Should the threat of foot-and-mouth disease be met by the destruction of all animals that might have been exposed to the virus

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LIFE SCIENCE

Should the threat of foot-and-mouth disease be met by the destruction of all animals that might have been exposed to the virus?

Viewpoint: Yes, all animals that might have been exposed to the foot-and-mouth disease virus should be destroyed.

Viewpoint: No, animals suspected of being exposed to the foot-and-mouth disease virus should not be destroyed because the threat of the disease to humans is minimal.

Foot-and-mouth disease (FMD) is a highly contagious viral disease that affects cloven-hoofed livestock animals like pigs, cattle, sheep, and goats. The foot-and-mouth disease virus (FMDV) is a member of the picornavirus family, which includes important human pathogens, such as poliovirus, hepatitis A virus, and rhinovirus. These viruses are characterized by a small RNA genome. Usually, the virus is transmitted by contaminated passive vectors, such as people and farming equipment and implements that have come in contact with infected animals. Because the virus is found in the upper respiratory tract of infected animals, aerosols may also play a role in the transmission of the disease. The movement of livestock, or livestock products, however, seems to be the most important factor in the management and control of the disease.

During the nineteenth century microbiologists used the term "virus" in a general sense to describe pathogenic agents with infectious properties. Eventually, the term was used in reference to submicroscopic infectious agents. Because the agents associated with certain diseases could be separated from visible microorganisms by special filters, they were often called "filterable, invisible viruses." Several important diseases of humans and animals were thought to be caused by these mysterious "filterable" and "invisible" microbes. Studies of a disease of the tobacco plant led to the hypothesis that certain disease were caused by entities that needed the living tissues of their hosts in order to reproduce.

Foot-and-mouth disease provided the first experimental demonstration of a filterable viral disease of animals. Friedrich Loeffler (1862-1915) and Paul Frosch (1860-1928) were able to culture bacteria from the fluid in lesions in the mouths and udders of sick animals. In 1897 they discovered that the filtered fluid could transmit the disease to experimental animals and that experimentally infected animals were capable of transmitting the disease to other animals. These experiments suggested that a living agent, rather than a toxin, must have been in the filtrate.

FMD was introduced into the Americas in 1870. FMD was reported in certain parts of the United States, Argentina, Chile, Uruguay, and Brazil. By the beginning of the twentieth century FMD had spread to the rest of Brazil, Bolivia, Paraguay, and Peru. During the 1950s the disease was introduced into Venezuela, Colombia, and Ecuador. Since an outbreak of FMD in 1929, the United States has not had a confirmed case. The last outbreak in Canada took place in 1952. Outbreaks of FMD occurred in Mexico in 1947, but no cases have been reported since 1954. The establishment of the Pan-American Foot-and-Mouth Disease Center (PANAFTOSA) in 1951 has been an important factor in the battle against FMD in the Americas. PANAFTOSA developed a plan for FMD eradication, which included vaccination, and established a highly successful surveillance system. Epidemiologists believe that this model could be used to control and eradicate other diseases as well.

The magnitude of the 2001 FMD epidemic was quite different from previous modern outbreaks, including an outbreak in Britain in 1967 and another in 1981. Many authorities thought that the disease had been effectively controlled, or perhaps even eliminated, by the end of the twentieth century. The outbreak of 2001 began in February with reports of FMD in pigs at slaughterhouses in England. The first outbreak was confirmed on February 20, and livestock movement restrictions were imposed on February 23. Despite harsh measures instituted to stop the spread of the virus, the disease quickly spread to many locations in Britain and to many European nations. Millions of cattle, sheep, and pigs were destroyed and the export of British meat, milk, and livestock was banned. These expensive measures devastated the British farming industry, which was already depressed because of damage caused by outbreaks of swine fever and international fear of mad cow disease. Other measures affected people who were not directly involved in agriculture, as the government banned parades, fox-hunting, horse racing, visits to farms, fishing in streams, and hiking through the countryside, forests, and bird sanctuaries. Whether or not the wholesale destruction of animals that might have been exposed to the virus is warranted remains a subject of active debate.

Although vaccines for FMD were available before the 2001 outbreak, European governments have opposed routine vaccination because vaccinated animals test positive for FMD antibodies. Vaccination for FMD, therefore, makes it impossible to qualify for official disease-free status, which results in trade restrictions. Some of the experimental vaccines developed in the 1990s might allow the authorities to distinguish between vaccinated and infected animals.

