information on the use of lufenuron for ringworm and the article took the company by surprise. Because this is off-label use, he can not recommend the drug for this indication. However, Novartis is very excited about the article and its potential. We extensively discussed the safety data Novartis had performed for registration, and then I reviewed the safety data on the FDA website made possible through the Freedom of Information Act. Safety studies in both dogs and cats showed the drug had a wide margin of safety. Reproduction studies were performed in dogs and cats and lufenuron did not to cause toxicity or congenital defects at the doses tested. Because this is off-label use and one article does not prove efficacy or safety, I can not recommend lufenuron for the treatment of ringworm in cats. However, for those breeders determined to try it, I have some suggestions.

Lufenuron is sold under the brand name PROGRAM. SENTINEL also contains lufenuron, but in addition contains milbemycin, which may be toxic when given at the overdoses suggested by the article. So do NOT use SENTINEL for ringworm. Lufenuron comes as a tablet, a suspension, or as an injectable. The injectable lasts 6 months, but can leave a lump (granuloma) at the injection site, and this may be a consideration for show cats. Novartis found that the tablet appears to have better efficacy than the suspension. I have not used PROGRAM, but understand that some cats do not like the taste of the suspension, so this may be a consideration when treating some cats.

If you are going to try it to see if it works on a few cats, try to keep them separate from other ringworm-infected cats. Ringworm, like many parasitic organisms, can become drug resistant and it will do neither the breeder nor the rest of the cat fancy any good to develop Lufenuron resistant ringworm. Similarly, once the decision to treat a cattery is made, break down, bite the bullet, and treat the whole cattery and not just a few cats. Couple treatment of cats with physically cleaning the cattery to get rid of the spores so the cats are not reinfected. Getting rid of the problem will be cheaper in the long run than living with it and will be much less likely to generate resistant forms.

Plan to treat once monthly for 2 to 3 months to make sure it is really gone. Yes, the article had good success after one treatment, but a cattery situation is very different than the average pet household. If it works please let other breeders know! Me? I currently have a ringworm-free cattery. But I sometimes show adjacent to cats that have ringworm lesions (yes, I notice those things) and have, on occasion, brought ringworm home from the shows. So I will probably treat my show cats with the lufenuron dose recommended for fleas. At those doses it is unlikely to hurt the cats and may help keep them from bringing ringworm home.