Epidemiological Studies of Foot-and-Mouth Disease in Northern and Southern Darfur States, Western Sudan

Abstract

Western Sudan includes Kordofan and Darfur areas. The former is flanking the Nile basin while the latter constitutes the whole of the Western and South Western border areas of Sudan. Darfur area shares border with Libya, Chad, Central African Republic (CAR) and the Republic of South Sudan (SSR). Administratively, it is divided into 5 States; North Darfur in the North and East, South, Central and West Darfur States in the South. Darfur is a chief pastoral area in Sudan where around 10 million cattle and 20 million sheep and goats are reared. During the programme ''Surveillances of Trade Sensitive Diseases'' (STSD), in 2016, two levels of foot-and-mouth disease virus (FMDV) circulation, as indicated by non-structural proteins (NSPs) serology, were recognized in cattle, the main target species for FMD infection, in Darfur. The higher level was recognized in North and East Darfur States and the lower one in South, Central and West Darfur States. The presented study meant to investigate FMD infection in Darfur area. Passive and active disease surveillances were utilized and NSPs positive bovine sera from the programme STSD from Darfur area were serotyped against the three current infections of FMD in Sudan; O, A and SAT2. Passive disease surveillance was carried out in South Darfur State; the State with the largest international border area with SSR and almost the whole international border area with the CAR. For logistic reasons, active disease surveillance was carried out in the White Nile State; the State with largest State border area with Western Sudan, rather than in Darfur States. For the serological study, NSPs positive bovine sera from two States were used; Northern and Southern Darfur States, representing respectively the higher and lower levels of FMDV circulating in the study area. Presumptive diagnosis of clinical FMD was reached in the two surveyed States (South Darfur and the White Nile States) late in 2017 and early in 2018. The clinical signs were mild, hardly detectable and seen in limited number of cattle in limited geographical area. Epithelial samples from these mild cases were barely available and laboratory diagnosis was not achieved; neither locally nor abroad at the World Reference Laboratory (WRL) for FMD where sensitive cell culture system and reverse-transcription polymerase chain reaction (RT-PCR) were used. In the serological study, 389 bovine sera representing the whole positive lot from North (243) and South (146) Darfur in the STSD stood for the tested sera. Accordingly, the procedure for determining sero-prevalence of FMD virus (FMDV) serotype-specific antibodies involved serial testing approach i.e. only sera positive in two test systems, ID ELISA and virus neutralization test (VNT), were considered positive. The approach is known to decrease sensitivity but increase specificity. The decrease in sensitivity is not expected to affect interpretation of results since NSPs negative structural protein (SPs) positive reactors are expected to be more associated with the predominant rather than the subordinate serotypes. These reactors might be encountered due to mild repeated infection with the same serotype which is likely to be the predominant one. Indices of FMD infection by sero-prevalence estimates of SPs (combined) antibodies compared to estimates of NSPs antibodies were found to be 55.4% - 66.3% (95% C.I.) compared to 69.1% - 78.6% (95% C.I.) in North Darfur, and 28.8% - 39.1% (95% C.I.) compared to 37.6% - 48.2% (95% C.I.) in South Darfur. They were overlapping where lower level of FMDV circulation prevailed in South Darfur State. Consistently, higher sero-prevalence rates were detectable in North rather than South Darfur State. The serological study revealed that serotype O was the predominant serotype in the North 43.3%-54.5% (95% C.I.) and in the South 22.3%-32% (95% C.I.) followed by A [21.3%-31.1% and 12.6%-20.7% (95% C.I.)] then SAT2 [9.7%-17.4% and 3.7%-12.4% (95% C.I.)]. It was the same order that was known in most parts of Sudan. The predominant serotype O exhibited a pattern of distribution where it showed statistically significantly higher sero-prevalence estimates in Northern rather than Southern localities in both states (North and South Darfur States); unlike serotype A and SAT2. The described pattern, compared to serotype A and SAT2, was consistent with the more regular circulation of serotype O and with its predicted circulation from the Nile valley to other parts of the country. Northern districts and urban centers that drive animal movements related to trade are important points for entry and circulation of FMDV in Darfur area. Simultaneously, no evidences of effective across border circulation were obtained. It was concluded that Darfur area was unlikely an important source of FMD viruses to other parts of the country. No particular risk seemed to be associated with animal movement across the Western and South Western border. On the other hand, animal movement to the North and back as part of the pastoral system or due to trade was likely associated with a considerable risk of introduction of the infection to Darfur area.