Epidemiological Studies of Foot-and-Mouth Disease in Northern Sudan

Abstract

The Nile basin north to Khartoum encompasses two administrative States: the Northern and the River Nile States. The area represents a geographical cluster characterized by an exclusive desert and semi-desert ecosystems and low animal density. Northern Sudan is an important corridor cluster between pools of foot-and-mouth disease virus (FMDV) in East and North Africa. It involves almost the whole border area with Egypt; an important trade partner of the country, and represents a considerable part of a projected disease-free zone in Sudan. The study monitored FMD infection between 2016 and 2018 in Northern Sudan. Passive and active surveillance of clinical FMD in cattle were, all, utilized. A total of 184 bovine sera proved to be positive to nonstructural proteins (NSPs) of FMD virus during the programme ''surveillance of trade sensitive diseases'' in 2016 were examined for antibodies against serotype O, A and SAT2 using virus neutralization test (VNT). These sera had originated from the River Nile (143) and the Northern State (41). Results largely confirmed previous reports that have described the relatively lower circulation of FMDV in the area than in other parts of the country. Clinical FMD was confirmed once in the three years period. Only serotype O of an unnamed lineage within the topotype East Africa 3 (O-EA3), like all other Sudanese O viruses, was typed during the study period in 2016 in cattle smuggled to Egypt and in resident cattle. It was closely related to Egyptian and Sudanese isolates of 2017 (phylogenetic identity ≥ 99.4%) rather than the Sudanese viruses that had been detected in the Northern State in 2012. These sequences formed a large temporal cluster that included in addition Israeli, Ethiopian and Palestinian isolates from 2017. Obviously, these sequences, like the sequences in 2012, were of transboundary nature. Antibodies against the structural proteins (SPs) of the 3 serotypes of FMDV in Sudan; O, A and SAT2, were detected. Consistently, sero-prevalence estimates were statistically significantly higher in the River Nile than in the Northern State. In the River Nile, the 95% C.I. for the estimates were 9.5%-16.4% for serotype O, 11.7%-19.1% for serotype A and 11.0%-18.2% for serotype SAT2. In the Northern State, corresponding estimates were 3.5%-9.3%, 4.3%-10.7% and 1.2%-5.6%. In the River Nile, sero-prevalence estimates were lowest in the most Northern district of Abu Hamad while in the Northern State the Western district of Al Goled proved to be negative (n = 64) for anti-NSPs activity. Results suggested a direction of infection from the South and East to the North and West; South Eastern districts in the River Nile and Eastern districts in the Northern State were showing higher sero-prevalences and were likely crucial points of entry of the infection. The serological study involved a serial testing approach i.e. sera positive by both test systems, ID Screen® FMD NSPs Competition ELISA and VNT, were considered positive. Sero-prevalence by the ID Screen® FMD NSPs Competition ELISA in the River Nile State was found to be statistically significantly higher (P = 0.000725) than that by the combined VNT (O, A and SAT2) but not in the Northern State (P = 0.106567). Earlier, it was expected that mild exposure (limited virus multiplication) to different serotypes could result in boosting immune response to NSPs but not to SPs and consequent difference in performance between NSPs and SPs serology. Mild exposure to different serotypes was clearly more expected in the River Nile than in the Northern State where very low levels of circulation of FMD virus prevail. Epidemiological pattern of FMD infection in Northern Sudan as recognized by the study presented the lowest level of circulation of FMD viruses in the country. Concurrently, unlike other parts of Sudan, no predominance of serotype O antibody was detected. It was concluded that low animal density and limited animal movement in Northern Sudan together with the high antibody levels against serotype O in immediately neighbouring States (Khartoum and Kassala) effectively decreased infiltration of endogenous O viruses.