The danger that FMDV poses to humans has been the subject of considerable debate and uncertainty. During the 2001 outbreak in Europe many reports assured the public that FMD is generally not regarded as a threat to humans who might consume meat or pasteurized milk from affected animals, although it was conceded that humans might contract the disease through close contact with infected animals. Some epidemiologists argued that the threat to humans must be evaluated more cautiously, even though the occurrence of the disease in humans is clearly quite rare. There is some evidence that FMD could be spread to humans through infected milk and dairy products. Proven cases of FMD in humans have been reported in Europe, Africa, and South America, and virologists have isolated and typed the virus in more than 40 human cases.

Scientists are developing novel vaccines and more rapid diagnostic tests for FMD. New diagnostic tests could cut the waiting time for the diagnosis of FMD from two days to 90 minutes. Better diagnostic tests might limit the need for destruction of whole herds of animals. New vaccines might make it possible to distinguish between infected animals and vaccinated animals. Reacting to the serious and widely disseminated outbreak of 2001, epidemiologists emphasize the importance of recognizing the fact that the human residents of our "global village" must understand the dynamics of infectious diseases in the twenty-first century "global farm."

—LOIS N. MAGNER

Viewpoint: Yes, all animals that might have been exposed to the foot-and-mouth disease virus should be destroyed.

Foot-and-mouth disease is so contagious and such a serious threat that all animals believed to have been exposed to the virus should be destroyed. It was the only way to eradicate the 2001 outbreak in the United Kingdom (UK), which had spread to France, Ireland, and the Netherlands before being stopped.

Foot-and-mouth disease (FMD), a highly contagious illness that has been likened to a biblical plague, is devastating to livestock herds.The disease gets its name from its symptoms: the virus causes blisters to form on the animals' tongues, lips, and nostrils and between their hoofs. The animals go lame, experience a tremendous amount of pain, stop eating, produce less milk, and are unable to fatten. Most could survive the disease but would be left debilitated and could not thrive. They would remain contagious and be an economic drain to their owners.

Outbreak in the UK

In February 2001 the worst outbreak in history devastated the livestock industry in the UK and threatened the livelihood of farmers in Europe. Outbreaks were reported in 300 British locations and in France, Ireland, and the Netherlands. Outbreaks also occurred on the continents of South America, Africa, and Asia. To prevent the further spread of FMD and in an attempt to eradicate the disease and preserve its meat export business, over six million cattle and sheep were destroyed in the UK at a cost of hundreds of millions of dollars.

Because the disease is so contagious, authorities in the UK had no viable alternative but to destroy not only infected animals, but also those believed to have been infected. The authorities had no real choice other than killing all suspected animals, because the FMD virus can be transmitted from animal to animal via contaminated humans or their clothing, through contaminated equipment or feed, or even blown through the wind. Nearly any cloven foot animal—cattle, pigs, sheep, and goats as well as wild animals including deer, antelopes, llamas, camels, giraffes, and elephants, but not horses—exposed to FMD will develop the infection. Once an animal is infected, the virus reproduces and spreads to the animal's mouth, feet, stomach, and teats as well as to the heart muscle. The incubation period can last for up to three weeks, a time when the animal is highly contagious yet demonstrates no symptoms.

How do you fight an invisible viral enemy that might float in the wind or be carried on someone's shoes? After less intensive methods failed to slow the disease, the UK instituted draconian methods, an aggressive policy of control known variously as "ring culling" or "slaughter on suspicion." All cloven hoof animals (except horses) within 3 km (1.9 mi) of a diagnosed case of the disease were sacrificed, and their carcasses were buried or burned.

It was a painful decision for the stakeholders involved, yet even leading farmers' organizations supported the government's mandate. At a March 2001 press conference announcing his support for the government policy, Ben Gill, president of the UK's National Farmers, told news reporters, "The pain and anguish members … are going through now with the announcement of all livestock within three kilometers of an outbreak are going to be slaughtered is a death penalty for their businesses that is so devastatingly cruel it is beyond description. I am absolutely heartbroken for them, but … we have to get rid of it."

