IELTS Academic Reading Practice Test 395

Rinderpest — the word is German for 'cattle plague' — was for centuries the most feared disease of livestock. It was caused by a virus that infected cattle, buffalo, yaks and other cloven-hoofed grazing animals, producing fever, discharges from the eyes and nose, sores in the mouth and severe diarrhoea. In a herd that had never encountered the infection almost every animal fell ill, and in the worst outbreaks nine out of ten of those affected died. Two features of the disease, however, made it a candidate for elimination. Animals that recovered did not remain infectious, so the virus had no hidden reservoir in the bodies of survivors; and it was not carried by insects or by contaminated water, but passed only through close contact with infected animals or their secretions.

The disease had been recognised in Asia for many centuries and had repeatedly advanced into Europe as soldiers, traders and cattle dealers moved animals across the continent. The crisis that finally pushed governments to act began in 1865, when the infection appeared in British cattle, almost certainly brought by animals unloaded from a ship that had sailed from northern Europe. The resulting outbreak led to the deaths or slaughter of hundreds of thousands of cattle. A commission of inquiry was appointed, and in 1866 Parliament gave officials the power to restrict the movement of animals and to order the destruction of infected herds. Other European countries adopted similar rules, and by the end of the century the disease had been driven out of most of the continent.

The most terrible damage was done in Africa. Rinderpest reached the Horn of Africa in the late 1880s, carried by cattle brought from India for an Italian military expedition in Eritrea. From there it travelled south with herds and with moving people, reaching Ethiopia and Sudan and, within a decade, the grazing lands of eastern and southern Africa. African cattle had no immunity whatsoever. In Ethiopia the great majority of cattle are thought to have died, and in parts of Kenya the herds of pastoralist communities were almost entirely destroyed. The epidemic coincided with drought and locusts, and the loss of ploughing animals and of milk contributed to a famine that killed people as well as livestock.

Wildlife suffered on the same scale. Buffalo, giraffe, wildebeest and antelope died in enormous numbers across the savannah. Ecologists have argued that the sudden removal of so many grazing animals changed the vegetation of parts of East Africa for decades, allowing bush to spread over land that had previously been kept open by herds. The epidemic also stripped pastoral communities of the animals on which their wealth, their diet and their social standing depended.

Rinderpest virus belongs to the morbilliviruses, a group that also includes measles, canine distemper and a disease of sheep and goats known as peste des petits ruminants. Genetic studies indicate that measles, one of the great killers of human history, evolved from a rinderpest-like virus that crossed into people, probably at a time when humans and cattle lived in close quarters. The rinderpest virus itself never established itself in people, and this mattered enormously for control, because there was no human reservoir to sustain it once cattle had been protected. The virus is also fragile, surviving only briefly outside the body and being destroyed quickly by sunlight and drying.

For most of the twentieth century, control depended on slaughter, quarantine and restrictions on movement, which were expensive, unpopular and impossible to apply in remote areas. The turning point was a vaccine developed around 1960 by Walter Plowright and his colleagues at a veterinary research laboratory in Kenya. They grew the virus in laboratory cultures of cattle cells until it had lost the ability to cause serious illness while still provoking a protective response. A single dose gave long-lasting immunity. The vaccine could be freeze-dried and stored, but it lost potency in heat, which made it difficult to use in the tropics without reliable refrigeration.

Mass vaccination campaigns followed. In the early 1960s, African governments and international agencies launched a joint programme that vaccinated many millions of cattle across the continent. It interrupted transmission over large areas, yet the disease returned wherever fighting, weak administration or a shortage of money allowed vaccination to lapse. The lesson drawn from that experience was that a campaign must be maintained until the very last case has been dealt with, not merely reduced to a level at which outbreaks become rare.

By the 1990s, rinderpest survived mainly in a few pockets of East Africa and the Middle East. The Food and Agriculture Organization of the United Nations coordinated a global eradication programme with the World Organisation for Animal Health, combining vaccination with intensive surveillance so that every suspected outbreak was investigated in a laboratory. The last confirmed outbreak occurred in Kenya in 2001. For a decade afterwards, veterinary services searched for any trace of the virus, testing samples and monitoring herds, and in 2011 the disease was formally declared to have been eradicated worldwide.

