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Camels may be source of MERS virus transmission

Various news sources today report that dromedary camels – “ships of the desert” as The Independent puts it – could be the source of the MERS (Middle East respiratory syndrome) virus that emerged last year. MERS is believed to be caused by a type of coronavirus.

Coronaviruses are found worldwide and cause respiratory illnesses of varying severity, ranging from the common cold to the severe respiratory illness SARS.

As of August 2013, there have been 94 confirmed cases of MERS, all in people with links to Jordan, Qatar, Saudi Arabia and the United Arab Emirates.

There has been some evidence of human-to-human transmission of MERS, but it is thought that the virus could have been spread through contact with animals. Animals are common “biological reservoirs” for coronaviruses.

In the current study, blood samples routinely collected from a group of camels in Oman were all found to be positive for antibodies against MERS virus, suggesting that the animals had been infected with the virus. Only 9% of samples from camels in the Canary Islands were positive for antibodies against MERS virus.

The researchers say that this does not mean that camels are necessarily the main animal reservoirs – they have not yet tested other livestock from the Middle East where MERS has occurred. Even if camels are the main reservoir for infection it remains unclear what level of contact with them could cause transmission.

Where did the story come from?

The study was carried out by researchers from the National Institute for Public Health and the Environment, Bilthoven, the Netherlands, and various other academic and research centres from countries worldwide. Funding was provided by the European Union, European Centre for Disease Prevention and Control and the Deutsche Forschungsgemeinschaft, which is a German research foundation.

The study was published in the peer-reviewed medical journal The Lancet Infectious Diseases.

The UK’s media reporting of this research is accurate.

What kind of research was this?

This research explores the possible animal reservoirs of the newly identified coronavirus – Middle East respiratory syndrome coronavirus or MERS-CoV.

In 2012, a new type of coronavirus – MERS – was identified in humans for the first time. It caused severe respiratory infection in a small number of people in Middle Eastern countries. As of August 2013, 94 laboratory-confirmed cases of MERS have been reported to the WHO, and 46 of these people have died.

All cases of MERS so far are reported to have been linked to Jordan, Qatar, Saudi Arabia and the United Arab Emirates. There have been a few reports of human-to-human transmission in hospitals, but overall it is currently thought that the virus does not easily spread between people. For this reason, animals are believed to be the main reservoir of MERS virus and responsible for most infections in humans.

Coronaviruses can have various animal hosts, including wildlife, livestock, poultry and pets. For the SARS virus, a particular species of bat was identified as a reservoir. For the current MERS outbreak, patient histories suggest that those affected may have been in contact with dromedary camels or goats. Therefore, the researchers have investigated the possible animal reservoirs of MERS virus by examining antibodies found in blood samples taken from livestock.

What did the research involve?

In 2012–13, blood samples were collected for routine veterinary purposes from 105 dromedary camels (also known as the Arabian and Indian camel, distinctive for having a single hump). These samples were taken from camels in two herds on the Canary Islands (half the camels were male, half were female). The herds had the same owner, but one was from a coastal dune habitat with no other nearby livestock, while the other herd was from an inland valley close to a tropical fruit farm. This second inland herd was in possible proximity to fruit bats, and was near horse and goat farms.  

In March 2013, samples were also taken from 50 female dromedary camels from Oman. These camels were retired racing camels now used for breeding who came from different owners and different locations.

The researchers also examined blood samples taken from various other animals for routine veterinary purposes:

  • two llamas, six alpacas and two Bactrian camels from the Netherlands
  • two Bactrian camels, 18 alpacas, five llamas and two guanaco in Buin Zoo in Chile
  • 40 cattle, 40 domestic goats and 40 sheep from samples submitted to the Dutch Animal Health Service
  • 40 Spanish domestic goats

In the laboratory the researchers tested the blood samples for IgG antibodies that would bind to MERS, the SARS coronavirus and to another strain of human coronavirus – OC43 (closely related to a bovine coronavirus found in cows, sheep, goats and camels).

The researchers hoped the results would show them whether the blood samples contained antibodies that would recognise these viruses. They also wanted to see how effective these antibodies were at neutralising a virus, particularly MERS (making it inactive and protecting a cell from being infected with it).

To do this, the researchers mixed MERS virus particles with different dilutions of blood serum from the animals, before adding the mixture to cells grown in the laboratory. They looked to see what was the greatest dilution (lowest concentration) of serum that could still protect the cells from being infected with MERS. This is called a neutralisation assay.

What were the basic results?

Blood samples from all 50 camels from Oman were positive for antibodies that reacted against MERS, while only 15 of 105 camels (14%) from the Canary Islands were positive. None of the other animals sampled were found to have antibodies to MERS.

Some of the samples from camels that contained antibodies that recognised and bound to MERS were further tested to see how ‘effective’ they were. Nine camels from the Canary Islands had antibodies able to neutralise MERS. The blood serum from these camels was able to protect cells from infection when it was diluted between one in 20 and one in 320. All camels from Oman had antibodies able to neutralise MERS.

The serum from these animals could protect cells when it was diluted between one in 320 and one in 2,560. This means that the serum from the camels in Oman could be diluted much more than the serum from the camels from the Canary Islands and still be effective. This in turn suggests that the camels from Oman had a greater degree of historical exposure to the MERS virus.

Blood samples from three llamas, four alpacas, one guanaco, one cow, one goat, two Bactrian camels and 16 of 105 (15%) Spanish dromedary camels were found to contain antibodies that reacted with human coronavirus OC43.

No antibodies bound to SARS (the virus that led to the outbreak of 2002 to 2003).

How did the researchers interpret the results?

The researchers say that “MERS-CoV or a related virus has infected camel populations”. The antibody levels from blood samples from camels from across Oman suggest that there is widespread infection in that country. 

Conclusion

This study is valuable in being reportedly the first to examine animals for the presence of antibodies against the newly discovered MERS coronavirus.

The study found that blood samples from all 50 dromedary camels from Oman contained antibodies able to neutralise MERS. Coronaviruses have various animal hosts, and some people who have caught MERS in the current outbreak in the Middle East are reported to have been in contact with dromedary camels or goats. Therefore, the findings suggest that camels could be a “biological reservoir” for MERS virus.

However, as the researchers say, they have not been able to study blood samples from other common livestock species in the Middle Eastern region where cases have occurred, including cattle, sheep and goats (samples from these species tested in the current study were not from the Middle East).

Therefore, they are not able to rule out the possibility of other animal reservoirs for this virus.

Also, only 9% of the camels sampled from the Canary Islands were able to neutralise MERS. Judging by these lower levels, the researchers speculate that exposure to other animal reservoirs of the virus in this area (which could include wild rodents, rabbits, pigeons, doves and possibly bats) may be rare. Alternatively, there may have been an outbreak among animals there in the past. Whichever is the case, the situation in the Middle East is clearly different from that in the Canary Islands.

As the researchers say, targeted studies are needed to confirm their findings and examine their relevance to the people who have contracted MERS.

The WHO currently states that with only a small number of cases reported so far, there is very limited information available on the likely transmission, severity and clinical impact of MERS. Nine countries in the Middle East are so far reported to have had cases.

The WHO is not currently (as of August 9 2013) advising trade or travel restrictions or entry screening, though they advise routine measures for assessing sick travellers. The WHO has provided personal hygiene and food safety advice for pilgrims, including advice to avoid unnecessary contact with farm, domestic and wild animals.

 

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