7 October 2013
Unwelcome Guests: Human Diseases
Professor Chris Whitty
There has always been a tendency to assume that infections can be stopped at national borders. This perception is long-standing, natural and almost always wrong. When Shakespeare wrote of Britain as 'this fortress built by nature for herself against infection and the hand of war' he may have been pandering to a popular prejudice. He was well aware this was incorrect; there had been multiple major imported infectious epidemics in his era and the previous centuries, including plague and syphilis, both of which appear regularly in his writings, and both spread rapidly and with deadly effect. Estimates of plague mortality in Europe vary from 30 to 60% of the population. Syphilis, a disease which first appeared in Europe in 1497 was well established in the UK within 10 years with a much higher mortality than in later centuries. The reality is that wherever there is trade, travel or war disease will follow. Foreigners are often blamed for infectious outbreaks, sometimes reflecting the prejudices of the time. Syphilis was variously referred to as the French pox, the Spanish pox and the Neapolitan disease. More recently most people will be used to German measles, Spanish 'flu and Mexican pig 'flu.
Influenza is a good example is of the extraordinary destructive power epidemics can sometimes have, and the remarkable speed with which they will travel. Influenza killed more people in the 20th-century that conflict deaths from all wars combined- 22 million in the 1918-22 pandemic alone. Even in those days of more restricted travel it took the epidemic in America less than a month to sweep across the entire country. The most recent major infection to emerge, HIV, went from a rare disease to up to 40% of the population infected in some countries in Africa over less than two decades. The complexity, speed and volume of travel globally is steadily increasing. Given that, and a rising global population, it would seem logical to assume that the risk of major imported epidemics is bound to be increasing.
Infectious diseases often cause panic and attention which is wholly disproportionate to the real risk. Consider the 2001 anthrax scare in the United States, a disease which cannot be practical purposes be transmitted from person to person, which infected 17 people, killed 5, and closed down the government. Around 650 people die a week on the roads in the USA. With new diseases this is more rational because the risk is uncertain. SARS, which spread rapidly between November 2002 and July 2003 eventually caused less than 10,000 cases and less than 800 deaths globally (compared to around 100,000 smoking-related deaths in the UK a year). Around $40 billion was why two of the global economy with Southeast Asia and Canada particularly badly hit. New variant CJD from BSE is a recent UK example. In both cases because of the uncertainty when the epidemic emerged, vigorous (arguably Draconian) public health measures which had to be taken given the lack of knowledge of the possible outcomes have profound impacts on segments of the economy.
The risk of catching infections, particularly in areas of policy or which loosely described as tropical, is not trivial, and some of these are imported. A large proportion of these however have essentially no risk of being transmitted on in the UK either because they have to be transmitted by vectors such as mosquitoes or biting flies which do not live in the UK, or because they were caught from eating exotic foods or using preparation methods which do not occur here. Examples include malaria, dengue, sleeping sickness, many tapeworms and flukes of the gut, the liver the lungs and the bladder. In biological terms the person who catches them becomes a dead-end host as soon as they touch down in London or Manchester airports.
There are however many things which transmit infections which citizens of the UK are liable to do pretty well as much those of any other country including breathing, eating, drinking, touching and having sex. Infections imported by these routes should be possible to transmit onward through the population. The basic mathematics of transmission are relatively easy to understand and in epidemiology can be summarised by R0. If 1 person gives it on average to 1 person, who gives it to 1 person the infection is stable in the population and R0 = 1. If one person gives it to 2 people, who then give it to 4 people R0= 2 and the diseases is increasing in the population. If it is less than 1 the disease is dying out.
The reality for the great majority of infectious diseases in the UK is that they were very common once, and have steadily been dying out over the last two centuries as our country developed. Multiple factors, each of which has a small but significant contribution, add up to making the society we live in much less open to infectious diseases than that even our grandparents were born to. This lecture will illustrate this with a number of infections including tuberculosis, measles, and diphtheria. The correlation between the wealth of the country and the probability of dying is extraordinary, and initial improvement in mortality is in large part mediated by reductions in infectious diseases. Things that have contributed to hardening the UK against infectious epidemics include better access to safe water and sanitation, better housing, less crowding, better and more varied nutrition. More recently vaccines have reduced still further the risk of many viral and some bacterial infections, and antibiotics reduce the risk of mortality, and onward transmission, of bacterial diseases. Each step is incremental but the overall effect is dramatic. Compounding this is the herd effect; if 80% of the people around me protected from infection, then my risk of infection is massively reduced. The chance of a waterborne infectious epidemic such as cholera again infecting the population in the UK is essentially zero. Globally the World Health Organisation also considers the chance of a plague pandemic to be zero. Several diseases are reducing globally partly because of socio-economic factors but also vigorous attempts to control them, and in a few cases such as smallpox and polio, to eradicate them for all time.