Rinderpest thus became the second disease, after smallpox, to be eliminated from the planet. Live virus remains in a small number of highly secured laboratories, and deciding whether to keep it there or destroy it is a continuing responsibility. The campaign also left a set of lessons. Eradication is realistic when a vaccine gives solid protection, when there is no animal or insect reservoir, and when the international community is willing to pay for surveillance long after the visible threat has gone.

A virus that lives in a bat, a pig or a chimpanzee is, for the most part, poorly suited to life in a human being. The cells it must enter are different, the temperature is different, and the immune defences it meets are different. Occasionally, however, one of these viruses makes the crossing, and the consequences can be enormous. Diseases that pass from animals to people are known as zoonoses, and estimates suggest that around six in ten of the infections that afflict humans are of this kind. Among infections appearing for the first time in recent decades, the proportion is thought to be higher still.

The first obstacle is the surface of the cell. A virus gains entry by attaching to a protein on the outside of a cell, and the shape of that protein varies from species to species. Small changes in a virus's genetic material can alter the part of its coat that does the attaching, and a variant that happens to fit the human version of the protein gains a foothold. Beyond the receptor lies a second line of defence. Cells respond to invading genetic material by producing interferons, which interfere with replication, and many animal viruses are stopped there. Temperature matters as well: birds maintain body temperatures several degrees above ours, so avian viruses often multiply poorly at 37 degrees Celsius.

Spillover is therefore a matter of opportunity as much as of biology. The practices that bring people into contact with animal blood and tissue — hunting, butchering, farming and the sale of live animals in crowded markets — are the situations in which most crossings are thought to have occurred. Bats are hunted for food in parts of Africa and Asia, primates are butchered in forests, and poultry and pigs are reared in enormous numbers, frequently close to where people live and work. In every case the virus needs only a single successful infection, in a single person, to begin.

Most such infections stop there. A virus that has just crossed into a person may replicate well enough to cause illness yet never pass to anyone else, so that the patient becomes what epidemiologists call a dead end; rabies behaves in this way, killing the person it infects but not spreading between people. Sustained transmission requires that the virus be shed in a form, and by a route, that allows it to reach another host. Respiratory droplets, blood and sexual contact are the routes that have allowed animal viruses to establish themselves in human populations.

Influenza is the classic example of a pathogen that crosses the barrier repeatedly. Its genome is divided into eight separate segments, so when two influenza viruses infect the same cell their genes can be mixed into new combinations, a process known as reassortment. Pigs can be infected by both avian and human influenza viruses and have long been regarded as mixing vessels in which such combinations arise. The pandemics of 1957 and 1968 both involved viruses with genes of avian origin, and the 2009 pandemic was caused by a virus with swine ancestors.

Some crossings have consequences that unfold over decades. HIV, the virus that causes AIDS, is a descendant of viruses that infect African primates. Several transfers from animals to people are thought to have occurred during the twentieth century; the strain responsible for the global pandemic, HIV-1 group M, is most closely related to a virus found in chimpanzees in central Africa. A virus that spreads through blood and sexual contact can persist indefinitely in a small population, and from a slow beginning it may spread across continents long before anyone recognises that a new disease exists.

Certain hosts appear to carry a disproportionate share of the viruses that concern us. Bats are the leading example. They harbour a wide range of coronaviruses, several viruses of the family that includes Nipah and Hendra, and they are suspected of being the natural reservoir of the Ebola viruses. Several explanations have been proposed: bats live in dense colonies that may number millions, flight raises their metabolism and body temperature, and their immune response appears unusually tolerant of viral infection. It is also possible that they are simply studied more intensively than other groups of mammals.

Human behaviour determines how often these opportunities arise. Clearing forest brings people, livestock and wildlife into new contact; new roads open remote areas to hunters and traders; keeping large numbers of genetically similar animals together makes it easy for a virus to spread once it has arrived; and the movement of people and goods can carry an infection far from its origin before its symptoms are understood. Climate change is expected to shift the ranges of many host species, although how much difference this will make to human health is uncertain.

