September 2011 - Australian Veterinary Association

Euthanasia of Dusty the Dog
Letter from Dr Peter Reid as requested by Honorary Editor
I hope the following short piece is relevant and suitable,
and I haven't included or discussed the historical information
on relapsed Hendra and Nipah human patients
which is a very big subject in itself.
Polymerase chain reaction (PCR) results reported to the
Office International des Epizooties on Dusty were negative
on 4/7/11 and 25/7/11. Serology results reported
were the Enzyme-linked immunosorbent assay (ELISA)
test and Virus Neutralisation Test (VNT) which were
positive on 25/7/11. There was no report of an ELISA
antibody test being performed on 4/7/11. Serology was
reported to have been performed at the Australian Animal
Health Laboratory (AAHL), Geelong and a subsequent
serological test performed after 25/7/11 was reported
to confirm a positive result before Dusty was
euthanased by the owners’ private Veterinarian on
31/7/2011 although appearing well at the time.
We've known since 1995 that dogs are capable of being
infected experimentally, although they did not appear to
show symptoms (Westbury et al 1995 AAHL experimental
data), by mounting a serologically detectable immune
response. In addition, a survey of dogs in Malaysia
after the Nipah outbreak in 1999 confirmed seropositives
and apparent deaths attributed to acute infection.
Therefore in my view it wasn't unexpected to find a dog
naturally infected or seropositive during an outbreak
situation of HeV here in Australia.
What is unknown is whether dogs are infectious at
some stage after initial exposure. The exposure capable
of causing a detectable infection could occur after
the dog became infected by oro-nasal and/or mucous
membrane contact with an infected horse shedding virus,
or by the body fluids or excreta on the ground from
an infected horse, or even presumably by contact with
an infected bat shedding virus, e.g. mouthing and/or
eating an infectious bat, or by licking bat excretions containing
an infectious dose of virus.
Whether dogs are likely to relapse, and, even if they do
relapse, whether they are likely to be infectious, is also
unknown based on our current level of knowledge and
requires urgent scientific investigation and clarification.
We also need to know the minimum infectious viral dose
capable of causing infection, the pattern of antibody response
and duration of immunity and it is hoped that
these studies will be undertaken at AAHL following the
announcement of State and Commonwealth funding for
further research.
If we look at what has occurred with the closely related
Nipah virus (NiV) overseas, in one study published in
2000 after a Nipah outbreak, Parashar et al concluded
that direct, close contact with pigs was the primary
source of human Nipah infection, but other sources,
such as infected dogs and cats, could not be excluded.
Mills et al in another Nipah study published in 2009
found that the absence of NiV antibody in dogs less
than 15 km from the epidemic area, and the low prevalence
in populations nearer the epidemic, provided evidence
that the virus was not spreading by dog-to-dog
transmission and that the dog population was not acting
as an amplifying reservoir for NiV in the absence of infected
pigs. In addition, there were no reports of unusual
numbers of dead or sick dogs outside the immediate
disease-endemic area.
However, Mills et al concluded that they could not exclude
the possibility that dogs may have remained infectious
for some period after infection or that other dogs or
even humans may have become infected through contact
with infected dogs. In their view, the results indicated
that such infection was rare and was insufficient
to maintain and spread NiV in dog populations in the
absence of infected pigs.
Dusty the dog was challenged with live virus and seroconverted.
The difference between Dusty and vaccinated
horses is that it looks from the experimental data
so far that horses vaccinated with the sG (soluble G glycoprotein)
which are subsequently challenged with live
virus, will be protected and will not shed virus, that is not
only do the horses not succumb to Hendra disease, but
they are not infectious for humans. AAHL are also developing
a test to differentiate naturally acquired immunity
from vaccine induced immunity, i.e. the vaccine will
have so called DIVA capability.
_ References
Westbury HA, Hooper PT, Selleck P W, Murray P K.
(1995) Equine morbillivirus pneumonia: susceptibility of
laboratory animals to the virus. Aust Vet J 72:278
Parashar UD, Sunn LM, Ong F, Mounts AW, Arif MT,
Ksiazek TG, et al. Case-control study of risk factors for
human infection with a new zoonotic paramyxovirus,
Nipah virus, during a 1998–1999 outbreak of severe
encephalitis, Malaysia. J Infect Dis. 2000;181:1755–9
Mills JN, Alim ANM, Bunning ML, Lee OB, Wagoner KD,
Amman BR, et al. Nipah virus infection in dogs, Malaysia,
1999. Emerg Infect Dis [serial on the Internet]. 2009
Jun [date cited]. Available from http://www.cdc.gov/EID/
content/15/6/950
Honorary Editor’s Note:
We are indebted to Dr Reid for this information. Peter is
the Divisional representative on the Horse Industry Biosecurity
and Market Access Liaison Group (acronym
“Hamburger”).
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