Indirect evidence for excess deaths due to chikungunya infection

The current issue of Emerging Infectious Diseases carries an article by Dileep Mavalankar and coauthors at the Indian Institute of Management, Ahmedabad, India. The article, reported by PromedMail as well claims that 3056 more deaths than expected that occured in 2006 in Ahmedabad, India are due to the chikungunya epidemic. The authors arrived at this conclusion by estimating the expected number of deaths in 2006 based on the number of deaths reported in 2002-2005, and comparing it with the actual number of cases reported. About 2944 additional deaths occured in the period August-November when the chikungunya epidemic was at its height in 2006 in the city.  Statistically significant difference was shown between the observed number of deaths and the expected number of deaths for July, August and September of 2006. Overall, 60,777 suspected chikungunya cases were reported in 2006 in Ahmedabad. The authors generalize this case-fatality-ratio of around 5% to the chikungunya epidemic in the whole of India in 2006 with 1.39 million suspected cases, estimating around 50,000 [see the ProMedMail report] deaths due to chikungunya infection in the whole of India in 2006. [According to my calculation, 70,000 deaths would be more appropriate estimate for a CFR of 5% with 1.39 million cases] It seems reasonable to assume some mortality with any infection in the elderly or immuno-compromised patients, as the article claims. However, no mechanism has been found so far by which chikungunya infection would lead to mortality in itself. Another recent article in the Virology Journal shows that the chikungunya virus produces acute arthritis by large area of necrosis and collagenosis or fibrosis, damaging the cartilage and connective metabolism and releasing the degraded products from the tissue, increasing levels of proline, hydroxyproline and mucopolysaccharides in affected patients. Whether this mechanism in itself is life-threatening in elderly or immunocompromised patients, or other opportunistic infections cause the mortality, seems to be an open question.