First recognized in July 2013
TAIPEI, Taiwan––A new rabies strain identified in Taiwanese ferret badgers may have the potential to exponentially increase the risk of rabies transmission by dogs. But even if the new rabies strain does not behave in dogs as it does among ferret badgers, it has ignited unprecedented public controversy in Taiwan over the value of animal testing.
First recognized in July 2013, the ferret badger rabies strain had by September 17, 2013 been found in 131 ferret badgers, one Asian house shrew, and an unvaccinated six-week-old puppy who was known to have been bitten by a ferret badger. Cases were found in nine different cities and counties.
“A total of 556 wild carnivores, 273 wild animals of other types, 714 dogs, 49 cats, and 42 bats have been tested,” the Central Epidemic Command Center told the Taipei Times.
The first human victim
The first human victim, a male adult, was bitten on his right index finger and right ankle by a ferret badger on September 15, 2013. He received post-exposure rabies vaccination and human rabies immune globulin two days later, Taiwan Center for Disease Control deputy director general Chuang Jen-hsiang said. The man was expected to avoid rabies infection.
Taiwan had been officially free of all forms of rabies since 1961. But a retrospective examination of 13 ferret badger carcasses preserved since July 2010 found that five were rabid, the Council of Agriculture and Central Epidemic Command Center disclosed on September 6, 2013.
Commented Program for Monitoring Emerging Diseases moderator Craig Pringle, “Although rabies virus infection of domesticated animals has not been detected for at least 52 years, it cannot now be concluded that rabies was ever eradicated from the island. It is still a possibility that the virus was reintroduced into wild animals more recently,” Pringle conceded.
Conversely, “The shy and retiring habit of ferret-badgers and their lack of contact with both wild and domesticated animals and humans may have allowed rabies virus infection to have persisted in Taiwan for a long period,” Pringle said.
Outbreaks in China
Three research papers published by mainland Chinese scientists between 2009 and 2013 warned that ferret badgers may be a vector species for rabies, as are dogs, foxes, raccoons, skunks, and bats. Wrote S. Zhang, Q. Tang, and Wu X in a 2009 edition of the journal Emerging Infectious Disease, “Rabies in ferret badgers occurred during two alleged epizootics (1994-1995 and 2002-2004) in southeastern China. Our preliminary data suggests another probable epizootic of rabies in ferret badgers during 2007-2008. Rabies in ferret badgers is becoming a greater public health threat to humans in eastern Anhui, middle to western Zhejiang, and northern Jiangxi provinces in China.
“Because no practical rabies vaccine has been developed for wildlife in China,” Zhang et al warned, “a rabies epidemic in ferret badgers is almost inevitable without intervention, and the threat to public health is immediate. Lack of communication and cooperation among the Chinese Center for Disease Control and Prevention, Ministry of Agriculture, and wildlife services from the Bureau of Forestry makes the situation more complicated than canine rabies control. Whether rabies in ferret badgers is a spillover event from rabid dogs, or whether ferret badgers serve as a natural reservoir remains to be addressed.”
Genetic sequencing done by the Taiwan Council of Agriculture indicated that the similarities between the rabies virus found in Taiwanese ferret badgers and the strain found in China appeared to be less than 90%, meaning that they would be classed as different strains, albeit closely related.
Any mammal may become infected with rabies, and may transmit rabies during the “furious” phase of infection that immediately precedes terminal muscle contractions, paralysis, and death.
Only the “vector,” “host,” or “reservoir” species, however, have evolved enough resistance to rabies to carry and transmit it before displaying active symptoms. As actively rabid animals are suffering from a high fever and gradually losing mobility, they tend to be recognized and avoided.
Animals carrying “dumb” rabies, however, behave almost normally for weeks or months before symptoms become evident, and may infect many others. As rabies is usually transmitted in warm saliva, social grooming behavior that involves licking fur is perhaps a more common mode of transmission than bites.
Nocturnal solitary dwellers
The most perplexing aspect of the appearance of rabies among Chinese and Taiwanese ferret badgers is that few mammals are believed to be less potentially susceptible to infection. Ferret badgers are nocturnal solitary dwellers, who normally have little contact with either others of their own species or other mammal species.
Since ferret badgers are not known to have much if any opportunity to infect each other, except at birth and mating, they would appear likely to be able to carry rabies in the “dumb” stage for much longer than any of the other known hosts other than bats, in whom the ancestors of all rabies strains are believed to have evolved.
The slow emergence of rabies among ferret badgers in a recognizable form may reflect maternal transmission of an exceptionally slow-developing rabies strain, which may have been spreading for many generations before enough ferret badgers were infected and behaving abnormally to attract notice.
Usually born in May and June, in litters of two or three, young ferret badgers disperse a month to two months later, coinciding with when the first rabid ferret badgers of 2013 were found.
Of greatest concern from a public health and zoonotic disease control perspective is that an exceptionally slowly emerging rabies strain which behaves the same way in dogs might markedly extend the length of time when dogs might infect other animals––and humans.
The quarantine period necessary to establish whether a dog who is infected with the ferret badger rabies strain is rabid might be years, rather than the conventional two weeks if the dog has been vaccinated or two months if not. Meanwhile, the dog might be able to infect other victims for considerably longer before being recognized as ill. Humans and other non-host species with low resistance to rabies would be at greater risk of dying from unidentified cases––and at greater risk of infecting others, brief though the terminal phase in most victims is.