UK prime minister Tony Blair, who ultimately made the decision to rigorously cull the animals, declared, "There is no alternative to this policy." Nick Brown, the UK minister for agriculture, stated repeatedly in the House of Commons that the government's policy was based on "The latest scientific and veterinary advice. Killing healthy sheep and pigs in a three-kilometer zone around infected sites in the worst affected areas is vital. This is necessary for the whole of the livestock sector. It is not being done for any other reason than to follow the explicit veterinary advice on what is necessary to control the disease."

Vaccination

Some critics of the slaughter on suspicion policy have argued that vaccination against the disease might be a viable alternate. But critics of the vaccination approach point out that vaccination cannot alone completely eliminate the disease. Furthermore, because the vaccine only confers about six months of immunity, an immense mechanism would have had to be established for continual repeat vaccinations. Those against the widespread use of vaccines cautioned that such intensive use of antiviral serums would, in turn, pose an increased risk of the virus escaping from laboratories. They also warned of the danger of further FMD spread if the vaccine was administered improperly.

"Moves to vaccination would be a diversion of resources and could mean the disease was not completely stamped out," stated Dr. Alex Donaldson of the Institute for Animal Health. A computerized study later backed up Donaldson's opinion. Ferguson et al. did a computerized mathematical model of the transmission of FMD that simulated the impact of various control strategies. Writing in Science, they endorsed the preemptive slaughter of the animals, saying, "Culling is predicted to be more effective than vaccination."

Indeed, objective evidence and recent experience indicates that the vaccine approach falls short, whereas "slaughter on suspicion," although painful, works. For example, both Japan and South Korea suffered FMD epidemics in the year before the British outbreak.Japan instituted and stuck with a vigorous culling program and regained its FMD disease-free status by 2001. South Korea, however, chose to vaccinate. By mid-2001 South Korea still was not disease free, according to Donaldson.

The experience of Greece, in contrast, supports vigorous destruction of all animals believed to be exposed. Greece, the last European Union country to suffer an FMD outbreak before the UK was hit, chose not to vaccinate when FMD struck there in 2000. By 2001, after a vigorous slaughter on suspicion program, Greece was certified disease free.

That is the ultimate goal of the ring culling program. The United States and other countries do not allow the import of meat or animals from countries not certified as disease free. Billions of dollars are at stake.

"FMD is the most infectious animal disease and its eradication is of crucial importance, … not only for the farming industry in terms of resumption of normal trade, including experts, but for all parts of the rural economy," emphasized UK Secretary of State Margaret Beckett in a May 2001 notice to the public calling for continued cooperation to prevent the spread of the disease.

Possible Threat to Humans

Animal FMD should not be confused with hand, foot, and mouth disease, a human infection fairly common in children caused by an entirely different virus. A telling argument for the slaughter on suspicion program, however, is the fact that animal FMD has jumped the species barrier and infected humans. Although health authorities maintain that FMD causes no real danger to humans, a review of the medical literature conducted by Paul Greger, a physician in Jamaica Plain, Massachusetts, revealed over 400 reported cases of FMD in humans during the twentieth century. A 1923 report told of one patient who developed a serious heart infection, but the vast majority of the cases were considered mild. Most people recovered within two weeks. However, the results of the disease must have been uncomfortable. There were reports that the "skin of [the] soles peeled off like sandals, in one piece, in some of those people," Greger said.

Conclusion

In summary, all animals believed to have been exposed to the virus should be destroyed. The policy puts the animals out of their misery and has the highest odds of eradicating even large numbers of localized FMD outbreaks. In the long run, the policy may also be protective of human health, because the disease has crossed the species barrier.

—MAURY M. BREECHER

Viewpoint: No, animals suspected of being exposed to the foot-and-mouth disease virus should not be destroyed because the threat of the disease to humans is minimal.

The 2001 epidemic of foot-and-mouth disease (FMD) in the United Kingdom (UK) began in late February. By early July, nearly 3.5 million animals had been destroyed in an effort to halt the disease: 2.8 million sheep, 548,000 cattle, 129,000 pigs, and 2,000 goats, according to statistics from the UK Department for Environment, Food and Rural Affairs (DEFRA).Few scientists dispute that the limited slaughter of infected and exposed animals is a vital first line of defense against the disease. For many, however, destruction on this massive scale is deeply upsetting. More than that, it may be ineffective at stemming widespread epidemics. Over four months after the mass slaughter started in the UK, for example, new confirmed cases were still cropping up at an average rate of three per day. Large-scale slaughter also may be economically counterproductive. By some estimates, the economic benefits of the UK policy may be far exceeded by the costs to the government for compensating farmers and disposing of carcasses and the costs to society at large for lost revenues from farming, farm-related businesses, and tourism in the affected regions. In addition, the disposal of such a large number of carcasses presents its own problems. Burning them can release highly toxic compounds called dioxins into the air, and burying them could lead to the pollution of water supplies. Finally, and perhaps most importantly, the disturbing specter of animals being killed by the tens of thousands and burned in massive pyres may be unnecessary. Today, many scientists believe a combination of limited slaughter and mass vaccination is a better way of fighting FMD.

