Via the epidemic blog, Andrew Rambaut offers some well-inforrned thoughts on MERS-CoV and the hypothesis of a camel source. Excerpt from a long, often technical argument:
1) Camels can be infected with MERS. A few studies have shown that some populations of camels have antibodies that strongly cross-react with MERS-CoV spike proteins but not with other related coronaviruses (Reuskken et al 2013; Meyer et al 2014). The Qatar 3 patient in the study above, owned and looked after a herd of 14 camels and 3 of them were shown to be PCR positive for MERS-CoV with sequence isolated from one of them (Haagmans et al 2013).
To my knowledge no other livestock animal has tested positive for MERS-like antibodies and none of the Qatar case's other animals were positive. The camels have subsequently tested negative so have possibly cleared their viruses (suggesting this is a mild, acute virus in camels).
2) Despite causing human cases for nearly 2 years, this virus has resolutely stayed geographically constrained to the Arabian Peninsula. The few cases outside this area can all be traced directly to the affected countries. There is clear evidence of chains of human-to-human transmission but these are not sustained. If the nearly 200 (mainly severe) cases represented the tip of a large sustained human transmission network, the virus would not likely remain so geographically constrained. Most other acute viral epidemics of humans travel the globe in airplanes.
3) At least some of the human cases have had documented contact with camels. This information is not reliably reported so it is not possible to calculate if this is a risk factor (I also don't know what the denominator on this would be - i.e., the expected exposure to camels non-MERS patients with similar demographic profiles).
4) The incidence of (apparently primary) cases does not seem to be growing over the last 6 months. This can be seen in the timeseries data here but this data is not reported with consistent clinical dates so it is difficult to test this formally. There is perhaps an increase in incidence from April last year but this may be due to increased awareness (perhaps due to the large nocosomal outbreak in Al-Hasa about that time). Or it may represent increased exposure due to an increase in prevalence in the reservoir host.