Dromedary camels in northern Mali have high seropositivity to MERS-CoV

Falzarano, Darryl; Kamissoko, Badian; de Wit, Emmie; Maïga, Ousmane; Cronin, Jacqueline; Samaké, Kassim; Traoré, Abdalah; Milne-Price, Shauna; Munster, Vincent J.; Sogoba, Nafomon; Niang, Mamadou; Safronetz, David; Feldmann, Heinz

Camels from Nara showed no reactivity at all dilutions in the MERS-CoV S1 ELISA and as such were used to set up cut-off values for all other samples (Figs. 1A, 2) . Subsequent analysis of neutralizing activity of the sera from the Nara camels revealed no neutralization titers in a microneutralization assay, further supporting their lack of reactivity (Fig. 1B) . At the lowest dilution tested in the MERS-CoV S1 ELISA (1:400), nearly all samples assayed (88%) were positive for antibodies that react to S1. While the positivity decreased with increasing dilution, 70% and 67% of samples respectively were positive at 1:1600 and 1:6400 (Table 1) . Even at a dilution of 1:25,600, 24% of samples remained positive. There was no variation in seropositivity between different sampling locations in Kidal province (Fig. 1A) . In terms of seropositivity and approximate endpoint dilution, there were no notable differences when animals were categorized by age or sex. When camels were grouped by age (1-2, 3-8 and 9-16 years) neither the seropositivity rate (83, 91, 88%) nor the endpoint dilution were significantly different. Similarly, the seropositivity rate in male (n = 245) and female (n = 328) camels of 86 and 92% respectively was comparable. A small number (n = 10) of serum samples from Malian cattle and sheep were also tested in the S1 ELISA but consistent with reports from other countries [8, 13] were negative. To confirm data from the S1 ELISA, as subset samples (n = 147) were assayed for neutralizing antibodies against MERS-CoV in a microneutralization assay. Of the animals tested, 78% had neutralizing antibodies with a reciprocal titer ≥ 20, with titers up to 1920 (Fig. 2B) .