Demodex is a microscopic mite that lives inside hair follicles and is present on nearly every dog. Puppies pick up the mites from their mother during the first few days of life through close contact, and in most dogs these mites remain at low numbers without ever causing problems. When a dog’s immune system fails to keep the population in check, the mites multiply and cause a skin disease called demodicosis, commonly known as demodectic mange.
How Dogs Get Demodex Mites
Mites transfer from mother to puppy during cuddling in the first week of life. After roughly one week of age, a puppy’s developing immune system is strong enough that it no longer accepts new mites onto its body. This means demodex is not considered a contagious disease in the way sarcoptic mange is, and dogs with demodicosis generally don’t need to be isolated from other pets.
That said, recent observations have complicated the picture slightly. Occasionally, multiple unrelated dogs in the same household develop demodicosis, suggesting some transfer between adult dogs may be possible under certain circumstances. Current thinking is that mites can move between dogs, but in a healthy animal they simply blend into the existing mite population without causing disease.
Why Some Dogs Get Sick and Others Don’t
The key factor is immune function. Skin cells normally recognize the mites and keep their numbers low through an immune response triggered at the level of the hair follicle. Dogs that develop demodicosis are thought to have an inherited, Demodex-specific immune deficiency. The exact genetic defect hasn’t been identified, but it appears to selectively impair the body’s ability to control this one parasite.
Once the mite population gets out of hand, the immune system enters a state of T-cell exhaustion, a kind of immune fatigue that allows the mites to proliferate further. The good news is that this exhaustion is reversible with treatment.
In adult dogs that develop demodicosis for the first time, an underlying cause is often suspected. Hormonal disorders, long-term steroid use, and cancer treatment can all suppress the immune system enough to let mites flourish. However, a specific underlying cause is only identified in about 50% of adult-onset cases.
Breeds at Higher Risk
A large study by the Royal Veterinary College identified seven breeds with increased risk of juvenile-onset demodicosis: British Bulldogs, Staffordshire Bull Terriers, Chinese Shar-Peis, Dogues de Bordeaux, Pugs, French Bulldogs, and Boxers. This breed clustering supports the idea of an inherited immune vulnerability. Any dog can develop demodicosis, but these breeds appear significantly more likely to do so.
Localized Demodicosis
The mild form of the disease typically appears in puppies under six months old. It shows up as one to five small, well-defined patches of hair loss, usually around the lips, the area around the eyes, or the front legs. The skin in these patches may look pink or slightly scaly. Itching is usually absent or very mild, which helps distinguish it from other skin conditions.
Most cases of localized demodicosis resolve on their own as the puppy’s immune system matures. It’s the canine equivalent of a minor setback during development rather than a serious illness.
Generalized Demodicosis
When mite overgrowth spreads beyond a few small patches, the disease becomes generalized. This is a moderate to severe condition. Hair loss can cover large areas of the body or appear as multiple patches that merge together. The skin often becomes red, oily, thickened, and darkened. Crusty bumps and swelling are common.
One of the most frequent complications is secondary bacterial infection. When mites damage the hair follicles at scale, bacteria invade the weakened skin, leading to deep infections that cause boils, draining sores, and significant discomfort. Dogs with severe secondary infections can become systemically ill, developing swollen lymph nodes, fever, and lethargy. Paw involvement is also common, with swollen, painful feet that can make walking difficult.
Generalized demodicosis can appear as a juvenile-onset form in young dogs or as an adult-onset form, which looks clinically similar but carries the added concern of a possible underlying immune-suppressing condition.
How Demodicosis Is Diagnosed
The standard diagnostic tool is a deep skin scraping. A veterinarian uses a blade and mineral oil to scrape the surface of the skin at an affected area. The goal is to reach the depth of the hair follicle where mites live. The procedure looks like a mild abrasion afterward, similar to skinning your knee. It’s not intended to cut the skin. The collected material is placed on a glass slide and examined under a microscope, where the mites are clearly visible.
Multiple sites are typically scraped to get an accurate picture of mite numbers and to check for live mites versus dead ones, which helps gauge treatment progress later.
Treatment and What to Expect
Treatment for generalized demodicosis has improved dramatically in recent years. Oral medications in the isoxazoline class, originally developed as flea and tick preventatives, have proven highly effective against Demodex mites. In clinical studies, one such medication reduced live mite counts by 97% within two weeks and by 99.8% within a month of the first dose. No live mites were detected after that point in treated dogs.
Treatment typically involves monthly oral doses for several months. Your vet will perform follow-up skin scrapings to confirm the mites are gone. The standard for calling a dog “cured” is two consecutive negative scrapings, meaning no live mites are found. This usually takes a few months of treatment depending on the severity of the case.
If secondary bacterial infections are present, those need to be treated as well, often with antibiotics and medicated baths. The skin damage from a severe case can take time to heal even after the mites are eliminated. Hair regrowth is typically the last visible sign of recovery, often lagging weeks behind the clearing of active infection.
Localized vs. Generalized: Knowing the Difference
The practical distinction matters because the two forms require very different levels of concern. Localized demodicosis, with its handful of small bald patches on a puppy’s face or legs, often resolves without aggressive treatment. Generalized demodicosis requires active, sustained treatment and monitoring.
If your dog has a few small hairless spots and is otherwise acting normally, that’s worth a vet visit but not a cause for alarm. If hair loss is spreading, the skin looks inflamed or infected, or your dog seems unwell, that points toward generalized disease and warrants prompt attention. For adult dogs developing demodicosis for the first time, the vet will likely recommend additional testing to look for an underlying condition driving the immune suppression.