Coronaviruses
Human coronaviruses were first isolated in the mid 1965 from volunteers at the Common Cold Unit. The coronaviridae are a monogeneric group of RNA-containing viruses that are associated with respiratory infections in animals, including pigs, cats, dogs, mice and chickens. The group was so named because of the crown-like projections on its surface. At present, at least 10 species are recognized, of which human coronavirus is one. The other are found in animals.
A. Properties
ssRNA enveloped viruses of pleomorphic morphology
60 to 220nm in diameter.
Positive stranded RNA; helical symmetry
characteristic club-shaped projections of 20nm, involved in neutralization, fusion and in some coronaviruses, haemagglutination.
Electronmicrograph of corona virus particles.
Some Human Coronavirus strains, namely 229E and serologically
related strains, are generally readily propagated in human cell
culture. However, other human coronavirus strains, namely OC43
and some serologically related strains, are considerably more
difficult to propagate in cell culture. Three antigenic molecules
are found in the virions ie. nucleocapsid, surface projection and
transmembrane proteins. The NP antigens may be common to all
coronaviruses, while the main antigenic determinants of
individual viruses reside on the surface projections. The surface
projection antigens are used for the serological grouping of
coronaviruses. One avian and two mammalian serological groups
have been established. Human coronavirus strains fall into each
of the mammalian groups which are named the OC43 and 229E
serological groups. It is not clear how closely related are the
viruses within a serological group.
B. Epidemiology
Generally, human coronavirus infections occur during the winter and early spring but the peak period may vary by several months. The periodicity of infections caused by 229E and OC43 group viruses follows a complex pattern, although they usually cycle with an interval of 2 or 3 years. In general, high infection rates in any particular year are caused by either 229E or OC43 group viruses with only the occasional sporadic human coronavirus infection belonging to the other group. This pattern is observed throughout the world.
C. Clinical Features
Human coronaviruses are responsible for 10 - 30% of all common colds. All age groups are affected, and infection rates have been shown to be uniform for all age groups. This is different from other respiratory viruses such as RSV, where there is a distinct decrease in infection rates with an increase in age. The incubation period is short, being 2 to 4 days. Infection may also be subclinical or very mild. There have been some reports of more severe lower respiratory tract involvement in young children and old people. Reinfection of individuals with the same human coronavirus serotype often occurs within 4 months of the first infection, suggesting that homologous antibodies are protective for only 4 months. Although many people have high antibody levels after infection, reinfection with the same or related strains is common. Antibodies to one human coronavirus group may not be protective against infection with viruses from another group. There are no differences in pathology observed between the OC43 and the 229E strains.
Other possible infections - coronavirus-like particles
are often seen in the faeces of children and adults suffering
from diarrhoea. These particles have a different morphological
appearance to those seen in respiratory infections. As yet, there
is no firm evidence associating the presence of these particles
to diarrhoea. Also human coronavirus particles have been observed
in tissue form patients suffering form multiple sclerosis, there
is no evidence for an aetiological role.
D. Diagnosis and Treatment
Diagnosis of human coronavirus infections is not attempted in many routine laboratories. They have fastidious growth requirement in cell culture, and the conditions caused by them are of minor clinical significance. The routine diagnostic procedures comprises of cell culture and serology.
Virus isolation - 229E and related strains can be isolated in roller culture monolayers of human embryonic lung fibroblasts, such as W138 and MRC5 cells. A virus CPE of small, round, granular cells is seen throughout the monolayer. Isolates can be confirmed by virus neutralization tests. OC43 related strains usually cannot be grown in cell cultures. Isolation has to be performed on organ cultures of human embryonic tissue such as trachea.
Serology - virus neutralization are the most frequently used tests. HI, CF and ELISA tests have been used. Most of these tests are not carried out in routine diagnostic laboratories. There is little, if any, antibody cross-reaction between strains of 229E and OC43.
Direct detection of virus - indirect immunofluorescence and ELISAs have been developed to detect the presence of coronavirus antigen in nasal secretions with differing results. The usefulness of indirect immunofluorescence has still to be established.
No antiviral drugs against coronaviruses are available and little research is being taken to produce any. However, vaccines against certain animal coronaviruses are available.
