Public health experts have a problem.

Until they can be more certain of what causes SARS, where it comes from and where it spreads, it is difficult to reassure people about the level of threat it poses to them.

Until they can be more certain of what causes SARS, where it comes from and where it spreads, it is difficult to reassure people about the level of threat it poses to them.The almost unprecedented co-operation between scientists around the world trying to identify the virus has produced results extremely quickly. In Britain the laboratories at the Health Protection Agency (formerly the Public Health Laboratories Service) have been working ‘flat out’ according to officials. They are not just examining specimens from patients in the UK and abroad. They are sharing information over the internet and by email on the molecular structure and genetic sequencing of the material. The Corona Virus family which up to now has been associated with the common cold is the scientists best bet for what is causing this respiratory disease.Animal origin The World Health Organisation believes the most likely explanation for its origin is that it has jumped species from animals to humans. We know already the Corona Virus is carried in animals and birds, in some cases in more severe forms than it is found in humans. In the area in which the SARS outbreaks are thought to have first occurred, the Guangdong province in Southern China, and in Hong Kong, it is not unusual for forms of influenza to jump species. Although the Corona Virus is different from influenza this ‘species jump’ hypothesis is pretty credible. The next question is how it spreads. To date it remains the case that the vast majority of cases can be explained by close contact. That suggests that the virus is passed on in droplets, either when an infected person coughs on another and those droplets are breathed in to the upper respiratory tract, or when they cough an object such as a tea cup or a door handle which is then touched by someone else. In Hong Kong though the large number of SARS cases has led to concern there that transmission is more virulent. Environmental factor The clutch of cases inside an apartment building suggested it was also passed on in the environment, through the water system or sewage perhaps. That seems quite plausible. The virus has been found in secretions from infected patients so it would not be hard for it to get into either system. However although the virus does seem to pass through the air across short distances – people have been infected by people they have sat next to for example – there does not appear to be much evidence yet that it is spread further through the air in the same way that influenza is. If someone suffering from flu coughs everyone in the room is at risk. So far there is little to suggest SARS is so virulent. Hong Kong may be a special case. Its population density and climate mean it is one of the few places in the world where we see influenza epidemics all the year round. In the Northern Hemisphere we tend to see them just before Christmas. In the Southern Hemisphere in July and August. Different carriers The unique characteristics of Hong Kong may explain why SARS appears to have spread more rapidly. And if it did originate in Guangdong, it is possible that it was in fact brought to Hong Kong by several different carriers and so as a result there were more different outbreaks to cope with, leading to a greater spread of infection. What is of ‘great concern’ to those leading the fight against the disease at the WHO is the fact that many of those who have died have been young otherwise healthy people. Respiratory illness usually affects more vulnerable individuals; those whose immune system is weak or whose respiratory track is damaged such as elderly people. Yet many of those who died were health workers between the ages of 20 and 40. The doctor who first alerted the world scientific community to the threat SARS posed was one of them. The twin priorities now are infection control in affected areas – trying to stop the spread of the illness – and attempting to develop a diagnostic test. Sensitivity Laboratory tests can recognise antibodies to fight the virus in a patient’s blood or pick up the virus in their secretions. But they are not yet robust or sensitive enough to be used as a diagnostic test to try to measure the spread of the virus in the general population. That is vital. At the moment all scientists can do is try to spot the virus in people who are already displaying symptoms. A decent diagnostic test could give them a better idea whether everyone who is infected becomes ill, and whether as might be expected some people carry the virus but do not fall ill That date will help them to gain a better understanding of the threat it poses. Senior WHO officials tell me they hope such a test may become available within weeks rather than months. In the meantime British officials at the Health Protection Agency are drawing up contingency plans to deal with any further outbreaks. They say their level of response is ‘appropriately cautious’ but flexible. Possibilities They are working on three possible scenarios. Firstly, that it is snuffed out after a few weeks and poses no further threat. Secondly, that it rumbles on causing severe problems elsewhere in the world in the existing hotspots but aside from the odd spark does not affect British citizens apart from the travel restrictions. Thirdly, that we suffer a major outbreak in Britain. There appear to be no plans however to screen travellers entering Britain from affected areas. Partly this is down to logistics. Screening at Heathrow airport in London would be very difficult. Several thousand people from South East Asia fly in each day. Many of them do not fly here direct but change at other European hubs and fly in on other carriers. The approach the public health officials are taking is to give airlines as much information as possible and encourage them not to let anyone displaying symptoms or who has been in contact with anyone displaying symptoms to board a flight. Secondly, they are trying to get as much information out as possible about the symptoms of the disease; high temperature, cough, shortness of breath, out to the public so that people returning from affected areas are aware of the threat and the need to contact their doctor at the earliest opportunity if they fall ill. Extent of the threat So how worried should we be? Less worried while the outbreaks are contained in Hong Kong and other parts of South East Asia for the reasons of population density and climate I have already mentioned. Far more worried if there is a major outbreak in a Northern European country with a lower population density like Denmark or the UK. That would suggest the virus had mutated to spread more easily and become more dangerous. And for those people considering whether or not to travel to Hong Kong, despite the understandable fear of those living there, there is perhaps some solace in the mathematics of the risk you face. There are 7 million people in Hong Kong. Around 50 have died of SARS. Around a thousand or so are in hospital, perhaps ten times that number are carrying the virus but have not been taken ill. So your chances of meeting one of them are pretty small, unless you are taken to hospital for some other reason. As ever, assessing the actual risk you face as an individual is almost impossible. Until we have more answers from the WHO, it will not get any easier. (Source: BBC Online)