The objective of this study was to determine if prevalence and risk of canine diseases associated with immune function were affected by neutering. There have been many reports on the benefits of neutering [18] and although several studies have looked at the change in prevalence of various conditions in relation to neuter status [1, 15, 16, 45], there are few reports associating neuter status with immune disorders [19, 21]. For several diseases evaluated in the present study, neutered dogs showed higher prevalence than their intact counterparts and intact females had a reduced risk for most of the diseases with the clear exception of pyometra, a disorder specifically chosen to have greater incidence in intact females. For conditions other than pyometra, neutering did not reduce the risk of immune disease. Considering both female sex categories together, the proportion of females diagnosed with these immune conditions was slightly greater than when compared to the aggregate male data. A 1999 review by Pedersen reported the greatest prevalence of immune conditions occurred in intact females and the lowest with intact males [32] an assertion supported by the human literature [46] although those previous reports focused solely on intact individuals. In the canine literature, diagnoses of atopic dermatitis [47], CMG [20], IBD [48], lupus erythematosus [49], and PEMC [50] do not exhibit sex differences whereas for ADD, AIHA, IMPA, and ITP females have approximately a twofold higher risk than males [24, 51, 52]. A sex differential for those diseases did not show significant differences in the present study although there was a tendency of greater prevalence in females for AIHA, ADD, and ITP but not at a twofold higher risk as indicated in the literature possibly due to the different populations assessed in each study.

Hoffman et al. noted that neutered dogs had longer lifespans and were less vulnerable to death by infectious disease than intact dogs [18]. The authors postulated that gonadal steroid profiles [53] present in the intact dog act as immune-suppressants making the intact dog susceptible to infectious disease. Although gonadal steroids are known to impact immune responsivity as noted above, additional factors likely influence incidence and severity of disease [54]. The present study evaluated conditions that have a different etiology and progression than pathogenic infectious diseases; it is reasonable that the immune conditions evaluated in the present study would respond differently to gonadal steroids. Furthermore the current study looked only at disease diagnosis and expression but not mortality associated with disease.

Some conditions were diagnosed at earlier ages in the intact dogs, more frequently for females than males. Possible explanations for this observation include that the health status of intact dogs may be more heavily scrutinized prior to inclusion into breeding programs, hormonal cyclicity related to gestation or the estrous cycle [55], and an impact of a gravid uterus on expression of immune conditions. Despite that for some conditions intact females were diagnosed at earlier ages, intact females showed the lowest risk in many of the diseases assessed, suggesting a health benefit for not neutering female dogs. Females have higher immunoglobulin levels (reviews include [23, 24]) partly due to increased immunoglobulin production prompted by estrogen. In mice, estrogen increases immunoglobulin production and enhances antibody response, while testosterone inhibits humoral immunity [26]. Perhaps this permits a stronger humoral and cell-mediated immune response than seen in males and a stronger response may benefit intact female dogs by detecting threats more quickly and efficiently or be better able to distinguish between self and non-self.

However, in previous studies of other species, intact females also tend to be more prone to many autoimmune diseases. In the present study, females had a slight, though not significant, greater prevalence than males for autoimmune conditions. An increased prevalence in autoimmune diseases may reflect estrogens driving immunoglobulin production and/or permitting autoantibodies to reach maturity whereas testosterone, through its inhibitory action on immunoglobulin production, may prevent autoantibody production.

One might predict that ablation of estrogens would reduce the risk for these conditions; that hypothesis was not supported by the present data. Similarly, androgens have been found to be immunosuppressors [56]; neutering may release this inhibitory action and promote a more active immune response. Further evaluation is needed to assess whether a more robust immune response is a benefit or detriment with respect to risk for autoimmune disorders in the dog. Results from the present study, which did not show intact females to be at greater risk for autoimmune disorders, may be due to the particular autoimmune diseases assessed, the methodology of experiments (e.g. retrospective or case study), species evaluated, or whether the parameter of neuter status was included. Most studies that have demonstrated a preponderance of female autoimmune disease expression have focused upon intact animals.

In the few studies evaluating gonadectomy, neutering of male and female mice resulted in thymic and lymphoid tissue hyperplasia [57]. In normal development, the thymus begins to involute following puberty as a consequence of elevated gonadal steroids; the involution is presumably responsible for the age associated decline in immune function [58]. The withdrawal of the steroids by gonadectomy may reverse the involution although older animals may be less responsive [59]. Additionally, thymic hyperplasia has been associated with autoimmune conditions [60] suggesting gonadectomy may potentiate the expression of immune-mediated diseases. Ovariectomy in rats was shown to increase autoantibodies to thyroglobulin [61] and, in agreement with the present data, a study correlated neutering in dogs with hypothyroidism in both sexes [62] which was speculated to have been associated with autoantibody destruction of the thyroid gland although autoantibody levels were not assessed. Furthermore, gonadectomy in both sexes of mice potentiates Sjorgren’s syndrome, an autoimmune condition [63]. In contrast, canine diabetes mellitus is associated with pancreatitis or autoimmune destruction of the pancreas [64] and a study by Pöppl et al. demonstrated recovery from the condition upon neutering [65]. Taken together these findings suggest differential regulation of autoantibody production in response to gonadal steroids; this view is supported by human literature that both demonstrates differences in tissue specific response to steroids for females and males as well as differences in expression of autoimmune conditions [66].

