Poliomyelitis in Oman. I. The last outbreak?

Since 1988, the Sultanate of Oman has experienced three outbreaks of paralytic poliomyelitis. The last outbreak occurred in December 1993 and involved two children aged 10 months and 4 1/2 years. The children had received five and four doses, respectively, of trivalent oral polio vaccine (OPV) and lived in the same village. Serum neutralizing antibody tests suggested that paralytic polio in these children was due to poor antibody response to OPV. Wild poliovirus type 1 was isolated from both patients, as well as from seven of ten close contacts of the older child, and one of eight contacts of the younger child. All contacts had received three to six doses of OPV. Genomic sequence studies indicated that the virus isolates belonged to a genotypic group prevalent in southern and western Asia, but differed markedly from virus isolated during the 1988/89 outbreak, suggesting another importation of poliovirus. In response to the outbreak, supplementary immunization with OPV was given to children <6 years of age, initially in the affected district, and subsequently to children in the whole country. This study demonstrates that immunization with three to six doses of OPV did not prevent infection with wild poliovirus. In those children with sub-optimal response to OPV, infection resulted in paralytic poliomyelitis. The outbreak remained localized in one village, indicating that the outbreak control measures were effective.

Poliomyelitis in Oman. II. Toward eradication.

In the past decade, the Sultanate of Oman has experienced three outbreaks of paralytic poliomyelitis--a widespread polio type 1 epidemic in 1988/1989, four cases of polio type 3 in three different regions in 1991, and a localized type 1 outbreak in 1993. The lessons learnt from each of these epidemics have guided us to modify and improve our polio eradication activities. Currently, these activities include administration of five primary and three booster doses of trivalent oral polio vaccine, yearly national immunization campaigns (NIDs) since 1995 with coverage of >90%, localized immunization campaigns, acute flaccid paralysis (AFP) surveillance which involves reporting of all cases by facsimile to the Department of Surveillance within 24 h of detecting a case and weekly zero reporting from 22 sentinel sites, and virological testing of stool specimens of all AFP cases and their close contacts at the national, World Health Organization accredited laboratory. The cumulative success of these activities has resulted in Oman being free from polio for the past 6 years. However, the possibility of importation of wild poliovirus, particularly from southern and western Asia still exists.

Bluetongue in the Sultanate of Oman, a preliminary epidemiological study.

A group specific agar-gel immunodiffusion test was used to demonstrate that there is a frequent and widespread distribution of bluetongue virus throughout the Sultanate of Oman. The Culicoides midges C. imicola and C. schultzei, both capable of transmitting bluetongue group viruses, were recorded throughout the year. Although these studies did not establish that bluetongue is enzootic in Oman, type-specific neutralizing antibody results supported previous evidence for the existence of a Saudi Arabian bluetongue ecosystem. Variations in antibody evidence of virus activity within a restricted locality suggested a hot-spot theory concerning the perpetuation of the virus.

Isolation and identification of arboviruses from the Sultanate of Oman.

Sentinel herds and a vector surveillance system were used to identify the presence of arboviruses in Oman. Two strains of bluetongue virus (BTV) serotype 4 and two strains of Akabane virus, were isolated and identified. Both BTV isolates and one Akabane virus isolate came from goats while the second Akabane isolate came from Culicoides imicola. This is the first isolation of an Akabane virus from Culicoides in Arabia. Vector competence studies with the Oman viruses in laboratory reared C. variipennis showed that after oral infection both viruses replicated in Culicoides and were maintained at high titre for at least 10 days post infection.

Epidemiology of bluetongue and related orbiviruses in the Sultanate of Oman.

Sentinel herds at 34 farms were used to study the epidemiology of bluetongue and related orbiviruses in Oman. The results indicate that bluetongue virus (BTV) is widespread and is enzootic in Northern Oman. At least three BTV serotypes (3, 4 and 22) were present at the time of the study. Antibodies to epizootic haemorrhagic disease of deer virus (EHDV) type 2 and EHDV-318 were also detected but were less prevalent. Entomological investigations identified the presence of 16 species of Culicoides. The peak seasonal incidence of the BTV vector C. imicola and the EHDV vectors C. schultzei (group) midges at Rumais in Northern Oman correlated closely with the spring rains in that area. However, both species of midge were also present in lower numbers throughout the year. four species of Oman midge, C. arabiensis, C. ibriensis, C. neoschultzei and C. buettikeri are new to science.

Prevalence of neutralising antibodies to Akabane virus in the Arabian peninsula.

Serum-neutralising antibodies to Akabane virus were found in a wide range of domestic animals in all countries of the Arabian Peninsula but the virus does not seem to be endemic there. Sentinel herds in Oman and N. Yemen did not detect any Akabane activity between August 1984-November 1986 and May 1983-November 1984, respectively. However, there is strong evidence to suggest that Akabane virus incursions have recently taken place in Kuwait, Saudi Arabia and Bahrain as neutralising antibodies were detected in 1-year-old cattle bled during 1986 in each of these countries. The possibility of windborne infected vectors, from virus-endemic areas, initiating these incursions into the Arabian Peninsula is discussed.