Combatting the Myth of Neuropathology

The central nervous system (CNS) represents a highly complex immune-privileged organ with compartmentalization, region-specific peculiarities and properties, as well as unusual cell types. It consists of highly specialized structures in various neuroanatomic regions, including gray and white matter areas, hippocampus, and circumventricular organs. Although gray and white matter can be easily distinguished in some areas, such as cerebrum, cerebellum, and spinal cord, this is not the case in other regions, such as the brainstem. In addition, CNS cells—such as astrocytes, neurons, oligodendrocytes, microglia, cerebellar external germinal cells, progenitor cells, and epiplexus cells—are present at varying quantities during embryogenesis and in the adult CNS as resident cells or during disease. Moreover, cellular plasticity during organogenesis and during disease initiation and progression represents a unique tissue-specific response. Furthermore, reaction patterns of cells—including chromatolysis of neurons, spheroid formation of axons, gemistocytic forms of astrocytes, and gitter cells of microglia/macrophages—will result in modified functions and morphology. ^32,36

This complexity of the CNS, with its neuroanatomic, neuropathologic, and neurologic implications, engenders the “myth of neuropathology,” in which the pathologic investigation of brain diseases is difficult, arcane, and frustrating. However, as with the skin and eye, the tissue-specific terminology should be considered a helpful manual to aid in precise thinking, instead of an insuperable hurdle. Most CNS structures and cell types can be identified with hematoxylin and eosin–stained sections, with assistance from a textbook on neuroanatomy and neuropathology. ^32,36 Factors causing CNS diseases include infectious pathogens, immune-mediated processes, gene defects, and noninfectious environmental factors resulting in inflammatory and degenerative changes. As for other body systems, pathologic evaluation often allows a morphologic diagnosis that, even if not indicating a specific diagnosis, suggests several causes or pathogeneses that form the basis for further investigation.

This issue of Veterinary Pathology includes a series of 10 articles on degenerative and inflammatory CNS diseases due to various pathogens and noninfectious factors impressively displaying the plethora and diversity of causes, consequences, and reaction patterns of diseases of the nervous system.* This collection combats the myth of neuropathology by presenting a comprehensive insight into a range of morphologic CNS changes, including their interpretation, diagnosis, pathogenesis, and clinicopathologic correlation.

The CNS is exposed to various environmental factors, including dietary imbalances, trauma, infectious pathogens, and toxins, or it may suffer from genetic conditions resulting in acute and chronic lesions. Most frequently, host cell–pathogen interaction is discussed under the assumption of a black-and-white response or, in other words, cell death or survival. However, especially in the CNS, cellular functions may remain impaired despite cell survival. A reduction of cellular “luxury functions,” defined as a loss of key elements that are essential to maintain organ homeostasis, may include reduced myelin production or disturbed axon formation. ^2,15

Although the overall stereotypic reaction pattern in the CNS allows a morphologic diagnosis, in most cases a definitive etiologic or pathogenetic interpretation may await further studies. In this respect, the study by Valberg et al on the pathogenesis of “shivers,” a progressive equine movement disorder, represents an excellent example of a thorough and successful analysis of a well-known but poorly understood disorder by combining neuropathology, immunohistochemistry, transmission microscopy, and neuroanatomy. ³¹ This well-structured study sheds, for the first time, some light on the underlying mechanisms of this disease by revealing the existence of calretinin-negative, calbindin-positive, and glutamic acid decarboxylase–positive spheroids in Purkinje cell axons. The study by Ogawa et al on canine degenerative myelopathy—a progressive neurodegenerative disease frequently found in Pembroke Welsh Corgi dogs—and its clinical and pathologic similarities to human amyotrophic lateral sclerosis represents another example of modern and innovative neuropathology. ²¹ The authors investigated abnormalities of autophagy, resulting in cell death through what is called type II programmed cell death, an important mechanism in this neurodegenerative disease. Their detailed study indicates that altered autophagosome degradation may result in LC3 and p62 accumulation in the degenerative myelopathy spinal cord. Both studies revealed possible pathogenetic mechanisms in most likely genetic diseases by relying on classical and novel methods in neuropathology.