The Culling Fields

Some animal lovers might question whether any animals at all need to be destroyed to combat a disease that is itself rarely fatal. FMD usually runs its course in two or three weeks, and most animals recover. Even after recovery, though, the animals may grow more slowly, making it more expensive to bring them to market for meat. In dairy cattle, the ability to produce milk may be markedly decreased. Because the disease is highly contagious, infecting nearly 100% of exposed animals, the result of an uncontrolled outbreak could be a devastating loss of valuable meat and milk. As a result, most scientists concede that it is necessary to destroy infected animals along with other livestock on the same farm, a process known as stamping out. The real debate is over whether to move beyond the farm to destroy apparently healthy animals from nearby areas that may have been exposed to the FMD virus—but then again, may not.

In the UK, the slaughter of a selected group of livestock, known as culling, was applied to well animals on the same farm as infected ones as well as those on neighboring farms. In some of the hardest hit areas, such as the county of Cumbria in northwest England, the cull was extended to all livestock within a 1.9-mi (3-km) radius of infected farms. Proponents say a wider cull was needed in these hot spots because the FMD virus travels so easily. It can be spread not only directly by contact with an infected animal, but also indirectly by contact with people, clothing, vehicles, equipment, dogs, and vermin that happen to be carrying the virus. This explains why movement restrictions also were imposed around infected farms. In addition, though, the virus can spread through the air. Critics say this is one reason that the 1.9-mi cull was doomed to failure. The FMD virus has been known to travel up to 37 mi (60 km) over land and as far as 186 mi (300 km) over water. Some of the longest airborne transmissions on record occurred under weather conditions very similar to those found during the 2001 UK epidemic.

Another problem with a slaughter-only policy is the sheer number of animals that must be killed in modern farming regions very densely populated with livestock. The time between the infection of one animal and its ability to infect other animals, called the generation interval, is quite short for FMD. Studies have shown that sheep with the type O strain of the disease, the type involved in the 2001 outbreak, are able to infect other sheep and pigs in as little as two days. For culling to work, therefore, it must be started immediately and finished quickly. Unfortunately, the size of the UK cull soon overwhelmed authorities, causing them to fall well behind the government's own timetable for slaughtering animals and disposing of the carcasses. Two months into the crisis, the nation's agriculture minister admitted that the goal of destroying livestock on neighboring farms within 48 hours still was being met only 70% of the time. This time lag was another important reason that the cull alone was unlikely to stop FMD.

Money Matters

It is no surprise, then, that the UK crisis soon turned into a catastrophe. The World Organization for Animal Health—also known as the OIE for its French name, Office International des Epizooties—is the international group charged with collecting data about FMD worldwide. It also decides which countries can claim its coveted FMD-free status. Most other countries will not import animals or meat from places without this designation. By early July 2001 more than 1,800 cases of FMD had been confirmed in the UK. The disease also had spread elsewhere in Europe, with a small number of cases reported in France, Ireland, and the Netherlands. As a result, these four countries all had their FMD-free status suspended, temporarily shutting off exports.

The potential economic losses from such a situation are staggering. Thus, once FMD occurs, the strategy used to get it under control is dictated as much by economic concerns as by animal health needs. The main alternative to mass culling is mass vaccination combined with smaller scale slaughter. European countries have resisted this option, however, because standard tests for FMD cannot tell the difference between antibodies from a vaccinated animal and those from an infected one. To be safe, other countries also ban the import of animals and untreated products from places that vaccinate. The chief argument against large-scale vaccination, then, is the cost in additional lost exports. In the UK, the best estimate of the value of threatened exports, adjusting for predicted changes in the domestic market, is $423 million (£300 million). The cost of a vaccination program itself would run about $21 million (£15 million), bringing the total cost of a vaccination-based strategy to about $444 million (£315 million).

