Editorial: Koch's postulates and slow infections of the nervous system

Managing the deluge of newly discovered plant viruses and viroids: an optimized scientific and regulatory framework for their characterization and risk analysis

The advances in high-throughput sequencing (HTS) technologies and bioinformatic tools have provided new opportunities for virus and viroid discovery and diagnostics. Hence, new sequences of viral origin are being discovered and published at a previously unseen rate. Therefore, a collective effort was undertaken to write and propose a framework for prioritizing the biological characterization steps needed after discovering a new plant virus to evaluate its impact at different levels. Even though the proposed approach was widely used, a revision of these guidelines was prepared to consider virus discovery and characterization trends and integrate novel approaches and tools recently published or under development. This updated framework is more adapted to the current rate of virus discovery and provides an improved prioritization for filling knowledge and data gaps. It consists of four distinct steps adapted to include a multi-stakeholder feedback loop. Key improvements include better prioritization and organization of the various steps, earlier data sharing among researchers and involved stakeholders, public database screening, and exploitation of genomic information to predict biological properties.

Challenges in determining causation in structure-function studies using molecular biological techniques

The use of molecular biological techniques for determining the levels and types of different microbial populations in bioreactors has led to the emergence of the microbial community 'structure-function' paradigm that is often used in research. Typically, lab- or full-scale systems are monitored for the relevant parameters, and these parameters are related to the changes in microbial populations. Research in activated sludge phenomena, such as filamentous bulking, filamentous foaming, nitrogen removal, and phosphorus removal, are replete with many examples of this 'structure-function' paradigm, most commonly those that involve 16S rRNA gene-based analysis of the microbial populations. In many cases, such studies assume a causal microbial population (e.g., a species that causes bulking or foaming), or conclude in identifying a causal population. However, assigning cause to specific organisms and populations is problematic in a complex environment such as wastewater bioreactors. The Koch-Henle postulates, the gold standard in evaluating causation of disease, have limitations when applied to systems with mixed microbial communities with complex interactions, particularly if pure cultures are not available. Molecular techniques that allow specific identification and quantification of organisms have been used by researchers to overcome the limitations of culture-based techniques, and at the same time, raised new questions on the applicability of causation postulates in environmental systems. In this paper, various causation criteria improving on the Koch-Henle postulates are presented. Complicating issues in assigning cause in wastewater bioreactors are identified. Approaches for determining cause-effect relationships are illustrated using 16S rDNA-based investigations of filaments that cause bulking and foaming in activated sludge. The hope is that a causation framework that accounts for the assumptions in molecular studies, as applied to wastewater treatment research, will lead to improved experimental design and analysis of data.

A specific viral cause of multiple sclerosis: One virus, one disease

"Multiple sclerosis is an autoimmune disease," is heard so often that it is widely accepted as fact by the current generation of students and physicians. Yet, although it is undisputed that multiple sclerosis (MS) is immune mediated, an autoimmune mechanism remains unproven. Immune-mediated tissue damage can also result from viral infections in which the host immune response is directed to viral rather than self proteins, or as a consequence of nonspecific or bystander immune responses that change the local cytokine environment. Increasing evidence suggests that poorly controlled host immune responses account for much of the tissue damage in chronic infections, and it has been postulated that a similar mechanism may underlie many chronic diseases with features suggestive of an infectious causative factor, including MS. A recent study suggesting that oligodendrocyte death accompanied by microglial activation is the primary event in new MS lesion formation, rather than lymphocyte infiltration, could change the current mindset almost exclusively focused on autoimmunity. This review presents the rationale for considering MS a single disease caused by one virus, as well as the anticipated pattern of a persistent central nervous system infection, the application of Koch's postulates to viral discovery in MS as the causative agent, and tissue culture-independent genotypic approaches to viral discovery in MS.

Causality in medicine: the case of tumours and viruses

Clarification of the aetiology of chronic human diseases such as atherosclerosis or cancer is one of the dominant topics in contemporary medical research. It is believed that identification of the causal factors will enable more efficient prevention and diagnosis of these diseases and, in some instances, also permit more effective therapy. The task is difficult because of the multistep and multifactorial origin of these diseases. A special case in contemporary aetiological studies is definition of the role of viruses in the pathogenesis of human cancer. Virus-associated cancer develops only in a small minority of infected subjects, which implies that, if the virus does play a role in the pathogenesis of the malignancy, other factors must also be involved. In this paper the author attempts to review the present methodological approaches to aetiological studies of chronic diseases, discusses the role of criteria for identifying causal relationships and proposes guidelines that might help to determine the role of viruses in human cancer.

Sequence-based identification of microbial pathogens: a reconsideration of Koch's postulates

Over 100 years ago, Robert Koch introduced his ideas about how to prove a causal relationship between a microorganism and a disease. Koch's postulates created a scientific standard for causal evidence that established the credibility of microbes as pathogens and led to the development of modern microbiology. In more recent times, Koch's postulates have evolved to accommodate a broader understanding of the host-parasite relationship as well as experimental advances. Techniques such as in situ hybridization, PCR, and representational difference analysis reveal previously uncharacterized, fastidious or uncultivated, microbial pathogens that resist the application of Koch's original postulates, but they also provide new approaches for proving disease causation. In particular, the increasing reliance on sequence-based methods for microbial identification requires a reassessment of the original postulates and the rationale that guided Koch and later revisionists. Recent investigations of Whipple's disease, human ehrlichiosis, hepatitis C, hantavirus pulmonary syndrome, and Kaposi's sarcoma illustrate some of these issues. A set of molecular guidelines for establishing disease causation with sequence-based technology is proposed, and the importance of the scientific concordance of evidence in supporting causal associations is emphasized.