The result of all these improvements is that infectious diseases causing mortality in UK now only a significant risk at the extremes of life. Once through the early part of childhood, the really big risks of dying from infection in this country are in late old age, and that will not be an exotic infection but usually a bacteria you carrying on or in you now. The 1918 flu pandemic and HIV, for different reasons, were the exception where young adults were at significant risk.
Within the UK where is the greatest risk of infection being imported likely come from? One obvious but misleading answer is those in migrant communities with close links to overseas countries. These are highly concentrated, depending on which migrant group we are talking about- for example the great majority of African migrants live in London. The risk of this however easily greatly overstated; with the exception of a few chronic diseases (tuberculosis, HIV and hepatitis) the increased risk from this group compared to the general population lasts a matter of two or three weeks after arrival. The less obvious but more accurate answer is wherever humans and animals interact. Both wholly new-to-human diseases such as HIV (chimpanzees) and SARS (probably bats), and dangerous strains of old enemies such as influenza (pigs, chickens) jump the species barrier into humans. Occasionally this will occur in the UK as happened with BSE/nvCJD. Statistically it is far more likely to happen elsewhere and either be imported in the animals (unlikely, but if so it will first appear in agricultural areas) or in humans. Predicting where this will happen is virtually impossible, although how closely humans live with animals, and the density of animal populations have a bearing. As a result of development a global transformation of agriculture and the rapid rate of urbanisation, human animal interactions are likely to decrease for most species. Humans who have animal interactions are in many countries also substantially better nourished making them less susceptible to any infection. The risk of new diseases, and new strains of old diseases to which humans have limited immunity causing epidemic will however never go away. We can confidently predict there will be more during the lifetime of members of this audience, but reliably predicting when, and from where they will come is impossible.
New diseases are dramatic but infrequent; the steady erosion of the effectiveness of antibiotics and antimi crobials is arguably a much greater risk. This has been highlighted as a major threat to this country recently by the Chief Medical Officer, Dame Sally Davies. There are now multiply antibiotic resistant bacteria, drug resistant tuberculosis, anti-viral resistant HIV and influenza, and drug resistant malaria around the world. The greatest avoidable cause is overuse or incorrect use of antibiotics by doctors, patients, vets and farmers. There is however a significant risk of importing drug-resistant disease from other countries. An example is drug resistant tuberculosis, where the rates are highest in parts of the former Soviet Union and Eastern Europe. The probability of drug resistance spreading depends on a number of factors, including whether acquiring drug resistance has a survival disadvantage to the organism. Antimicrobial resistance is now rightly seen as a global threat, but for many of the key drug resistant organisms defending the U.K.'s borders against infections from outside is much less important (and realistic) than practical measures like better hand washing in hospitals. The UK has reversed the rising tide of MRSA for example with this simple approach. The threat needs to be taken seriously, but also not got out of perspective; in the developed United Kingdom of 2013 for the reasons outlined earlier the rapid spread of disease in the general relation is not likely in the way that it was previous generations.
It remains the case that the sinks of infection, the harbours of exotic infections and the places where new infectious diseases and drug resistance will emerge, potentially to threaten the UK, are mainly in areas of poverty. Additionally, statistically new infections will emerge were the greatest populations are found; now and in the future these are not in Europe and North America. When it comes to infections no man is an island, and the best defence for us as individuals and as a country is the development, sanitation, housing and improved medical services of others. The reason to undertake international development is to alleviate suffering among the poorest in the world, but in the area infectious diseases a welcome secondary gain is that it will diminish the risk to the rest of the world. The United Kingdom has a very proud history of tackling the great infectious diseases of poverty not only at home but in Africa and Asia. Historical achievements such as the second Nobel Prize for medicine going to Sir Ronald Ross for his work on malaria, or Florence Nightingale's work on sanitation in India, are matched by current work for example by the great schools of tropical medicine in London, Liverpool and Oxford. The development currently being seen in Africa, Asia and Latin America is heartening and in places dramatic. To quote a recent Economist headline "Africa is experiencing some of the biggest falls in childhood as it ever seen, anywhere." Behind this lies the steady retreat by major infectious diseases, and this will benefit everyone.
This Gresham lecture has painted a broadly optimistic picture of the risks of imported infections to this country and indeed other countries. Infections have always been imported, and some have been devastating. Travellers increasing and drug resistance is a significant threat. Better, water, sanitation, housing, nutrition and wealth mean however that the risks of infectious diseases are much smaller than for previous generations. Our society is hardened against infections in a way unforeseeable 200 or even 70 years ago. Preventive and curative medicine reduces the risks further and currently most infectious deaths are at the extremes of age. Many imported infections cannot be transmitted, but new infections or strains of infection which jump from animals will continue, and sometimes will be imported into the UK. Development in the rest of the world reduces the risk to us when it comes to infectious disease, both at an individual and the country level; what is good for my neighbour is good for me.
© Professor Chris Whitty 2013