Yet the economic toll of mass culling was probably much higher, according to Peter Midmore, an agricultural economist at the University of Wales in Aberystwyth, who made these calculations. Much of the income in rural areas of the UK comes not from farming but from tourism. This tourist trade was sharply curtailed by the travel restrictions and bad press that went along with the mass cull. By Midmore's estimates, the cost of lost tourism alone amounted to $846 million (£600 million). Other costs, including payments to compensate farmers, expenses for carcass disposal, and uncompensated losses from farming and farm-related businesses, brought the total cost of mass culling to about $1.1 billion (£781 million)—more than double what a vaccination-based plan might have cost.

The Dioxin Dilemma

Clearly, mass culling is a far from perfect solution to FMD. It also creates a brand-new problem of its own: what to do with all those dead bodies. In the UK, many thousands of carcasses were burned in giant pyres. Residents were forced in some areas to live for weeks with the sight of smoke-filled skies and the stench of burning animals. Their greatest worry, however, was about the possible health effects of dangerous chemicals released into the air by the fires. Among these chemicals are dioxins, a group of highly toxic compounds that can increase the risk of cancer and cause a severe skin disease called chloracne. Some evidence indicates that dioxins may also affect reproductive health, weaken the immune response, and cause behavioral problems. Manufactured sources of dioxin in the air include fuel burning, waste incineration, the manufacture and use of certain herbicides, and the chlorine bleaching of pulp and paper. In the case of the FMD pyres, dioxins and other harmful chemicals were emitted by objects, such as wooden railway sleepers, coal, and old tires, used to light the fires. According to the UK government's own statistics, pyres lit during the first six weeks of the FMD crisis released 63 g of dioxins into the air—18% of the country's average annual emissions. It is still unclear whether this level of exposure to dioxins poses a public health threat.

Burial is another option for disposing of carcasses, but it carries its own pollution risks. The UK Environment Agency, the government agency that oversees environmental protection in England and Wales, says the main concern is the possibility that breakdown products of carcass decomposition might leach into groundwater over time. Potential pollutants include ammonia, chlorides, phosphates, fatty acids, and bacteria. Metal concentrations in the water might increase, and the taste and smell of the water might be affected. Once groundwater has become contaminated this way, it can be difficult or impossible to fix the problem. In addition, the carcasses of older cattle cannot be buried, because they might be infected with bovine spongiform encephalopathy (BSE), a fatal neurological condition in cattle commonly called "mad cow disease." BSE is thought to be caused by feeding cattle material derived from diseased sheep. People, in turn, seem to get a rare human brain condition, a form of Creutzfeldt-Jakob disease (CJD), from eating certain parts of meat from BSE-infected cows. Scientists say the danger of anyone getting CJD from drinking water contaminated by buried cattle carcasses is very low. For caution's sake, though, the UK government has prohibited the burial of cattle born before 1996, when a ban on risky material in cattle feed went into effect.

A third option for carcass disposal, and the one most favored by the Environment Agency, is processing the bodies in carefully monitored rendering plants. Scientists say rendering is likely to be very effective at destroying the FMD virus while causing less damage to the environment. During the 2001 outbreak, however, burning and burial still were practiced widely, partly because there were not enough rendering plants in some places to handle the huge number of carcasses produced by mass culling.

Vaccination Facts

If mass culling is not the answer to FMD, what is? A growing number of scientists favor a program of mass vaccination combined with more limited slaughter. Although livestock from infected farms might still have to be killed, vaccination rather than culling could be used on animals in a several-mile radius around the farm to form a kind of firebreak against the disease. This strategy, often called ring vaccination, would greatly reduce the number of animals needing slaughter and the carcasses needing disposal. It would decrease the spread of FMD virus both from direct and indirect contact and through the air. Some people argue that, although vaccination might buy time, vaccinated livestock would still have to be slaughtered to protect a country's ability to export its animals and animal products. However, even if the vaccinated animals were not killed, European Union rules say an emergency vaccination program would mean a 12-month ban on exports, not a permanent one. Because exports are banned as long as the disease lasts anyway, an effective policy to end it quickly would make both scientific and economic sense.

