Winter is mating and birthing season for dromedary camels. Therefore, animals kept for milk and meat production gather in areas that provide access to salty plants and other minerals. This environment provides an optimal opportunity to catch and examine large numbers of animals from different herds and origins, given that the camels that roam freely through the desert the rest of the year congregate simultaneously.

Figure 1

Figure 1. MERS-CoV prevalence in dromedary camels, Tunisia. A) Overview of seropositive and PCR-positive dromedaries from 4 different sampling areas (Appendix), by sex and age group. Blues boxes represent seropositive...

In January 2020, we collected serum samples and nasal swabs of 382 gathered animals in Tunisia. We sampled an additional 119 camels used for transport or patrol purposes, all of which were males and kept enclosed. The specimens were collected from 20 different locations within the Kebili Governorate (Table; Figure 1, panel A). Furthermore, serum samples of 22 camel keepers and 2 veterinarians were obtained. We have compiled details of our sampling and testing methods (Appendix).

We analyzed all 501 dromedary serum samples for MERS-CoV–specific antibodies by ELISA and found 80.4% to be seropositive for MERS-CoV IgG. At 85.7%, MERS-CoV seropositivity was higher in female than in male camels (65.6%) (Table; Figure 1, panel A).

Although none of the calves (0–6 months of age) and 4.9% of the juvenile camels (6–24 months of age) were seropositive for antibodies against MERS-CoV, relative seropositivity increased with age (Table). None of the camel keepers or veterinarians was seropositive, indicating no previous MERS-CoV infection.

Screening the dromedary nasal swab specimens for active virus infections with real-time reverse transcription PCR revealed MERS-CoV RNA in 19.8%. However, cycle thresholds >30 (for all but 6 samples) indicated low virus concentrations. Female animals (23.0%) actively shed MERS-CoV RNA, whereas only 10.7% of the male camel specimens were PCR-positive. In contrast to the immunologic findings, a high percentage (40%) of juvenile camels (<2 years of age) shed MERS-CoV RNA, compared with 17.8% of the adult camels that tested positive (Table, Figure 1, panel A)

In summary, 433 of 501 dromedaries tested positive for MERS-CoV. A total of 334 animals were seropositive for MERS-CoV IgG but did not shed MERS-CoV RNA. Of these, 30 dromedary swab specimens contained MERS-CoV RNA, but no specific antibodies were found in the respective serum samples. Sixty-nine PCR-positive dromedary camels also had MERS-CoV antibodies, indicating reinfection (Figure 1, panel B).

Figure 2

Figure 2. Phylogenetic analysis of MERS-CoV samples from dromedary camels in Tunisia, conducted by using the spike RBD. We used 720-bp fragments of the MERS-CoV spike RBD amplified from nasal swab samples...

Attempts to cultivate the virus from all respective PCR-positive swab specimens were unsuccessful, most likely because of the low virus concentrations in the samples. Presumably, whole-genome sequencing did not work for the same reasons. However, we performed Sanger sequencing of cDNAs obtained from PCR-positive samples with the highest viral concentrations and subsequently conducted phylogenetic analysis with a 720-bp fragment of the spike receptor-binding protein. The analyzed nucleotide sequences from the dromedaries in Tunisia differ from previously published MERS-CoV sequences and therefore form a separate group distinct from strains found in Arabia. Two MERS-CoV isolates in Egypt, however, cluster in the same clade (Figure 2).

On the continent of Africa, active surveillance, longitudinal studies, and epidemiologic monitoring are scarce, and little is known about the prevalence and circulation of MERS-CoV in many regions. Whether MERS-CoV lineages in Africa have a lower tendency to cross the species barrier and infect humans is not fully understood. Therefore, closing the gaps in surveillance and virus prevalence data remains a focus for all regions with dromedary camel populations.

Seroprevalence studies in Egypt, Ethiopia, Nigeria, and Kenya all indicate MERS-CoV circulation within camel herds, reporting seropositivity rates ranging from 30% to 100% (10). For dromedaries in Tunisia, only 2 studies have been published, reporting 49% and 87.3% MERS-CoV seropositive animals and only 0.7% active viral shed (8,9).

However, most studies focus on locations where large numbers of camels congregate (e.g., abattoirs, large-scale farms, harbors, or livestock markets). At these locations, dromedaries are kept at a substantially increased population density compared with their normal habitats. This practice, referred to as crowding, increases stress for individual animals (11). Under these circumstances, increased intensive animal contact can lead to higher transmission rates of various microorganisms. Crowding, in combination with animal transport, is known to promote infections of the upper and lower respiratory tract, especially in cattle (12). In contrast, the camels investigated in our study represent a rare example of MERS-CoV prevalence in animal groups with a natural herd structure in northern Africa.

We found extensive MERS-CoV IgG seropositivity (80.4%) and high ratios of MERS-CoV RNA (19.8%) among dromedaries in Tunisia. Compared with adult animals, juvenile camels were more likely to have active MERS-CoV infections and less MERS-CoV IgG in their serum samples. Furthermore, some dromedaries appeared to have MERS-CoV reinfections, explained by the fact that coronaviruses tend to establish endemic infection patterns with high seroprevalence and low but continuous viral shedding in their natural host (13). Waning antibodies in combination with antigenic drift of the virus fosters reinfection events (14).

A limitation of our study is that the low sample size of humans tested, comprising 22 camel keepers and 2 veterinarians (data not shown), precludes drawing generalized conclusions. Furthermore, no nasal swab specimens were collected from camel keepers to check for active MERS infections. Also, no phenotypic or whole-genome analysis of MERS-CoV strains from the dromedary camels was possible because virus growth and next-generation sequencing were not successful because of low viral concentrations.

In conclusion, the high seroprevalence of MERS-CoV antibodies and the active shed of MERS-CoV RNA indicate the widespread nature of the virus in dromedaries in Tunisia. However, more extensive studies in the human and dromedary camel populations and in-depth whole-genome sequence analysis of circulating MERS-CoV strains are required to increase epidemiologic understanding of the disease and its infection dynamics.