Infections of the nervous system

In 2004, research into infectious diseases of the nervous system was shaped not only by the diverse pathogens, outcomes, and mechanisms of neural dysfunction but also by progress made in therapy and prevention. The role of infections continues to change, epidemiological studies show the emergence of new or the re-emergence of old maladies, and other diseases continue unabated. There are therapeutic interventions for some diseases; for others there are limited possibilities to change the disease course or ameliorate clinical signs and symptoms. Over the past decade, more than at any other time, we have seen changes in disease patterns that might result from changes in environmental cues or in microbial evolution. Infectious diseases of the nervous system now include severe acute respiratory syndrome, West Nile virus, Ebola, and variant Creutzfeldt-Jakob disease (vCJD). New instances of virus transmission of ebola haemorrhagic fever from non-human primates to hunters are emerging in Zaire, the Republic of Congo, and Gabon and have caused more than 264 deaths.¹

vCJD, a lethal disease that affects young people, has rapidly emerged as part of the family of transmissible spongiform encephalopathies. These rare fatal neurodegenerative diseases of human beings and animals include kuru, scrapie, and chronic wasting disease, and are characterised by the accumulation in the brain of an insoluble protease-resistant isoform of the host-encoded cellular prion protein. In the past 20 years these diseases have become a major agricultural and economic threat. In the late 1970s, production of protein supplement for livestock from animal carcasses was linked to transmission of bovine spongiform encephalopathy; after an outbreak of which in dairy cows, 153 human beings developed vCJD. Health concerns were raised because the disease crosses the “species barrier” between cattle and human beings and adapts to different mammalian species.² Diagnostic criteria for vCJD are being formulated. MRI shows high signal in the posterior thalamus in most people with vCJD. As subclinical infection precedes overt disease, research focuses on the development of sensitive assays for detection of minute amounts of prion protein in suspected cases. Proteomic, immunological, pathological, and molecular approaches to diagnosis are being developed. Refinements in such research have led to the identification of a second transmissible spongiform encephalopathy in cattle, which is characterised by the presence of prion-protein-immunopositive amyloid plaques, similar to those in sporadic CJD.²

Bioterrorism is a potential cause of lethal neurological infections.³ Concern increased after the deliberate release of Bacillus anthracis in the weeks after the September 11, 2001, attacks in the USA. This act highlighted the need to research the pathogenesis of and immune responses to organisms that are possible terrorist weapons. However, the need for joint efforts among the pharmaceutical and biotechnology industries with parallel governmental responses towards antiviral discovery has yet to be realised.³

On a more positive note, action has been taken to eradicate several deadly infections of the nervous system, on three pivotal research fronts: therapeutics, preventative vaccines, and microbial detection. The most remarkable advances in the prevention of nervous system infections have been in vaccine delivery and global eradication. The campaign for the eradication of poliomyelitis involved the vaccination of more than 2 billion children and reduced the global incidence of polio from 350 000 cases in 1998 to less than 800 in 2002. Polio will likely be eliminated within the next 1–2 years.⁴ Introduction of vaccines for serotypes of pneumococcus and Neisseria meningitis (the meningococcus) have substantially reduced the cases of meningitis for 2004. For the former, the incidence of invasive pneumococcal disease in children under age 2 years in Europe was 14–90 cases per 100 000.⁵ The meningococcus is a leading cause of meningitis and fulminant septicaemia worldwide. During epidemics in sub-Saharan Africa, incidence of 1000 cases per 100 000 people occurred. In 1996, an epidemic in several west African countries caused nearly 25 000 deaths. Another major epidemic happened from 2000 to 2001. Meningococcal serogroups A, B, and C are associated with the most morbitiy and mortalities. Meningococcal polysaccharide vaccines are either bivalent (groups A and C) or tetravalent (groups A, C, Y, and W135). Conjugate vaccines against meningococcus serotype C have noticeably reduced disease incidence, and global elimination of bacterial meningitis may be achievable⁶ when vaccines against all serotypes are available. The large-scale synthesis, pharmaceutical development, and clinical assessment of a conjugate Haemophilus influenzae type b (Hib) vaccine is now a possibility.⁷ Such research shows that access to synthetic complex carbohydrate-based vaccines is feasible and provides a platform for vaccine delivery in the developing world. Indeed, more than 600 000 infant deaths occur annually in developing countries as a result of Hib-induced meningitis or pneumonia. The use of Hib polysaccharides requires technology that restricts global access to much needed vaccines.

In the face of a “revolution” in infectious disease occurrence, persistence in research to advance diagnostics and therapeutics have produced some of the most substantial accomplishments in global public health.