Viral diseases of the CNS can be caused by a variety of neurotropic and nonneurotropic viruses. ^10,29,30 Some have been known for decades to infect animals (eg, rabies virus, canine distemper virus, arthropod-borne viruses, and herpesviruses). ^2,35 In addition, emerging viral CNS diseases have been recognized recently, commonly due to mosquito-borne infections. Examples of mosquito-borne viruses with recently expanding geographic ranges include West Nile, Schmallenberg, and bluetongue viruses. They are transmitted by arthropod vectors and/or free-living birds, including migratory species. In addition, some viruses—including the bat henipaviruses Nipah virus and Hendra virus as well as Japanese encephalitis virus—crossed the human species barrier to cause neurologic disorders in humans. Others, such as the Usutu virus, bear the potential risk of human disease. ³³ Factors contributing to the emergence of new pathogens include changes in human demographics and behavior, intensification of international travel (tourism) and commerce (global trade), increased economic development and land use, increasing importation of infected animals and the exotic pet trade, altered migration of vectors such as birds and arthropods due to trade and their adaption in new environments (in part facilitated by global warming), as well as microbial adaption and changes. Faced with complex patterns of global changes, analyzing the interconnections among humans, companion animals, livestock, and wildlife requires integrated approaches to study human and animal health in their social and environmental contexts. These complex interactions and their conceptual interpretation require a multidisciplinary and cross-sectoral approach involving veterinary and human medicine, as envisioned by the one health–one medicine concept. ^8,13,37 Moreover, new detection systems, including deep-sequencing analysis, will improve our ability to rapidly diagnose and recognize emerging and reemerging pathogens and host genetic factors involved in disease outbreaks. This concept applies also to many of the emerging CNS diseases. One could assume that some of these new pathogens would have spread unrecognized for some time in the past and could have caused epidemics or may have disappeared unrecognized.

Infection of the CNS by any pathogen and its interaction with resident and infiltrating cells and the innate and acquired immune responses requires a multidisciplinary effort by experts from various fields with a common understanding about neuropathology, neuroscience, immunology, infection medicine, molecular microbiology, epidemiology, and biostatistics. New approaches to investigate CNS infection are therefore needed in pursuit of a more detailed understanding of the cellular and molecular mechanisms in CNS diseases and to design specific intervention strategies, such as vaccination and treatment concepts. Neuroinfectiology may be defined as the interaction between CNS host cells and pathogens. The most obvious sensu stricto definition refers to a direct pathogen–host cell effect resulting in meningitis, encephalitis, and myelitis. However, more commonly, derailments of the humoral and cellular immune responses, genetically mediated susceptibility, triggered autoimmunity, and epitope spreading, as well as interactions with infectious and noninfectious environmental or dietary factors, can cause CNS diseases. For example, vasculitis in the CNS is another common sequel of viral, bacterial, mycotic, or parasitic infection and may be immune mediated or due to a direct action of the infectious agent. Alternatively, delayed sequelae—including degeneration, malformations, behavioral changes, and autoimmunity—may occur long after the initial infection and virus clearance. ^1,19 Thus, infectious CNS diseases may be the direct result from pathogen-mediated damage to CNS-resident cells or may develop as an unfortunate consequence of the host response to infection.

The different clinical and neuropathologic manifestations of virus infection are well documented by the 3 articles about Schmallenberg virus, West Nile virus (WNV), and equine coronavirus. The study of Peperkamp et al displays the results of such an unusual viral infection in great detail by combining macroscopic and light microscopic findings with epidemiologic and serologic data. ²² Following natural infection with Schmallenberg virus, congenital abnormalities of the CNS and the musculoskeletal system were observed in ovine and bovine offspring. Interestingly, detection of the causative agent varies among animals, indicating a variable course of infection in individuals. Furthermore, the lack of virus detection does not necessarily rule out a potential etiologic role of the suspected pathogen. The study by Toplu et al describes the clinicopathologic findings in naturally occurring WNV infection, a potential zoonotic disease, in a foal. ²⁸ Provided data suggest an immunopathogenetic mechanism in WNV in horses, indicating that direct virus-triggered and immune-mediated processes both contribute to the observed neuropathology. Equine coronavirus infection represents an emerging disease in various parts of the world. Using immunohistologic and molecular approaches, Giannitti et al provide a detailed description of this new equine disease. ⁹ Surprisingly, 1 horse suffering from necrotizing enteritis developed CNS lesions reminiscent of hyperammonemic encephalopathy.

Besides viruses and bacteria, parasites can cause CNS infection. ⁶ A very unusual CNS manifestation of cytauxzoonosis, a tick-borne disease of felids caused by the protozoan Cytauxzoon felis, is described by Clarke and Rissi. ⁷ Interestingly, the bulk of CNS lesions of affected cats were attributed to vascular occlusion and secondary ischemia caused by protozoal infection, indicating another mode of pathogen-triggered CNS lesion.

