New MERS-CoV case with known camel contact

UPDATE 22-Mar-2014: There are now 11 cases with reported camel contact (5 in KSA) but the details of this post still stand.

The WHO have just released a bulletin describing a new case of MERS-CoV in a 66 year old man living in Abu Dhabi, UAE. Interestingly it reports he owns camels and had visited Oman where he had been in contact with camels. This is the 7th case with recorded contact with camels and the 11th with recorded contact with farm animals (whether camels or not). This adds evidence supporting the role of camels in the transmission of MERS-CoV across the Arabian Peninsular. There are 94 confirmed human cases in all that have no recorded link to other human cases so may be primary human infections. This means that only 7.4% of these have reported exposure to camels. Taking these numbers at face value (i.e., gnoring factors like absence of detailed reporting for many cases and a possible bias towards reporting of camel links in recent months) this seems a low rate of exposure if camels are the primary source.

However, if camels were not the source and a link to camels was random, how likely would we to see 7 out of 94 cases with a link?  This paper reports an estimate of 1.6 million camels in the Arabian Peninsular with about 0.84M in Saudi Arabia. The CIA World Fact Book (via Wikipedia) report a total human population of the Arabian Peninsula of 78M. Very crudely, if every one of the 1.6M camels was looked after by exactly 1 human, we would expect 2% of the humans in the area to have a camel. On that basis, 7 out of 94 does seem higher than we would expect by chance (under a binomial distribution with a 0.021 probabililty of success the probability of having 7 or more out of 94 would be 0.0033). In fact, camels will not be as evenly distributed as that so the individual rate of contact is probably lower depending on how many clusters of camels (herds) there are and how many people it takes to look after each. 

Following the same logic and just considering Saudi Arabia, there are 0.029 camels for every human (0.84M camels, 29.2M humans), 73 confirmed primary cases of which 3 (4.1%) have recorded links to camels. This has a probability of been observed by chance if all camel contacts are reported of 0.36 so not unlikely. However, KSA has been consistently less forthcoming with details of individual cases. All 4 of the additional cases with reports of unspecficied animal cases are from KSA and most of the remaining cases simply do not specify one way or the other.

Overall, this admittedly very simplistic analysis suggests that 7 camel-linked cases would be unlikely to be seen by chance if having contact with camels were not a risk factor for MERS-CoV.

See this page for some wider speculation on camels as a source of human MERS-CoV cases.

Tables collating the evidence for a link with camels are here.