MUO, the Brain Inflammation Without a Known Cause: Which Dogs Get It
In 1,121 English referral cases, MUO peaked at about four years old. Maltese, Papillons and French Bulldogs were among the breeds at highest odds.
Written by thecanineledger
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If your dog has a seizure, walks in circles, tilts its head, becomes wobbly or suddenly behaves differently, contact a vet the same day. A seizure lasting several minutes, or one seizure after another, is an emergency.
Our note: These signs are ours. The paper describes diagnosis at referral centres and gives no owner advice.
→ First Seizure: What to Do, and What Counts as an Emergency
In 1,121 dogs diagnosed with MUO at 13 English referral centres, diagnosis peaked at about four years old (Choi et al., Frontiers in Veterinary Science 2025). Maltese, Papillons and French Bulldogs were among the breeds with the highest odds.
While a named diagnosis sounds definite, MUO is a label vets give when tests point to brain inflammation and they can’t find a cause. Only a brain biopsy or post-mortem confirms the exact type.
1. The condition
What it is. The authors describe MUO as “an umbrella term to indicate a presumed autoimmune inflammatory disease of the central nervous system (CNS) in dogs”. In other words, the immune system is thought to attack the brain or spinal cord.
How it’s diagnosed. Specialists usually combine an MRI scan with a sample of the fluid around the brain and spinal cord, and they test for infections that can look the same.
2. The study
The cases. 1,121 dogs diagnosed at 13 referral centres in England from 2017 to 2021. MUO made up about 2% of new neurology referrals.
The comparison. About 750,000 dogs from UK practice records, using a large database run by the Royal Veterinary College.
Funding. A UK veterinary research grant, with support from a medical research academy and a biomedical research charity, the Wellcome Trust. The authors declare no commercial or financial conflicts.
3. Which dogs
| Breed | Odds of MUO, compared with crossbreeds |
|---|---|
| Welsh Springer Spaniel | about 24 times |
| Maltese | about 21 times |
| Papillon | about 17 times |
| Boston Terrier | about 17 times |
| French Bulldog | about 9 times |
Flat-faced and small dogs. Flat-faced breeds had about 2.6 times the odds of breeds with medium-length faces. Dogs of 15 kg or more had lower odds than dogs under 10 kg.
Sex. The study found no difference between males and females. Neutered dogs had higher odds, but the authors say this needs cautious interpretation.
Age. Pugs and French Bulldogs tended to be diagnosed younger, at around three years.
Our note: The paper mentions a DNA test for one breed. We don’t cover it.
→ French Bulldog Health: Strengths, Weaknesses, and What the Records Show
4. What diagnosis involves
Both tests. About 92% of the dogs had both an MRI and a spinal fluid sample.
When a test was skipped. The authors write that “financial constraints were the primary reason for not performing MRI scans”. The fluid sample was skipped mainly for safety.
Normal results. A few dogs were diagnosed with MUO even though both tests were normal. Vets relied on the history or on how the dog responded to treatment.
Our note: The paper doesn’t report treatment or outlook, so we leave them out.
5. What the study can’t tell you
Referral cases only. Dogs that weren’t referred, often for cost or distance, are missing.
Different specialists. The 13 centres may have defined cases differently.
Small slips. The abstract says 961 dogs had abnormal results on both tests, but the results say 916. The abstract gives the share with both tests normal as 1.5%, but the results say 1.9%.
Our note: Older dogs may be under-counted, because owners often choose not to refer them.
6. What we could not verify
| Commonly said | What we found |
|---|---|
| “MUO mostly affects females” | Not shown. This large study found no difference between the sexes. |
| “Only toy breeds get MUO” | No. Welsh Springer Spaniels had the highest odds, though small dogs had higher odds overall. |
| “An MRI always shows it” | No. About 3.5% of dogs who had both tests had a normal MRI but abnormal spinal fluid. |
Sources
- Choi Y, O’Neill DG, Ives E, et al. Meningoencephalitis of unknown origin in dogs under veterinary referral care in England (2017–2021): a multicenter case control study. Front Vet Sci 2025: 1,121 referral cases at 13 centres, about 750,000 VetCompass controls
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