Résumé de : CASALS (J) - 1975 - Arenaviruses. Yale J. Biol. Med., 48 (2): pp. 115-140.
TEXTE PARTIEL :
I. INTRODUCTION
Arenavirus is the proposed designation for a set of viruses which have a unique morphology. The virions are round, oval, or pleomorphic with diameters. between 60 and 350 nm, an electron-dense membrane with spikes or projections, and a number of inclusion-like, dense particles that give the virion an aspect of having been sand sprinkled (arenosus). Arenaviruses have an RNA genome, are inactivated by lipid solvents, and share antigenic components. Since the first recognized member of this group was lymphocytic choriomeningitis (LCM) virus, it is considered the prototype.
II. HISTORICAL BACKGROUND
LCM virus has been known since 1934 and soon after it was etiologically associated with a disease syndrome of man, acute benign aseptic meningitis. Tacaribe virus, isolated from bats in Trinidad in 1956 and first described in 1963, has not been associated with human disease. The next virus to be isolated and characterized was Junin virus, in 1958; the virus was isolated from patients with Argentinian hemorrhagic fever (AHF). Five years later another virus, Machupo, was isolated from a fatal case of an illness, Bolivian hemorrhagic fever (BHF), clinically similar toAHF. Other viruses, antigenically placed in the Tacaribe group, were discovered soon after: Amapari in Brazil, 1964, Latino in Bolivia , Parana in Paraguay, Pichinde in Colombia (88), and Tamiami in USA , in 1965. None of these five is known to cause human illness. The last member of the group, Lassa VIrus, was recovered from patients suffering from a severe disease first seen in Nigeria.
A serological relationship among members of this set was first observed between Junin and Tacaribe viruses in 1963, resulting in the creation of the Tacaribe antigenic group ; the other viruses, except LCM and Lassa, were easily shown to be related to Tacaribe and Junin and placed in the group. The similarity of morphology and morphogenesis between LCM and the Tacaribe group viruses was noted in 1969 and morphogenesis between LCM and the Tacaribe group viruses. was noted In 1969 and soon after it was shown that there was an antigenic connection between them . Finally, it was observed in 1970 that Lassa virus was antigenically related to LCM and to some of the Tacaribe group agents and that its morphology conformed to that described for LCM.
III. METHODOLOGY.
A. Mortality.
The case fatality rate of some members of this group is sufficiently high that deaths from the disease gives a good idea of the morbidity rate provided an accurate diagnosis can be made. For example, the mortality from Lassa fever ranges from 30-60%, Argentine hemorrhagic fever 3-15%, and Bolivian hemorrhagic fever 5-30%. However, deaths from lymphocytic choriomeningitis are rare. For the most part thes disease are are, limited to localized outbreaks, and confined to a very few geographic areas. For those reasons, the official mortality records would be unlikely to reflect their occurrence in a country.
B. Morbidity.
Epidemiological studies on arenavirus infections of man are hindered by the fact that disease reporting is uncertain and incomplete. The diagnosis of sporadic cases of LCM is, most likely, not made or is guesswork in nearly all instances, unless laboratory aid is sought. Diagnosis of AHF in the area and season where the disease is anticipated is confirmed by laboratory studies in about 70% of clinically diagnosed cases ; how many clinically undiagnosed infections go undetected is not known. It is doubtful whether an effective reporting system has been devised and implemented for these diseases, with the possible exception of AHF. Owing to the highly specialized type of diagnostic work required for identification of the viruses and antibodies resulting from infection, and due to the risk associated with certaini aspects of the laboratory procedures, the knowledge of the prevalence of the infections and illnesses caused by the arenaviruses is limited.
C. Serological Surveys.
Due to the difference in duration of different antibodies, seroepidemiological surveys are carried out mainly by means of the neutralization test even though this is a more cumbersome test than complement fixation; it should be noted that not many extensive surveys have been carried out, except with LCM virus. The complement-fixation test is extremely useful as an aid to the diagnosis of a recent iIlness; the fact that it is less type specific than the neutralization test, to the point that it hardly differentiates between Junin and Machupo viruses infections of man, does not detract from its value due to the limited geographic distribution of these viruses.
D. Laboratory Diagnosis.
While a clinical diagnosis of fully developed, typical cases of AHF and BHF can be made with considerable accuracy in the districts where the diseases are endemic at the time of year when they prevail, particularly if several similar cases appear simultaneously, it is most doubtful whether a sporadic case of LCM can be accurately diagnosed. A presumptive diagnosis of Lassa fever can be entertained in known endemic areas by experienced physicians faced with a severe or moderately severe case.
A specific diagnosis of these illnesses requires that either the virus be isolated and identified from the patient's blood, excretions, or secretions, usually early during the disease; or that development of specific antibodies is shown to occur late in the disease or In convalescence.
The complement-fixation (CF) test has been most useful as a diagnostic aid with these illnesses; it is not, however, an early means of diagnosis since from 15 to 30 days from onset are required for it to become positive with most patients, perhaps a shorter time with LCM. The CF is not a specific test within the arenaviruses, certainly not between Machupo and Junin viruses ; but since these viruses occur in different areas a diagnostic error between the two is unlikely to occur. On the other hand, difficulties in the interpretation of CF results have arisen in an area where Junin and LCM viruses have infected man, simultaneously or sequentially. In fact, in view of the known wide distribution of LCM virus, it is conceivable that diagnostic problems may arise when the CF test is used to diagnose Machupo or, if in Africa and if LCM virus is there, with Lassa viruses. Since LCM virus occurs in many European countries, introduction of an exotic arenavirus in that continent might result in diagnostic difficulties if the CF test alone were used.
The fluorescent antibody technique (FAT) has been used advantageously for an . early diagnosis of LCM virus infection of man . The neutralization test has been used less for diagnosis of current illnesses and serological surveys-LCM exceptedÑthan for characterization of the viruses themselves; as will be seen in another section, it is sharply specific.