By the Pethocyn editorial team. Published October 28, 2026. Updated November 1, 2026. Sources are listed at the end of the article.

Claws grow from the skin at the end of each toe, so infections, immune diseases and tumors of the skin can reach the claw and the skin around it, the nail bed. A dog with a toe problem often limps, has a lump or an open sore on the toe, or has claws that break or split. [1]

This guide covers the claw problems that published studies and reviews describe: fungal infection, immune diseases that target the claws or the claw folds, tumors that start in the nail bed of dogs, and toe lesions in cats that come from a tumor elsewhere in the body. It does not cover nail trimming. For problems between the toes and on the pads, see dog paw problems; for scaly, flaky skin, see dog dandruff and dry, flaky skin.

Infections of the claw and claw fold

Ringworm. The fungi behind ringworm feed on keratin, the tough protein in skin, hair and nails, in people and in animals. [2] So although ringworm usually shows on the skin and coat, the claws can be involved too. See ringworm in dogs and cats.

Crusting and pus in the claw folds of cats can be a sign of pemphigus foliaceus, an immune skin disease. A 2025 review shows cats with heavy crusts and thick, pus-like discharge built up in the skin folds around the claws, and notes that vets can sample that discharge to look for the typical cells. [3] See pemphigus foliaceus in dogs and cats.

Symmetrical lupoid onychodystrophy (dogs)

Symmetrical lupoid onychodystrophy (SLO) is a disease that exclusively affects the claws of dogs. [4] A 2026 review of 31 dogs examined at a veterinary hospital over 20 years reported: [4]

  • The average age at onset and diagnosis was 7 years.
  • Nineteen breeds were represented.
  • Most dogs presented with onychodystrophy (abnormally formed claws) and onychomadesis (claws separating and coming off).
  • Thirty of the 31 dogs were diagnosed from their clinical signs; a biopsy was taken in one. The authors write that a biopsy of the toe bone was historically the recommended test, but that the distinctive presentation of SLO supports a clinical diagnosis.
  • Many different treatment combinations were used. The authors concluded that medical treatment seems to support a favourable long-term outcome, despite the wide variation in protocols.

A 2010 genetic study describes SLO as an immune-mediated disease with a suggested autoimmune cause. Comparing 98 Gordon Setters with SLO with 98 healthy controls, it found that certain immune gene types (DLA class II) were significantly associated with the risk of developing SLO. In smaller samples of Bearded Collies and Giant Schnauzers, the same or similar gene types were over-represented among affected dogs. [5]

Your vet will choose the treatment; this guide does not name medicines or doses.

Tumors that start in the nail bed (dogs)

Two kinds of cancer often start in the toe or nail bed of dogs.

Squamous cell carcinoma of the toe is the most common cancer found in amputated dog toes. It usually affects older dogs. [1]

  • Studies point to a breed tendency in Schnauzers, Rottweilers, Standard Poodles, black Labrador Retrievers, Briards and Beaucerons. [1][6]
  • Black-coated dogs are more prone to it, and more than one toe can be affected. Because the toes get little sun, researchers suspect an inherited cause; a gene linked to coat color intensity has been tied to this cancer in black Poodles and black Giant Schnauzers. [6]
  • One reported Schnauzer had a tumor that had caused loss of the claw and damage to the toe bone. [1]

Melanoma can also start in the toe or under the claw. A 2026 review says melanomas under the claw usually show up as a swollen, painful toe or a sore next to the nail that does not heal, and the dog often limps or keeps licking that toe. Melanomas of the toes tend to behave aggressively and can spread. [7]

Infection and cancer can look alike on a toe. In a study of 2,912 dog toes removed in surgery and sent to a lab, about half of the lesions were tumors, and most of the others were inflammation. [1] That is why a vet usually needs x-rays and a tissue sample before saying what a swollen or sore toe is. Have any change in your dog’s toes or claws checked promptly.

Toe lesions in cats that start in the lungs

In cats, a lesion on a toe can be a cancer that started in the lung and spread there. This rare pattern is called feline lung-digit syndrome, and it can affect one or more toes, sometimes on more than one leg. [9] For other ways cancer can spread to the skin, see cancer that spreads to the skin in dogs and cats.

