Discussion
Previous studies have demonstrated that, besides dromedaries, several
other members of the Camelidae family are susceptible to MERS-CoV
infection, such as Bactrian camels, alpacas, and llamas , although human
contact infections have only been reported from dromedary camels .
However, the existing data on the susceptibility of non-camelids are
conflicting. Our results are in line with a series of publications
demonstrating that non-camelids do not play a major role in the
transmission cycle . For instance, in a seroepidemiological study in
Saudi Arabia (2010-2013), no antibodies were detected in sheep, goats,
cattle, or chickens, but a high prevalence was found in camels .
Similarly, after an experimental challenge of sheep, goats, and horses
by intranasal MERS-CoV inoculation, none of the animals effectively shed
the virus, and neutralizing antibodies were only found in goats .
Further regarding the family Equidae , including horses, donkeys,
and mules, although viral replication has been observed in primary horse
kidney cells, no antibodies have been found in any of 1,053 equid sera
collected in the UAE and Spain . A study on livestock cell lines from
the Arabian Peninsula showed that MERS-CoV can replicate in goat,
alpaca, and dromedary cells, in contrast to sheep, cattle, rodent, and
insectivore cells . In a study experimentally challenging animals from
France and Spain, MERS-CoV nucleic acid in the respiratory tract as well
as seroconversion were detected only in llamas and pigs, but not in
sheep and horses . A serosurvey in a MERS-CoV-affected area in Jordan
identified MERS-CoV neutralising antibodies in the sera of all camels
but none in goats and cattle, while 4.8 % of sheep sera reacted with
MERS-CoV antigen . In a monitoring study of several mammalian species in
direct contact with infected dromedaries in Egypt, Tunisia, and Senegal
(2015-2017), antibodies were identified in an astonishing 55.6 % of
sheep from Senegal, 1.8 % of sheep from Tunisia, and 0.9 % of goats
from Egypt. MERS-CoV RNA was detected in nasal swabs from 1.2 % sheep
and 4.1 % goats from Egypt and Senegal, as well as 1.9 % cattle and
7.1 % donkeys from Egypt . In conclusion, it seems that several
non-camelid species may become infected with MERS-CoV during viral
spillover events when they are in direct contact with virus-shedding
dromedaries. Maybe this susceptibility is due to the close relatedness
of the Betacoronaviruses (specifically their receptors) infecting
camels and especially bovids, but also humans, porcines, and equids .
However, whether these animals are implicated in the MERS-CoV
transmission cycle remains unknown, but seems to be highly unlikely.
The fact that none of the sheep, goats, and cattle in our survey tested
positive for antibodies indicates that none of them had previously
experienced a MERS-CoV infection, although with an age range of three
months to six years, all animals could have encountered the virus in the
past. Moreover, as the average time of stay at the market for goats is
usually 7-10 days, 10-14 days for sheep, and between 14 and 30 days for
cattle, there is also a not negligible risk of infection at the market.
However, the nasal swab of one sheep (1.8 %) contained MERS-CoV nucleic
acid (Supplementary Table 1), suggesting either a current infection, or
a spillover event from nearby dromedaries, which appears to be more
likely since no antibodies were detected and the sheep was kept in a pen
in close proximity to the camel pens (approximately 25 m away; Fig. 1).
Of note, we additionally sampled 90 dromedaries at the same market and
at the same time as the non-camelids, of which 35.6 % were positive for
MERS-CoV nucleic acid, and another 24.4 % tested borderline positive
(Ct values 38.6-41.2). Moreover, 91.1 % of the 90 camel sera were
MERS-CoV antibody-positive by ELISA, and 96.7 % by IFA (data can be
provided upon request), further supporting the high prevalence of
MERS-CoV infections in dromedaries. This hypothesis is also supported by
our latest study, in which we screened 76 dromedaries during spring and
autumn 2019 at the same market, and found MERS-CoV nucleic acid in
57.9 % of their nasal swabs .
The positive sheep from this study was a two-year-old female, presumably
locally raised Nuaimi breed. However, due to the fact that sex was not
equally distributed across the animals we sampled (115 females, 75.7 %,
compared to 37 males, 24.3 %), no conclusions can be drawn regarding
the susceptibility of the different sexes. The same is true for the
different breeds, with only one positive animal, we cannot infer any
breed-specific genetic predispositions to MERS-CoV infection.
In conclusion, our study shows that non-camelids such as sheep, goats,
and cattle do not play a major role in MERS-CoV epidemiology. The one
sheep that tested RNA-positive in our study most likely reflects a viral
spillover event from infected dromedaries in nearby pens. Furthermore,
serology revealed that the sheep did not develop an antibody response,
suggesting that the infection was very recent, or the sheep only carried
viral particles on its nasal mucosa without actually being infected.