THINGS were looking bad for the lions of the Serengeti. A strange sickness was sweeping through the population, claiming lions of all ages, from young cubs to adults in their prime. Their paws twitched. They seemed disoriented and depressed. Some had seizures, their limbs flailing and their faces twisted into a lopsided sneer. Before the year was out, the mysterious disease had killed a thousand lions-a third of the population in Tanzania鈥檚 Serengeti National Park-before spreading north across the border into Kenya.
That was 1994. Within a few months, blood and tissue tests revealed that the lions had died from a disease they weren鈥檛 supposed to get: canine distemper, a dog disease caused by a morbillivirus related to measles. Careful molecular detective work identified the virus as a strain from domestic dogs, and a search for the source led to villages on the western edge of the Serengeti.
Distemper is spread in fine droplets from an animal鈥檚 breath and transmission requires close contact with an infected individual. Village dogs rarely get close enough to a lion to pass on the virus, so there are likely to have been several intermediaries. Spotted hyenas are high on the list of suspects as the final link in the chain because they mix with lions at a kill.
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By the end of 1994, the epidemic had burnt itself out. 鈥淚n the long term, we are sure the lions will recover-as long as it doesn鈥檛 hit them again,鈥 says Sarah Cleaveland, a wildlife vet with the Institute of Zoology in London who spends much of her year in the Serengeti.
While the lions may have weathered the epidemic, the disaster highlights the dangers posed by pathogens of domestic animals. As a species, Africa鈥檚 lions are in good shape: had the disease hit a rarer animal, the outcome could have been catastrophic. The likelihood that one day a disease of domestic animals will drive an endangered species to extinction is forcing wildlife biologists to think about how to stop the spread of these diseases. It has also prompted a fierce debate about the rights and wrongs of interfering in the lives of wild animals, either by treating sick animals or vaccinating them.
鈥淪ome people would say that if you have a population that is so small, then sooner or later something will get it. But infectious diseases are significant candidates for finishing them off,鈥 says David Macdonald, head of the Wildlife Conservation Research Unit at Oxford University. 鈥淚n that event, intervening to control the disease might be judged the only option.鈥
Wildlife biologists and vets have every reason to worry. As the human population increases, people are moving into ever more remote areas, coming closer to the refuges of endangered species. Where they go, their animals go-and multiply. Dogs especially have the potential to see off some of the world鈥檚 rarest carnivores. Among their own relatives, they pose a special risk to the Ethiopian wolf, now down to about 400 animals, and the African wild dog, of which fewer than 5000 survive.
And as the lion epidemic illustrates, you don鈥檛 have to be canine to contract a dog disease. If the much rarer Asiatic lions caught distemper their prospects would be poor: at the last count, there were just 304 of them in their final refuge in the forest of Gir in Gujarat. And the threat is real. 鈥淭here are stray dogs everywhere,鈥 says Peter Jackson, who chairs the World Conservation Union鈥檚 cat specialist group. Mustelids, the group that includes otters, ferrets and polecats, are also extremely susceptible to canine distemper, a fact that has prompted fears for the safety of the few remaining giant otters of the Amazon and the last surviving European mink in the former Soviet Union.
Pushed to the brink
Canine distemper may have played a part in the extinction of the marsupial wolf early this century, and it certainly pushed the black-footed ferret of North America to the brink in the 1970s. But it is not the only killer disease that can be contracted from dogs. For wild canines, the threat of rabies is even worse, and parvovirus, a highly contagious cause of enteritis, can take a heavy toll of puppies. Even a common and rarely fatal disease such as mange can become a killer in animals that have never been exposed to it before.
Wildlife biologists have already seen how rabies can race through a population. In 1990 and 1991, it claimed more than half of the Ethiopian wolves in the Bale Mountains National Park, home to the largest surviving group of wolves. In Israel, an outbreak in the late 1980s put the future of the tiny Blanford鈥檚 fox in doubt almost as soon as the species was discovered. Rabies was also responsible for the deaths of African wild dogs in the Masai Mara reserve in Kenya in 1989 and in the Serengeti in 1990.
The latest incursions of dogs are in some of the most remote rainforests of South America, where there are still isolated tribes that have only recently come into contact with other people. The Madre de Dios region of southeastern Peru has the continent鈥檚 largest population of giant otters, with around 200 animals. Between 60 and 70 of these live in Manu National Park, where growing trade between forest villagers and townspeople has led to an influx of dogs.
鈥淭he problem is that when new dogs come in from the cities they are normally infected,鈥 says Christof Schenk of the Frankfurt Zoological Society, who studies the Manu otters. In 1994, Schenk found that most dogs in the forest villages carried antibodies to canine distemper and parvovirus. Both pathogens can kill otters.
Nor are dogs the only problem. Domestic cats, particularly those that live in large, semi-wild colonies in the countryside, also put their wild relatives at risk. Take the rare Scottish wildcat, for example. 鈥淪cottish wildcats are so closely similar to feral domestic cats that most of their genome is the same,鈥 says Macdonald. 鈥淪o you would expect the pathogens that bedevil domestic cats to bedevil wildcats too.鈥 And feral cats are plagued by a long list of pathogens.
Two other feline contenders for extinction are the Florida panther, a subspecies of mountain lion now reduced to about 50 animals, and the Iriomote cat, a species with about 80 survivors hanging on in the forests of Iriomote Island at the southernmost tip of the Japanese archipelago.
But just why are wild animals so much at risk from the diseases of domestic animals? In some cases it is simply a question of unfamiliarity. Just as common European diseases, such as measles, flu and smallpox, cut a swathe through the native peoples of the Americas in the years following the Spanish conquest, so common diseases of domestic animals threaten isolated animal populations that have never encountered them before. Long association with a virus or other agent of disease leads to the evolution of some degree of immunity but a population that has never encountered it before has no defence at all.
Battle for supremacy
Added to this, pathogens are constantly changing in their battle to outmanoeuvre their host鈥檚 evolving immune system. A new version, perhaps only slightly different, may have what it takes to infect a wider range of hosts. In fact, old blood samples from long-dead Serengeti lions contained antibodies to canine distemper, indicating that this population had encountered the virus before. But while lions had resisted the earlier version of the virus, the 1994 model cut them down with unnerving speed.
The more contact there is between domestic animals and wild ones, the greater the risk of new infections reaching rare species. As populations of domestic dogs-and cats-become larger, they form reservoirs for pathogens. Where infection was once sporadic, it becomes a persistent presence, circulating continuously and flaring up when conditions are right. While wild animals can be susceptible to the same pathogens as domestic animals, they usually live at such low densities that they cannot keep the disease going indefinitely. After an outbreak, the pathogen disappears, and reinfection must come from outside.
The big question for those responsible for conserving wildlife is whether anything can be done to protect rare species from the diseases that spill out from human settlements. Trying to keep the animals apart is probably impossible. While banning the introduction of any more domestic cats to Iriomote Island might work, it is neither practical nor politically feasible to ban people and their animals from parks such as Manu or the Serengeti.
Is the answer to go for the high-tech fix and vaccinate wild populations at special risk? This question is currently a matter of fierce debate in wildlife circles, not only because it means interfering in the lives of wild animals-and the natural process of disease-but also because it carries risks that may be unacceptable for a species on the edge.
Banana vaccine
In the 1960s, some of the world-famous chimpanzees at Gombe in Tanzania contracted polio, almost certainly from local villagers. Faced with the prospect of an epidemic, Jane Goodall and her team at Gombe dosed the chimps鈥 bananas with human polio vaccine. Twenty years later, several mountain gorillas living on the border of Rwanda and Zaire fell sick with measles, again contracted from humans. This time, a small test run with a human vaccine known to protect a range of primates was carried out before launching into a wider vaccination programme.
Today, wildlife vets are warier, after events that began in 1990. An outbreak of rabies in the Serengeti prompted fears for the safety of the small number of wild dogs that still lived there. A team of vets gave 34 animals rabies jabs, either by dart or by hand. Within 10 months, four of the vaccinated dogs were dead. Worse, since June 1991, there have been no sightings of any wild dogs at all. No one knows if the vaccinations played a part in the disappearance of the dogs. The population had been in decline for 25 years and the open plains of the Serengeti are not ideal for wild dogs, which have a tough time competing with lions. But critics argue that either the vaccine or the stress of being handled made the dogs vulnerable to disease, perhaps rabies, perhaps something else.
The whole notion of providing wild animals with an instant but artificial form of immunity is fraught with difficulty. For a start, so little is known about the epidemiology of diseases in wildlife, what course an epidemic might take or what impact vaccination might have, that it is difficult even to weigh up the risks. And hardly any research has been done into vaccines for wild animals. Most modern vaccines are based on weakened forms of the virus, which are too weak to cause disease but still trigger immunity. However, not all species respond to weakened viruses in the same way, and even close relatives may react differently. In one of the most famous conservation disasters of the 1970s, the black-footed ferret, an almost extinct species from North America, was nearly wiped out by distemper-given in a vaccine. The live vaccine had been tested in European ferrets, but it proved fatal to their American cousins. Added to these problems, some animals are so sensitive to disturbance that the trauma of being caught, restrained and dosed with vaccine may be as harmful as the disease the vaccine is intended to save them from.
鈥淎nother argument against vaccinating wild animals is that it interferes with natural selection,鈥 says John Harwood of the Sea Mammal Research Unit in Saint Andrews. 鈥淵ou reduce exposure to the selective forces of disease and end up with a population potentially more vulnerable than before.鈥 Without vaccination, susceptible animals will die and those that resist the disease will go on to breed, producing future generations with better defences. 鈥淪ometimes there is the temptation to do something when the correct strategy is to do nothing,鈥 says Macdonald. But for rare species, leaving things to nature may condemn them to extinction.
If the outlook is this bleak, then intervention might be the right option. 鈥淏ut you have to be extremely sure that the situation warrants it,鈥 says Macdonald. In the case of the Mednyi arctic foxes, devastated by an outbreak of mange, there was no real choice, he argues.
This subspecies of arctic fox was once common in the Commander Islands, at the Russian end of the Aleutian chain. Reduced by hunting, one last group survived on Mednyi Island, maintaining a population of around 600 until 1975, when it crashed. Hunters visiting the island had brought dogs infected with mange. The mange mite, a parasite the foxes had probably never encountered before, suddenly catapulted them towards extinction, killing around 90 per cent of each year鈥檚 cubs. By 1978, the population was down below 120.
鈥淪ome people argue that extinction is natural and mange is a natural disease of foxes,鈥 says Macdonald. 鈥淏ut something dramatic had happened to the foxes, and it was brought by humans. The consequences of doing nothing were likely to be serious, and the consequences of intervention were not likely to be harmful, so the balance was in favour of trying.鈥
In 1994, he and two Russian colleagues, Elena Kruchenkova and Mikael Goltsman of Moscow State University, decided to treat the cubs with antiparasitic drugs. The drugs are well-established and an early test on two cubs had been encouraging. After treatment, fewer cubs died, and a census the following year suggested that the treatment may have produced a small improvement in numbers.
A strategy that has found more favour than direct treatment of wildlife is to vaccinate the domestic animals that form the reservoir for the disease. Such an approach worked for rinderpest, a disease of cattle caused by another morbillivirus. Rinderpest is generally mild in cattle but if it gets into wild ungulates such as antelope and buffalo, it becomes a killer. In the 1950s, after half a century of outbreaks, cattle in a buffer zone around the Serengeti were vaccinated against rinderpest, and have been ever since. With the exception of an outbreak in 1982, the area has been free of the disease for four decades.
Popular support
Taking this as a model, last September Cleaveland began a programme to vaccinate all the dogs on the western border of the Serengeti against both rabies and distemper. The task is a challenging one. Conditions in the villages are ideal for keeping a disease in circulation: the dogs have a high birth rate and a low life expectancy, so turnover is high, and there is always a large pool of susceptible animals.
But local villagers are enthusiastic about the programme, mainly because they fear rabies as a human disease. 鈥淚t is going very smoothly,鈥 says Cleaveland. 鈥淏ut we won鈥檛 see any measurable impact until later this year when we should see a significant drop in infection in the western zone.鈥
A key requirement for a programme of this sort is to know how many dogs must be vaccinated to prevent outbreaks of disease. For rabies, the WHO puts the figure at 70 per cent. For distemper, the figure is lower. 鈥淲e are getting to treat 80 to 90 per cent of the adult dogs, but only 20 per cent of the young puppies,鈥 says Cleaveland. 鈥淥verall, we are getting around 70 per cent coverage.鈥
Following Cleaveland鈥檚 example, Karen Laurenson, a vet with the Institute of Zoology in London, and Fekadu Shiferaw of the Ethiopian Wildlife Conservation Organisation have begun a similar project with Claudio Sillero-Zubiri of the Wildlife Conservation Research Unit, in the villages of Ethiopia鈥檚 Bale Mountains National Park.
In Bale, dogs and wolves already come into contact too often. Dogs may be owned by villagers but they generally have to fend for themselves. This brings them into competition with wolves for prey, and occasionally male dogs mate with female wolves, producing offspring with a mixture of dog and wolf genes. Both activities endanger the wolf. 鈥淏ut disease from dogs is the biggest threat,鈥 says Laurenson. Outbreaks of rabies in 1990 and 1991 killed more than half of the park鈥檚 wolves, which were already in dramatic decline, and distemper is also a threat.
As in Tanzania, the local people like the idea of vaccination for their dogs, mainly because rabies can kill their cattle, causing heavy financial losses. But Laurenson has run into one unexpected problem. Most of the Bale villagers are Muslim, and it is taboo for them to touch dogs. 鈥淭heir dogs are quite friendly until you try to touch them,鈥 she says. 鈥淭hey just aren鈥檛 used to being handled.鈥 Nevertheless, she hopes to have vaccinated 80 per cent of the dogs in the park this spring.
Schenk would like to see a similar programme in and around Manu to protect the giant otters, but admits that the logistical problems are immense. 鈥淚t is a very difficult area to work in. And the people are very poor. It is hard to vaccinate dogs and cats when not even human children are vaccinated,鈥 he says.
Too many puppies
Such programmes would be more effective if they were combined with some sort of population control, but this is a sensitive subject in the villages. 鈥淲e have to tackle this slowly and carefully,鈥 says Cleaveland. 鈥淪ome people ask for help to stop the puppies but people want dogs, so control is not a popular option at the moment.鈥 If the dog population is going to keep on growing then it is vital that vaccination campaigns are continued, otherwise the level of immunity will fall below the threshold to prevent outbreaks.
Sustaining any vaccination campaign could prove a major problem in countries where money is short and the health of dogs is a low priority. With such a high turnover of dogs in the Serengeti villages, Cleaveland reckons campaigns must be repeated every six to eight months to keep the level of immunity at 70 per cent. So far, she has relied on contributions from charity, and vaccine donated by the manufacturer. The money will last another three years. 鈥淚n the long term, I would like to see routine vaccination with funding from wildlife revenues-from hunting and tourism,鈥 she says. 鈥淚t wouldn鈥檛 cost much if you only had to top up by vaccinating the puppies.鈥
The health of dogs, however, is fairly low on the list of priorities in the countries that are home to the rarest species. 鈥淕overnments and researchers put time and money into studying the epidemiology of diseases like rabies and rinderpest because they affect people and livestock. No such work has been done on canine distemper,鈥 says Rosie Woodroffe, a member of the World Conservation Union鈥檚 canid specialist group, and author of its action plan for wild dogs. 鈥淎lthough local people want their dogs to stay healthy, governments have little interest in protecting dogs in general.鈥 Some sort of monitoring scheme is needed to watch out for the first signs of disease, but again, this costs money.
And while it might be feasible to create disease-free zones around endangered populations confined to relatively small and manageable areas, an animal like the African wild dog would pose more of a challenge. 鈥淲ild dogs range over very long distances. They don鈥檛 always stay in parks,鈥 says Woodroffe. For species like this, there is no better solution than making sure the population is as big as possible, and that means having large protected areas that take into account the dogs鈥 nomadic lives. 鈥淭hat way,鈥 says Woodroffe, 鈥渋f there is an outbreak of disease, they stand a chance of recovering from it.鈥