As of early October 2013, examining brain tissue slides under a microscope––negribody fluoroscopy––has identified the disease in ferret badgers as an apparent rabies strain, but has not definitively identified the specific strain. The paucity of cases found in other species may indicate either low transmissibility or just that ferret badgers tend to avoid other animals.
There is no information as yet about how the ferret badger rabies strain might behave in mature animals of other species. The one infected puppy was bitten by a ferret badger on August 14, 2013; fell ill on September 6; and was euthanized on September 8. The progression of that case appeared to be typical of other rabies strains, but a puppy still weeks too young to have been vaccinated successfully does not necessarily model accurately what would happen in adult dogs.
The conventional way to find out how the ferret badger rabies strain behaves in dogs would be to inject the virus into dogs, observe the dogs’ behavior until they fall terminally ill, kill them, decapitate them, and examine their brain tissue under a fluoroscope, looking for the bullet-shaped tiny black “cinders” called negribodies that are indicative of rabies infection.
Taiwan Bureau of Animal & Plant Health Inspection and Quarantine director Chang Su-san in mid-August 2013 announced that researchers would inject the ferret badger rabies virus into 14 beagles. A Facebook protest page posted by Animal Rescue Team Taiwan reportedly generated more than 3,000 endorsements within just a few hours.
Taiwan Animal Health Research Institute director-general Tsai Hsiang-jung defended the proposed study in media statements, but Bureau of Animal & Plant Health Inspection and Quarantine official Liao Mei-hui on August 19, 2013 told Taipei Times staff reporter Alison Hsiao that the experiment had been indefinitely postposed, pending discussion with a visiting team from the U.S. Centers for Disease Control and Prevention.
“Experimentation on animals still has to be conducted”
A month later the beagle experiment was again imminent, despite attempted intervention by both Physicians Committee for Responsible Medicine founder Neal Barnard and Humane Society International president Andrew Rowan.
“Experimentation on animals still has to be conducted, because the virus strain found in the ferret-badger is idiosyncratic,” Bureau of Animal & Plant Health Inspection & Quarantine director Chang Su-san told the Taipei Times on September 20, 2013.
Countered Rowan, “As far as we have learned, the U.S. CDC experts made no recommendations for experimentation on dogs. It was the impression of the American experts that Taiwan was not in a position to even consider tests on live animals because there does not exist a competent laboratory capacity to work with a live culture of the virus,” a surprising claim in view that Taiwan has a $4.4 billion pharmaceutical industry competing in the international marketplace, and attracted 90,000 “medical tourists” in 2012, many of whom visited to undergo more advanced treatments than they could obtain or afford in their home nations.
“Mass vaccination is the most advisable approach”
Continued Rowan, focusing on the transmissibility of the ferret badger rabies strain rather than the unique behavior of it, “The experts also pointed out to us that while some animals are the reservoir animal for a particular variant, all variants can be transmitted to other mammals. Live experimentation is therefore unnecessary. We agree that determining if current rabies vaccines protect against the strain recently found in ferret badgers is important,” Rowan added. “This determination can be achieved through the use of virus neutralization assays with serum from previously vaccinated dogs.”
But that would not tell how long the ferret badger rabies strain might persist in the “dumb” phase in unvaccinated dogs.
“Mass vaccination of dogs and ferret badgers is the most advisable approach to go,” Rowan concluded.
Barnard three days later asked PCRM supporters to contact Taiwanese embassies with essentially the same talking points.
As yet lacking either a vaccine specific to ferret badgers, a means of distributing it, or even reliable information about how many ferret badgers might inhabit Taiwan, the Taiwan Center for Disease Control has since July 2013 focused on encouraging vaccination of dogs and cats in the areas where suspected rabid ferret badgers have been found.
Former Taiwan Animal Health Research director Liou Pei-pai in an August 14, 2013 guest column for the Taipei Times argued meanwhile that the ferret badger disease might be a lookalike disease, not actually rabies.
Tunghai University life science professor Lin Liang-kong contended ferret badgers might be not hosts but victims of either rabies or another rabies-like disease spread by bats.
Though the disease afflicting ferret badgers appears to be rabies, there is a chance that it might be a similar but as yet unknown virus in the lyssavirus family. Lyssavirsuses include both rabies and the also deadly hendravirus, discovered in Australia in 1994 when it killed a man named Vic Rail and 14 of his horses.
Four of the 14 known human hendravirus victims have died.
Hendravirus is carried by flying foxes, a type of fruit bat, whereas rabies is carried primarily by insectivorous bats.
Mutated lyssavirus strain?
Thirteen fruit bat species inhabit China; several are also found in Taiwan, along with one species not found anywhere else. But insectivorous bats, usually smaller and much more numerous, are believed to be almost routinely blown back and forth by the prevailing winds between Taiwan and mainland China. Either fruit bats or insectivorous bats might have evolved a mutated lyssavirus strain that could have been separately passed to ferret badgers on either side of the straits that divide Taiwan from the mainland.
There is a precedent for the theory that the ferret badger disease spread from bats. Ferret badgers are mustelids, more closely related to skunks than to other rabies host species. Skunks, also nocturnal insectivores, are believed to host a rabies strain that came directly from bats, whereas, the fox, dog, and raccoon rabies strains are more closely related to each other than to bat and skunk rabies.