Healthy pets typically do not have an offensive odor, but unhealthy pets often do. This means medical care can help resolve many odors. Pet odor is most often caused by bacteria and yeast metabolizing secretions, especially skin oils. Places where skin is moist and dark–mouth, ears, skin folds, and under the tail—have the highest concentration of yeast and bacteria and the strongest smell. Yeast and bacteria are present on the skin of all pets, even healthy pets, but the number of these organisms is kept to a minimum when skin is healthy. With infirm, diseased, or aged pets, the skin is less healthy and the number of yeast and bacteria increase. As the numbers increase, malodor increases.
Yeast and bacteria cause pruritus or itching. Pets scratch and this increases blood flow to the area. Increased blood produces inflammation, swelling, and heat so that pet odors become even more offensive.
Odors also worsen when pets are wet. This is because the number of compounds causing odor increases from 16 compounds in a dry coat to 22 compounds in a wet coat. In addition to increasing in number, odor-causing compounds chemically change when they become wet so that their odor becomes stronger. For example, the natural coat compound benzaldehyde is 30 times more malodorous when the coat is wet than when it is dry.
The ears, skin, mouth, anal area, and vulva have normal odors, but in healthy pets these odors are not offensive. In sick pets, however, these areas can emanate offensive odors.Footpads have glands that secrete oils, which some people find offensive, even when pets are healthy.
Ears can be malodorous when infected with bacteria or yeast. These infections are common in:
Generally, ear infections smell worse the longer they persist. It is difficult for the body to clear these infections because pus, debris, and cerumen collect in the ear canal and form a waxy plug that blood, infection-fighting white blood cells, and medication does not penetrate. Nor can antibiotics in the blood penetrate into debris in the canal.
To resolve ear odor problems, ears must be cleared of debris, and medication must be put directly into the canal. If the infection involves the inner ear, as well as the outer ear, systemic antibiotics are used because blood penetrates the inner ear.
Healthy skin does not have an offensive odor. Unhealthy skin often has a thick layer of oil that holds dirt, clogs pores, and supports odor-causing bacteria and yeast. In addition to skin oil contributing to malodor, the skin itself can contribute. Normal skin forms new cells every 3 weeks. When the skin is abnormal, skin cells divide more rapidly, or more slowly, than normal. When skin cells divide rapidly, as with seborrhea, there is a flaking or a thick crusting that traps oils and bacteria. When skin cells divide too slowly, pets have dull, easily damaged, slow-healing skin that is ripe for yeast, fungal, and bacterial invasion.
Skin serves as a protective barrier to keep pathogens like bacteria out of the body and keep moisture in, but few realize the skin also serves as an excretory organ that discharges wastes. For example, pets with kidney disease expel uremic wastes and salts through the skin. These pets smell like urine and their skin is intensely itchy.
Some medications, such as DMSO, give skin a peculiar garlicy odor because volatile byproducts of DMSO are released through the skin and breath. Hormones influence skin health and skin odor. Dogs with under-active thyroid glands (hypothyroidism) have dull, thin hair coats and thick, flaky skin over a thick layer of fat. The skin is slow to heal and is easily invaded by malodorous yeast and bacteria. Fungal infections, such as ringworm, are also common in hypothyroid dogs.
Cats with hyperactive thyroid glands have scruffy, oily, unkempt coats. The coats mat and stink because these cats don't groom themselves. Many cancers affect skin and leave the foul odor of decaying flesh. Among cancers that can be malodorous are anal sac tumors, osteosarcoma, mammary tumors, squamous cell carcinoma, and lymphosarcoma.
Pet odors from the mouth are usually caused by bacteria that colonize the junction of tooth and gum. This gingival space enlarges as bacteria multiply within it until the space is large enough to trap food particles and develop fetid-smelling abscesses. Weak pets may have fetid breath because they lack the strength to swallow food and clean their mouths so the food sits in the cheeks and rots.Mouth odor is also caused when pets eat feces (coprophagia), onions, and garlic.
In addition to the odors coming from the mouth itself, odors caused in other areas of the body can be released through the mouth. For example, pets with bleeding stomach ulcers have offensive breath when the odor of digested blood rises from the stomach. Pets with kidney disease exhale waste materials from the blood through their breath and have uremic breath. Pets with uncontrolled diabetes metabolize fat and muscle which gives the breath a peculiar acetone odor. Pets on medications, such as DMSO, have a garlicly breath.
All dogs have a skin fold in the lower lip that can trap bacteria, yeast, food particles, and moisture leading to an offensive odor. This lip fold is deepest in Cocker Spaniels.
The area under the tail can be malodorous due to a mixture of anal gland material, fecal material, bacteria and yeast. Odors are strongest in pets with anal sac inflammation (anal sacculitis), anal fistulas, or fecal incontinence. Pets with long hair develop malodorous rear ends because feces mats in their coats. Pets with diarrhea have offensive anal odors because diarrheic stool is often fetid. Pets with heavy folds of skin and deep wrinkles around the base of the tail (obese pets, Shar Peis, Pugs) can have offensive odors.In addition to problems arising from skin around the anal area, some pets qualify as malodorous for the quality and quantity of gas they pass. Flatulence reaches crescendos of noise and malodor in pets with maldigestion, malabsorption, and food allergies.
Female dogs in season have vaginal discharge for 7-10 days; the discharge has a strong odor, but it is not fetid. Post delivery there is a vaginal discharge for several days in dogs and cats, but it is not fetid. When vaginal discharges become fetid, there is a medical problem: pus in the uterus (pyometra), retained placenta, etc.
Pets with urinary incontinence that dribble urine have an offensive odor around the vulva or prepuce. The odor extends to cover large areas if incontinence occurs when sleeping, causing a pet to sleep in urine.
Fat pets and pets with thick skin folds or wrinkled skin around the vulva have an offensive odor because moisture, bacteria and yeast congregate in the fold. This condition is "intertrigo."