Therapy for FASS
Allergen-specific immunotherapy
To formulate ASIT, the offending allergens need to be identified, and allergy testing is recommended (via skin and/or serum allergy testing) if the clients choose to manage their cats with this option. Approximately 60-75% of patients have a good to excellent response to ASIT, but it may take up to a year before a clinical response is seen, and adjunctive symptomatic therapy is often needed during the first 6-12 months of treatment. Once ASIT successfully controls the clinical signs, other therapies can be tapered and often discontinued. Premature treatment discontinuation is a relatively common cause of treatment failure, and in our practice a year’s supply of ASIT is dispensed, making early discontinuation less likely.
Whilst ASIT is a relatively safe and well-tolerated treatment for cats diagnosed with FASS, client education and routine follow ups are essential for treatment success; allergy companies are a good resource for client education brochures and technical assistance. The clinic’s veterinary technicians can also provide excellent support to clients during their pet’s treatment. Referral to a veterinary dermatologist for testing and ASIT implementation is always a good option.
Symptomatic therapy
Most commonly used symptomatic therapies include fatty acids, antihistamines, glucocorticoids, and cyclosporine. Fatty acid supplementation and antihistamines have a success rate of approximately 25%, although given together they have a synergistic effect which may increase the success rate. Chlorpheniramine (2-4 mg PO per cat q12h), hydroxyzine (10 mg PO per cat q12h); clemastine (0.68 mg PO per cat q12h) are the antihistamines I use more commonly. Amitriptyline is a tricyclic antidepressant, with antihistaminic, anti-inflammatory and sedative actions, which can be given orally or intradermally; the latter may be beneficial for cat/pet owners who have difficulty medicating their cats. I start amitriptyline at 10 mg per cat q24h. Although not common, adverse effects to antihistamines in cats include sedation, hypersalivation, urinary retention, anorexia, vomiting/nausea and dysrhythmias, so it is important to evaluate for any pre-existing problem that may increase the risk of developing these side effects and monitor during treatment. Pre-treatment bloodwork and urinalysis and monitoring every 6 months is recommended.
Glucocorticoids are used frequently in various formulations, and glucocorticoid-induced adverse effects are less common in cats than in dogs, although marked and unique problems may occur. Several studies have documented cardiovascular risks associated with glucocorticoid use in cats; for example, an association between long-acting depot corticosteroids (e.g., methylprednisolone acetate) and the development of congestive heart failure in cats without pre-existing cardiac disease has been seen 4. Diabetes is also a relatively common side effect; in one study, up to 75% of cats showed hyperglycemia after a single 5 mg/kg subcutaneous injection of methylprednisolone acetate 5. Diabetes may be transient or permanent after discontinuation of therapy.
Prednisolone or methylprednisolone can be started at 0.5-1 mg/kg PO q24h for 5-7 days, then on alternate days at the lower effective dose. If the cat cannot be well-controlled with≤ 0.5 mg/kg q48h or less, alternative therapies may need to be evaluated. If prednisolone is not effective, triamcinolone or dexamethasone can be tried. Triamcinolone can be started at 0.2 mg/kg PO q24 for 5-7 days, then tapered to lowest effective dose, ideally≤ 0.08 mg/kg q48-72h. Dexamethasone is started at 0.25-1 mg per cat PO q24h for 5-7 days, then tapered to lowest effective dose, ideally≤ 0.125 mg q48-72h. Complete blood count (CBC), chemistry profile and urinalysis are recommended to be performed at baseline and every 6-12 months in cats receiving maintenance glucocorticoid dosing.
For long-term control of pruritus in a cat with FASS, I prefer cyclosporine (CsA); studies have shown it to be an effective and safe treatment option 6, and I usually start at 5-7 mg/kg every 24h; a recent study suggested 7 mg/kg as the optimal dose 7. Initial improvement can be seen by the second week of treatment, but it may take 4-6 weeks for a full response. Treatment is usually well-tolerated; vomiting and/or diarrhea may occur, but in most cases signs resolve without discontinuation of treatment. Although a rare complication, fatal toxoplasmosis has been reported 8. It is recommended to exclude seronegative outdoor cats from treatment; cats seropositive to Toxoplasma seem to be protected from acute fatal disease. The decision to treat seropositive cats should consider possible relapses, and potential complications need to be discussed with the client. The same survey tests (at the same interval) recommended for cats on glucocorticoids are also recommended for those receiving maintenance CsA therapy.
