The Amazing Story of a Mammal Virus That Became a Bird One

This is one the most extraordinary and convoluted evolutionary tales that I have ever heard. It’s the origin story of a group of viruses called REVs. It’s the tale of how naturalists and scientists inadvertently created a bird virus out of a mammalian one through zoo-collecting and medical research.

To understand it, we need to go back to 1957, when the very first REV was isolated from a turkey in America.

Or we should travel back to the years after World War II, when REVs spread around the world in vaccines that were meant to stop poultry diseases rather than cause them.

Better still, we should go back to the 1930s, when scientists analysed the blood of a beautiful Asian pheasant (captured from New Guinea and living in what’s now the Bronx Zoo) and found a new species of malaria parasite (which was a huge boon for malaria research but is now lost to science).

Actually, forget all that. Let’s start with the mongoose.

The mongoose and the turkey

The ring-tailed mongoose, a native of Madagascar, looks like a cross between a ferret and a red panda. It has a sinuous, rusty body and a fuzzy tail with black and red stripes. A few years ago, virus hunters Anna Maria Niewiadomska and Robert Gifford were scanning the mongoose’s DNA when they noticed the complete genome of an ancient virus.

Many viruses—the retroviruses, in particular—have a habit of inserting their genes into the genomes of their hosts. These sequences sometimes get passed down the generations and turn into genetic “fossils”—remnants of ancient infections that are now permanent parts of their hosts. An incredible 8 percent of your genome consists of these “endogenous retroviruses” or ERVs.

Niewiadomska and Gifford specialise in studying these fossilised viruses, and they’ve found more than anyone ever expected. Finding another one in the ring-tailed mongoose wasn’t odd. But its closest relative turned out to be a bird virus known as reticuloendotheliosis virus (REV), which was first isolated from a turkey in 1957. REV and its relatives infect a wide range of poultry, including chickens, ducks and geese. It stunts their growth, weakens their immune system, and occasionally causes cancer.

Niewiadomska and Gifford screened the genomes of other animals for similar fossilised viruses, but they couldn’t find any in birds. The only hits came from the narrow-striped mongoose (another Madagascan mammal), and the short-beaked echidna—a spiny, egg-laying mammal from Australia.

Now, that was odd. Why is a widespread bird virus absent from the DNA of any birds, but present in the DNA of three mammals that are separated by the Indian Ocean? And how does it move from mammals to birds (or vice versa) anyway? Retroviruses shouldn’t be able to do that. They might occasionally jump between distantly related animals, but sustained transmission in the new host is incredibly rare, if it ever happens. What was going on?

To find out, Niewiadomska and Gifford reviewed every published report of a REV outbreak, and got their hands on as many archived samples as they could. They sequenced everything and used those sequences (combined with the ERVs from the mongooses and echidnas) to create a family tree of the REV dynasty.

The tree unequivocally showed that REVs were originally mammal viruses. They arose no earlier than 25 million years ago. Between 18 and 8 million years ago, one of them entered the genome of a Madagascan carnivore and stayed there. The others continued to infect mammals and at some point, one of them hopped over into birds. This must have happened very recently—decades ago, rather than centuries. We know this because retroviruses change very quickly, but the genetic diversity of bird REVs is incredibly narrow.

Fortunately, Niewiadomska and Gifford’s viral family tree also gives us clues about when, where and how this jump took place.

The pheasant and the parasite

In 1935, a French scientist called Emile Brumpt identified a new parasite called Plasmodium gallinaceum, which causes malaria in poultry. Lowell T. Coggeshall, an American tropical disease specialist, saw the potential in this discovery. If the parasite could be easily raised in domestic birds, it would provide a convenient way of studying malaria. The problem was that he couldn’t get any—Brumpt’s parasite came from a Sri Lankan chicken and US laws frowned upon importing poultry diseases from foreign lands.

But the solution was already in America. Around 10 years earlier, an intrepid naturalist called Lee Saunders Crandall had travelled to New Guinea and, barring one unfortunate shipwreck, returned to the US with hundreds of captive animals. His collection lived in the New York Zoological Park, now Bronx Zoo. Coggeshall reasoned that one of these imported birds might be carrying a parasite similar to P.gallinaceum, and he was right. In June 1937, he found Plasmodium lophurae in the blood of a stunning Borneo firebacked pheasant.

That was the first and only time that P.lophurae has ever been isolated, but it was enough. Coggeshall and others kept it going by repeatedly injecting it into chicken, duck and turkey chicks. It became a mainstay of malaria research, especially in the post-war years when the search for anti-malarial drugs reached fever pitch. But eventually, scientists started using other malaria parasites instead, and P.lophurae left the limelight. The stocks were finally exhausted in the 1980s and no one has ever managed to find it again. It’s probably still out there, infecting wild birds in south-east Asia, but for now, this once-fashionable parasite is lost to science.  

But during its heyday, P.lophurae acted as a stepping stone for REV, on its crossing from mammals into birds.

As early as 1941, scientists suspected that P.lophurae stocks had been contaminated by… something. Infected animals were becoming anaemic, independently of their malarial symptoms. In 1959, William Trager identified the cause as a virus that was hiding in the stocks, which he called spleen necrosis virus (SNV). In 1972, a second contaminating virus was discovered—duck infectious anaemia virus (DIAV), found in P.lophurae from five different laboratories. Both of these were REVs.

