TAIPEI, Taiwan––Can dogs incubate and transmit Taiwan ferret badger rabies?
Would a mutant form of ferret badger rabies specific to dogs have the exceptionally long latency time that it apparently has in ferret badgers?
If so, this could exponentially increase the threat of rabies from dogs.
Any mammal may become infected with rabies, and may transmit rabies during the short “furious” phase of infection that immediately precedes terminal muscle contractions, paralysis, and death.
Just a handful of species, however, including bats, dogs, foxes, raccoons, skunks, mongooses, and ferret badgers, have evolved enough resistance to rabies to carry and transmit the disease in “dumb” form, meaning that the vector animals are infectious for a time without displaying obvious symptoms.
The longer an animal carries “dumb” rabies, appearing to behave normally, the greater the risk that the animal may infect many others. The normal latency interval for canine rabies is about two weeks, ranging up to several months, but the latency interval for Taiwan ferret badger rabies may be measured in years.
Known to science only since July 2013, Taiwan ferret badger rabies remains the most mysterious rabies strain, only slightly better understood as result of genomic identification than it was before. Findings published in the May 1, 2014 edition of Emerging Infectious Disease Journal suggest that Taiwan ferret badger rabies does not pass easily or often to dogs, but that it can, may be a mutated form that came originally from dogs, and that if dogs became a vector for it, rabies control could become much more difficult.
Six Taiwanese scientists contributed to the study “Molecular Characterization of Cryptically Circulating Rabies Virus from Ferret Badgers, Taiwan”: Hue-Ying Chiou, Chia-Hung Hsieh, Chian-Ren Jeng, Fang-Tse Chan, Hurng-Yi Wang, and Victor Fei Pang. The lead researcher was Pang, a professor at the National Taiwan University School of Veterinary Medicine in Taipei.
Lurking for 100 years
“After the last reported cases of rabies in a human in 1959 and in a nonhuman animal in 1961, Taiwan was considered free from rabies,” Pang et al reported. Indications of rabies among Taiwanese ferret badgers appeared in late 2012, but were not recognized until mid-2013. Pang and team discovered that the virus strain afflicting the Taiwan ferret badgers “is a distinct lineage within the group of lineages from Asia and that it has been differentiated from its closest lineages,” found in Chinese ferret badgers, dogs, and dogs in the Philippines, for 158 to 210 years.
This would mean the differentiation occurred between 1804 and 1856, coinciding with heavy immigration to Taiwan from Fujian and Guangdong on the Chinese mainland, and then a British invasion during the Opium War of 1840.
The Taiwan ferret badger rabies strain appears to have been circulating on Taiwan and exclusively among ferret badgers for 91 to 113 years, according to the genetic evidence. Thus the rabies strain became isolated among ferret badgers between 1901 and 1923––the time frame within which Japanese governors consolidated rule of Taiwan, after seven years of fighting frequent insurrections, 1895-1902; pursued rapid economic development; and redirected most commerce away from China, toward Japan.
Though ancestrally related to canine rabies, the Taiwan ferret badger rabies strain does not appear to have been influenced by dogs since that era.
“Although dogs are considered the principal host of rabies in developing countries, the virus is also dispersed among many species of wild carnivora and chiroptera [bats],” Pang et al noted. “In southeastern China, Chinese ferret badgers have been associated with human rabies for many years and are considered to be a primary host.”
This is a counter-intuitive finding because according to what is known of rabies in other species, few mammals should be less potentially susceptible to infection than ferret badgers. Whereas rabies is usually carried by highly sociable animals, 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 chance 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 Taiwan 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.
Rabies researchers at first believed that the Taiwan ferret badger rabies outbreak had somehow spread across the Straits of Formosa from the Chinese mainland. Pang et al investigated that hypothesis by studying rabies virus extracted from the remains of three dead Taiwan ferret badgers. Each ferret badger was found alive, but died under veterinary care. The ferret badgers came from relatively widely separated locations.
“One was in the Xitou nature education area at Lugu Township, Nantou County; one was in Gukeng Township, Yunlin County; and one was in Yuchih Township, Nantou County,” Pang et al wrote. This indicates that the rabies strain that the dead ferret badgers all carried is likely the only strain among Taiwan ferret badgers.
Through study of “the archived formalin-fixed and paraffin-embedded brain tissues of ferret badgers, kindly provided by various institutes,” Pang et al had already demonstrated that this rabies strain “could be traced back to 2004,” they recalled. That was a surprise. Discovering that the Taiwan ferret badger rabies strain has apparently been present on Taiwan throughout the time that the island was believed to be rabies-free was a scientific shock.
“Because previous rabies surveillance was mainly focused on dogs and bats, cases in remote areas might have gone unnoticed,”Pang et al allowed. “However, Taiwan is an island with a high population density; 23 million persons live in an area of 36,188 square kilometers, and for rabies cases to have gone unnoticed for more than 50 years would be very unusual.”
Taiwan ferret badger rabies may have emerged when it did, Pang et al offered, because “according to a recent survey about wildlife,” by Liang Kong Lin of the Laboratory of Wildlife Ecology at Tunghai University, “the ferret badger population has been increasing in the past five years.”
This would increase the numbers of potential hosts for the outbreak, and the likelihood of humans finding dying or dead ferret badgers, but usually an outbreak of an inevitably fatal disease among a growing population of animals suppresses the population growth.
“Therefore, despite the ancient history of the ferret badger’s association with rabies, the fact that its population is seemingly unaffected by infection with RABV is perplexing,” wrote Pang et al.
Taiwan ferret badger rabies is only known to have infected two other animals, a shrew from whose brain tissue the same strain was extracted, and a puppy, who was known to have been bitten by a ferret badger on August 14, 2013, fell ill on September 6, 2013, and was euthanized two days later.
The ability of Taiwan ferret badger rabies “to transmit across species is thus worthy of further investigation,”Pang et al concluded.
Animal experiments scheduled
“According to Tsai Hsiang-jung, director general of the Animal Health Research Institute under the College of Agriculture, experiments on mice will start within the next couple of weeks, followed by work on 5-month-old healthy badgers and then beagles,” reported Taiwan Today/United Daily News.
This may revive a controversy that exploded after 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. If the experiment proceeded afterward, it apparently was not reported.
“All of this leaves a big question standing in the room: Where has this virus been in the past 50-100 years? Saying it has been cryptically circulating merely describes the situation, but not how it hid,” commented Program for Monitoring Emerging Diseases ProMED-mail moderator Martin Hugh-Jones, a professor emeritus at Louisiana State University in Baton Rouge.
If the rabies virus can aesitivate [lurk in an inactive form] in bats, there may be a seasonal hibernation in ferret badgers. And what about congenital infections in utero, such as we see with leptospira and brucella? A stretch maybe. An interesting puzzle.”
Said Sean McCormack of The Sanctuary in Taiwan, who earlier cofounded Animals Taiwan and the Taiwan SPCA, “I just hope the research and findings are accurate. I have my fingers crossed that the unusual nature of this strain means it will stay relatively isolated.” Two studies, published in 2009 and 2013, have linked Chinese ferret badger rabies to cases in humans, first identified in 1994.
(See also “Newly found ferret badger rabies strain raises concern about dogs,” http://wp.me/p4pKmM-mP.)