In a recent study published in the CDC’s journal Emerging Infectious Diseases, researchers estimated the incidence and potential human transmission of the Middle East respiratory syndrome coronavirus (MERS-CoV) in dromedaries (nomadic camels) in northern Kenya. They found that the incidence of MERS-CoV among these animals was biphasic, peaking in October 2022 and February 2023. Further, slaughterhouse workers in contact with the dromedaries were found to show serologic signs of exposure to MERS-CoV.
Dispatch: Biphasic MERS-CoV Incidence in Nomadic Dromedaries with Putative Transmission to Humans, Kenya, 2022–2023. Image Credit: Hamady / Shutterstock
MERS-CoV is prevalent in dromedary camels in the Arabian Peninsula and Africa, with >75% seroprevalence. Zoonotic transmission to humans, mainly in the Arabian Peninsula, has resulted in >2,400 cases and >800 deaths so far. Although camel breeding is a major activity in Kenya, only three cases of MERS-CoV were identified in camel-exposed humans in 2019, suggesting regional epidemiologic differences.
MERS-CoV outbreaks in farmed dromedary camels are linked with annual camel parturition, with calves testing positive for MERS-CoV ribonucleic acid (RNA) after losing maternal antibodies. Nomadic camels in Africa, with fluctuating population densities due to seasonality and food availability, have shown correlations between high population density and MERS-CoV seropositivity in Kenya, indicating gaps in our understanding of MERS-CoV circulation.
Limited infrastructure hinders field studies on nomadic camels, but the regular transportation of these animals to slaughterhouses allows for continuous testing. Leveraging this setup, researchers in the present study conducted a year-long study at a northern Kenyan slaughterhouse hub to estimate the MERS-CoV incidence in dromedaries and their potential transmission to individuals working there.
About the study
The study was conducted at a slaughterhouse hub in Isiolo, northern Kenya. Sampling was conducted from September 2022 to September 2023. Samples were collected from 10-15 dromedary camels at a frequency of 4-5 days per week. The camels (n = 2,711) originated from various administrative wards (n=12), primarily from Laisamis and Burat.
MERS-CoV RNA detection was performed using quantitative reverse transcription polymerase chain reaction (qRT-PCR). Confirmation was done by open reading frame (ORF) 1ab qRT-PCR or sequencing. Phylogenetic analysis was additionally performed. Randomized camel serum samples (n = 369) were tested to assess MERS-CoV immunoglobulin G (IgG) levels using ELISA (short for enzyme-linked immunosorbent assay). Optical density ratio (ODR) values were obtained. Statistical analyses were conducted to explore the associations between MERS-CoV IgG levels, RNA-positivity, seasonality, camel sex, and age.
Sero-epidemiologic investigation was conducted among slaughterhouse workers in contact with dromedaries. MERS-CoV S1 IgG reactivity was assessed using ELISA. Potential cross-reactivity with SARS-CoV-2 antibodies was excluded by comparing ELISA ODRs between MERS-CoV S1 and SARS-CoV-2 S1 assays. Neutralization tests (NT) were conducted using green fluorescent protein (GFP)–encoding vesicular stomatitis virus pseudoparticles (VSVpp) carrying MERS-CoV S protein from two clades. Testing was performed on seven serum samples at a 1:20 dilution. A plaque-reduction neutralization test (PRNT) based on MERS-CoV EMC/2012 was conducted.
Results and discussion
MERS-CoV RNA was detected in 1.3% of camels. The cumulative RNA positivity rate was found to be higher in September-October 2022 (5.0%) compared to January-March 2023 (2.3%). Incidence showed biphasic peaks in October 2022 and February 2023. Phylogenetic analysis revealed high similarity (>99.93% nucleotide identity) with MERS-CoV strains from Akaki, Ethiopia, in 2019. The sequences clustered within clade C2.2, which includes strains initially identified in Kenya in 2018, indicating three putative MERS-CoV outbreaks in Kenyan camels.
MERS-CoV IgG levels had a median ODR of 2.14, with a seroprevalence of 80.76%. IgG levels were lowest in June and highest in March. A negative association was found between MERS-CoV IgG levels and RNA positivity. RNA-positivity was found to be negatively linked to the season. Compared to female camels, male camels showed a greater probability of being RNA-positive and a lower probability of being seropositive. Older animals (>3 years) had a higher (but statistically insignificant) seropositivity rate (86%) compared to animals ≤3 years (72%).
MERS-CoV S1 IgG reactivity was detected in 14.6% of Isiolo abattoir workers. The absence of MERS-CoV S1 IgG reactivity was noted in a control cohort (n = 12) without camel exposure despite high SARS-CoV-2 S1 IgG levels (92%). Notably, one serum sample showed a VSVpp-NT 50–90% reduction of foci-forming units. Additionally, results from PRNT confirmed MERS-CoV seroconversion for the sample. None of the MERS-CoV ELISA-negative samples demonstrated neutralizing capacity in VSVpp-NT and PRNT assays.
In conclusion, the present study revealed a biphasic incidence of MERS-CoV in dromedary camels, potentially influenced by increased animal interactions during transport and seasonal factors. The evidence of human transmission in the study highlights the need for enhanced surveillance and preventive measures to mitigate zoonotic transmission risk. Further research is warranted to investigate the dynamics of MERS-CoV circulation and formulate strategies for potential disease control and prevention.