Palyam serogroup orbivirus infections

Previous author: R SWANEPOEL

Current author: M QUAN, PhD, Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria, South Africa

GENERAL INTRODUCTION: REOVIRIDAE

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, 13 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.^{6, 7, 8, 33} Many of the remaining members were also obtained initially from haematophagous arthropods (Table 1).

The first indication that the members of the Palyam serogroup may be pathogenic was provided by the isolation of Nyabira, Gweru and Abadina viruses from aborted cattle foetuses in Zimbabwe^{38, 39, 43} and the detection of antibodies to Nyabira virus in the sera of aborted foetuses.³⁷ Antibodies to Nyabira virus were 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.^{2, 37, 38}

At least five serotypes of the group, Abadina, Nyabira, Gweru, Marondera and Apies River viruses (Table 1), are known to occur in southern Africa,^{43, 46} but further isolates from aborted cattle foetuses in Zimbabwe, and from Culicoides midges and aborted sheep foetuses in South Africa, have not been typed.^{12, 15, 36} Neutralizing antibodies to one or more of the five known serotypes were found in the sera of cattle, sheep, goats and humans collected at ten widely separated locations in South Africa, with the prevalence of antibodies being much higher in cattle (53,4 per cent) than in the other species (4 to 14,1 per cent).⁴⁶

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 affected by hydranencephaly and cerebellar hypoplasia.^{16, 17} 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 antibodies in pre-colostral sera of affected calves and pathogenicity tests. ^{16, 17, 29, 30, 31} 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.^{23, 27}

There have been over 100 isolations of Palyam serogroup viruses from blood samples of sentinel cattle in Australia, yaks in China and antibodies to the viruses have been demonstrated in the sera of cattle and Asian buffaloes in Australia, cattle in India and Nigeria, and goats in South Korea, all in the absence of evidence of disease.^{5, 14, 32, 33, 40, 41}

Aetiology

Palyam serogroup viruses have morphological and physicochemical properties typical of members of the genus Orbivirus of the family Reoviridae ^{3, 21, 25, 26, 27, 42, 44, 47, 48} (see Bluetongue, and African horse sickness). They are most closely related to African horse sickness virus and can be divided into two clades: an Afrotropical clade containing all the viruses isolated in Africa (except for Petovo virus) and another clade containing the Australasian and Oriental serotypes (and Petovo virus), with Palyam virus as the ancestor to this group.¹¹  

The viruses of the Palyam serogroup synthesize at least seven structural and four non-structural proteins during virus replication.¹¹ Virus protein (VP) 2 and VP5 constitute the outer capsid layer and have a variable antigenic structure that is specific to individual serotypes and responsible for inducing a protective immune response. The inter-relationships of the core proteins are poorly understood, but they tend to have a conserved antigenic structure that is cross-reactive within the serogroup.^{11, 42} 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. On this basis the members of the serogroup have been placed in six antigenic complexes.^{26, 43} (Table 2)

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. ^{26, 42} 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,³⁸ 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.²²

Table 1 Palyam serogroup orbiviruses showing original source and year of initial isolation of each serotype