Submitted by: Patricia Shea, DVM
Domest Anim Endocrinol. 2020;71:106389. DOI: 10.1016/j.domaniend.2019.106389. Epub 2019 Sep 5. PMID: 31731251.
Short-term biological variation of serum thyroid hormones concentrations in clinically healthy cats.
Prieto JM, Carney PC, et al.
Hyperthyroidism is a commonly diagnosed disease in older cats. Most cats are 12-13 years old at the time of diagnosis, but 5-30% of hyperthyroid cats receive their diagnosis at less than 10 years of age. Often the hyperthyroidism diagnosis is straightforward in that compatible clinical signs such as weight loss and polyphagia are present, and total T4, measured by an inexpensive assay included in first tier laboratory panels, is elevated beyond the laboratory’s reference range. Reference ranges established by laboratories for a specific analyte are obtained by compiling data from a large population of conspecific individuals considered to be normal.
In other cases, the hyperthyroidism diagnosis is unclear due to atypical or no clinical signs or a total T4 within the laboratory’s reference range. In healthy humans, there is significant variability in thyroid hormone concentrations among individuals, but substantially less variability of these hormone levels within a single person. In this study of 10 healthy cats, the concentrations of serum thyroid hormones (total T4, free T4 by equilibrium dialysis, and T3) and thyroid stimulating hormone (TSH) were measured weekly in each animal over a period of 6 weeks. Similar to findings in humans, these researchers found that serum concentrations of total T4, free T4, T3, and TSH had far greater variability among the subject cats than within individual animals.
The total and free T4 concentrations had an intermediate index of individuality (1.1 and 1.2, respectively), while the serum TSH and total T3 both had high indices of individuality (1.8 for both). In the case of total T4 and free T4, the authors suggest that the diagnosis of hyperthyroidism in an individual cat be based on a combination of the population-based reference intervals and reference change values in a specific animal. Due to the higher index of individuality of serum TSH and total T3, use of population-based reference intervals may be misleading, and instead the focus should be on reference change values in an individual cat when using these analytes for diagnosis of hyperthyroidism.
Over time, some cats may have significant changes in their thyroid hormone and TSH levels, even when these values stay within the population-based reference ranges. Although this was a small, short-term study, the authors suggest that baseline levels of thyroid hormones and TSH be measured in individual cats while these animals are relatively young (5-8 years), healthy, and unlikely to be hyperthyroid. Then, as these animals age, their individual reference change values can offer additional information regarding changes in thyroid function beyond that provided by population-based reference intervals for thyroid hormones and TSH.
Vet Dermatol 2021;32:8-e2. DOI: 10.1111/vde.12899
Feline allergic diseases: introduction and proposed nomenclature
Halliwell R, Pucheu-Haston CM, et al.
The etymology of the term ‘atopy’ comes from the Greek, meaning “strange disease.” Does this “strange disease” exist in cats, and if so, what similarities does it have to atopy in humans and dogs? Atopy is well characterized in the latter two species.
In humans, as originally conceived, ‘atopy’ referred to a familial predisposition to allergic disease affecting the skin, respiratory system, and/or gastrointestinal tract. In this paper, these authors review available published literature regarding non-flea allergic diseases of the cat and discuss how they compare with atopic diseases of humans, as well as propose a standardized nomenclature to identify these diseases.
Feline skin has a limited spectrum of reaction patterns associated with inflammatory skin diseases. Some of these reaction patterns are unique and species-specific, and each reaction pattern may have a number of inciting causes. This makes it impossible to assume that inflammatory skin diseases in the cat will produce similar dermatologic responses to those in dogs and humans experiencing such diseases.
Studies of atopy in man in the first half of the 20th century ultimately led to the discovery of an antibody class now known as immunoglobulin E (IgE), present in elevated levels in response to environmental and food allergens. Atopic diseases associated with IgE sensitization, including asthma, dermatitis, and rhinitis, were labeled as “extrinsic” or “allergic”. When any of the foregoing three diseases were not identified with detectable IgE sensitization, they were considered “intrinsic” or “nonallergic.” However, these so-called “intrinsic” varieties of asthma, dermatitis, or rhinits are observed to occur along with activation of many other inflammatory pathways , including the Th2 response, so they are not truly “nonallergic”, but do not have an association with elevated IgE levels to certain allergens. Also, in humans, an “atopic march” has been described in which predisposed individuals often develop dermatitis in childhood and later progress to asthma and/or allergic rhinitis.
Based on this information from the human field, the three questions regarding the etiology of some non-flea associated feline allergic diseases that this international panel of dermatologists asked were:
- Do these diseases have a genetic basis?
- Is IgE involved?
- Does an “atopic march” exist in cats?
A review of a limited number of published studies, including several involving cats with known familial relationships, does indicate that there is possibly a genetic basis for feline allergic skin diseases. Known breed predispositions to allergic skin diseases also support this hypothesis. In large studies of pruritic cats, purebred cats of Siamese, Persian, Abyssinian, Maine Coon, and Devon Rex lineages have been found to have a significantly higher predisposition to non-flea hypersensitivity dermatoses than mixed breed domestic cats.
In cats, studies of responses to atopy patch testing, positive serology results for allergen-specific IgE, results of intradermal allergy testing, responses of feline patients to allergen avoidance, and beneficial outcomes from allergen-specific immunotherapy, suggest that IgE is involved in feline allergic skin diseases. How significant a role IgE plays in these conditions is still not clear. Moreover, there also may be “intrinsic” forms of allergic dermatitis or asthma in cats, as there are in humans, that would not be correlated at all with the detection of allergen-specific IgE.
With regard to “atopic march” in cats, there is still no clear evidence that such a phenomenon is present in this species. However, there are published reports of more than one of the atopic spectrum diseases occurring in individual cats, including dermatopathy and enteropathy, or skin disease and allergic disease of the upper or lower respiratory tract, which may point to the presence of a feline “atopic march.”
The authors conclude by recommending detailed and in-depth investigations to determine if “intrinsic” variants of atopic diseases are present in cats, that do not correlate with the presence of elevated levels of allergen-associated IgE. They also propose the following standardized terminology for non-flea, non-mosquito bite hypersensitivity dermatitis of the cat:
Feline Atopic Syndrome (FAS): Includes allergic dermatitis associated with environmental allergens, food allergy, and asthma, any of which may involve IgE antibodies.
Feline Atopic Skin Syndrome (FASS): An inflammatory and pruritic skin condition of cats associated with a number of nonspecific skin reaction patterns, which may include IgE antibody production to environmental allergens. It is important to remember that the clinical signs of FAS and FASS may receive contributions from flea allergy and food allergy, as well as environmental allergens.
Feline Asthma: An eosinophilic inflammatory bronchiolar disease that leads to spontaneous, reversible bronchoconstriction and airway remodeling. Feline asthma can manifest with acute respiratory distress or chronically with coughing and expiratory wheezing. IgE antibodies to inhaled allergens may be present. Both FASS and feline asthma may have significant “intrinsic” variants in which there are no detectable levels of IgE present. “Intrinsic” FASS would be similar to canine atopic-like dermatitis.
Feline Food Allergy: Any clinical syndrome, including signs of FASS, that may be associated with immunological reactivity to an ingested food item.