Palyam serogroup orbivirus infections

Palyam serogroup orbivirus infections

R SWANEPOEL

Introduction

Palyam serogroup orbiviruses are arthropod-borne viruses that occur in Africa, Asia and Australia, and which appear to be associated with abortion and teratology in cattle and possibly other ruminants.

At present, 15 viruses are recognized as members of the serogroup. The original members, Palyam, Kasba and Vellore viruses, were isolated from mosquitoes in southern India in 1956, 1957 and 1966 in the course of investigation of Japanese encephalitis.7, 8, 9, 31 Many of the remaining members were also obtained initially from haematophagous arthropods (Table 107.1).

The first indication that the members of the Palyam serogroup may be pathogenic was provided by theisolation of Nyabira, Gweru and Abadina viruses from aborted cattle foetuses in Zimbabwe,35, 36, 38 and the detection of antibody to Nyabira virus in the sera of aborted foetuses.34 Antibody to Nyabira virus was found to be widely distributed in cattle sera in Zimbabwe, and rising titres or seroconversions were recorded on occasion in heifers and cows that aborted.1, 34, 35

At least five serotypes of the group, Abadina, Nyabira, Gweru, Marondera and Apies River viruses, are known to occur in southern Africa,38, 41 but further isolates from aborted cattle foetuses in Zimbabwe, and from Culicoides midges and aborted sheep foetuses in South Africa, have not been typed.11, 14, 33 Neutralizing antibody to one or more of the five known serotypes was found in the sera of cattle, sheep, goats and humans collected at ten widely separated locations in South Africa, with the prevalence of antibody being much higher in cattle (53,4 per cent) than in the other species (4 to 14,1 per cent).41

In Japan, an epidemic of congenital abnormalities in cattle was noted in Kyushu district between November 1987 and April 1988, with at least 2 463 calves being affected by hydranencephaly and cerebellar hypoplasia.15, 16 A Palyam serogroup virus, initially given the name Chuzan, was incriminated as the causative agent of the outbreak in epidemiological studies which included demonstration of antibody in pre-colostral serum of affected calves, and pathogenicity tests.15, 16, 27, 28, 29 Subsequently, it was found that Chuzan virus was the same as Kagoshima virus, which had previously been isolated from midges in Japan, and later both viruses were found to be the same as Kasba virus, which had originally been isolated in India.22, 25

There have been over 100 isolations of Palyam serogroup viruses from blood samples of sentinel cattle in Australia, and antibodies to the viruses have been demonstrated in the sera of cattle and Asian buffaloes in Australia, and in the sera of cattle in India and Nigeria, all in the absence of evidence of disease.6, 13, 30, 31

Aetiology

Palyam serogroup viruses have morphological and physicochemical properties typical of members of the genus Orbivirus of the family Reoviridae 3, 20, 23, 24, 25, 37, 39, 43, 44 (see Bluetongue, and African horse sickness).

The viruses of the Palyam serogroup have four major and five minor structural proteins, and at least six non-structural polypeptides are synthesized during virus replication.18, 20, 37, 39, 42, 45 Two of the major proteins constitute the outer capsid layer and have variable antigenic structure that is specific for individual serotypes and is responsible for inducing protective immune response. The interrelationships of the core proteins are poorly understood, but they tend to have conserved antigenic structure that is cross-reactive within the serogroup.37 The viruses are strongly cross-reactive in complement fixation and immunofluorescence tests, and although by definition serotypes react specifically in neutralization tests, there is some cross-reaction between closely related viruses and on this basis the members of the serogroup are placed in six antigenic complexes.24, 38

As with other orbiviruses, the genome of the Palyam serogroup viruses consists of ten discrete segments of double stranded RNA, with a total molecular weight of 11 to 11,5 × 106 . 24, 37 Little is known of viral attachment to susceptible cells and internalization, but replication occurs in the cytoplasm of infected cells and virus appears to be released by cell lysis.

The viruses of the serogroup are less resistant than reoviruses to lipid solvents,35 remain infective for at least a period of days at ambient temperature, and are very stable at temperatures below –60 °C. By analogy with other orbiviruses, it can be assumed that the infectivity of the Palyam serogroup viruses is unstable below pH 6,0 and is destroyed at temperatures above 60 °C. Salt- and pH-dependent haemagglutination has been demonstrated with Kasba virus.21

Table 107.1 Palyam serogroup orbiviruses showing original source and year of initial isolation of each serotype

VIRUS YEAR SOURCE COUNTRY REFERENCE
Palyam 1956 Mosquitoes India 8
Kasba 1957 Mosquitoes India 8
Vellore 1966 Mosquitoes India 31
Abadina 1967 Midges Nigeria 26
Apies River 1967 Febrile cow South Africa 41
D’Aguilar 1968 Midges Australia 10
Nyabira 1973 Aborted cattle foetus Zimbabwe 35
CSIRO Village 1974 Midges Australia 6
Marrakai 1975 Midges Australia 6
Gweru 1976 Aborted cattle foetus Zimbabwe 38
Bunyip Creek 1976 Midges Australia 6

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