Gonadectomy is an important reproductive management tool employed in many countries, and is highly prevalent in the US with an estimated 85% of dogs being neutered. Despite the societal benefits in pet population control, negative associations between neuter status, and health conditions have been reported in recent years. Most particularly observed are the consequences of early age neutering. Knowing that different physiological systems rely upon gonadal steroids during development and physiological maintenance, studies have been undertaken to assess the impact of neuter status on multiple body and organ systems. For some inherited conditions, neutering is associated with an increased risk of expression. Neutering has also been associated with altered metabolism and a predisposition for weight gain in dogs, which may confound the detected risk association between neutering and disease expression. This review summarizes the effects of neutering on cancer, orthopedic, and immune disorders in the dog and also explores the potentially exacerbating factor of body weight.
Cancer
For cancers having an inherited component, there is a generalized trend for an increase in risk associated with neutering across breeds and sexes. The physiological mechanism underlying that breed specific susceptibility for cancer is unknown although it may be related to breed-specific genetic signatures of the major histocompatibility complex (
47,
48), a component of the immune surveillance that can detect developing tumor cells (
49) as well as other ancestry related susceptibility.
Orthopedic Disorders
Given the interaction of gonadal steroids and normal musculoskeletal development (
24,
84), it is unsurprising that neutering impacts bone elongation in the dog (
23) and thus, inherited conditions related to bone maturation. In one large study across many dog breeds, neutered males were at risk for hip dysplasia and neutered females for cruciate ligament damage with dogs of large and giant breeds at the greatest risk (
85).
Immune-Mediated Disorders
Female dogs, as also seen in humans and mice, are more prone to immune-mediated disorders (
97,
98) and neutering appears to exacerbate that risk. The risk of certain immune diseases is elevated with neutering in both males and females: atopic dermatitis, autoimmune hemolytic anemia, hypoadrenocorticism, hypothyroidism, immune-mediated thrombocytopenia, inflammatory bowel disease, and systemic lupus erythematosus (
14,
28,
99–
102). For dogs diagnosed with hypoadrenocorticism, the relative risk of death is 2-fold higher than that seen for other dogs and in a large retrospective study in Sweden, breed by sex interactions were detected with some breeds exhibiting a female predisposition although in other breeds, both sexes were equally affected (
103).
Weight as a Co-Factor
Although there exists conflicting information as to whether neutering alters metabolism thereby causing weight gain in dogs (
129), neutering is viewed as a common predisposing risk factor for obesity, most especially for cats but also for dogs (
130–
137). Neutering associated with increased body weight is speculated to reflect energetic metabolic changes (
138), dysregulated feeding behavior (
139), and the removal of estrogen, a known regulator of food intake in cats (
140). Obesity is a risk factor for many disorders and that risk between disease and obesity is overrepresented in females (
141). This suggests a role for gonadal steroids across many inherited diseases and an interaction with body weight.
Summary
In many cases, neutering is promoted as the means to reduce the number of dogs euthanized in shelters, although one study (
148) reported that “no clear results were found demonstrating the impact of total spay/neuter procedures on shelter intake.” With 85% of the dogs in the United States neutered (
149) and yet ~3.3 million dogs enter a shelter annually in the United States (
150), there are reasons beyond a failure to neuter that account for the number of dogs relinquished to shelters. With that backdrop, neutering is not the sole answer to reduce shelter euthanasia. However, there are definite advantages to neutering beyond preventing unwanted pregnancies including offering substantial convenience for owners and a reduction or elimination of some reproductive disorders.
They have to remove the whole end of the "toe", including the bit that makes the claw grow.
