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A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
Date: Mon 4 May 2009
Source: National Geographic News [edited]
Fast-evolving rabies virus found -- and spreading
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Evolving faster than any other new rabies virus on record, a
northern-Arizona rabies strain has mutated to become contagious among
skunks and now foxes, experts believe. The strain looks to be spreading
fast, commanding attention from disease researchers across the United
States. It's not so unusual for rabid animals to attack people on hiking
trails and in driveways, or even in a bar as happened 27 Mar 2009, when an
addled bobcat chased pool players around the billiards table at the
Chaparral in Cottonwood. Nor is it odd that rabid skunks and foxes are
testing positive for a contagious rabies strain commonly associated with
big brown bats.
What is unusual is that the strain appears to have mutated so that foxes
and skunks are now able to pass the virus on to their kin not just through
biting and scratching but through simple socializing, as humans might
spread a flu. Usually the secondary species in this case, a skunk or fox
bitten by a bat -- is a dead-end host. The infected animal may become
disoriented and even die but is usually unable to spread the virus, except
through violent attacks.
Skunks have already been proven to be passively transmitting the strain to
each other, as documented in a 2006 study in the journal Emerging
Infectious Diseases [see comment below - Mod.CP]. Genetic studies suggest
foxes are also spreading the new strain to each other, though the results
have not yet been peer reviewed.
When a skunk in Flagstaff, Arizona, died of rabies in 2001, wildlife
specialists thought it was a "freak accident" due to a one-off,
run-of-the-mill bat bite said Barbara Worgess, director of the Coconino
County Health Department. Laboratory tests later showed that the virus had
adapted to the skunk physiology and become contagious within the species.
"It shouldn't have been able to pass from skunk to skunk," Worgess said.
Rabies has continued to crop up in skunks for 8 years now, despite periodic
vaccination campaigns. And so far this year [2009], county officials have
documented 14 [now 33] rabid foxes in the Flagstaff area. Now laboratory
studies at the US Centers for Disease Control and Prevention (CDC) in
Atlanta appear to confirm that the fox and skunk rabies viruses are mutated
forms of the bat strain. "We can see degrees of relatedness and patterns in
their genetic codes," said Charles Rupprecht, chief of the rabies program
for the CDC. This sort of rapid evolution is exactly what worries public
health officials when it comes to all manner of viruses. Virologists
haven't seen such fast adaptation to a new species in rabies before. That's
why Flagstaff is such an interesting story worldwide," said David Bergman,
the US Department of Agriculture's (USDA) state director for Arizona.
"We're watching evolution in action on the ground."
Could rabies become contagious in humans? The Arizona rabies situation is
risky, because the infected species live so close to people. Flagstaff's
sprawl in recent decades has created a perfect opportunity for rabies to
mutate into species-hopping forms, the CDC's Rupprecht said. New-home
construction, often in wooded areas, has actually increased habitat and
food sources for bats, skunks, and foxes. Skunks live under houses, for
example, and as diggers, make themselves at home on golf courses. Bats,
meanwhile, are adept at living in attics and under loose shingles. As more
rabies-susceptible animals congregate in the region, more infections can
take place. And each infection is an opportunity for the virus to mutate
into a more virulent form literally upping the odds of a new strain
developing. "That's a pattern that we see all over the United States,"
Rupprecht said. Similar suburban development in the eastern US in the late
1970s, he noted, led to the spread of raccoon rabies from the Canadian
border to the Deep South.
The risk of such a virulent strain jumping to people "should be a major
concern," said Hinh Ly, a molecular virologist at the Emory University
School of Medicine in Atlanta, who is not involved in studies of the
Arizona outbreak. But no one is expecting the rabies strain to become a
contagious, swine flu-like epidemic among humans. Flu viruses, for one
thing, tend to infect people fast, so "vaccination after exposure would be
too late to prevent infection," said Elisabeth Lawaczeck, the Arizona
Department of Health Services' public health veterinarian. Rabies takes its
time before going from incubation to infection, so post-exposure rabies
vaccinations tend to be effective at stopping the virus. If untreated,
though, rabies, which attacks the central nervous system, is often fatal in
humans.
