Possible Health Issues
Spay & Neuter
A dog’s behavior is influenced by four things: Genetics, early learning, the current environment, and the humans they live with. Genetics give the ability and environment provides the opportunity for these traits to develop to their full potential. Dogs nor humans “just snap” or have sudden behavior changes unless there is a medical reason or impactful experience. Behavior does not change without a reason. Often times behavior changes gradually but people are not aware of the changes until it has moved further across the continuum. If there is a sudden change in behavior it is advisable that dogs be evaluated by a veterinarian.
Diseases and physical issues can cause pain and irritability and contribute to reactive / aggressive behavior. A physical exam is a good place to start but it will not uncover more difficult to diagnose diseases or physical issues.
When evaluating dogs’ observe how the dog moves and uses its body to assess their physical abilities. When they sit are they slow, fast, tight, sloppy, rolled over on one hip, back onto pelvis, legs tucked or extended? How long before the dog changes positions? Do they prefer to lay down rather then sit? Does there appear to be any pain in the back, knees, hocks, or hips? Dogs like humans will move or shift weight to compensate for pain or discomfort.
Possible Health Issues
GI issues that do not result in diarrhea and/or vomiting (Food Sensitivity)
Since food sensitivities and thyroid problems are common those are two things I would rule out as contributing to irritability or other behavior problems. For both tests I recommend: Hemopet (Dr. Dodds) https://www.hemopet.org/ .
Behavioral pharmacology with animals differs from other areas of pharmacology because animal behavior seems to be more prone to environmental influences, rather than pathophysiology. There is no reason to believe that a neurotransmitter imbalance is commonly the cause for most problem behaviors. Rather than seek out a drug for a quick fix, the owner should seek out a professional who understands dogs and has the skills to bring about positive results. Using psychotropic drugs to correct behavior should always be a last resort and always under the direct supervision of a veterinary behaviorist.
Since the majority of problem behaviors are not caused by a neurotransmitter imbalance, these problems can be resolved without the necessity for drug treatment. Problems behaviors such as, anxiety, barking, hyperactivity, separation anxiety, whining or any other behavior considered disruptive should first be addressed by a knowledgeable individual who understands a dog′s development and behavior. When drugs are used the common approaches are typically to assist a behavior modification program that theoretically could work alone, or to correct an existing physiological abnormality such as deficiency in a neurotransmitter system.
Canine and Feline Behavior Therapy Second Edition Benjamin L. Hart, Lynette A. Hart, Melissa J. Bain
The Dog : Its Behavior, Nutrition, & Health Second Edition Linda P. Case
Spay or Neuter
The timing of spaying may have an affect on aggression.
“The calming effect of the high level of progesterone is also the reason why it is best to avoid spaying bitches for two months after estrus. Spaying during this time can result in a precipitous drop in progesterone levels with possible accompanying emotional disturbances, irritability, aggression and depression.” The Dog’s Mind By Bruce Fogle, D.V.M., M.R.C.V.S.
“Progesterone has calming influences, and because spaying removes the source of progesterone production, causing a precipitous fall in progesterone may increase irritable tendencies in young females. After a female has gone through some estrus cycles, and possibly been disciplined for aggressive behavior, the spaying may have less of an impact.” Canine and Feline Behavior Therapy Second Edition By Benjamin L. Hart, Lynette A. Hart, Melissa J. Bain
“Hart and Eckstein (1997) point out that female dogs are in a progestational state for 2 months following an estrus period and spaying them during that period creates a sudden removal of the source of progesterone (which tend to have a calming influence on animals). It is postulated that this removal of progesterone may promote irritability or aggression in some individuals.” Aggressive Behavior In Dogs By James O’Heare
Bifidobacterium longum BL999 has been reported to help with behavior and mood. You may need to inquire of the manufacturer to determine strain.
“In a study of 24 dogs, Dr. McGowan’s team found that, “from both a behavioral and physiological standpoint, increased intake of fish oil had a calming effect on anxious dogs.” 
Noise Sensitivities in Dogs: An Exploration of Signs in Dogs with and without Musculoskeletal Pain Using Qualitative Content Analysis
“In this study, concerns over musculoskeletal problems were confirmed using a range of procedures (some individuals having multiple procedures): four clearly demonstrated pain during physical examination in the clinic, eight were radiographed, and one underwent magnetic resonance imaging. The problems identified or inferred related to the hip (including dysplasia–five subjects), degenerative joint disease of the limbs (four subjects), and focal spondylosis in L2 and L3 (one subject). In six of these cases, the owner commented that the dog seemed to be in some pain and/or the pain worsened after exercise.”
“There was a large proportion of neutered dogs: 9 of 10 of both controls and “clinical cases,” whereas the Pet Animal Welfare Report (29) suggests that nationally only about 71% of dogs are neutered. A study by Spain et al. (30) found that decreasing age at gonadectomy in shelter dogs was associated with an increased risk of developing a noise phobia, but it could not be concluded that neutering is causative of noise phobias.”
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