D. A. HENDERSON

D.A. Henderson, M.D., is a professor of medicine andpublic health at the University of Pittsburgh. He isresident scholar at the Center for Biosecurity, an adviserto the U.S. Department of Health and Human Serviceson public health emergency preparedness, and formerlychief medical officer for smallpox eradication of the WorldHealth Organization (WHO).

During the past 20 years, a surprising number ofnew infectious diseases have been discovered,some afflicting only one or a few countries,while AIDS has spread inexorably to eventually become aglobal epidemic and the fourth leading cause of death inthe world. Others have also appeared—in all, at least 30new diseases. Many more may be anticipated because overrecent decades, there have been dramatic demographic,technological, and social changes that have markedlyaltered the potential for disease transmission, and thesechanges are progressing exponentially.Of special concern today is the global threat ofpandemic avian influenza, a new type of influenzavirus that poses a serious threat to every country.

Theavian influenza threat clearly shows how advances inglobal technology may help spread diseases, but italso demonstrates how global cooperation may lead toeffective countermeasures. Influenza outbreaks normallyrecur every year throughout the world. Although theseare capable of causing severe illness and death in theelderly and those with chronic lung or heart disease,most persons experience little more than fever andrespiratory symptoms for a week or so. About every 30years, however, a new and different strain of influenza hasemerged and spread across the world, causing widespreadepidemics, known collectively as a pandemic.

THE BIRD FLU THREAT
One of the most serious pandemics occurred in 1918,when a new type of influenza virus arose that proved tobe much more lethal than ever before. It resulted in thedeaths of at least 50 million persons worldwide.

Concernsabout the possibility of an equally serious pandemicwere reawakened in 1997 when a new, more threateningstrain of influenza (now identified as the H5N1 strainof avian influenza) was discovered in Hong Kong. It wasexceptionally lethal for poultry, especially chickens, but it also claimed 18 human victims,of whom six died. Never beforehad an influenza strain causedsuch a high human deathrate. The cases all occurredamong those working with sickchickens.Fortunately, at that time,the disease did not spreadfrom human to human. Public health authorities quicklydestroyed millions of chickens, and the virus seemedto have disappeared. Unfortunately, six years later itreemerged once again in chickens and soon began tospread throughout Southeast Asia. Tens of millions ofchickens have died from the disease or have been killed inattempts to control the further spread of the virus. Close to150 human cases have occurred, almost half of which werefatal.

Almost all patients had been in close contact with thesick birds or provided health care to one of the patients.Wild fowl arenow infected and, asa result of migration,the virus has spreadto western Asia,Eastern Europe, andAfrica. As the diseasecontinues to spread,there is justifiableconcern that, at anytime, the virus mightchange its characterand begin to spreadfrom person toperson. Because oftoday’s volume of airpassenger traffic, it iscertain that it wouldspread worldwidewithin weeks.A vaccine will beneeded to protect humans against the disease. However,the vaccine, to be effective, must closely resemble the virusthat is capable of spreading from person to person, thoughthat viral strain does not yet exist, as far as scientistsknow. Thus, an intensive international effort involvinglaboratories, public health staff, and industry is underway to obtain, as quickly as possible, the virus as soonas it begins to spread from person to person, and to usenew approaches in vaccine production that will permitlarge quantities of vaccine to beproduced rapidly.

THE NEED FOR GLOBAL COOPERATION
The influenza threat aptlyillustrates the need for greaterinternational cooperationto discover and counter disease threats, wherever theymay occur. Such cooperation is needed more urgentlytoday than at any time in history. In the microbialworld, countless species are continually multiplying atastronomical rates, each species mutating, adapting, andchanging to assure its own survival. Inevitably, from timeto time, microbes with different characteristics arise, somehighly lethal to humans and some with a capacity to growand to spread readily. In agrarian societies with scatteredpopulations and small towns or villages, new agents hadmuch less of a chance of being transmitted from person toperson, and soon they would die out.

Even if significantspread of a new disease did occur in one area or country,further dissemination often was curtailed because ofthe limitations of travel. The ease with which we travelthe globe may spread the most threatening diseases, butadvances in communications may also serve to facilitatecooperation to find cures—a global health connection.Today, cases and outbreaks of disease, whatever theircause and wherever they may occur, pose a threat to thehealth of people throughout the world. No major city inthe world is more than 36 hours distant from any other.In 2003, some 642 million international air travelersdisembarked at 750 different airports in 135 countries.Once-common border controls and inspections haveproved to be of no value in the prevention of disease,as was clearly shown during the 2003 SARS epidemic.More than 35 million passengers were screened with theintent of quarantining those with fever.

No cases werefound. If travelers had been infected, they were mostlikely in the silent, incubation phase of illness and couldnot have been identified, whatever screening measureshad been employed. We are now experiencing populationmovement of a magnitude and speed such as has neverbefore been witnessed.The likelihood of new microbial agents gaininga foothold is greatly enhanced by the rapid growth ofurban populations. As recently as 50 years ago, there wereonly two cities with populations of more than 7 millionpersons (New York and London); only 20 percent of the world’s population lived inurban areas. Today, there are 30cities with populations of morethan 7 million, and seven ofthese, in fact, have populationslarger than 15 million.Manysuch cities are in tropicaland subtropical areas wherecrowding, malnutrition, poorsanitation, and environmentalpollution are predominantfeatures. This is fertile soil indeed for establishing a newdisease.Another major influence in disease disseminationis the industrialization and internationalization of foodsupplies. Only a few decades ago, most foodstuffs weregrown locally on small farms and preserved or preparedfor commercial use in small establishments, few of whichengaged in international trade. If contamination occurredat any point, few persons were affected. With larger-scalefood production and processing and the potential forrefrigerated and air shipment of food, contaminationat any point in the food production chain can result inmassive epidemics extending across many countries. Asmall illustration of this was a severe diarrhea epidemiccaused by an organism called shigellosis in August2004.

It resulted from contamination in a United Statesairline food preparation kitchen.

In all, 241 cases werespecifically identified, but it was estimated that 9,000cases had actually occurred on 219 different flights goingto 24 states and four foreign countries.A seldom considered but major factor facilitating thespread of disease is the extensive proliferation of hospitals,especially in countries and areas where economic resourcesare taxed and professionally trained personnel are sparse.Many such hospitals have no provision for the isolationof contagious patients and little or no equipment topermit adequate sterilization of needles and syringes aswell as surgical instruments. Blood-borne diseases mayresult, and, indeed, this factor has been an importantcontributor to the spread of AIDS in some nations.

At thesame time, it is customary in such medical care settingsfor large numbers of family and friends from villages andtowns scattered over a very wide area to visit the patient.In consequence, sudden explosive epidemics of diseaseextending over a wide area are not uncommon. Recentexperience has shown thathospitals have been the primarysite for epidemic transmissionof measles and hemorrhagicdiseases such as those caused bythe Lassa, Ebola, and Marburgviruses.In this global age, thehealth of every human being onthe planet has become relevantto the health of every other.We have yet to fully grasp the implications of this fact,although both AIDS and avian influenza are provingto be importantin communicatingthis message. Thereis a need to attackinfectious diseaseproblems whereverthey occur.Anepidemic today inthe most remoteareas of Africaor the Americas,for example, cantomorrow result incases and perhapsoutbreaks almostanywhere else in theworld.

In practicalterms, the May2005 adoption ofupdated WHOInternational Health Regulations is a positive steptoward undertaking the shared research and developmentnecessary to deal with disease problems wherever theyoccur, and in fashioning effective international networksfor research and education such that important findingsand observations can be more rapidly and effectivelytransmitted and applied. «±