Science: Microbe of the Month: Doctor, my whole family has this giddy feeling ...

Why do groups of people suddenly suffer vertigo? The answer may lie in a virus, says Bernard Dixon

One of the most puzzling conditions reported from time to time in the medical journals is epidemic vertigo. For no apparent reason, people in a particular place develop giddiness - a feeling that either they or their surroundings are spinning out of control - which lasts for a few days and then goes away. Some outbreaks of this sort have been put down to hysteria or imagination. The much more plausible cause is an infectious microbe, affecting the organs in the inner ear that control our sense of balance. Yet good evidence to support this idea has never been forthcoming.

A study of a vertigo outbreak in Wyoming, described in this month's Epidemiology and Infection, now strongly indicates that a virus is indeed to blame. The investigators, from the US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, have recommended practical measures to be adopted in any future incidents to make the conclusion watertight.

The Wyoming epidemic came to light when a local hospital in Hot Springs County began to receive patients suffering from acute vertigo. Over a period of several months, about 1 per cent of the population of 5,000 people went to their family practitioner with the same complaint, some being sufficiently ill to require hospital treatment.

As the dizziness seemed more of a curiosity than a condition causing serious medical concern, it did not attract the attention of the CDC in time for an immediate on-the-spot investigation. The investigators worked retrospectively, interviewing people who had suffered from vertigo to determine whether there were any other significant ways in which they differed from those who had not been affected.

First, they eliminated any individuals whose vertigo could have been attributable to a well-recognised cause such as high blood pressure. The investigators asked them - and a control group that matched the patients in age and sex - a wide range of questions about activities and experiences that could have helped to account for the vertigo. Had they, for example, suffered from an acute illness during the previous month, or had any chronic health problems during the past 10 years? Had they been mountaineering, or bathed in hot springs? Participants also provided blood samples, which were tested for antibodies against several different viruses.

All the vertigo victims had experienced both a spinning sensation and problems with balance, accompanied in three-quarters of cases by nausea. Most also described symptoms such as buzzing in the ear or a temporary loss of hearing.

Some factors, such as mountaineering, were soon eliminated from the CDC analysis and two different types of evidence clearly pointed towards a microbe as the cause. First, the victims were significantly more likely to have shown symptoms of a recent acute virus infection: these included fever, sore throat, diarrhoea, lethargy, fatigue and a general feeling of "getting the flu".

Second, the blood tests revealed a striking difference between the victims and the controls in their evidence of recent infection with an enterovirus. Three-quarters of the former had enterovirus antibodies in their bloodstream, as compared with just over half of the controls. Enteroviruses are a group that includes the polio virus and others which cause relatively mild infections, rashes and respiratory symptoms. The test used in the Wyoming investigation did not allow the particular member of the enterovirus group to be identified.

Each of these findings contributes to the conclusion that an enterovirus was the cause of the epidemic vertigo. The much higher frequency of enterovirus antibodies, together with actual symptoms of virus infection, in the vertigo victims strongly suggests that the virus was responsible for the condition. This is supported by the results of another, smaller-scale investigation of a vertigo outbreak in Atlanta in 1994 which revealed that these victims too had had an enterovirus infection.

One of the CDC researchers' motives in publishing their findings is to alert doctors and scientists elsewhere to the need to confirm their observations by speedy investigation of any future outbreaks. Their own work has shown the general identity of a virus that is the cause - or a crucial contributory cause - of this bizarre condition of epidemic vertigo. Knowing that, investigators should be able to identify the precise enterovirus through tests of throat and stool samples taken from victims while they are still infected.

Unfortunately enteroviruses, like most other viruses, are not susceptible to drugs in the way that bacteria are to antibiotics. Nevertheless, identifying the cause of epidemic vertigo will bring several benefits. They include the likelihood of determining why certain people are especially vulnerable to this particular virus, and of developing a vaccine that could protect high-risk individuals.