Most people hold the misconception that human doctors are far more competent than veterinarians, thinking that veterinarians only need to give animals a few injections of veterinary drugs and that human doctors are more than qualified to diagnose and treat animals, such views are based on a serious lack of understanding.

The differences between human medicine and veterinary medicine are enormous. Take internal medicine as an example. I have seen many cases where drugs containing acetaminophen were administered to cats. Yet, cats lack the UGT1A6 gene, which renders them unable to glucuronidate acetaminophen—making the substance extremely toxic to felines.

Veterinary medicine is also a science. Below, I will discuss the differences between human medicine and veterinary medicine from several aspects including laboratory testing, surgery and internal medicine.

Human myocardium (the middle muscular layer) is relatively thick, specialized for continuous contractions and suited for endurance activities. Feline myocardium is thinner, but the right ventricle is larger and more flexible, enabling powerful contractions required for explosive speed.

Atherosclerosis is common in humans and a leading cause of cardiovascular diseases, but it is extremely rare in dogs and cats. A key reason is the absence of cholesteryl ester transfer protein (CETP) in canines and felines, which promotes the production of high-density lipoprotein (HDL₁) in their bodies, making them far less susceptible to atherosclerosis.

Essential hypertension (of unknown cause) is prevalent in humans. In contrast, the vast majority of hypertension cases in dogs and cats are secondary, meaning they are caused by underlying conditions such as kidney disease or hyperthyroidism.

In domestic mammals, the valve of the inferior vena cava is usually hypoplastic or absent. The valve of the coronary sinus is vestigial in domestic animals. In the left ventricular cavity of cats, false tendons—thin fibrous bands not attached to the valve leaflets—can be observed. The moderator band is a characteristic white band-like structure visible in the right ventricular cavity of dogs. Most tricuspid valves in dogs and cats have four leaflets.In humans with mitral valve prolapse, the posterior leaflet is most commonly affected, and the course of the disease is usually benign. The apical four-chamber view is not recommended for diagnosis. In dogs, the anterior leaflet (twice as large as the posterior leaflet) is the primary site of involvement. Degenerative valve disease has a higher incidence in dogs, often leading to severe heart failure, and its prognosis is less favorable than in humans.

In humans with hypertrophic cardiomyopathy (HCM), a significant resting pressure gradient is associated with a high risk of sudden cardiac death; this correlation has not been confirmed in cats. Feline restrictive cardiomyopathy is idiopathic and non-infiltrative (without amyloid deposition), which differs from the human form. In dogs, endocarditis most frequently affects the mitral valve, followed by the aortic valve.

The Teichholz formula—a method derived from human data to estimate ventricular volume—is invalid for cats; for example, it erroneously suggests that the feline left ventricle is significantly elongated. Turbulence should not be detected via color Doppler in healthy dogs, but it may be observed in the right ventricular outflow tract of cats without any cardiac abnormalities.Due to the high heart rate of cats, the diastolic E wave and A wave in tissue Doppler imaging (TDI) often fuse into a single EA wave, which limits the application of this technique in evaluating feline diastolic function. In humans with slower heart rates, the E wave and A wave are usually clearly distinguishable.

Animals possess unique coagulation factors: the human coagulation system includes factors I–XII, compared to I–VIII in dogs and I–IX in cats. Fetal red blood cells of most species have higher oxygen affinity than maternal red blood cells, but domestic cats are an exception. Feline hemoglobin molecules contain 8–10 reactive sulfhydryl groups (vs. 2–4 in most species), making them highly vulnerable to oxidative damage and Heinz body formation. Thus, a small number of Heinz bodies may be present in the blood of healthy cats. The feline spleen is non-sinusoidal, so its efficiency in clearing these bodies is relatively low. There are two types of feline reticulocytes: aggregate and punctate.

Feline platelets (megathrombocytes) are generally larger and more heterogeneous in size, which causes their size to overlap with that of red blood cells. This makes platelet counting using traditional impedance methods potentially inaccurate. In dogs, von Willebrand factor (vWF) is mainly stored in vascular endothelial cells, with minimal content in platelets.

Feline eosinophil granules have a unique rod-shaped or segmented appearance; canine granules are round (with variations in size and quantity across breeds), as are human eosinophil granules. Unlike dogs, healthy cats typically have no stainable iron stores in their bone marrow.

Humans have enclosed orbits completely surrounded by bone. Dogs and cats have open or incomplete orbits, with the lateral aspect closed by the orbital ligament, allowing for a wider jaw opening. Dogs and cats have specialized retractor bulbi muscles to retract the eyeball, as well as a fully functional nictitating membrane (third eyelid) for protection and tear secretion. Humans have a Bowman’s layer, a structure absent in dogs and cats.

