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| Fig. 2. Virological and serological evidence for MERS CoV in dromedary camels. |
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Just over a year after the first human infection with MERS-CoV was announced out of Saudi Arabia in 2012, dromedary camels were identified as a host species for the MERS coronavirus (see 2013's The Lancet Camels Found With Antibodies To MERS-CoV-Like Virus).
While bats are believed to be the primary host reservoir for MERS, SARS, and an array of other novel pathogens (see Curr. Opinion Virology: Viruses In Bats & Potential Spillover To Animals And Humans), camels - due to their close contact with people in the Middle East - are the main conduit by which the virus appears to spillover to humans.This news was not well received in the Middle East (see Saudi Camel Owners Threaten Over MERS `Slander’) - where camels are both loved and respected - and despite several years of warnings from public health authorities, many people (locals and tourists) continue to expose themselves to camels and camel products (meat, milk, etc.) without taking recommended precautions.
Despite six years of research, many questions remain unanswered about the MERS-camel connection, including:
- Why 100% of the known and suspected camel-to-human transmissions have been recorded on the Arabian peninsula, while the MERS virus and antibodies have been detected in camels from both Africa and South-Central Asia.
- Why roughly 90% of Middle Eastern human MERS infections have been reported by Saudi Arabia
- Why spillover from camels appears to peak in the winter and spring
- And why human cases were never identified before 2012, despite evidence of MERS circulation in camels going back 30 years.
I've only included some excerpts from this open-access review, so follow the link to read it in its entirety. When you return, I'll have a brief postscript.
Global status of Middle East respiratory syndrome coronavirus in dromedary camels: a systematic review
R. S. Sikkema (a1), E. A. B. A. Farag (a2), Mazharul Islam (a3), Muzzamil Atta (a3) ...
https://doi.org/10.1017/S095026881800345X
Published online: 21 February 2019
Abstract
Dromedary camels have been shown to be the main reservoir for human Middle East respiratory syndrome (MERS) infections. This systematic review aims to compile and analyse all published data on MERS-coronavirus (CoV) in the global camel population to provide an overview of current knowledge on the distribution, spread and risk factors of infections in dromedary camels.
We included original research articles containing laboratory evidence of MERS-CoV infections in dromedary camels in the field from 2013 to April 2018. In general, camels only show minor clinical signs of disease after being infected with MERS-CoV. Serological evidence of MERS-CoV in camels has been found in 20 countries, with molecular evidence for virus circulation in 13 countries.
The seroprevalence of MERS-CoV antibodies increases with age in camels, while the prevalence of viral shedding as determined by MERS-CoV RNA detection in nasal swabs decreases. In several studies, camels that were sampled at animal markets or quarantine facilities were seropositive more often than camels at farms as well as imported camels vs. locally bred camels.
Some studies show a relatively higher seroprevalence and viral detection during the cooler winter months. Knowledge of the animal reservoir of MERS-CoV is essential to develop intervention and control measures to prevent human infections.
(SNIP)
Worldwide distribution of MERS-CoV in dromedary camels
The first evidence of MERS-CoV in camels described so far is the detection of antibodies to MERS-CoV in camel sera from Somalia and Sudan from 1983 of which 81% tested positive [32]. Additional serological evidence of the widespread presence of MERS-CoV infection in camels, included in this review, has been found in 18 additional countries: Bangladesh, Burkina Faso, Egypt, Ethiopia, Iraq, Israel, Jordan, Kenya, KSA, Mali, Morocco, Nigeria, Oman, Pakistan, Qatar, Spain, Tunisia and the UAE (Fig. 2).
In addition, Promed mail reported that virus-positive camels had been found in Kuwait and Iran, the latter reportedly in imported animals (Archive number 20140612.2534919 and 20141029.2912385). In 11 countries, serological findings were complemented with the finding of viral RNA in dromedary camels: Burkina Faso, Egypt, Ethiopia, Iraq, Jordan, KSA, Morocco, Nigeria, Oman, Qatar and the UAE.
Investigations of MERS-CoV circulation amongst dromedary camels in Australia, Japan, Kazakhstan, USA and Canada did not find any proof of MERS-CoV circulation. All countries where MERS-CoV circulates in the camel population, with the exception of Spain (Canary Islands), Pakistan and Bangladesh, are located in the Middle East or Africa [4, 33].
One out of 17 camels that had MERS-CoV antibodies in Bangladesh was born in Bangladesh, 16 others were imported from India [34]. However, there have not been any additional reports of MERS-CoV in camels in India. There is no record of foreign origin of the seropositive camels from Pakistan [35]. Moreover, in previous studies there had already been evidence of seropositive camels that originate from Pakistan [37, 58].
(SNIP)
The increase in MERS-CoV circulation in winter and spring can have multiple explanations. Firstly, the winter is the calving season [10], which leads to a larger proportion of young animals that usually have a higher number of MERS-CoV infections and virus excretion. Moreover, in winter season, there is a major increase of camel and human movements due to camel racing competitions, camel breeding, trading and movements to grazing grounds, which increases the chance of virus spread. Additionally, cooler temperatures may facilitate coronavirus survival in the environment [57].
(SNIP)
Conclusion
Since the discovery of MERS-CoV in 2012, the dromedary camel has been identified as the animal reservoir of human infections with the MERS-CoV. However, the exact route of human primary infections is still unknown.
Moreover, the scale of the spread and prevalence of MERS-CoV in the camel reservoir is not fully known yet since there is still a lack of MERS-CoV prevalence data in some countries that harbour a very significant proportion of the world camel population.
However, knowledge of the animal reservoir of MERS-CoV is essential to develop intervention and control measures to prevent human infections. Prospective studies that include representative sampling of camels of different age groups and sex, within the different husbandry practices, are needed to fully understand the patterns of MERS-CoV circulation. Such studies are important as they may give more information on critical control points for interventions to reduce the circulation of MERS-CoV and/or exposure of humans.
(Continue . . . )
SARS, Ebola, Marburg, Rabies, Nipah & Hendra are just a few of the other zoonotic viruses that are believed to be of bat origin - but are often transmitted to humans via an intermediate host.
In 2017 researchers from EcoHealth Alliance published a letter in Nature (Host and viral traits predict zoonotic spillover from mammals) providing the first comprehensive analysis of viruses known to infect mammals.
From their website summary:
The study shows that bats carry a significantly higher proportion of viruses able to infect people than any other group of mammals; and it identifies the species and geographic regions on the planet with the highest number of yet-to-be discovered, or ‘missing’, viruses likely to infect people. This work provides a new way to predict where and how we should work to identify and pre-empt the next potential viral pandemic before it emerges.For more on bats and the zoonotic viruses they carry, you may wish to revisit:
Curr. Opinion Virology: Viruses In Bats & Potential Spillover To Animals And Humans
Back To The Bat Cave: More Influenza In Bats
EID Journal: A New Bat-HKU2–like Coronavirus in Swine, China, 2017
Emerg. Microbes & Infect.: Novel Coronaviruses In Least Horseshoe Bats In Southwestern China
SARS-like WIV1-CoV poised for human emergence
