Itching bites may limit Ross River virus infection

To the Editor: Reactions to insect bites are unpleasant and can be dangerous.1 Kumar2 commented that people who react to mosquito bites with local itching and inflammation appeared less likely to develop malaria than those with no reaction. In a later personal communication, he gave me unpublished data showing an inverse linear relationship between the severity of the reaction to mosquito bites and the incidence of clinical malaria.

Ross River virus infection is endemic in all Australian states. A specific serological test is available to confirm suspicious clinical illnesses. Some people have serological signs of past infection without any history of clinical disease. With Kumar's findings in mind, I asked people with a past history of clinical Ross River virus infection, proven by serology, whether they reacted to mosquito bites. All seven asked said that they had had no reaction. Their main complaint was the noise made by predatory mosquitoes. I then asked patients who were in the same age range and general social class, who lived in the same area and were attending clinics with other diseases, whether they had had any clinical illness diagnosed as Ross River virus infection. Of the 18 asked, none had had the clinical disease or serological tests for the disease. All 18 had moderate to severe reactions and itching with mosquito bites. The Box shows these results

Fisher's exact test gives the probability of this finding as 0.0000003. These observations have not explored all aspects of the problem, so this level of probability may be optimistic, but, even so, it makes pointless any further informal collection of data. These findings justify a formal epidemiological study, including antibody titres. It should include those who react to mosquito bites and those who do not, and those with and without a past history of the clinical illness.

This informal study suggests that reactions to mosquito bites protect against Ross River virus infection, and parallels Kumar's findings in malaria. There may be behavioural and biological explanations for this finding. People who itch with mosquito bites may take greater precautions to avoid them. Conversely, people who do not itch may spend more time outdoors and be more likely to be bitten. Biologically, reactions to bites may be examples of a generalised protective effect of local reactions against insect-borne diseases. The inflammatory reaction with itching may be a factor in defence against infection3 by limiting or destroying injected parasites and viruses locally or through a more vigorous generalised response that prevents disease or limits infection to a subclinical level. Investigation of local inflammatory response might provide clues to effective prevention and treatment.