Other treatment options
Oclacitinib, a Janus kinase inhibitor, has been used off-label for the treatment of FASS. Pharmacokinetic studies showed that this drug is absorbed and eliminated faster in cats than in dogs 9, therefore a shorter dosing interval and higher dose may be needed in cats. A safety study performed in healthy cats given 1 or 2 mg/kg q12h for 28 days reported vomiting and soft stools in the cats given 2 mg/kg 10. In a methylprednisolone-controlled study, oclacitinib given at 0.7-1.2 mg/kg q12h was as effective as methylprednisolone controlling the clinical signs of FASS 11. No long-term safety studies have been performed in cats receiving oclacitinib, so this and its off-label use should be discussed with clients before initiating treatment. Reported side effects include anemia, vomiting, and an increase in ALT, creatinine, BUN, and soft stools in Giardia-positive individuals 9. There is one report of acute fatal toxoplasmosis in a cat receiving oclacitinib 12.
Maropitant is a neurokinin-1 receptor antagonist (NK-1 R), which inhibits the action of substance P. Substance P activates receptors in mast cells and sensory neurons, causing itch. Maropitant has been used orally at 2.2 mg/kg q24h for the control of FASS in an open study 13, with owners reporting good to excellent response in 83.3% of the cases after a 4-week course. Oral maropitant citrate is not labeled for use in cats in the United States, and no long-term safety studies have been performed in cats receiving this drug, so again this and its off-label use should be discussed with clients before initiating treatment. A recent clinical trial indicated that transdermal application of maropitant in cats experiencing vomiting may be a good and effective alternative; however, further studies are needed to determine dosing and pharmacokinetics 14.
Cannabinoids are biologically active substances similar to the primary psychoactive compound found in Cannabis sativa. They can be plant-derived, synthetic, or endogenous (endocannabinoids). Endocannabinoids are naturally produced by the body and include arachidonoylethanolamide (AEA), 2-arachidonoylglycerol (2-AG), and N-palmitoylethanolamide (PEA). Endocannabinoids bind to cannabinoid (CB)1 and 2 receptors. The activation of CB2 receptors on mast cells decreases the release of inflammatory cytokines such as IL-2, and upregulates IL-10, an anti-inflammatory cytokine. Cannabinoid receptors can also be found in skin sensory nerve fibers, and activation of these receptors may reduce the sensation of pruritus. A recent study showed increased expression of CB1 and CB2 in cats with allergies when compared to healthy cats, suggesting that the use of cannabinoid receptor agonists like PEA may be useful in the treatment of FASS 15. Some countries now have a licensed cannabinoid available; for example, one product now marketed in the USA is a powdered, flavorless and odorless supplement, containing 60 mg of PEA per 2 mL scoop; the recommended dose is one scoop/day in cats up to 4 kg (9 lb.) and 1½ scoops/day in cats weighting more than 4 kg. This product can be sprinkled on food and is well-tolerated with minimal side effects.
Gabapentin is a neuroactive agent used to treat neuropathic pain. Chronic pruritus causes neuronal sensitization, with hypersensitivity of sensory neurons to itch stimuli. Gabapentinoids such as gabapentin and pregabalin have been used for neuropathic forms of chronic pruritus in people with good response. Gabapentin has been used in cats for treatment of feline hyperesthesia syndrome, either as sole therapy or in combination with other anti-pruritic therapy 16. I start this treatment at 10 mg/kg PO every 12 hours, often in combination with PEA, or as a glucocorticoid-sparing treatment. Reported side effects, although uncommon, include sedation, ataxia, weakness and muscle tremors.