The virology of demyelinating diseases

Infectious agents have been postulated as causes of multiple sclerosis for over a century. The possible role of a virus or viruses is supported by data that (1) a childhood exposure is involved and "viral" infections may precipitate exacerbations of disease, (2) experimental infections in animals and natural infections in humans can cause diseases with long incubation periods, remitting and relapsing courses, and demyelination, and (3) patients with multiple sclerosis have abnormal immune responses to viruses. The pathogenesis of three human demyelinating diseases of known viral etiology is discussed. In progressive multifocal leukoencephalopathy, a papovavirus selectively infects oligodendrocytes and causes focal areas of demyelination. In postmeasles encephalomyelitis, the virus is lymphotrophic and disrupts immune regulation that can result in an autoimmune perivenular demyelinating illness without evidence of infection of the central nervous system. In human immunodeficiency virus-encephalopathy and myelopathy virus is present in macrophages and microglia and the myelin abnormalities apparently are caused by soluble factors such as viral proteins, cytokines, or neurotoxins. These findings may have implications on how, when, and where to seek viruses in multiple sclerosis.

Isolation of an HTLV-1-like retrovirus from patients with tropical spastic paraparesis

Tropical spastic paraparesis (TSP) is a slowly progressive myelopathy associated with increased serum and cerebrospinal fluid antibodies to the human T-lymphotropic retrovirus type I (HTLV-I) (ref. 1), and has been observed in many regions of the world. A similar condition known as HTLV-I-associated myelopathy occurs in the Kagoshima prefecture of Japan. Recent but controversial reports suggest involvement of virus related to HTLV-I in multiple sclerosis. Magnetic resonance imaging and electrophysiological studies indicate that TSP lesions are like multiple sclerosis in that they are disseminated throughout the nervous system. Complete virus from patients with TSP has proved difficult to isolate using techniques successful in adult T-cell leukaemia cases associated with HTLV-I. Here we report the isolation of an HTLV-I-like virus from T-cell lines derived from the peripheral blood and cerebrospinal fluid of TSP patients. The monoclonal antibody OKT3 was used to generate non-transformed T-cell lines that express HTLV-I antigens. Infectious virus was demonstrated by co-cultivation and complete, replicating virions were visualized ultrastructurally.

Conceptual/operational criteria of causality: relevance to systematic epidemiologic theory

Emphasis has recently been placed on the possibility of developing "unified" criteria of causality applicable to both the infectious and chronic, non-infectious diseases. Yet, contemporary efforts are essentially compilations of criteria established over the past 140 years, without provision of the conceptual basis allowing such a unification. From hierarchical views of the natural history of infectious and neoplastic (i.e., chronic non-infectious) disease, it is possible to identify discipline and natural history-related concepts of causation from which it is possible to derive conceptual criteria of causality and complementary operational criteria of causality. These criteria are entirely relevant to the comprehension of disease as both product and process and applicable to both the infectious and neoplastic diseases.

[Virologic studies in patients with amyotrophic lateral sclerosis]

The effects of intracerebral inoculation of cerebrospinal fluid in mice from 9 patients with amyotrophic lateral sclerosis are reported. There were 704 animals inoculated. The results were considered positive when the animals presented difficulties in walking and equilibrating. Cases 1, 2, 6, 7 and 9 were positives in the first passage. In case 1, the positivity was reached from first to fourth passage and in case 6, to third passage. At sight of this results, the possibility of that disease to be caused by slow-virus is discussed.

Is the polio virus responsible for late central nervous system tumors?

A hypothesis is advanced concerning the relationship of poliomyelitis viruses and late developing neoplasia in the central nervous system. The literature is surveyed, and several experimental approaches based upon known evidence of viral and neoplastic behaviour are advanced. The present status of world polio incidence is discussed as it applies to this possible relationship.

Serological response of multiple sclerosis patients and controls to 6/94-parainfluenza virus

The serological responses of 195 multiple sclerosis (MS) patients and 251 controls were tested against 6/94-parainfluenza virus, which was previously isolated from brain tissue of two patients with MS. The hemagglutination-inhibition titers of greater than or equal to 1: 128 were found more frequently in MS patients (21.5%) than in controls (14.0%). However, the geometric mean titers did not differ between these two groups. The present study concludes that a causal relationship of 6/94-virus to MS, based on a specific immune response, is improbable, although it does not exclude the possibility of a pathogenetic significance of the agent in the cases from which the autopsy material was derived.

Subacute sclerosing panencephalitis: experimental infection in primates

Measles virus isolated from the brain of a 12-year-old boy with subacute sclerosing panencephalitis caused a chronic, progressive encephalitis in experimentally infected rhesus monkeys. The infection was eventually fatal in spite of pre-existing measles immunity and a vigorous secondary antibody response in serum and cerebrospinal fluid of the infected animals. The findings provide a basis for studies into the pathogenesis and possible treatment of the human disease.

Intranuclear "paramyxovirus-like" material in multiple sclerosis, adreno-leukodystrophy and Kuf's disease

Detailed comparative ultrastructural examination of multiple sclerosis (MS) plaques, inflammatory CNS lesions from adreno-leukodystrophy(A-LD), tissue from a case of chronic granulomatous meningitis, biopsy samples of necrotic cerebral cortex and CNS tissue from a case of Kuf's disease (adult-type ceroid lipofuscinosis), has revealed that the intranuclear filamentous material previously thought to be related to a viral infection in MS is a non-specific finding. These intranuclear strands were, however, found in greatest frequency in the acute lesions of MS and were absent from chronically demyelinated areas. The macrophages, lymphocytes and fibrocytes containing filamentous material in the nuclei were mainly perivascular. In A-LD, some macrophages in active lesions contained similar nuclei, and in Kuf's disease they were present in some glial cells in the cerebral cortex.