I'm sorry that your question wasn't answered in a timely manner. After receiving the injectable lufenuron injection, a small lump or tissue reaction at the injection site has been noted in some cats. A few weeks may be required for this to dissipate. Systemic reactions appear to be rare. The manufacturer reports that the adverse rection rate is less than 5 animals in one million doses. My initial thought is that a severe flea saliva allergy is present that hasn't responded to the steroids yet perhaps due to improper dosing or another allergic dermatitis exists. Please note, too, that lufenuron doesn't kill adult fleas. Fleas must feed on your cat to then prevent flea eggs from hatching. I'm going to post my entire synopsis of the pruritic (itchy) cat for you...
Pruritic cats are suffering from an allergic dermatitis in the great majority of cases.. Allergies to flea saliva, environmentals such as pollens, molds, dust and dust mites, and foods should be considered. In rare instances the mange mite Demodex might be responsible and is diagnosed with skin scrapings examined microscopically. Bacterial skin infection (pyoderma) is under-diagnosed. Your vet can perform a cytology (microscopic exam of a small sample of your cat's skin surface) looking for abnormal numbers of either bacteria or yeast.
Our dermatologists tell us to apply an effective over the counter flea spot-on such as Advantage or a fipronil-containing product such as Frontline or one of the newer prescription products available from your vet even if fleas aren't seen. It would be prudent to switch to one of the newer flea products in a different class of insecticide such as Activyl (indoxacarb), Cheristin (spinetoram), Comfortis (spinosad), or Vectra (dinotefuran & pyriproxyfen). These are all prescription products available through your vet. You can purchase the oral Capstar (nitenpyram) over the counter in pet/feed stores, however, which will kill any fleas on your cat within minutes of your dosing her. Be sure to treat your premises with an over the counter area treatment spray that contains an insect growth regulator (IGR) such as Siphotrol Area Treatment Spray containing the IGR methoprene. The IGRs don't allow flea eggs and larvae to develop into adult fleas and so the life cycle of the flea is broken. Cats can be such effective groomers so as to eliminate all evidence of flea infestation. Indoor cats can contract fleas because we walk them in on us and flea eggs and larva can remain viable in your home for months. Turning on the heater or as the weather warms at this time of year then hatches the eggs. Flea saliva allergy is usually most evident on the saddle area – the area between the edge of the rib cage and tail. In severe cases, an anti-allergenic prescription glucocorticoid (steroid such as prednisolone) will work wonders for cats allergic to the saliva of the flea. Your other pets may not be allergic to the saliva of the flea.
Environmental allergies are usually addressed with prednisolone as well. In some cats an over the counter antihistamine such as chlorphenamine (Piriton) dosed at 2mg/cat daily may be effective but antihistamines aren't reliably effective.
Food intolerance/allergy is addressed with prescription hypoallergenic diets. These special foods contain just one novel (rabbit, duck, e.g.) animal protein or proteins that have been chemically altered (hydrolyzed) to the point that her immune system doesn't "see" anything to be allergic to. The over the counter hypoallergenic foods too often contain proteins not listed on the label - soy is a common one - and these proteins would confound our evaluation of the efficacy of the hypoallergenic diet. There are many prescription novel protein diets and the prototypical hydrolyzed protein diet is Hill’s Prescription Diet z/d ultra (I prefer a hydrolyzed protein diet because it removes the possibility of my patient being intolerant to even a novel protein diet.). We usually see a positive response to these foods within a few weeks if we’ve eliminated the offending food allergen. A food intolerance can appear at any age and even if our cats have been eating the same food for quite some time. Food intolerance is not as steroid-responsive as is either a flea saliva allergy or atopy.
Please respond with further questions or concerns if you wish.