Diagnostic Trials

Diagnostic trials are well accepted tests in veterinary dermatology. They are performed when a certain problem is suspected and the trial is either the only or the best way to diagnose the possible underlying disease. A response to the trial confirms the diagnosis in some instances (such as the scabies treatment trial), but in other instances a relapse after discontinuing the trial with subsequent resolution on restarting the trial is diagnostic (such as in elimination diets). If there is no response to a well-conducted diagnostic trial, the suspected disease is extremely unlikely (which helps the veterinarian, owner, and patient, and needs to be emphasized to clients frustrated by the lack of response).

Elimination Diet

Indication

An elimination diet is used to evaluate food adverse reaction which can occur with any food fed over a period of time. As a general rule food adverse reactions present infrequently. Any dog with nonseasonal pruritus (particularly if the face, feet, or ears are affected) or recurrent pyoderma, or any cat with miliary dermatitis, noninflammatory alopecia, eosinophilic granuloma complex, or head and neck pruritus could possibly have an underlying food adverse reaction.

Procedure

♣ An elimination diet for dogs consists of one protein source and one carbohydrate source previously not fed! This means that the elimination diet for a particular patient is determined by the diet fed so far to this animal. Cats are fed only one protein without the carbohydrate source to enhance compliance.

> Possible options for proteins are chicken, turkey, duck, venison, mutton, beef, horse, buffalo, rabbit, hare, kangaroo, emu, various sorts of fish, among others. Carbohydrates may consist of rice, potatoes, sweet potatoes, beans, or others.

> The diet chosen needs to be fed exclusively! Concurrent heart-worm prophylaxis or supplements must not contain food flavor extracts.

> It may take 6 to 8 weeks before a response becomes evident.

♣ After initial improvement, a rechallenge with the normal diet previously fed is essential because improvement may result from other factors such as seasonal or environmental changes or concurrent medication. If a relapse occurs within 2 weeks and clinical signs resolve again after reinstitution of the elimination diet, the diagnosis is confirmed.

Tips to increase compliance

> Warming the food may improve patient compliance.

> Spices such as garlic or salt (in small amounts) may also be beneficial to improve palatability.

‡ If the animal (and owner) is used to treats, the habit should be continued in a modified fashion to prevent feeding of inappropriate proteins. Little pieces of the selected meat protein can be fried and kept in the fridge for use as treats. The selected meat can be dried (in the oven or microwave) and given as treats. If an animal is receiving potatoes in the diet, then fried pieces of potato may be used (so long as they are not fried in butter, but in a plant-derived oil). If rice is chosen, rice cakes may be an additional option.

> If bones are part of the normal diet, bones of the meat selected for the elimination diet may be fed if available.

> Good client communication is essential. It must be made clear that an occasional slip in feeding habits (as little as once or twice weekly of a very small amount of a different protein) may destroy all the effort.

> It may be worthwhile to advise neighbors about the diet as well.

> If a home-cooked diet is not an option, a commercial diet consisting exclusively of a protein source and a carbohydrate source not previously fed may be considered. The same principles apply to commercial as to home-cooked elimination diets. However, some animals with food adverse reactions may be missed when using commercial diets.

After a diagnosis of food adverse reaction is confirmed, the client has two options: 1. To continue a commercial elimination diet forever - the more convenient option; 2. A home-cooked diet. It should be properly balanced (the help of a veterinary nutritionist may be indicated).

> The identifying of the offending allergen allows a more varied diet and is achieved through a sequential rechallenge with proteins formerly fed. Beef, lamb, chicken, or cheese and milk products are added to the elimination diet one at a time for 2 weeks each. If a relapse occurs within the first 2 weeks (many patients show symptoms within the first 2 days), the protein is discontinued until the patient's condition settles. That particular protein is avoided in the future. After 2 weeks of a given protein without clinical symptoms, a reaction to this protein is ruled out and it may be fed in the future. Some dogs will tolerate any home-cooked diet, but relapse on commercial diets may be caused by a reaction to additives or preservatives.

Insect Control Trial

Indication

An insect control trial may be used in any patient with suspected insect-bite hypersensitivities. Most animals with insect-bite hypersensitivities will be allergic to fleas. Clients generally accept these trials more readily when they are labeled "insect control trials" because many do not believe fleas cause the problem, whereas most will accept ants or mosquitoes as a possible cause. Any dog with pruritus, alopecia, and/or a papular or crusty rash in the tailbase or inguinal area, and any cat with miliary dermatitis, non-inflammatory alopecia, or eosinophilic granuloma complex may benefit from an insect control trial. Mosquito bite hypersensitivity in the cat is characterized by papules and crusts on the nose, pinnae, and foot pads. A trial using insect repellents may be beneficial to these animals.

Procedure

> The patient should be treated regularly with an insecticide. In a diagnostic trial, I often increase the frequency of administration above the manufacturer's recommendations. Fipronil spray, imidacloprid, permethrin, and selamectin spot-ons are administered every 2 weeks. Pyrethroid sprays are administered daily depending on the product. Nitenpyram tablets are given either daily or every other day. Which products to use depends on the individual circumstances. More details are provided in The cat with Non-inflammatory Alopecia.

> At the start of the trial, treat the animal's environment with an insect-development inhibitor such as methoprene, fenoxycarb, or pyriproxyfen. More details are provided in The Cat with Non-inflammatory Alopecia.

> Contact animals (either living in the same household or those that visit on a regular basis) must be treated as well, although the frequency between adulticide applications may be increased to the manufacturers' recommendations.

> At the start of the trial, I often prescribe 5 to 7 days of prednisolone at 1 mg/kg bodyweight daily to hasten clinical response.

If there is a good response to the trial, insect-bite hypersensitivity is present and insect control may be tapered to the minimum required.

‡ Remember that the required minimum treatment typically varies seasonally, as does the insect load.

Scabies Treatment Trial

Indication

Any pruritic dog or cat could possibly be infested with Sarcoptes scabiei or Notoedres cati, respectively, particularly if the pruritus was of sudden onset or if pinnae, ventrum, and elbows are pruritic. With spot-ons used for flea control, I have seen patients with pruritus and lesions limited to ventrum and lower legs. Negative superficial skin scrapings do not rule out scabies (see Specific Tests in Small Animal Dermatology) so trial treatment is indicated in any patient with suspected scabies irrespective of negative skin scraping results. In as much as Cheyletiella spp. and Otodectes cynotis are sensitive to the same antiparasitic agents, a scabies treatment trial will be useful for these parasites as well.

Procedure

> Several treatments for scabies are available but many of them are not labeled for this use.

> Topical treatments include ivermectin, lime sulfur dips, amitraz, and other antiparasitic rinses. They are used weekly for 4 weeks. More details are given in The Cat with Non-inflammatory Alopecia.

> Systemic therapy may be undertaken with selamectin, ivermectin or milbemycin. Treatment details are outlined in The Cat with Non-inflammatory Alopecia.

> All animals in contact with the patient need to be treated as well!

> Initial deterioration during the first days of treatment may occur. Treat with glucocorticoids daily for 3 to 4 days at 1 mg/kg body weight.

> Remission should be achieved within 4 weeks although some patients may need extended treatment for up to 8 weeks.