Would that change put neospora at the top of your differential?

Masticatory muscle changes on magnetic resonance imaging of dogs with Neospora caninum compared to meningoencephalitis of unknown origin

Jessica Zilli 1,*, Kathryn Fleming 1, Chloe Fisher 2,3, Tim Sparks 4, Tom Harcourt-Brown 2, Edward Ives 1

Background
Infectious meningoencephalitis, including Neospora caninum infections, and meningoencephalitis of unknown origin (MUO) are differential diagnoses in dogs presenting with central nervous system inflammation. MUO is an immune-mediated condition often treated with immunosuppressive therapy, which can worsen infectious diseases like neosporosis if misdiagnosed. This study aimed to determine if masticatory muscle changes on magnetic resonance imaging (MRI) could help distinguish neosporosis from MUO and enable earlier, targeted treatment.

Methods
Study Design: Retrospective analysis of 45 dogs (22 with neosporosis, 23 with MUO) from two referral hospitals (2015–2023).

Inclusion Criteria: Dogs with neurological signs compatible with focal or multifocal brain disease, brain MRI (including masticatory muscles), and serologic or polymerase chain reaction (PCR) testing for Neospora caninum.

MRI Evaluation: A blinded team of specialists assessed the masticatory muscles for:
-Signal intensity changes (T2-weighted, FLAIR, and T1-weighted sequences).
-Atrophy, symmetry, and contrast enhancement patterns.

Data Analysis: Statistical comparisons included clinical history, MRI findings, and cerebrospinal fluid (CSF) analysis.

Results
Masticatory Muscle Changes:
-Observed in 27% (6/22) of neosporosis cases, appearing as bilateral, multifocal lesions with T2 and FLAIR hyperintensity and contrast enhancement.
-Only 4% (1/23) of MUO cases exhibited muscle changes, which were focal, unilateral, and mild.

Clinical Differentiation:
-Dogs with neosporosis had significantly elevated CK and AST levels compared to MUO dogs (P < 0.001).
-Masticatory muscle changes correlated with higher CSF white blood cell counts and protein levels in neosporosis cases (P = 0.017 and P = 0.025, respectively).

Specificity and Sensitivity:
-The presence of masticatory muscle changes had a specificity of 96% and sensitivity of 27% for diagnosing neosporosis.
-The absence of muscle changes did not exclude neosporosis, emphasizing the need for additional diagnostic testing.

Limitations
The study was retrospective, with potential bias from variable MRI protocols. Small sample sizes limited statistical power. Additionally, histopathological confirmation of muscle lesions was not performed, and overlapping conditions, such as immune-mediated myositis, were not investigated.

Conclusions
Characteristic bilateral, multifocal masticatory muscle lesions observed on MRI may indicate Neospora caninum infection in dogs with brain inflammation, aiding differentiation from MUO. However, their absence does not exclude infection. MRI findings should be considered alongside clinical history, serological/PCR testing, and CSF analysis to guide treatment decisions. Further prospective studies with histopathological validation are recommended to confirm these findings.

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