FMD is caused by an RNA aphthovirus of the family picornaviridae. There are seven immunological serotypes (A, O, C, SAT 1, 2, 3 and Asia1) that exist and over 60 subtypes of the virus are circulated Worldwide (Wernery and Kinne, 2012). Four (A, O, SAT 1 and 2) out of this seven serotype exist in Nigeria (unpublished data). Foot-mouth-disease virus (FMDV) is a small non-enveloped virus and has a genome of 8.5 kb which encodes for structural as well as non-structural proteins (NSPs) (Yousef et al., 2012). The viral capsid composed of four structural proteins, VP1, VP2, VP3 and VP4 (Fry et al., 2005).

A structural protein produces antibodies to FMDV in vaccinated animals, whereas infected animals produce antibodies to both the structural and non-structural proteins (Yousef et al., 2012). Meanwhile, assays to demonstrate antibodies against non-structural proteins have potential to differentiate infected animals from vaccinated (Berger et al., 1990; Rodriguez et al., 1994; De Diego et al., 1997; Clavijo et al., 2004). The disease is endemic in some parts of Europe, Africa, Middle East and Asia, but places like North America, Australia, New Zealand and many countries in Western Europe are free from the disease and have stringent regulations preventing the introduction of the virus (Wernery and Kinne, 2012). 

The first reported case of FMD outbreak in Nigeria was in 1924, which was attributed to type O virus (Libeau, 1960). Subsequently, other serotypes (A, SAT 1 and SAT 2) were reported, and each of these introductions was associated with trade cattle entering Nigeria from neighboring countries, (Nawathe and Goni, 1976; Owolodun, 1971; Durojaiye, 1981; Abegunde et al., 1988). It should be understood that trans-humance production system is the predominating system of animal management in the Sub-Saharan Africa and many of these individuals  traverses  national  borders  (especially those of Central African Republic, Chad, Niger, Cameroun, Benin and Nigeria) in search of feed and water resources for their livestock without any recourse to quarantine and control measures. Furthermore, camels are frequently moved across the desert of Niger, Chad, Benin and Central African Republic to Nigeria across areas that FMD outbreak is endemic in cattle and other small ruminants. Therefore, it is possible that camels may play a possible role in the maintenance and transmission of FMDV and may carry FMDV over a very long distance and across the borders (Yousef et al., 2012).  However, because of the limited information concerning FMDV in camels in West Africa, Nigeria in particular, this study was aimed to investigate the serological evidence of natural exposure of camels (Camelus dromedaries) to FMD virus. The investigation was to detect the presence of antibodies against non-structural proteins (NSP) using competitive ELISA and solid-phase competitive ELISA (SPCE) for serotype specific FMDV antibodies, to evaluate the role of camels in the epidemiology of FMD in Nigeria and by extension West Africa.

The overall result indicated 10.83% (95% CI: 7.93 to 14.37) of the total serum samples collected from Maiduguri, Kano and Sokoto abattoirs to be positive for antibodies against FMD NSP (Table 1). All sera that tested positive for FMD NSP were further analyzed for antibodies against FMD structural proteins for serotypes A and O using a SCPE and the results is presented in Table 2. The optical density of camel sera was read on a MultiSkan spectrophotometer reader at 450 nm wavelength.

FMD is endemic in most of Sub-Saharan Africa, in particular with unrestricted movement of susceptible animals across the border with no or less control measure instituted to the susceptible animals. Because of the poor veterinary services in most African countries and limited information on the role of camels (dromedary camels) in the epidemiology, few documented evidence exists on its epidemiology of FMD. Camels however, move frequently across the Sahara desert for grazing and trade purposes thereby mixing freely with susceptible animals in endemic countries like Niger, Nigeria, Chad and Central African Republic. These results indicate serological evidence of FMD non-structural proteins in dromedaries camel which may be as a result of exposure to FMDV since camels are not vaccinated against FMDV. It could be that during FMD outbreak, camels come in contact with infected susceptible animals like cattle, sheep and goats which are often herded together by most pastoralists.  

These results contradict several reports by researchers that tested camel sera in Africa and the United Arab Emirates for evidence of FMD with negative results (Wernery and Kaaden, 2004).  Also, the finding of Moussa and Yousef (1998) that the antibodies identified by Richard (1979) were non-specific inhibitory substances frequently observed in camel sera may be correct but with the use of ELISA test, it is confirmed that dromedaries develops antibodies against FMDV and this results is in agreement with studies by Moussa et al. (1987) in Egypt which indicated the susceptibility of dromedaries to natural FMD infection. Similarly, Metwally et al. (1986) did experimental studies in dromedaries in Egypt with FMD serotype O1/2/72 Egypt with no clinical signs observed, but the virus was re-isolated from one camel between one to three weeks post-inoculation (PI), both dromedaries sero-converted but with low titers, which lasted for six weeks and this contradicted the studies by Moussa et al. (1987) where the dromedary did not develop any antibodies to the artificial infection which suggest the lack of antibodies development is as a results of route of inoculation.

From these results, it is clear that dromedaries can contract the FMDV by contacts with FMD infected animals, but may not pose risk of transmitting the disease to susceptible animals. According to Wernery and Kaaden (2004) it seems that dromedaries do not become FMD carriers because FMDV has not been isolated from oesophageal-pharygeal fluid (OPF) 14 days after viral exposure and the statement agrees with the study of Farag et al. (1998) where they were unable to isolate FMDV   from   30   probang   samples    harvested    from dromedaries on different farms in Saudi Arabia where FMD was said to be endemic. This study was limited because of the availability of the ELISA kit that could test only for two FMD structural protein serotypes A and O. Therefore, other serotypes might have been remained undetected as a result of the non-availability of a kit that could test for other FMDV serotypes that circulates within this region.

However, from the results obtained dromedaries appear to be susceptible to infection with FMDV but may likely not play a significant role in the epidemiology of FMD. However, further research on the epidemiology of FMD in dromedaries in West Africa is necessary where the disease is endemic.  

The authors have not declared any conflict of interest.

The authors are grateful to the management of the National Veterinary Research Institute, Vom for providing an enabling environment for conducting this study. The authors also acknowledge The International Atomic Energy Agency (IAEA) for partly funding the study and the staff of the Foot-and-Mouth Disease, Research Laboratory and field officers that participated in the field work and sample collection during the study.