Overview of bluetongue disease, viruses, vectors, surveillance and unique features: the Indian sub-continent and adjacent regions.

The occurrence of bluetongue (BT) disease in India was initially confined to exotic breeds of sheep and subsequently became endemic in native breeds. BT virus (BTV) antibodies are common in cattle, buffaloes and goats although clinical disease has not been reported. Exotic breeds of sheep and their cross-breeds are more susceptible to disease than native breeds. Overall, morbidity, mortality and case fatality rates of 9.3%, 2.7% and 28.8%, respectively, have been reported in rural flocks; these rates are higher than in organised farms. The disease is mostly cyclical in occurrence. Outbreaks usually occur between June and December during the monsoon period when livestock biting midges greatly increase. BTVs have been isolated from native sheep, and sentinel herds have been used to demonstrate virus activity. A total of 21 serotypes of BTV have now been reported in the country. Major impediments to control the disease include the presence of multiple virus serotypes, the broad vertebrate host range of the virus and a lack of detailed knowledge of vectors. Inactivated vaccines prepared from local isolates are currently under field trials. BTV occurs in regions adjacent to India. An antibody prevalence of 48.4% has been reported in Pakistan with serotypes 3, 9, 15, 16 and 18 identified. BTV antibody, but not disease, has been reported in Bangladesh and Sri Lanka.

The expected economic impact of selected exotic diseases on the pig industry of Australia.

The authors assess the expected economic impact of three exotic diseases on the pig industry of Australia. An integrated epidemiological/economic approach was used to assess the effects of classical swine fever, Nipah virus and porcine reproductive and respiratory syndrome. Scenarios involving either an epidemic event, in which the outbreaks were confined to selected regions and were eradicated, or an endemic situation, in which the diseases became established in Australia, were studied. Based only on loss of sales and disposal costs, epidemics resulted in regional losses in income of the order of AUS$10 million-AUS$30 million (16%-37%) depending on disease and region. If any of these diseases became established, opportunity losses in gross national pig income of 5%-11% per year would occur, with classical swine fever the most serious of the three diseases. Establishment of any of the diseases would lead to rapid structural change in the pig industry, with concomitant social and economic dislocation in regional Australia.

Isolation of multiple serotypes of bluetongue virus from sentinel livestock in Malaysia.

Sixteen isolations of bluetongue virus (BTV) were made from the heparinised bloods of 4 groups of cattle and sheep in Peninsular Malaysia. These viruses were typed as BTV serotypes 1, 2, 3, 9, 16 and 23. Multiple serotypes of BTV are apparently endemic in Malaysia and in other countries in the region.

Koolpinyah: a virus related to kotonkan from cattle in northern Australia.

Two closely related viruses were isolated from the blood of bovines near Darwin, Northern Territory, Australia. When studies of virus morphology indicated that these were rhabdoviruses, serologic studies were done. These isolates are closely related or identical and are related to, but distinct from, the rabies-related kotonkan virus. Other serologic studies showed that these are two isolates of a newly recognized virus, for which the name Koolpinyah virus is proposed.

A monoclonal antibody blocking ELISA detects antibodies specific for epizootic haemorrhagic disease virus.

The isolation of a monoclonal antibody (1G9/C9) with specificity for the epizootic haemorrhagic disease (EHD) serogroup has enabled the development of a highly sensitive and specific blocking ELISA (B-ELISA) for the detection of serum antibodies to EHD viruses. The assay was sensitive to blocking antibodies present in hyperimmune reference antisera to all six EHD serotypes tested but was unaffected by reference antisera to 19 South African and eight Australian serotypes of the related orbivirus bluetongue virus (BTV). The sensitivity of the EHD B-ELISA exceeded that of an indirect ELISA (I-ELISA) for EHD-specific antibody detection. Serum antibody titres to BTV and EHD in experimental and field sera, including a sentinel herd from which virus isolations were made, were examined in both the BTV and EHD B-ELISA tests. These results showed the B-ELISA was only sensitive to antibodies specific for the homologous serogroup in each case, even where sequential and mixed infections with each virus type occurred.

Failure to establish congenital bluetongue virus infection by infecting cows in early pregnancy.

Two groups of 10 pregnant cows were inoculated with bluetongue virus type 11 at either 40 or 60 days of gestation. All the cows became infected as judged by the detection of viraemia and seroconversion but they showed no clinical signs. Seventeen of the cows produced live calves none of which showed any evidence of prenatal infection. After challenge with the same virus all the calves became viraemic and seroconverted. The response to challenge of the two groups did not differ from that of a control group challenged at the same time. It was concluded that the infection of pregnant cows in early gestation with this virus did not result in the transplacental infection of the fetuses and did not produce immunotolerant, latently infected calves.

Investigations of bluetongue and other arboviruses in the blood and semen of naturally infected bulls.

