Abstract. Astrocytes, once relegated to a mere supportive role in the central nervous system, are now recognized as a heterogeneous class of cells with many important and diverse functions. Major astrocyte functions can be grouped into three categories: guidance and support of neuronal migration during development, maintenance of the neural microenvironment, and modulation of immune reactions by serving as antigen-presenting cells. The concept of astrocytic heterogeneity is critical to understanding the functions and reactions of these cells in disease. Astrocytes from different regions of the brain have diverse biochemical characteristics and may respond in different ways to a variety of injuries. Astrocytic swelling and hypertrophy-hyperplasia are two common reactions to injury. This review covers the morphologic and pathophysiologic findings, time course, and determinants of these two responses. In addition to these common reactions, astrocytes may play a primary role in certain diseases, including epilepsy, neurological dysfunction in liver disease, neurodegenerative disorders such as Parkinson's and Huntington's diseases, and demyelination. Evidence supporting primary involvement of astrocytes in these diseases will be considered.Key words: Astrocyte; functions; morphology; pathology.Neuroglia were recognized as distinct cellular elements in the central nervous system (CNS) in the late 1 8 0 0~.~~~ Mihaly von Lenhossek in 1895 proposed the name astrocyte to identify a specific class of these cells and suggested that the term neuroglia be used to encompass all supporting cells in the CNS.256 Astrocytes have long been considered as playing structural and supporting roles, reacting sluggishly and in a largely stereotypical manner to injury and disease. Rarely have astrocytes been identified as playing a primary part in diseases of the CNS. Mainly because of advances in cell culture and more specific ways to identify these cells, knowledge of astrocytic development, form, function, and roles in disease has increased dramatically. Astrocytes are estimated to comprise as much as 20-25% or even up to 50% of the total volume in some brain a r e a~. * I .~~~ Although with increasing brain weight there is a decrease in the number of neurons across several species, in the same species the ratio of glia to neurons increases. Astrocytic density then remains relatively constant or may even increase in more phylogenetically advanced species.256 These findings indicate the importance of astrocytes in the CNS.As is true of many aspects of cell biology and medicine, knowledge gained from studies in one species may not apply to other animals or to human beings. Likewise, information from in vitro studies may not be representative of astrocytes in situ8' In an organ as biochemically and functionally complex as the brain, knowledge gained from the study of astrocytes in specific areas may not apply to other regions of the CNS. The concept of astrocytic morphologic, biochemical, and functional heterogeneity is important to remem...
Relatively few PI cattle arrive at feedlots. However, those cattle are more likely to require treatment for respiratory tract disease and either become chronically ill or die than cattle that are not PI. In addition, they are associated with an increase in the incidence of respiratory tract disease of in-contact cattle.
Abstract.A fatal enteric syndrome was identified in American bison (Bison bison) at a large feedlot in the American Midwest in early 1998. An estimated 150 bison died of the syndrome between January 1998 and December 1999. The syndrome was identified as malignant catarrhal fever (MCF), primarily the alimentary form. Clinical onset was acute, and most affected bison died within 1-3 days; none recovered. Consistent lesions were hemorrhagic cystitis, ulcerative enterotyphlocolitis, and arteritis-phlebitis. Vasculitis was milder and more localized than that in cattle with MCF, and in contrast to the situation in cattle, lymphadenomegaly was minimal. Virtually all affected bison examined were positive for ovine herpesvirus 2 (OvHV-2) by polymerase chain reaction (PCR) assay. A retrospective study of archived tissues established that MCF occurred in the yard as early as 1993. A prospective study was undertaken to establish the importance of MCF relative to other fatal diseases at the feedlot. The fate of a group of 300 healthy male bison in a consignment of 1,101 animals was followed for up to 7 months to slaughter. At entry, 23% (71/300) of bison were seropositive for MCF viruses, and 11% (8/71) of these seropositive bison were PCR positive for OvHV-2. Forty seronegative bison were selected at random from the group, and all were PCR negative for OvHV-2. There was no change in seroprevalence in the group during the investigation. The minimum infection rate for MCF virus was 36.3% (93/256). Twenty-two (7.3%) of the 300 bison in the feedlot died. Of these, 15 had MCF, 4 had acute or chronic pneumonia, and 3 were unexamined. Losses in the entire consignment were higher (98/1,101; 8.8% death loss); 76% of deaths were attributable to MCF. The study failed to reveal a relationship between subclinical infection and development of clinical disease.
Hepatitis E is recognized as a zoonosis, and swine are known reservoirs, but how broadly enzootic its causative agent, hepatitis E virus (HEV), is remains controversial. To determine the prevalence of HEV infection in animals, a serological assay with capability to detect anti-HEV-antibody across a wide variety of animal species was devised. Recombinant antigens comprising truncated capsid proteins generated from HEV-subgenomic constructs that represent all four viral genotypes were used to capture anti-HEV in the test sample and as an analyte reporter. To facilitate development and validation of the assay, serum samples were assembled from blood donors (n ؍ 372), acute hepatitis E patients (n ؍ 94), five laboratory animals (rhesus monkey, pig, New Zealand rabbit, Wistar rat, and BALB/c mouse) immunized with HEV antigens, and four pigs experimentally infected with HEV. The assay was then applied to 4,936 sera collected from 35 genera of animals that were wild, feral, domesticated, or otherwise held captive in the United States. Test positivity was determined in 457 samples (9.3%). These originated from: bison (3/65, 4.6%), cattle (174/1,156, 15%), dogs (2/212, 0.9%), Norway rats (2/318, 0.6%), farmed swine (267/648, 41.2%), and feral swine (9/306, 2.9%). Only the porcine samples yielded the highest reactivities. HEV RNA was amplified from one farmed pig and two feral pigs and characterized by nucleotide sequencing to belong to genotype 3. HEV infected farmed swine primarily, and the role of other animals as reservoirs of its zoonotic spread appears to be limited.Hepatitis E virus (HEV), the causative agent of hepatitis E, is a nonenveloped, single-stranded, positive-sense RNA virus that belongs to the Hepeviridae family (11). Among the mammalian HEV, at least four genotypes have been recognized (47). In addition, two putative genotypes of HEV, one genotype from the Norway rat (Rattus norvegicus) (25) and the other from a wild boar (56), were recently reported. In addition to mammalian HEV strains, avian HEV (38) and the newly described cutthroat trout virus represent new genera (3). Among mammalian HEV, genotypes 1 and 2 are primarily associated with fecal-oral transmission among humans which in developing countries can lead to waterborne epidemics of jaundice. Genotypes 3 and 4 circulate in humans and several animal species, and are associated with sporadic infection among humans in industrialized countries (57). Hepatitis E is mostly self-limiting but can progress to fulminant liver failure especially in pregnant women, and chronic HEV infection resulting in cirrhosis has been observed among solid-organ-transplant recipients and other immunosuppressed people (32).HEV infection and hepatitis E in industrialized countries, including the United States, are more common than previously recognized (33,58). A study conducted in a large civilian, noninstitutionalized U.S. population found an average anti-HEV-IgG seroprevalence rate of 21%, with a strongly positive
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