Low Seroprevalence Indicates Vulnerability of Eastern and Central Sudan to Infection with Chikungunya Virus

Chikungunya virus (CHIKV) causes febrile illnesses in humans. The virus is a zoonotic pathogen transmitted through bites of Aedes mosquitoes especially Aedes aegypti. CHIKV is maintained in complex zoonotic cycles in Africa. Phylogenetic analyses have identified three different genotypes of CHIKV: West African, East/Central/South African (ECSA), and Asian lineages (Higgs and Vanlandingham 2015, Halstead 2015). There is evidence that chikungunya fever outbreaks occur occasionally in Sudan. For instance, in 1989, 12% of the sera in an outbreak of arthropod-borne viral infections in the Northern Province were IgG-positive for CHIKV (Watts et al. 1994) and laboratory-confirmed cases of acute chikungunya fever have been reported from August 1988 in the Khartoum Province and from 2005 in South Kordofan (McCarthy et al. 1996, Gould et al. 2008). In addition, clinical CHIKV infections are occasionally being diagnosed in South Kordofan (F.E. Shadad, personal communication). More epidemiological data of CHIKV infection are needed to help with the differential diagnosis of febrile illnesses in Sudan and to determine the immunity to CHIKV in the population. To examine what percentage of the population has been infected with CHIKV, we conducted a seroepidemiological study of CHIKV infection in central and eastern Sudan.

A cross-sectional study was conducted in the Red Sea state, Kassala state, and North Kordofan between December 2012 and January 2013. Blood samples were obtained from patients with a febrile illness who visited the healthcare facilities in Elmawani Hospital, Port Sudan City, Red Sea state, Khashm El Girba Teaching Hospital, Khashm El Girba, Kassala, and El Obeid Teaching Hospital, El Obeid, North Kordofan. Of the samples, 43.8% were from male and 56.2% were from female patients. And 11% were children and adolescents under 15 years, 31.3%, 30.6%, and 26.9% were from the age groups 15–30, 31–45, and above 45 years, respectively. Five milliliters blood was obtained from each participant. Sera were separated by centrifugation at 4000 g for 5 min and immediately stored at −20°C. CHIKV IgG ELISA was performed with 379 sera using a commercial anti-CHIKV IgG ELISA (EUROIMMUN Medizinische Labordiagnostika AG). Anti-CHIKV-IgG-positive ELISA results were further evaluated by an in-house immunofluorescence assay (IFA) and a virus neutralization test. For the IFA, Vero cells were infected with CHIKV. Serum samples were serially diluted, added to the cells, and incubated for 90 min at 37°C. Slides were washed and incubated with FITC–labeled rabbit anti-human IgG (SIFIN, Berlin, Germany) (Schwarz et al. 2012). The neutralization test was performed with a lentiviral vector that was pseudotyped with the CHIKV glycoprotein (Weber et al. 2014).

One hundred and thirty of the 379 serum samples were from the Red Sea state, 125 from Kassala, and 124 from North Kordofan. Seven samples were positive for anti-CHIKV-IgG in the ELISA. One of the positive donors (0.75%) was from the Red Sea state, one (0.8%) from Kassala, and five positive sera (4.0%) were found in North Kordofan. All samples that were positive in the CHIKV ELISA were positive by IFA and in the neutralization test (Table 1). Although the sera have been collected from patients with fever attending outpatient clinics and, therefore, the data show the seroprevalence in this frame population, we think that it is plausible to assume a similar seroprevalence in the general population. The low seroprevalence indicates that CHIKV infections can be considered as rare causes of febrile illnesses in the differential diagnosis of acute fever. In contrast, the low seroprevalence indicates that the population in these parts of Sudan is susceptible to CHIKV infections. This may predispose to larger outbreaks of the disease. All samples positive for CHIKV were also found to be positive for dengue virus IgG antibodies.

Large outbreaks of chikungunya fever can occur in a country if the vector is present and the virus is introduced in the population. This has recently been observed in the Americas. In December 2013, the Asian genotype of CHIKV caused an epidemic on the Caribbean island Saint Martin. The infection subsequently spread to other Caribbean islands, the mainland of Central America, and South and North America. Until the mid of 2015, CHIKV infections have been identified in 44 countries and territories in the Caribbean and the Americas (Higgs and Vanlandingham 2015) (www.cdc.gov/chikungunya/geo/index.html). Transmission of the virus involved primarily A. aegypti. This mosquito species is endemic in eastern and central Sudan because we found a high seroprevalence of dengue virus in the population, a virus that is similarly transmitted by A. aegypti (unpublished data). Based on this observation, it is unclear why CHIKV infections are rare in Sudan. A limiting factor may be that the mosquitoes in Sudan have low vector competence for CHIKV. This might be similar to the low susceptibility of Aedes strains in Africa to dengue virus (Amarasinghe et al. 2011). The important role of viral adaptation to particular mosquito species has been documented for the outbreak of CHIKV infections in the Indian Ocean that started in 2004: An amino acid mutation in the E1 envelope glycoprotein gene (A226V) enabled transmission of CHIKV by A. albopictus (Higgs and Vanlandingham 2015). Thus, the lack of an efficient vector may have prevented a CHIKV epidemic in Sudan.

In conclusion, CHIKV infections are rare in eastern and central Sudan despite the presence of Aedes mosquitoes. The observation of outbreaks in the past and of occasional clinical cases in South Kordofan suggests that the area is at risk of a CHIKV epidemic. The best protection of the population would be a vaccine against CHIKV. Since this is not yet available, CHIKV seroprevalence and clinical cases in the southern part of Sudan should be closely monitored and the susceptibility of the endemic Aedes species for CHIKV transmission should be determined to be optimally prepared for or to prevent a CHIKV epidemic in Sudan.