Rabies cases among animals are expected to increase as the spring and
summer mating seasons bring potential pairs and rivals together. Already,
Flagstaff has declared a 90 day pet quarantine all dogs on leashes and all
cats indoors which began in April [2009]. A wildlife vaccination plan could
stem the virus's spread. Local and state officials enacted vaccination
programs in northern Arizona in 2001 and 2005 but discontinued each effort
after 2 years without rabies, reports the World Health Organization's
standard for declaring an area rabies-free. Now state vaccination funds
have been reallocated, the USDA's Bergman said, and emergency funds are
increasingly rare due to the recession. Adding to the worries, Lawaczeck,
the Arizona veterinary official, said she and other public heath officials
were "very unsettled" when the 1st rabid fox reports came in from Flagstaff
this year and not just because of the evolutionary implications for rabies.
"This means a much wider spread of rabies," she said, "because [foxes]
travel so much farther."
[byline: Anne Minard]
communicated by:
ProMED-mail rapporteur Mary Marshall
[The following extracts from the publication referred to above -- Mira J
Leslie, Sharon Messenger, Rodney E Rohde, Jean Smith, Ronald Cheshier,
Cathleen Hanlon, et al. Bat-associated rabies virus, skunks. Emerg Infect
Dis 2006; 12(8): 1275 (Aug)
account of the background to the current situation in Arizona.
"In North America, more than 90 per cent of cases of rabies in animals
occur in wildlife; several mammalian taxa harbor characteristic rabies
virus variants. In Arizona, skunks (_Mephitis mephitis_) and gray foxes
(_Urocyon cinereoargenteus_) maintain independent rabies enzootic cycles,
and in indigenous bats, rabies has been diagnosed in 14 of 28 species.
"Although skunks live throughout Arizona, until 2001, rabid skunks had been
found only in the southeastern quadrant of the state. In the United States,
bat variant rabies viruses are a source of infection for humans and other
mammals. Typically, interspecies infection produces a single fatal
spillover event; secondary transmission has rarely been observed. Antigenic
typing of rabid carnivores in Arizona from 1996 through 2000 identified bat
variant rabies viruses in one domestic dog and 2 gray foxes. This report
describes the largest documented rabies epizootic among terrestrial mammals
infected with bat variant rabies virus, with perpetuated animal-to-animal
transmission. Coincident with the zoonotic disease significance, this
report provides contemporary insight into pathogen evolution."
"Investigation of this novel outbreak showed evolution in action with the
emergence of a rabies virus variant that successfully adapted from
Chiroptera to Carnivora. Previously documented clusters involving 3-4 to
terrestrial mammals infected with a single insectivorous bat rabies virus
variant did not corroborate sustained transmission. Although more than one
skunk may have been exposed to a single rabid bat, it is highly unlikely
that each skunk was exposed to the same bat or that multiple bat-skunk
exposures occurred. We could not ascertain the complete scope of this
outbreak or whether it was the index event. Phylogenetic analyses support
the evolution of 2 independent lineages, suggesting establishment for
months or years. Additionally, virus isolation from salivary glands of 5
affected skunks and the reappearance of rabid skunks with the same variant
rabies virus in 2004 support the probability of independent transmission."
Rabies in skunks and foxes is now widespread in Arizona (see
result of sustained transmission of distinct variants of bat rabies virus
genetically adapted to different mammalian hosts. The National Geographic
News article uses the term contagious in an inappropriate manner. The virus
has become modified for tranmission in a new host, but infection still
occurs via exposure to rabies virus-containing saliva. The virus is no more
or no less contagious; it has become genetically adapated for sustained
transmission in an alternate host. Sustained transmission in humans has so
far never been observed. - Mod.CP]
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