At the time, everyone thought that SNV and DIAV were natural duck viruses that had somehow got into the parasite stocks. Niewiadomska and Gifford think otherwise. Their analysis says that SNV and DIAV came from the stocks themselves, which provided repeated opportunities for these mammalian viruses to sneak into birds.

Which mammal did they come from? No one knows. Laboratory mammals like mice or rabbits don’t harbour live REVs. Instead, Gifford suspects that “maybe one of the small south-east-Asian mammals that was housed in the Bronx Zoo was the source of the avian isolates.” Since REVs have never been found in a wild mammal, it could have been any of the zoo’s residents, although the smart money’s on bats. “The paper highlights the risks we create by bringing animals from disparate geographical origins into close proximity,” says veterinary virologist Glenn Browning.

Maybe this mystery mammal infected the firebacked pheasant that Coggeshall examined, so that the P.lophurae stocks were contaminated right from the start. Alternatively, it had plenty of opportunities to sneak in later, when the parasite was being bred and passaged from chick to chick. Either way, every time scientists injected P.lophurae into a bird, they gave the mammalian REVs a new opportunity to switch hosts.

The pox and the vaccine

REV genes turn up in the strangest places. They’ve also been found in the DNA of two completely unrelated bird viruses—fowlpox and gallid herpesvirus-2 (GHV-2). The latter causes Marek’s disease, which was a huge problem for the intensive poultry farms of post-war America and prompted an intense search for a vaccine. Progress was slow until the 1960s, when scientists developed ways of growing bird cells in laboratory cultures. Soon, we had created vaccines against both fowlpox and Marek’s, both using weakened versions of the respective viruses. They have saved countless numbers of domestic birds.

At the same time, malaria research was ramping up, and P.lophurae samples were whizzing around the country. Niewiadomska and Gifford think that these parallel streams of research allowed REVs to hop from the contaminated parasite stocks into the two bird viruses. “It’s likely that there were people working with both organisms,” says Gifford. “It’s circumstantial, but there was at least one company laboratory in the US that produced a commercial vaccine against fowlpox and worked with lophurae.”

When they got the chance, the REVs did what retroviruses like to do—they inserted their genes into a foreign genome. Only, in this case, those genomes belonged to the two other viruses—the weakened ones that were used to create the fowlpox and Marek’s vaccines. Aboard these vaccines, REVs hitchhiked around the world—a virus stowing away inside another virus, stowing away inside poultry, like the world’s worst turducken.

Occasionally, as earlier work has shown, they pop out to create REV outbreaks among birds that are vaccinated against fowlpox or Marek’s. This explains why all modern REVs are so genetically similar. They’re not circulating freely among the world’s birds, evolving as they go. They represent repeated incursions from the same stable staging ground.

There’s one possible exception. A REV that was recently isolated from China looks a bit different to the others, and might actually be an independently circulating virus. If that’s the case, it would be the icing on this extraordinary tale—of a mammalian virus that became confined with a malaria parasite, insinuated itself into other viruses, hitchhiked round the world in vaccines, and finally resumed its free-living existence, but this time as a bird virus.

Unintended consequences

“What an amazing paper!” says Vincent Racaniello from Columbia University. “Not only is the work an incredible detective story but it’s another example of how we can be so blind to exactly what viruses can do, even when we are as careful as possible. What we don’t know will always come back to haunt us.”

There are other similar stories of viruses arising or spreading through unexpected and ironic means. Last year, I wrote about a chicken virus called ILTV, which arose when two vaccines made from weakened viruses merged to create a new live one.

That’s unlikely to ever happen in humans (read the post for why) but our history isn’t short of accidents either. While trying to treat Egyptian people for schistosomiasis, a disease caused by parasitic worm, well-meaning healthcare workers accidentally injected them with needles contaminated by hepatitis C, kickstarting an epidemic that continues to this day. HIV has spread through Africa through similar means.

“I wonder how many other viral pathogens have we inadvertently spread?” asks Racaniello. “Fortunately, as this story shows, we now have the tools to determine what is in every virus stock that we produce, so such inadvertent infections should be a thing of the past.”

Gifford agrees that the story he discovered is unlikely to play out again but he points out that none of the scientists at the time could have anticipated what happened. They didn’t really know how retroviruses worked, or their propensity for smuggling themselves into their hosts, or even other viruses. “There could be things that we aren’t anticipating now that could be a threat,” Gifford says.

But let’s not throw the baby out with the bathwater. There are unquestionable benefits to doing basic research with pathogens like P.lophurae, or developing vaccines using live weakened viruses. The risks may be hard to assess, but there are ways of mitigating them.

Better surveillance—virus-hunting, in other words—tops Gifford’s list. “You’d have a few sentinel sites in key locations around the world that would routinely sequence viruses in their local environment and share their data.” This exists for some prominent infections like HIV and influenza, but Gifford thinks it’s time to expand such efforts to other viruses, including those that affect other animals and plants. “You wouldn’t need a lot of data to realise that something really strange like this is going on.”

Reference: Niewiadomska & Gifford. 2013. The Extraordinary Evolutionary History of the Reticuloendotheliosis Viruses. PLOS Biology

Credit: Thanks to Niewiadomska and Gifford who took the time to lay out the rich historical detail in their paper and a blog post.

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