TL;DR: Circular hair loss in dogs is usually caused by non-emergency conditions like ringworm or mites, but you should schedule a routine vet visit unless the skin is raw, bleeding, or your pet is lethargic.
What causes circular patches of hair loss on my dog?
Noticing circular patches of hair loss on your dog can be unsettling, but it is a common reason for veterinary visits. These bald spots, often called alopecia, are frequently caused by ringworm, which is a fungal infection rather than a literal worm. Other common causes include bacterial infections of the hair follicles (folliculitis), localized demodectic mange (mites), or even localized allergic reactions. While the hair loss itself isn't painful, the underlying cause might be itchy or irritating for your pet.
Is circular hair loss in my dog considered a veterinary emergency?
- Schedule a routine appointment with your veterinarian within the next few days, as this is typically a low-urgency condition.
- Seek immediate veterinary care if the affected skin appears raw or is bleeding.
- Contact a vet urgently if your dog is lethargic or refusing food in addition to the hair loss.
How can sending photos help with my dog's veterinary triage?
- Provide a high-quality photo of the affected area to help the veterinarian see the shape of the patch and the condition of the surrounding skin.
- Ensure the image captures details like redness, scaling, or crusting to help determine the likelihood of contagious conditions like ringworm.
- Use visual information to help the veterinary team provide better guidance on how to manage the area until your scheduled appointment.
Clinical Context (Merck Veterinary Manual)
Focal areas of alopecia, 0.5-2 cm in diameter, may indicate dermatophytosis or bacterial pyoderma. At the margins of hair loss due to pyoderma, mild epidermal collarette formation may be present; however, follicular pustules and erythema are often absent in shorthaired breeds, making diagnosis difficult. Collies and Shetland Sheepdogs often have areas of widespread alopecia with mild erythema and epidermal collarette formation at the leading edge of the expanding area. Pattern baldness affecting the pinna has been reported in Dachshunds, Chihuahuas, Italian Greyhounds, and Whippets and is thought to have a hereditary predisposition. The age of onset is >1 year of age, starting as thinning of the hair coat with complete pinnal alopecia occurring by 8-9 years of age. Other commonly affected areas are the ventral neck and thorax and the caudal medial thighs. Diagnosis of alopecia requires a careful history and physical examination to recognize breed predispositions for congenital or hereditary alopecias, as well as the duration and progression of hair loss.
Chapter: Dermatology
Source: The Merck Veterinary Manual, 11th Edition (Page 842)