One of the hallmarks of a complete epidemiological investigation is the case control study. This type of study compares people with a condition of interest with those without the condition with the aim of identifying factors that may contribute to acquisition of the condition. For example, a case control study comparing those infected with hepatitis C to those without hepatitis C would likely determine injection drug use was more common in those with the infection compared to those without the infection. Analyzing the results of such a study will produce an important statistic known as the odds ratio. If this ratio is greater than 1, it illustrates an association between the attribute and the condition.
Such a study for Middle East Respiratory Syndrome (MERS) cases in Saudi Arabia--highly anticipated and long awaited - was just published in the journal Emerging Infectious Diseases.
MERS, a severe respiratory infection caused by a novel coronavirus, has infected over 1600 people since 2012, killing nearly 600. While 26 countries have been affected, the vast majority of cases (85%) originate in Saudi Arabia. The leading hypothesis regarding the viral ecology at play is that the virus originated in bats and spilled into camels—who have demonstrated antibodies to the virus—and then into humans. Human-to-human spread seems restricted to hospital-based superspreading events as well as infection control lapses. Sustained community spread has not occurred.
This study of primary MERS cases—those contracted from the environment and not from another person—occurring in Saudi Arabia in 2014 involved studying 30 cases and 116 age, sex, and neighborhood matched controls.
Important findings included:
- Case patients were more likely to have a higher income than controls
- Cases patients were more likely to have exposure to camels than controls (OR = 3.73)
- Case patients were more likely to keep camels in or around the home than controls (OR = 3.34)
- Case patients were more likely to have visited a farm with camels than controls(OR = 11.57)
- Case patients who visited farms with camels were more likely to have milked a camel (OR = 10.36)
- Underlying medical conditions were more likely in cases than controls (OR = 5.11)
- No association was found with consumption of camel urine
Multivariable analysis revealed that exposure to a camel within the past 6 months and having diabetes, having heart disease, or tobacco smoking were the most highly significant associations.
The study’s implications are important for future control efforts and lend more evidence to the hypothesis linking camels to primary MERS infections. Control efforts can now be more exactly focused on delimiting exposure to camels, especially amongst those with underlying medical conditions. Additionally, vaccination of camels against MERS will become an important strategy to be pursued.