In veterinary medicine, the awareness that infectious pathogens are causing CNS disease in animals is well known. Yet, the cause of most lymphocytic or granulomatous CNS lesions in domestic animals remains undetermined. ^11,25 This raises the question about the existence of unrecognized agents or pathogen-triggered CNS diseases due to epitope spreading and molecular mimicry. Lack of virus detection could be due to the low sensitivity of methods applied so far. Nowadays metagenome sequencing is increasingly being used to identify viral pathogens. ^4,27 Although detection of pathogens and their correlation with observed lesions is a strong indicator for a direct causal relationship, final proof would require the fulfillment of Koch postulates. This is, unfortunately, lacking for most recently discovered pathogens. Alternatively, virus-triggered autoimmune disease due to molecular mimicry and/or epitope spreading may develop after initial virus infection and clearance. Canine granulomatous meningoencephalitis and Pug dog disease, to name just a couple, might represent entities caused by such mechanisms.

Other neuropathogenetic mechanisms are recognized in human medicine. Viral diseases such as JC virus–associated progressive multifocal leukoencephalopathy in humans are triggered by the application of novel immunomodulatory therapeutic agents. ^16,29,30 Whether similar activation of otherwise nonpathogenetic agents may also play a role in some CNS diseases in veterinary medicine is a question to be kept in mind in future studies. Furthermore, recent observations suggest that infection (or infections) may be a driving force for neurodegeneration and, in particular, Parkinson disease in humans. ^12,17,24 The question remains whether “purely” degenerative diseases of domestic animals are also initiated, triggered, or restimulated by infectious pathogens. Similarly, the gut microflora has been shown experimentally to play an important role as a trigger of CNS inflammation. ³ Furthermore, T cells become licensed in the lung to enter the CNS. ²⁰ Therefore, the lung could contribute to the activation of potentially autoaggressive T cells and their transition to a migratory mode as a prerequisite to entering the CNS and inducing autoimmune disease. These different modes of pathogen–host or immune system interaction might represent future avenues to improve our understanding of the pathogenesis of inflammatory and degenerative lesions in the nervous system.

The current concept for terminology and underlying cause in epilepsy refers to 3 categories: genetic, structural/metabolic, and unknown. This replaces the previously used terms idiopathic, symptomatic, and cryptogenic. Epilepsies resulting from various processes, including trauma, neoplasms, cardiovascular disturbances, and an infection, belong to the structural/metabolic category. ^23,26 In most cases, the underlying pathology and pathogenesis remain obscure even after detailed neuropathology. Hence, they represent “unthankful” submissions for neuropathologists in many cases. The article by Klang et al refers to an unusual case consisting of hippocampal alterations and meningioma in a cat, providing a wonderful example of a double lesion associated with epilepsy. ¹⁴

Regeneration and repair represent important research topics in CNS diseases in humans and animals. Especially the lack of regeneration in the CNS compared with the peripheral nervous system is a continuous challenge for neuroscientists. Recent years revealed the existence of a variety of progenitor cells in the CNS and their ability to proliferate and differentiate into specialized CNS cells. Still, functional integration is lacking, and their successful application has not been achieved yet. The study by Moore and Oglesbee points to the spinal cord ependymal layer as another highly interesting cell population that may be of great interest in spinal cord injury (SCI) research. ¹⁸ These cells might be a source of canine endogenous neural precursor cells, which could be useful for future clinical interventions, especially after SCI. Furthermore, based on the similarities between canine and human SCI, obtained results will be important for translational research by using the dog both as patient and as a highly suitable animal model for the human disease.

Various dietary factors, either deficient or abundant, could result in CNS lesions pre- and postnatally. The study by Capo et al represents an excellent example of long-term effects of vitamin C depletion in prenatal guinea pigs. ⁵ Observed dysplastic changes are reminiscent of lissencephaly type II, indicating that morphologic manifestation induced by dietary imbalances may mimic malformations due to gene defects. The role of neurotoxins as a trigger of neuropathologic changes in the developing and adult CNS is well known but difficult to prove in spontaneous cases. The study by Vieira et al investigated the cell tropism of domoic acid, a naturally occurring excitotoxin produced by the marine red alga Chondria armata and by various subspecies of marine diatoms, after intraperitoneal injection in rats. ³⁴ Interestingly, domoic acid was detected exclusively in pyramidal neurons of the hippocampus at early time points by immunohistochemistry. In contrast, lesions characterized by apoptosis and necrosis as well as calcification were observed at later time points and in various CNS compartments.

Future studies in neuropathology should address questions about the manner in which pathogens enter the CNS, the mechanisms of direct and indirect as well as acute and long-term damage, the role of misdirected immune responses in lesion initiation and progression, as well as the prevention of CNS infection, by developing appropriate prevention and intervention strategies and demonstration of potential beneficial approaches for tissue regeneration. Neuropathology, together with auxiliary investigations, represents an ideal tool and the most efficient approach to recognize emerging and reemerging CNS diseases as a first-line detection system initiating further etiologic and pathogenetic studies.