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Meningoencephalitis and Meningomyelitis
Christopher L. Mariani, DVM, PhD, DACVIM (Neurology)

College of Veterinary Medicine, NC State, Raleigh, NC, USA

Etiology/Pathophysiology more likely to develop infectious CNS disease than dogs,
although documentation of infectious agents is still diffi-
The meninges comprise three connective tissue layers cult in this species. In dogs, the vast majority of menin-
that surround the brain and spinal cord: the pia mater, goencephalitis and meningomyelitis cases appear not to
arachnoid mater (these collectively known as the have a direct infectious cause, and are often termed “ster-
leptomeninges), and the dura mater (or pachymeninx). ile” as a result. In these animals, invading immune cells
Inflammation of the meninges, known as meningitis, lead to CNS inflammation and resulting clinical signs.
may remain confined to these structures, or may also Whether these represent a response to an as yet unde-
involve the underlying brain and/or spinal cord, termed tected infectious organism, an overactive immune‐medi-
meningoencephalitis and meningomyelitis respectively. ated response to an unknown environmental trigger or a
There are a whole host of infectious diseases that can true autoimmune process remains to be determined.
cause inflammation of the meninges or underlying However, broad screening studies aimed at detecting pre-
central nervous system (CNS) in small animals. These viously unknown infectious agents have thus far been
include viral, bacterial, fungal, rickettsial, protozoal, and relatively unrewarding, and an immune‐mediated pro-
parasitic organisms (Table 73.1). These organisms may cess appears more likely for the majority of canine cases.
infect the CNS as part of a systemic, multiorgan process There are several well‐recognized syndromes that occur
(e.g, Rickettsia rickettsia, canine distemper, Blastomyces in dogs where infectious agents have not been identified
dermatitidis) although some agents display a notable (Table 73.2). Many of these conditions are highly breed
neurotropism with CNS entry via the bloodstream or nasal associated and a genetic role in their pathophysiology has
passages (e.g., Cryptococcus neoformans, Cladophia­ long been suspected. Such a role has been best investi-
lophora bantiana). Within the CNS, these infectious gated in pug dogs with necrotizing meningoencephalitis
agents may cause clinical signs through a variety of (NME). Several studies in this breed have shown a strong
mechanisms. Their presence often results in an immune association of disease with a specific haplotype localized
response with resulting inflammation. Such an inflam- in the major histocompatibility complex (MHC) II region
matory response can occasionally involve the vascula- of chromosome 12. This finding resembles what is seen in
ture supplying the CNS, leading to vascular injury and many immune‐mediated diseases in humans, including
resulting ischemic or hemorrhagic disease. Inflammation multiple sclerosis. How ever, this association probably
of the ependymal surface and changes in the viscosity of does not identify a strictly causative mutation, but likely
the cerebrospinal fluid (CSF) can result in disruption of confers an increased risk of disease development, and it is
CSF flow and obstructive hydrocephalus (e.g., with feline suspected that other factors, such as the presence of a
infectious peritonitis [FIP]). Some organisms can infect “trigger” to set the disease in motion, also play a role.
neurons or glia, causing direct dysfunction or death of Interestingly, other dog breeds with NME that develop a
these cells (e.g., canine distemper virus). histologically identical lesion do not seem to share the
Despite this large number of organisms that can cause same at‐risk haplotypes as pugs and clearly more work to
CNS disease, documented infections comprise a very define these underlying susceptibilities needs to be done.
small proportion of small animal meningoencephalitis The pathophysiology of most canine and feline
and meningomyelitis cases. Cats are generally considered meningoencephalitides is relatively poorly understood,

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical