The Killer Cat Virus That Doesn't Kill Cats
Can we learn from a feline? Over millions of years, wild cats have learned how to live with a virus quite similar to one that's killing us.
Craig Packer, who has studied the lions of the Serengeti for more than a decade, remembers getting the bad news in the spring of 1990. The report came from his collaborator Stephen O?Brien of the National Cancer Institute. Many of Packer?s kingly beasts, O?Brien had just learned, were infected with a feline equivalent of the virus that causes AIDS in humans. ?All I could think was ?Whoa, not my guys!? ? says Packer, a behavioral ecologist at the University of Minnesota. Feline immunodeficiency virus, or FIV, was new. It had been discovered only three years earlier in domestic cats, and in them it was often lethal. Alarmed by the possibility that his lions might be on the threshold of death, Packer alerted his field assistants in the Serengeti to watch for warning symptoms.
Today, after five years of close scrutiny, Packer and O?Brien are cautiously hopeful. Although O?Brien has now shown that 84 percent of the Serengeti lions harbor the virus, there is as yet no evidence that the animals develop AIDS. ?As far as we can tell,? says O?Brien, ?there seems to be a balance between the host and the virus.?
To O?Brien, the lions? apparent resistance to AIDS means that these infected cats may hold a key to fighting the epidemic in humans. At the very least, the cats are giving us an unprecedented understanding of how immunodeficiency viruses evolve. FIV, O?Brien?s team has now shown, has infected 25 species of cats around the world, from the lions of the Serengeti to the cougars of Wyoming, the snow leopards of the Himalayas, and the Pallas?s cats of the Siberian steppes. So far
O?Brien has studied four of these species closely--the lion, puma, leopard, and domestic cat--and has determined that each is infected with its own particular strain of FIV. He wouldn?t be surprised, he says, if that holds true for the other cat species also. That degree of variety and prevalence, coupled with the cats? seeming good health, has important implications.
?It basically means that FIV is an old virus,? explains O?Brien. ?And to me, an old virus is a good virus. It hasn?t killed off its host, or it wouldn?t be here. And so it gives us a beautiful model for understanding these coevolutionary events.?
By comparing the genetic sequences of each strain, O?Brien can now begin to trace out a genealogy of the pathogen back millions of years-- the first time anyone has managed such a feat for an immunodeficiency virus. From O?Brien?s work arises the possibility that the human immunodeficiency virus, HIV, which is no more than 200 years old, may in fact be the grandchild of the cat virus. Lions and other cat species have managed over the millennia to come to a peaceful arrangement with FIV, and what has happened in the wild cats, O?Brien and other researchers believe, will eventually happen in people: HIV will attain a symbiosis with its human host. But that is a long process--and one that O?Brien ultimately hopes to circumvent by understanding the evolutionary history of FIV and of immunodeficiency viruses in general.
O?Brien, 50, tall and slim, with hair more gray than sandy, is a driven man. He arrives early at his lab, stays late, works weekends, and for lunch gets a junk-food hit at whatever fast-food outlet happens to be closest. While his drive is innate, it has been compounded by the agonizing experience of watching his elder brother suffer with and finally die from AIDS last year. O?Brien says that he didn?t begin his research into FIV because of his brother?s illness, but the memory of that suffering clearly informs his work. ?No one should have to go through that,? he says, his mouth forming a tight line. ?AIDS is a terrible, terrible disease. So any clue we get to how it might be cured--or as in this case, how another species dealt with it in the past--should not be overlooked.?
Lining one hallway of O?Brien?s many labs are six deep freezers, of which he is justifiably proud. ?That?s my frozen zoo,? he says, nodding toward the blocky white containers. ?It?s the most valuable thing I have here.? The freezers are filled with over 5,000 vials containing the tissue, blood cells, serum, and semen of animals from around the world. ?I?ve got everything from Antarctic elephant seals to koalas to humpback whales from the North Pacific and the North and South Atlantic,? says O?Brien. Thirty- two of the 37 cat species in the world are here as well.
O?Brien has collected these biological resources over the last 15 years, often in conjunction with colleagues at the National Zoological Park?s Center for New Opportunities in Animal Health Sciences in Washington, D.C. The samples let O?Brien and others bring the latest biomedical techniques to wildlife conservation, allowing them to gauge the genetic diversity of a species or track a disease in a wild animal population. His quest has taken him to China, India, Africa, and South America, where he has helped collect specimens with teams of wildlife biologists; other samples arrive frozen in liquid nitrogen tanks from collaborators around the world. And there are others that hail from far less exotic locales: O?Brien scooped up one species--a skunk--from the road near his farm outside Frederick, Maryland, one morning on his way to work. ?It was a fresh roadkill, so I brought it in,? he says. ?And it turned out to be an animal we didn?t have.?
O?Brien?s well-chilled library also supports his long-running obsession with understanding how animals evolve resistance to viruses. ?I realized that there were episodes of epidemics in wildlife populations that we weren?t tracking. Or if we were, we were concentrating only on the virus, not on how the species responded, and so we were losing out on a lot of evolutionary lessons.?
Now when there is an outbreak in wild animal populations, O?Brien?s samples, dating back a decade or more, often make it possible to trace its history. For example, last year Packer got more startling medical news: many Serengeti lions were falling ill with canine distemper--a disease that primarily affects dogs--and some were dying. Packer and his fellow biologists worried about how many of the lions would ultimately be killed. But the majority survived, and an analysis of O?Brien?s samples showed why: they had already survived an outbreak of distemper in the early 1980s that had gone unobserved.
There are limits to what O?Brien can do. It?s not possible simply to scan a sample of blood for any new unspecified virus that it might harbor. ?You have to have that isolated virus to serve as a target,? O?Brien explains. ?There has to be someone who?s paying close attention to the animal who sees some kind of illness he hasn?t seen before.?
In 1987 one woman in Petaluma, California, was doing exactly that. Worried about one of her cats, she brought it to Niels Pedersen?s lab at the University of California at Davis. ?She was frustrated; she?d taken her cat to several veterinarians, but no one had been able to help her,? says Pedersen. ?She thought her cat had AIDS, and it certainly had AIDS- like symptoms--an infection of the gut, skin lesions, a respiratory tract infection, and wasting.? At the time there was only one virus other than HIV known to attack the immune system?s T cells: simian immunodeficiency virus (SIV), which affects primates such as chimps and monkeys and is suspected to be the immediate ancestor of HIV. Pedersen, who was studying SIV at the time, discovered that the woman?s cat was indeed sick with an immunodeficiency virus of its own.
Pedersen?s discovery was big news, but it raised a host of questions for researchers such as O?Brien. How common was the illness in domestic cats? How long had they had it? And did it exist in wild cat populations? ?We really didn?t expect to find very much evidence of it in the exotic species,? says O?Brien. Immunodeficiency viruses belong to a class known as retroviruses, which insert their genetic code into their hosts? DNA. Since they thus have to be finely attuned to the vagaries of a host species? genome, most retroviruses stick to one species. Some researchers had looked for feline leukemia virus, another retrovirus that infects house cats, in wild species and found no trace. There wasn?t much reason to think that FIV would be different.
The first clue that FIV was indeed different came from Margaret Barr, a virologist at Cornell. In 1989 her lab reported finding antibodies to FIV in several captive exotic cats and in wild Florida panthers. O?Brien meanwhile searched his own store of frozen cat serum. ?We ran over 2,000 samples, and the prevalence was enormous,? O?Brien says. ?FIV was almost everywhere we looked. It was absolutely counter to what we had expected.?
Yet the virus had a tantalizing geographic pattern. For example, although the East African and South African lion populations were heavily infected, none of the 44 lions tested in Namibia--a country separated from South Africa by the Kalahari Desert--carried antibodies to the virus. Neither did Asian lions or tigers. Perhaps this distribution encoded the virus?s history. Namibian cats might not have FIV, for instance, because it had originated elsewhere and hadn?t managed to cross the Kalahari.
Researchers had noticed similar patterns among SIV-infected monkeys, not only in the virus?s distribution but in its virulence. SIV is carried by numerous species of African monkeys that, like the lions, do not get sick. But when SIV jumps into a new species of primate--such as ourselves or, more recently, Asian macaques kept in American labs--the virus is fatal. ?The macaques get sick, just as we do, because they don?t have any genetic resistance to the disease,? says O?Brien. The African monkeys, on the other hand, seem to be resistant. Like the Serengeti lions, they have apparently reached a truce with the virus.
?Everybody thinks that SIV evolved in the African monkeys and that some time ago there was what evolutionary biologists call an adaptive episode,? O?Brien explains. ?In other words, some of the monkeys were resistant to the virus for genetic reasons and they survived the SIV epidemic, while all the others died off. Or the virus became attenuated; it became less virulent. Or some version of both things happened. Either way you end up with a standoff between the host and the virus. We think the same thing has happened in the Serengeti lions.?
The lions thus offer an almost identical parallel to SIV, except for one pleasant difference: studying their infection in the wild is far easier than tracking SIV-positive African monkeys and chimps. ?That is just very, very difficult,? says Preston Marx, a virologist at the Aaron Diamond AIDS Research Center in New York. Part of the problem has to do with where the primates live. Marx, who has been collecting SIV samples from monkeys in Sierra Leone and Liberia for the past seven years, has watched both projects collapse under the strain of war and political turmoil. He?s now starting over again in Gabon. The problem with SIV is also bound up with the endangered status of the virus?s hosts. Researchers want to study SIV in wild chimpanzees, but since chimpanzees are endangered, they cannot collect the needed blood samples. ?You can?t shoot them with tranquilizer darts,? explains Marx, which is necessary to get a blood sample. ?That would disrupt their society and could hurt them? by causing falls from tall trees.
Packer, on the other hand, can sneak up on a lion napping on the ground and dart it without ill effect. ?They just nap harder,? he says. Since wildlife biologists sometimes need to handle the animals--to fasten radio collars on them for tracking purposes and to inspect their general health--they have developed techniques and tranquilizers that are usually not harmful to the cats. Of the 800 cats that veterinary teams working with O?Brien have immobilized, only 1 has died.
O?Brien does not generally collect the samples himself but instead relies on a team of wildlife biologists and veterinarians. ?I?m the coordinator and facilitator for these trips. I go when I can, and I have been there right alongside the guy who puts one of these big cats on the ground. Sometimes I?ve even handled the cat--but usually there?s someone along who does it a lot better than I do.? O?Brien sometimes helps process the blood samples on-site, using a technique he and his National Zoo collaborators adapted from conventional biomedical research. In a makeshift lab, he uses a centrifuge to isolate the red and white blood cells, as well as the serum (the pale, straw-colored liquid that remains when all the other cells in the blood are removed). O?Brien then decants these three components into individual plastic vials, freezes them in a liquid nitrogen tank, and ships them back to his Maryland lab.
At the lab, researchers test the serum for FIV antibodies to identify the cats that are carrying the virus. They then extract the DNA of infected white blood cells, where the FIV has inserted its own genes. To this they add genetic probes that can seek out a segment of the virus?s polymerase gene (the most slowly evolving and thus most informative part of the virus?s genome). The researchers can then prepare the segment so that a computer can determine its sequence of base pairs.
O?Brien can use the information the computer provides to create a family tree of FIV. Over time the virus has split into different strains, each of which has a distinctive gene sequence. But the more closely related different strains are, the less time they will have had to drift apart, and the more similar their genes will be. O?Brien can explain the pattern of variation in the different FIV strains by arranging them on an evolutionary tree, each strain occupying a branch of its own. In this tree one can read the history of the virus as it has evolved with the cat--a history that is helping to link genetically all the immunodeficiency viruses.
?All the cat viruses look as if they diverged from a single common ancestor,? O?Brien says. ?The differences and similarities you see in the sequences usually have something to do with time. They reveal a steady accumulation of divergences--of changes. And while we can?t assign precise dates to those changes, we can still use them to see what happened in the past. One of the first things we saw is that FIV has been with the cats a long time, long enough for them to have evolved their own strains of the virus.? O?Brien thinks that the virus might first have entered the felid family when it began diverging into separate species, roughly 6 million to 3 million years ago. In other words, an ancestor of all the wild cats today became infected with the original strain of FIV. Since then the disease has mutated time and again, evolving along with new species of cats.
O?Brien can?t say with certainty when the virus jumped from one species to another, but judging from the profound differences in each strain?s structure (the sequences from lion and puma strains are 25 percent different), these dates must also be ancient ones. ?That tells us that it is a rare event, that the virus doesn?t often jump from its host to a new species.? Such transitions are rare because there is a long list of things FIV (or HIV or SIV, for that matter) must do to infect the host. It?s got to find the right cell in the host animal, for example, infect the cell, take over the cellular machinery, make a copy of itself, release that copy, escape the host?s immune surveillance, and infect another cell. It?s a lockstep process, and if at any point the virus makes a mistake, the host will almost certainly kill it. Thus FIV seldom infects a new, even closely related, species.
Yet it has made that transition: the many infected species are evidence of that. Domestic cats, O?Brien thinks, must be a new conquest for FIV, since in them infection leads to AIDS. ?The transfer is rather recent, maybe within the last 1,000 years. All the animals that get sick and die haven?t disappeared yet from their population. They--like us--are in the middle of an adaptive episode.?
O?Brien cannot tell which of the other cat species infected the domestic cat; that will have to wait until someone finds a close relative to its strain of the virus. But there are some clues about how the virus made the jump in the means by which it spreads among lions. ?It?s probably not passed sexually or during birth or when the cubs are nursing,? says O?Brien. In fact, at birth the lion cubs do not even harbor antibodies to the virus. But within a year or so, the majority of young lions will test positive. ?There?s a lot of biting, a lot of aggressive behavior in the prides, so a lion that may test negative for FIV one year is likely to be positive the next.? This idea is supported by the high infection rate in lions, compared with a much lower infection rate in more solitary cats like leopards and cheetahs. The limited interactions with their peers translate to fewer chances for infection. O?Brien suspects that the jumps FIV has made between species depended on an infected cat?s being bitten by another- -or perhaps even eaten (lions, for example, sometimes eat cheetahs).
O?Brien?s FIV trees also reveal a surprising amount of variation within each strain of the virus--so much that in the lion strain he has identified three distinct lineages of FIV. They all evolved from a single ancestor, probably in a bid to confuse the leonine immune system. ?The virus mutates almost every time it replicates,? O?Brien explains. ?The same is true of HIV, which is why it is so hard to fight and why it?s so difficult to make a vaccine against it.? O?Brien thus thinks it?s dangerous to assume that lions are never harmed by FIV. Moreover, because the virus is ?mutating like mad, it?s a ticking time bomb. One of these new strains could prove lethal to the lions; we just don?t know enough about it yet to say that FIV in the lions is completely harmless.? Recently O?Brien persuaded a number of zookeepers to quarantine their FIV-positive cats.
O?Brien?s trees not only show him how FIV has evolved among cats but also give him provocative clues about its relationship to other viruses--like HIV. HIV belongs to a special group of retroviruses called lentiviruses, meaning ?slow-moving viruses,? because they can take years after infection to develop into a fatal disease. So far scientists have found lentiviruses in horses, sheep, goats, cattle, old-world monkeys, cats, and humans. Closest among the lentiviruses to HIV are SIV, FIV, and BIV--bovine immunodeficiency virus, which attacks cattle. O?Brien?s team has compared certain sequences of the polymerase gene in FIV with those in these three lentiviruses, and from the several scenarios that could explain the variation, one particularly tantalizing (although completely hypothetical) story has emerged.
Several million years ago, perhaps in Africa or the Middle East, the first link was forged in the chain that has led to HIV. An ancestor of today?s lions killed and fed on a BIV-infected bovid, such as a buffalo. The virus made a devilish twist in the cat--it figured out how to work the immune system of a new family of mammals and became FIV. Subsequently that ancestral cat passed the disease through biting to its fellow cats; some of them in turn passed it to other cat species. Eventually one of those species bit a monkey, which escaped the attack and survived. This time FIV worked its way into the primate body and became SIV. Some thousands of years later, perhaps because of humans? hunting and butchering monkeys, SIV devised a way to infect humans.
?I?ve made this argument and it makes sense,? says O?Brien. ?It explains the geography of the disease--why you see it in Africa and not in certain parts of Asia--and it explains some of the genetic differences you see among these viruses.? For example, FIV has only half the number of genes that are found in SIV and HIV. To O?Brien, this discrepancy suggests that FIV is the more primitive virus. ?We think that?s one of the things viruses do when they jump to a new host: they acquire new genes. It gives us some insight into the steps a virus must take to escape the new host?s immune surveillance.?
The links in this theoretical chain, of course, represent only those few immunodeficiency viruses that scientists have identified. O?Brien suspects many more are waiting to be found. ?We have no idea how common lentiviruses are, or how far back in time they go,? O?Brien says. ?I suspect that they are probably ubiquitous and that all we?ve seen is the tip of the iceberg.? Again, researchers cannot simply begin searching a population of animals--bears, for instance--for an unknown lentivirus. They must first find a sick animal, look for a telltale retrovirus enzyme, and then isolate the virus. It can then be used as a guide for testing other animals of the same species for antibodies to the disease.
The great hope of FIV research is that these evolutionary studies of lions and other cats will give scientists a lead on combating HIV. ?These, after all, are the species that got away,? says O?Brien. ?Lions haven?t gone extinct because of FIV. Some natural genetic engineering has taken place--either in the cats or in the virus. We?d like to find out what that is.?
One tactic is to discover how HIV, SIV, and FIV actually destroy the immune system. ?We know that they cause T cell depletion, that the immune system finally falters and the individual dies,? says O?Brien. ?But we don?t know the steps involved in that process.? Presumably the sequence is the same in all the diseases--certainly their progression is nearly identical. Domestic cats, like humans, for example, first fall ill with flulike symptoms; they then seem to recover and are fine for about seven years. But then most fall ill again and soon die. ?If you could understand these different stages, then you might be able to find a way to block one of the steps,? says O?Brien.
Researchers also want to know why the lions are apparently unharmed by FIV while domestic cats die from it. O?Brien?s team is constructing a map of both species? genomes, as well as their separate strains of FIV. O?Brien suspects the maps will be superimposable, gene for gene, and that this will allow his team to compare them for differences in disease resistance. There may be some unknown difference between how a receptor molecule on a lion?s T cell functions and how a domestic cat?s does. Receptor molecules let the killer T cells sense the presence of the invaders and set in motion the immune response. But in domestic cats--and in humans--something goes wrong, and although the T cells fight hard, they are eventually killed off.
The lions? T cells, however, successfully hold up against the virus. That difference is what O?Brien would like to find. ?We could take the version of the lion?s resistant gene, which confers protection, and insert it in the domestic cat to see if it does the same thing.? Such research is highly experimental and must be done on animals before humans-- which is another reason that domestic cats are such an important model.
A lion might also come down with AIDS. It may be born with slightly different genes that don?t confer protection, or it may harbor a mutated, lethal FIV strain. ?I really do think that that?s just a matter of time,? says O?Brien. If such an event occurs, O?Brien?s team could then compare the genes involved with disease resistance of the sick and healthy lions, again searching for dramatic differences. ?We?re doing the same thing with HIV,? he adds. Not everyone who contracts HIV develops AIDS; some individuals have lived more than a decade with HIV yet remain symptom free. O?Brien and others suspect that somewhere in their genomes lies the ultimate cure for AIDS.
?We think some people have it, and the lions seem to have it. The lions got there by going through an adaptive episode--and eventually we would reach the same point.? Those who are naturally resistant to HIV will tend to survive and pass on their genes, while those who are not will tend to die off. ?But we don?t want to wait a million years to work through this epidemic. That?s why we?re working through the history of FIV and why we?re looking at the lion?s genome, and why we?re looking at the long-term HIV survivors. We want to find the genetic solution. The lions give us hope that that is possible.?
The lions should also give us another kind of comfort. They demonstrate that AIDS is not some unnatural horror unique to modern civilization. Immunodeficiency viruses strike not just humans but many animals, and our suffering is typical of this stage of virus-host coevolution. In short, the lions teach us that viruses are just another phenomenon of nature--and that nature may yet show us a way to master them.