In fact, for ADD, neutered males were at greater risk than intact males and in females neutering only minimally altered the risk of disease. The data indicate a protective role for androgens in ADD, a view supported by an early study of a female child with hypoadrenocorticism whose condition improved with the administration of androgens [67] and other human studies that indicate a correlation between hypogonadism and severity of the hypoadrenocortical condition most particularly evident in males [68]. Additionally, studies using mice have shown that gonadectomy affects the hypothalamic-pituitary-adrenal axis in a sex-specific manner: neutered males produce larger quantities of corticosterone from the adrenal [57]. That alteration may account for the higher risk for hypoadrenocorticism observed in the neutered male dogs along with a role for androgens in adrenal function. The finding also illustrates that specific tissues respond to the gonadal steroids differentially possibly accounting for the sex specific risks seen for immune diseases with neutering.

Lupus in mice has been found to be accelerated with estrogen administration and blunted with androgen administration [23, 24]. Interestingly in mice prone to systemic lupus erythematosus, neutering reduces mortality in female mice yet guarantees mortality in males; treatment of intact females with testosterone and dihydrotestosterone extends their lifespan [69]. Although in the present study, the risk of canine lupus erythematosus was higher in neutered females than either male category or intact females, caution is needed in interpreting this risk because the prevalence of lupus in intact females in the present study was very small and other studies in multiple species, including dogs, show a correlation between estrogen and increased morbidity of lupus [69].

For conditions with an elevated risk associated with neutering, the gonadal steroids may serve a protective role. Diseases such as atopic dermatitis and IBD, where the mechanism of disease onset involves inflammation, higher concentrations of sex steroids present in intact dogs may exert an anti-inflammatory effect to prevent disease progression similar to what has been reported in mice [70]. The anti-inflammatory effects of estradiol are believed to be mediated by estrogen receptor blocking intracellular trafficking of the p65 transcription factor, that would normally be translocated to the nucleus to drive transcription of proinflammatory proteins such as cytokines and chemokines [71]. A similar anti-inflammatory effect may occur in the dog. Additionally, estradiol has an antioxidant effect in mice [70] and a recent study in dogs reported that gonadectomized females have a greater risk for oxidative stress [72] which may lead to greater susceptibility for immune diseases.

Canine atopic dermatitis is a complex disease influenced by genetic predisposition and environmental factors. Nearly half of the risk for atopy is attributed to the genetic background of the dog, with as many as 54 different genes exhibiting differential transcription in atopy cases [73]. Environmental factors vary with atopy risk influenced by geographic location, rainfall, proximity to other animals, and rural versus urban habitat [73] though no published studies assessed the influence of sex steroids on atopy. It is a common disorder with estimates of up to 10% of dogs affected with both sexes equally impacted [47] and even though the present study population prevalence was only ~1.8%, it was one of the most prevalent diseases under study consistent with previous reports of conditions that prompt veterinary visits [74]. The lower prevalence may reflect the population under study representing patients at a referral hospital with only the most severe cases seen. Neutered females had a much greater risk than the other sex categories indicating a strong suppressive effect of estrogens on the expression of the disease. Atopic dermatitis involves the production of IgE or IgGd, a subset of IgG antibodies, in response to antigens in the environment with IgE being viewed as more diagnostic and implicated in causality though there exists conflicting reports on the latter [75–77].

Differences in sensitization of a biological pathway in erythrocytes, thrombocytes, or immune cells of the dog by estrogen or progesterone in the female may play a role in the hematologic diseases. AIHA and ITP involve the destruction of erythrocytes and thrombocytes, components of blood that have a high degree of regeneration. If estrogen or progesterone control cell surface markers on thrombocytes or erythrocytes, removal of estrogen or progesterone might change cell surface markers and cause the thrombocytes or erythrocytes to be recognized as non-self. A comparable hypothesis was proposed for colon cancer in women where it was postulated that estrogen sensitized and inhibited colon cells from becoming cancerous because following estrogen withdrawal, there was an increase in risk for having tumors [78]. Similar hypotheses have also been postulated for hemangiosarcoma and mast cell tumor development in the dog [1].

It is important to note that even though the study population included more than 90,000 dogs and expression of the diseases were statistically prevalent, the actual prevalence of the conditions studied was not high and the prevalence of diagnosis declined over the 15 years evaluated. Additionally, this was a retrospective study limited to the dogs seen at a referral veterinary hospital and may not reflect the prevalence of the population at large but rather may be biased to complex or more severe cases. However, the prevalence of disease seen in the present study does concur with other studies of much larger numbers of dogs, such as a study done using records from the Banfield hospital population which found 1.7% of dogs were diagnosed with atopy out of a population of 1,345,697 dogs in 2007 [79], a similar prevalence to that seen with the present study. Finally the study indicates a correlation of neutering and disease status but because of the retrospective nature does not prove causality. This is especially true given the known genetic contributions to some canine autoimmune disorders [80]. Thus, although the current results indicate a significant risk associated with neutering, additional prospective studies are warranted to confirm the relationship of sex steroids and autoimmune risk seen in this study.