Small groups of bulls were exposed to natural infection with arboviruses. The bulls were bled and ejaculated regularly and the blood and semen were processed for virus isolation. Over a 5-year observation period, virus isolation and serology indicated that the 29 exposed bulls had experienced 79 viraemic episodes with the viruses of the bluetongue, epizootic haemorrhagic disease, Palyam and Simbu serogroups and an incompletely characterised rhabdovirus. In no instance was there unequivocal evidence of bluetongue virus contamination of semen, despite 18 infections in the study period.

Arboviruses recovered from sentinel livestock in northern Australia.

Over 700 arboviruses were recovered between 1981 and 1987 from the blood of sentinel livestock near Darwin. Twenty-three isolates were made from sheep, goats, swamp buffalo (Bubalus bubalis) and horses, and the remainder were from cattle. The isolates have been typed as 27 separate viruses belonging to the bluetongue, epizootic haemorrhagic disease, Palyam, Simbu, bovine ephemeral fever, Tibrogargan and alphavirus groups. Ten of these viruses have not been isolated elsewhere in Australia and four have been isolated only in Darwin. Considerable annual variations in virus activity and in the durations of detectable viraemia were observed.

Arboviruses recovered from sentinel cattle using several virus isolation methods.

A group of 20 sentinel steers was bled weekly for 5 months in 1986 and the blood samples were examined for arboviruses by inoculation firstly into embryonated chicken eggs (ECE), baby mice, Aedes albopictus cells and BHK21 monolayers. A second group of cattle was similarly examined for virus in 1987, except that baby mice were not used. Viruses were recovered from 26% of the 878 weekly bleeds. The viruses identified consisted of 14 types belonging to the bluetongue, epizootic haemorrhagic disease (EHD), Palyam and Simbu groups with a single isolation of bovine ephemeral fever virus. The ECE system was found to be the best for isolating bluetongue and Simbu viruses, though the eggs were not usually killed by the inoculum. The ECE and A. albopictus systems were equally sensitive for recovering EHD viruses, while Palyam group viruses were most efficiently isolated in BHK21 monolayers.

Isolation of a new rhabdovirus in Australia related to Tibrogargan virus.

A virus isolated from the blood of a healthy steer and designated DPP53 was shown to have rhabdovirus morphology. Although DPP53 virus was antigenically related to Tibrogargan virus by reciprocal immunofluorescence and neutralization tests, the viruses were distinguishable by neutralization tests. DPP53 virus contained RNA and was sensitive to both ether and chloroform. The geographical distribution of neutralizing antibody to DPP53 virus in Australian cattle corresponded to the distribution of Culicoides brevitarsis indicating that this virus may be arthropod-borne with this midge as a possible vector. Antibody to DPP53 virus was detected in serum from cattle, buffalo, dogs and one horse, but not in serum from deer, pigs, humans or wallabies. Highest virus titres were obtained by growth in Vero and BHK21 cell cultures, but the virus could also be grown in Aedes albopictus cell cultures. Higher virus titres were obtained when the multiplicity of infection was low. The name advanced for DPP53 virus is 'Coastal Plains' virus.

Clinical variations of border disease in sheep according to the source of the inoculum.

Non-cytopathogenic pestivirus obtained from lambs with border disease, with or without nervous signs, was inoculated into pregnant ewes at 57 to 65 days of gestation. Live lambs born to inoculated ewes were clinically identical to the lambs from which virus was obtained, ie, either a hairy birth coat with central nervous system disturbance or a hairy birth coat without central nervous system disturbance.

Pathology in the ovine foetus caused by an ovine pestivirus.

Homogenised tissues or tissue culture supernatant fluid containing a noncytopathic pestivirus obtained from a lamb with a neurologic form of border disease, were inoculated into ewes at different stages of pregnancy. Foetal death occurred in 9 ewes of those inoculated between 19 and 47 days of pregnancy while 3 ewes did not lamb. Eight of the foetuses were aborted between 77 and 132 days of pregnancy; of these 6 were autolysed or mummified and one had arthrogryposis. The one full-term dead lamb had a hairy birth coat and lissencephalic micrencephaly. Foetal death occurred in only 7 of 14 ewes inoculated between 57 and 72 days of pregnancy. Four of these ewes aborted between 77 and 108 days of pregnancy and 3 gave birth to full-term, dead, hairy lambs. The remaining 7 ewes gave birth to live hairy lambs with severe inco-ordination. All lambs carried to term and aborted foetuses or lambs that could be examined had a range of intracranial malformations including focal leucomalacia, micrencephaly, hydranencephaly, porencephaly, lissencephaly and cerebellar hypoplasia. Some lambs also had skeletal abnormalities including arthrogryposis, scoliosis and brachygnathia inferior. The pestivirus isolate used in these trials produced more severe effects on the ovine foetus than previously observed in similar inoculation trials using pestivirus isolates from border disease lambs without nervous signs.