The pectinate ligament in dogs and cats is a fibrous bundle connecting the iris root to the limbus; this structure is vestigial in humans. The human iridocorneal angle contains Schlemm’s canal for aqueous humor drainage. Dogs and cats have a reflective layer behind the retina (the tapetum lucidum) that enhances night vision (causing eyes to glow in the dark)—a feature not found in humans. Humans have a highly specialized fovea centralis (macula) composed solely of cones, which is responsible for sharp central vision.

Dogs and cats lack a macula but have a functionally equivalent central area. Humans, dogs, and cats all have holangiotic retinas, but the blood supply origins differ. In dogs, optic nerve axons are myelinated at the optic disc; in cats and humans, myelination begins posterior to the lamina cribrosa. Humans have trichromatic vision.

Dogs and cats have dichromatic vision, which is analogous to human red-green color blindness. The human standard is emmetropia (normal vision). Up to 25% of dogs are myopic (especially in certain breeds). Cats tend to be myopic in juvenility and approach emmetropia in adulthood.

Human skin has an acidic pH, with an average value of 5.5. Canine skin is neutral to alkaline, with a pH range of 5.5 to 7.2—even reaching 9.1 in the dorsal region. Human skin is covered with eccrine sweat glands for thermoregulation. Most of the skin of dogs and cats contains apocrine glands, which mainly secrete pheromones for individual recognition. Small numbers of eccrine sweat glands are only present in non-hairy areas such as paw pads and the nose.

Humans synthesize most of their vitamin D through skin exposure to sunlight. Canine fur blocks ultraviolet rays, so dogs cannot effectively synthesize vitamin D via the skin and must obtain it from their diet. Additionally, dogs undergo phases of significant shedding.

Cats are obligate carnivores and cannot synthesize taurine, vitamin A, or arachidonic acid—these nutrients must be obtained from their diet. The saliva of dogs and cats contains almost no amylase for starch digestion. The entire canine esophagus is composed of striated muscle (voluntary muscle), while the distal one-third of the feline esophagus is made up of smooth muscle (involuntary muscle).

Fasting dogs exhibit a powerful emptying pattern called the migrating motor complex (MMC) to clear residual debris. Cats lack an MMC and instead have a weaker pattern known as the migrating spike complex (MSC).

In humans, intrinsic factor for vitamin B12 absorption is secreted by gastric parietal cells. In dogs, it is secreted by the pancreas (and partially by the stomach), while in cats, it is produced exclusively by the pancreas.

Feline hyperthyroidism is pathologically most similar to human toxic nodular goiter, rather than Graves’ disease—the most common cause of hyperthyroidism in humans, which is autoimmune in origin. Furthermore, hyperthyroidism in cats more frequently leads to hypertension than it does in humans.

The causes of hypercortisolism are more diverse in humans; Cushing’s syndrome caused by ectopic ACTH secretion (e.g., from lung cancer) is common in humans but extremely rare in dogs. The canine liver produces a glucocorticoid-induced alkaline phosphatase (ALP) isoenzyme, which is unique among species. Consequently, ALP levels are significantly elevated in affected dogs. Cats do not have this isoenzyme, so increased ALP levels are uncommon in feline patients.

Primary hyperparathyroidism in humans is a feature of several genetic disorders. In dogs, an inherited form has only been confirmed in the Kerry Blue Terrier. Multiple endocrine neoplasia is a well-defined genetic disease in humans. In animals, the term concurrent endocrine neoplasia (CEN) is preferred because its genetic basis remains unclear. Over 50% of central diabetes insipidus (CDI) cases in humans are idiopathic.

CDI is a rare disease in dogs and cats. Congenital nephrogenic diabetes insipidus (NDI) is the primary cause in humans. Congenital NDI is extremely rare in dogs and has never been reported in cats. Canine mammary glands are capable of producing growth hormone.

The estrous cycles and ovulation patterns of dogs and cats differ completely from those of humans: dogs and cats are induced ovulators, while humans are spontaneous ovulators. The tomcat’s penis has spines that stimulate ovulation in the queen during mating. The dog’s penis contains a os penis (penile bone). Humans have a simplex uterus, whereas dogs and cats have a bicornuate uterus.

In dogs, the CD4 antigen is not only expressed on helper T cells but also at high density on neutrophils. The therapeutic targets for atopic dermatitis differ: canine treatments primarily target the IL-31 cytokine, while human therapies focus on antibodies against IL-4, IL-13, and other cytokines.

Autoimmune polyendocrine syndrome (APS) is a well-defined disorder in humans. However, the existence of true APS in dogs remains controversial, and it is extremely rare in cats. Acetaminophen and ibuprofen are highly toxic to cats because they lack the UGT1A6 gene and cannot metabolize these drugs via glucuronidation. Pyrethroid insecticides are also highly toxic to cats and must never be used on them.

MDR1 gene mutations are common in herding breeds such as Collies, rendering them highly sensitive to multiple drugs including ivermectin and loperamide. Differences in the distribution and function of adrenergic receptors in the iris sphincter and dilator muscles of dogs and cats result in their distinct responses to certain ophthalmic drugs (e.g., timolol) compared to humans.