  • A 2024 case report described a cat with a lung tumor that spread to several toes. The cat had an unusual lameness that shifted between legs and came and went, which the authors suspected was an early sign of the toe spread before any lesions could be seen. The authors advised that this kind of lameness in an older cat should raise suspicion of a lung tumor and that chest x-rays be considered. [8]
  • A 2026 case report describes an older cat whose first sign was a limping front leg with a swollen toe that later ulcerated. The authors note that lung cancer is rare in cats and the outlook is often poor, especially once it has spread to the toes. [9]

When to see your vet

  • A claw that is misshapen, splitting or falling off. [1][4]
  • A swollen, sore or ulcerated toe, or a toe with a lost nail. [1][7]
  • Limping, or licking one toe over and over. [7]
  • Crusts or pus in the skin folds around the claws in a cat. [3]
  • An older cat with lameness that shifts between legs or comes and goes. [8]

FAQ

Why is my dog’s nail falling off?
There are several possible causes, including symmetrical lupoid onychodystrophy, which mostly shows as abnormally formed claws that separate and come off, [4][5] and nail-bed tumors, which can destroy the claw. [1] Your vet will need to examine the toe to tell them apart.

Can ringworm affect the nails?
Yes. The fungi that cause ringworm live on keratin, which is found in nails as well as skin and hair. [2]

Is a swollen toe in an older dog serious?
It needs a vet visit. Squamous cell carcinoma and melanoma of the toe mostly affect older dogs and can look like a swollen or infected toe, so a vet usually needs x-rays and a tissue sample. [1][7]

Why would a lung problem show up in my cat’s toes?
In feline lung-digit syndrome, a cancer that starts in the lung spreads to one or more toes. It is rare, and limping may be the first sign. [8][9]

The bottom line

Claws grow from the skin, so ringworm and immune diseases can involve them. [2][3] Symmetrical lupoid onychodystrophy affects only the claws of dogs and mostly shows as misshapen claws that come off. [4][5] In dogs, squamous cell carcinoma and melanoma can start in the toe or nail bed, so have any change in the toes or claws checked promptly. [1][6][7] In cats, toe lesions may spread from a lung tumor. [8][9]

This article is for general information and is not a substitute for a veterinary exam. If you are worried about your pet’s claws or toes, see your veterinarian.

Sources

  1. Grassinger JM, Floren A, Müller T, et al. Digital Lesions in Dogs: A Statistical Breed Analysis of 2912 Cases. Vet Sci. 2021;8(7):136. doi:10.3390/vetsci8070136. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC8310350/
  2. Silva LBBD, Oliveira LS, Furlan TBP, et al. Dermatophytosis: An Update on Global Epidemiology. J Fungi (Basel). 2026;12(7):503. doi:10.3390/jof12070503. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC13413057/
  3. Banovic F, Gomes P, Trainor K. Feline immune-mediated skin disorders: Part 1. J Feline Med Surg. 2025;27(4):1098612X251323413. doi:10.1177/1098612X251323413. Open access (CC BY-NC 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC12033501/
  4. Calesso JR, Souza CP, Bruner SR, Bicalho APCV. Symmetrical Lupoid Onychodystrophy in Review: Lessons Learned From 31 Cases. Vet Dermatol. 2026. doi:10.1111/vde.70114. PMID 42559714.
  5. Wilbe M, Ziener ML, Aronsson A, et al. DLA class II alleles are associated with risk for canine symmetrical lupoid onychodystrophy (SLO). PLoS One. 2010;5(8):e12332. doi:10.1371/journal.pone.0012332. Open access (CC BY). https://pmc.ncbi.nlm.nih.gov/articles/PMC2925901/
  6. Aupperle-Lellbach H, Heidrich D, Conrad D, et al. Comparative Study of Digital Squamous Cell Carcinoma in Giant, Standard, and Miniature Schnauzers. Animals (Basel). 2023;13(12):1990. doi:10.3390/ani13121990. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC10295616/
  7. Jeong H, Shin M, Kim Y, et al. Canine and Feline Melanoma: Diagnosis, Treatment, and Prognosis. Vet Sci. 2026;13(10):1053. doi:10.3390/vetsci13101053. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC13709743/
  8. Iqbal MM, Snead E, Cotter B, Philibert H, Sato-Takada K, Ricard M. Feline lung-digit syndrome: A differential diagnosis for shifting, waxing and waning lameness in a cat. Can Vet J. 2024;65(7):655-660. PMID 38952750.
  9. Cabral I, Matos G, Fernandes Silva G, Carvalho F, Amorim I. Treatment of Feline Lung-Digit Syndrome with Toceranib Phosphate: Prolonged Survival and Novel Metastatic Findings. Animals (Basel). 2026;16(5):839. doi:10.3390/ani16050839. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC12985166/