Can We Realistically Stop Infectious Diseases at the Border?
Everyone is pretty concerned about the possible spread of Ebola virus around the world, especially with the news of cases in Texas, USA, and Madrid, Spain. The question that often arises is why don’t authorities stop these people at airports or other points of entry when they arrive.
Well, there are really only three things you can do at a point of entry to screen people for illness. When I was Director of Emergency Preparedness for Canada during the 2003 SARS outbreak we tried all of them.
A mandatory questionnaire for arriving passengers about any symptoms and their country of origin can identify people who might have been exposed or who are having early symptoms of an infectious disease. The problem with this method is that it depends entirely on the truthfulness of the responder. People might elect to provide false information to avoid being detained for further examination.
We tried this in Canada during the 2003 SARS outbreak. A questionnaire with 3 questions was mandatory for all incoming passengers. A total of 678,000 questionnaires were filled out, and 2,478 people answered yes to one or more of the questions. A specially trained nurse referred each one of these people for in-depth questioning and temperature measurement. The result? None of the 2,478 people had SARS. Some passengers had responded yes to having a cough, but in a few instances, the cough was due to the fact that the individuals were habitual smokers.
The questionnaire approach is clearly not effective.
2. Thermal Scanning Machines
A thermal scanning machine can be installed to try to detect arriving passengers who might have a fever. The machines are infrared detectors which are pointed toward passengers at head level to pick up elevated temperatures. They are usually setup along the passageway leading to the customs inspection room. The reason for this is that the machines are fairly expensive and it would be cost prohibitive to set them up at every gateway to catch travellers as they leave the airplane.
While the machines are certainly exciting for any tech enthusiast, they have a number of shortcomings. First, some of these machines have to be constantly re-calibrated as environmental conditions change throughout day. For example, hallways receiving a lot of sun tend to throw off the relative measurements of the passengers’ temperatures. Second, there are many conditions or situations in which a traveller might have an elevated temperature reading, but not have an infectious disease at all. This could be anything from someone being hot and sweaty as they rush through the airport to a menopausal woman having a hot flash. The point is that simply recording elevated temperatures is not necessarily an effective way to find infectious diseases.
We also tried this in Canada during the 2003 SARS outbreak. Thermal scanning machines were installed in the 6 major airports that handle approximately 95 percent of all international arrivals. The results from the Toronto and Vancouver airports? Among the 468,000 people who were screened, only 0.02% (95) were referred to a nurse for further assessment. None of these people actually had an elevated temperature, and all were cleared.
3. On-The-Ground Support
Captains of vessels (e.g. airplanes and ships) are required by the International Health Regulations to report individuals on board who are sick to authorities at the point of arrival. At this time, quarantine officers are notified and meet the vessel to assess the situation before anyone is allowed to disembark. These days, quarantine officers are on call at many international airports. This measure has been in place in Canada and many other countries since the SARS epidemic.
This approach makes perfect sense because the sick individual is already displaying enough symptoms en-route to cause concern. While the passenger might simply have an upset stomach or the flu, it is better to assess this person and try to eliminate the possibility of a serious infectious disease. If the quarantine officers suspect a serious infectious disease, then the patient can be immediately isolated to prevent any potential spread. The remaining passengers can either be placed in quarantine or asked for contact information for further follow-up.
The big drawback to this approach is cost. On-call personnel are required. A quarantine officer working overtime, possibly with hazard pay can be fairly expensive. Given the number of people who are actually reported is generally pretty small, the cost per incident is quite large.
The main problem with all of these methods is that they don’t catch anyone who might be incubating a disease or someone who has taken measures to reduce their symptoms.
The incubation period of a disease is the time between when an individual contracts an infecting organism and when the first symptoms appear. With SARS, it took an average of 10 days for someone to start having symptoms. With Ebola, the range is anywhere between 2 and 21 days. Most people start showing symptoms of Ebola between 2 and 5 days.
Think about how far you can travel in 48 hours. Anywhere in the world. Do you think it’s likely that you’ll catch someone with symptoms at the airport? Spending the majority of resources at the point of entry might be a futile effort.
The reason why these measures don’t work very well is because we are trying to find a needle in a haystack.
Here is the challenge. We are trying to find a few people who might have Ebola among the huge number of people who are travelling by air or other means. How big is this number? In Canada, in July this year, there were 2.6 million passengers arriving across the country on two of its main carriers, Air Canada and Westjet. In other words, a very large number of people month after month come to Canada, not to mention the USA or European or Asian countries. At Canada’s largest airport, Toronto’s Pearson Airport, there are approximately 86,000 passengers in and out of this airport every day. Finding that one infected person among all these people is the proverbial “needle in a haystack.”
Public health officials will say that the positive predictive value in a low prevalence population is almost zero. In other words, the chance that you will actually find a true positive case of Ebola in a large population which generally doesn’t have Ebola is quite low. This seems pretty intuitive. In Canada, during the 2003 SARS outbreak, we estimated that about 1 in 1.2 million passengers actually had SARS. Therefore, we had a lot of false positives because the probability of finding that single person out of the total was already quite low. This is why the screening methods such questionnaires and thermal scanners generally don’t work.
So what is the best way to find and handle a serious infectious disease?
Screening At Hospitals
Most people will go to their closest hospital when they start to get really sick. The most important screening process takes place in the emergency room where anyone must be questioned regarding not just their travel history, but also their family’s travel history. Hospital staff need to be aware of current outbreaks throughout the world and how to respond when they suspect an individual might be infected. This single point of contact is of the utmost importance and where the majority of our time and resources should be spent.
So what do you think? Do you think it is realistic to stop an infectious disease at the border? Should we spend our time and money at points of entry? Let us know in the comments below.