Compelling evidence indicates that a vaccination-based program can work. When FMD broke out in Macedonia in 1996, for example, at least 18 villages in two districts were involved, and 120,000 cattle had to be vaccinated. Yet only 4,500 had to be destroyed, and the outbreak was controlled within three weeks. The main drawbacks to vaccination are the limitations of the vaccine itself. The vaccine used in this kind of emergency is potent, but it still can take up to a week to work. However, critics have noted that it took longer than this for authorities in the UK to mobilize a mass cull. Also, the vaccine is only effective for a period of months, and it only works against one particular strain of FMD. Nevertheless, this may be all that is needed to halt an epidemic.

The strongest argument against vaccination is that approved tests cannot distinguish between vaccinated animals and those with the disease. Yet that soon may change. The current FMD vaccine is made of deactivated live virus. Animals given the vaccine make antibodies to the virus's protein coat, but they are never exposed to other viral proteins produced when the virus infects a cell. In 1999 a scientific commission of the European Union concluded that experimental tests for noncoat proteins can tell the difference between vaccinated and infected animals 90% of the time. Although not perfect, a test with 90% accuracy is probably good enough to declare a herd uninfected. Also under development are experimental synthetic vaccines that likewise might make it easier to tell vaccinated animals from infected ones. With better diagnostic tests and vaccines, vaccination may become an even more attractive option in the near future.

—LINDA WASMER ANDREWS

KEY TERMS

BOVINE SPONGIFORM ENCEPHALOPATHY:

Fatal neurological condition in cattle, commonly called "mad cow disease."

CLOVEN FOOT:

Divided hoof of cows, sheep, and other ruminants (animals that chew a cud and have a stomach containing four chambers).

CULL AND RING CULLING:

To cull is to pick out substandard, weak, or sick members of a group or herd. Ring culling, the practice of destroying even apparently healthy animals in a 3-km ring around the central point where an infected animal was confirmed, was instituted by the British government in March 2001 to combat FMD.

CULLING:

Slaughter of a selected group of livestock in an effort to stem the spread of disease.

DIOXINS:

Group of highly toxic compounds that can increase the risk of cancer and cause a severe skin disease called chloracne.

DRACONIAN:

Harsh or very severe measures. Named after the Athenian lawgiver (621 b.c.) who drew up a code of laws prescribing fixed punishments.

GENERATION INTERVAL:

Time between the infection of one animal and its ability to infect other animals.

RING VACCINATION:

Vaccination of livestock in a several-mile radius around an infected farm to create a firebreak against disease.

STAMPING OUT:

Slaughter of infected animals along with other livestock on the same farm in an effort to stem the spread of disease.

VIRUS:

Submicroscopic agent smaller than bacteria. Composed of a core of RNA or DNA and an outer coat of protein. Viruses grow and multiply within living cells and are able to infect almost all types of organisms. Besides FMD, viruses cause mumps, rabies, and a host of other ills including the common cold.

FMD AND THE UNITED STATES

The United States has been free of foot-and-mouth disease (FMD) since 1929, and efforts are under way to try to keep it that way. In 2001, when the epidemic of FMD first broke out in the UK, then spread to a few other European nations, the United States quickly banned imports of livestock and related products from the entire European Union. As of July, that ban had been lifted for countries where no cases of FMD had been reported, but it was still in place for infected countries.

Import bans are just the start, however. Although FMD is not a health threat to humans, people can carry the virus from place to place on their clothing, shoes, personal items, and even in their throats and noses. As a result, travelers entering the United States from an infected country are warned to take special precautions:

Launder or dry-clean all clothing before traveling, and remove all dirt and soil from shoes. Any soiled luggage or personal items, such as watches, cameras, mobile phones, and laptop computers, also should be wiped clean.
Avoid visiting farms, stockyards, animal laboratories, packing houses, zoos, and other animal facilities for at least five days before leaving the infected country. Also, avoid contact with livestock and wildlife for five days after arriving in the United States.
Be honest with customs officials. Travelers arriving from an infected country are asked whether they have visited a farm, been near or around animals, or are carrying any live animals, meat, meat products, fruits, vegetables, plants, or other items that might harbor pests or disease. If the answer to any of these questions is yes, the person's baggage is x-rayed or physically inspected, and any prohibited items are taken and destroyed. Shoes worn around a farm are checked for dirt or manure, and dirty shoes are cleaned and disinfected before being returned to the person.
For more information, travelers can call the U.S. Department of Agriculture's information line at (866) SAFGUARD.

—Linda Wasmer Andrews

Science in Dispute