By Raina MacIntyre
March 2nd 2017
Every day when you check the news, you read of a new epidemic. An epidemic of ice, diabetes, obesity, antimicicrobial resistance or some other pressing problem. I searched the news today and came across the following new "epidemics": tooth decay, prescription pain pills, carer abuse, bomb threats and distracted driving. Journalistic misuse of the term is understandable, but even health professionals, researchers and public health experts regularly misuse the term "epidemic". The term conveys an immediacy, emergency and the need for urgent action, hence the desire of every party with a vested interest to use the term to describe their particular issue.
So what exactly is an epidemic? It is an outbreak of disease that attacks many people at about the same time and may spread through one or several communities. It is defined by rate of growth of the epidemic curve (see the red curve in my illustration, which is a typical epidemic curve), and usually has immediate impact on health systems and requires immediate surge capacity. An outbreak refers to a smaller scale event, and epidemic to a larger scale and a pandemic to the global spread of an infection meeting this definition. The transition from outbreak to epidemic is arbitrary and has no precise numeric cut off point.
The two common epidemic origins are point source and propagated. A point source outbreak is a single source of contamination affecting many people around the same time – food borne outbreaks are an example, where many people may consume a contaminated food item and become ill at around the same time. A propagated epidemic spreads from person to person, and this is usually caused by a contagious infection. There is also a mathematical definition of an epidemic, which applies to propagated outbreaks and is related to the reproductive number, R0. R0 is the number of secondary cases resulting from a first case of infection in a population that has no immunity to the infection. For example, if I have measles and I infect 15 unvaccinated people in a room, then the R0 of measles is 15. Each of those people in turn will infect on average another 15 people, giving rise to a rapid rise in cases and an epidemic. Whilst we do see measles epidemics in Australia, we don’t see this scale of measles epidemics in Australia because of high vaccination rates against measles. R=1 is the epidemic threshold, and any R value >1 creates the necessary conditions for an epidemic. From a disease control point of view we use interventions (such as vaccines, drugs, quarantine, PPE) to reduce the R below 1.
So, a large number of cases of diseases does not equal an epidemic. There must be a rapid rise in case numbers over weeks or days, not months or years. Diseases such as diabetes, and malaria are endemic diseases. Endemic refers to a disease that exists permanently in a particular region or population. Cases may exist at low or high numbers, and there may even be changes in trends over years – a rising or declining incidence. Yet these are not epidemics. What about antimicrobial resistance, which has been called a public health emergency and an epidemic at various times? The slow rise in resistance against antibiotics over years and decades, driven by simple Darwinian evolution and selective pressure on bacteria through antibiotic usage, is an endemic problem. Yes, it is serious, but it is endemic. The situation in which AMR can complicate an epidemic is when an epidemic disease which can be treated by a drug, develops resistance. So, for example, if a pandemic of influenza occurs and that strain of influenza is resistant to available antiviral drugs, then the impact of the pandemic will be affected by the inability to treat successfully with the drug. In a pandemic, many people die of bacterial secondary infections, so this too may be a problem if the common bacterial infections that complicate influenza are resistant to antibiotics. There are few bacterial infections with true epidemic potential, and these include plague. The current landscape is also complicated by quantum advances in science, more accessible science and the abilty to genetically engineer organisms - it is possible to alter the characteristics of viruses and bacteria in a lab, including conferring AMR and enhanced transmissibility potential. Scientists can also create a designer virus in a lab with advances in synthetic biology. In 2016 the director of National Intelligence in the US called the new gene editing tool CRISPR Cas9 a weapon of mass destruction, and more recently Bill Gates has highlighted the risk of bioterrorism. These developments influence the global risk of epidemics and pandemics of an altered nature.
So, moving back to patterns of disease, the other major pattern of disease is sporadic – where we see few cases of disease now and then, that do not meet definitions of endemic or epidemic. Typically these are caused by spread of zoonotic infections from animals to humans. An example is human cases of H5N1 avian influenza.
The illustration I have provided shows the difference in disease patterns – in red is an epidemic pattern, with a rapid rise, peak and decline in cases. The green is an endemic patrtern with high case numbers, but relatively little change over a period of weeks. The blue line is a sporadic pattern - low numbers and occasional cases, fitting neither the epidemic or endemic patterns.
An endemic disease may show a trend of rising incidence over years, but this is not an epidemic. But it just doesn't sound compelling in a headline, does it, to say "slow creep in cases of disease X over years" or "Disease X is endemic". The term endemic can also be correctly applied to occurrences other than diseases, because it does not have the requirement for a rapid rise in time - only that the occurrence is present permanently in a population. Whilst the term "epidemic" is used much more frequently, a search of the news today did identify some headlines using the term endemic - racial profiling in Germany, killings in Kaduna, and kudos to the South China Morning Post for correctly using the term in a disease context to describe a risk associated with Zika virus.
So, next time you read a headline about an epidemic, ask yourself what pattern it really is.