What remains unclear is why one virus and not another succeeds. Some that attach readily to human cells have caused only small outbreaks; others have spread around the world. Surveillance has improved. Programmes that sample wild animals, livestock and the people who work with them, and that sequence whatever viruses they find, have discovered an extraordinary variety of previously unknown pathogens. But knowing that a virus exists is not the same as knowing whether it poses a threat, and no method yet available can reliably predict which of them will cause the next pandemic.

In the first weeks of 2020, as a new coronavirus spread through the Chinese city of Wuhan, attention focused on a market where live animals, including wild species, had been sold. The market was closed, and politicians and campaigners around the world called for wildlife markets to be shut permanently. The precise route by which the virus first reached people has not been established, and the part played by the market remains debated. What is not in dispute is that the trade in wild animals carries risks. The question the outbreak raised is whether closing markets is the most effective way to reduce them, and that question has not been settled.

Much confusion arises from the words themselves. A wet market is simply a market selling fresh produce: vegetables, fish, poultry and meat, usually displayed at ambient temperature because many customers have no refrigeration at home. Most such markets contain no wild animals at all. A wildlife market is a different institution, where wild-caught species and farmed wild species are sold, often alive, for food, for medicine or as pets. Campaigns that condemn wet markets in general therefore attack something far broader than the practices that create disease risk.

The case for closing wildlife markets is not a sentimental one. When animals of many species are stacked together in small cages, stressed and in contact with one another's waste, the conditions favour the transmission of viruses. In such conditions a virus encounters hosts it would never meet in the wild, and each new host is an opportunity to adapt. Markets also act as amplifiers: an infection that might have died out in a single hunting household can be passed to dozens of customers and traders within a day. The SARS epidemic of 2002 and 2003, which killed several hundred people, was linked to markets in southern China where civets and other animals were sold, and viruses very similar to the one responsible were found in civets there.

Yet a blanket closure is a blunt instrument, and it would fall hardest on people with the least choice. For many households in parts of Africa, Asia and Latin America, wild meat is an important source of protein and income; where domestic livestock is scarce or expensive, it is not a luxury but part of the food supply. A trade driven underground would be harder to inspect rather than easier. And closing a market does nothing about the hunters, the transporters or the restaurants that create the demand in the first place.

The record of bans is mixed. In February 2020 China's legislature prohibited the consumption of wild animals and tightened the law on trading them, and Vietnam announced comparable measures later the same year. Both were significant steps, but enforcement has been uneven, and exemptions for farmed species and for the use of wild animals in traditional medicine leave gaps through which the trade continues. Where a prohibition has not been accompanied by compensation or by affordable alternatives, traders have carried on illegally, and inspections have been too few to stop them.

International law offers limited help. The Convention on International Trade in Endangered Species regulates cross-border trade in species whose survival is threatened, and it was not designed with disease in mind: a species may be abundant, and therefore unlisted, while still carrying a virus of concern. More importantly, almost all of the trade that matters takes place within national borders, where that convention has no authority. No international body has the power to inspect a market in a provincial town.

What would work better? Veterinary and public-health specialists tend to converge on a set of unglamorous measures. The sale and slaughter of live mammals and birds in urban markets should be prohibited, while fresh meat continues to be sold chilled. Animals should be traceable to their source, premises should be inspected without warning, and traders who lose a legitimate business should be compensated. Each of these costs less than the surveillance needed to detect the next outbreak after it has already begun.

It is also a mistake to treat markets as the root cause. Demand for wild meat grows with urban wealth; the logging and road-building that open forests make hunting easier and bring hunters into contact with species they rarely met before; and the intensification of livestock production creates risks of its own. A government that closes a market while continuing to permit the destruction of forest has dealt with the most visible part of the problem and left the rest of it untouched. The pressure to hunt comes from distant cities, not from the forest itself.

My own view is that the sale of live wild mammals and birds for food in crowded urban markets should be prohibited, and that the prohibition should be enforced with the money and the compensation that make it credible. That is a narrower demand than the one usually made. Calls to abolish wet markets confuse a food system on which billions of people depend with a much smaller trade in wildlife, and they risk turning a public-health argument into a campaign against the poor. Regulate the trade that carries the danger, and leave the market that sells vegetables alone.