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Letter
Nipah Virus Strain Variation
To the Editor: AbuBakar et al. described strain variation in Nipah
virus during the 1998–1999 outbreak in Malaysia (1).
They found an isolate from pigs in Perak, as well as from a flying fox,
that differed markedly from pig and human isolates from the main epidemic
in southern Malaysia. AbuBakar et al. proposed that this finding indicates
2 separate spillover events from bats to pigs occurred, the first in Perak
in 1998 and the second in southern Malaysia in 1999. However, investigations
at the time of the outbreak showed that many pigs were moved from Perak
onto southern farms in early 1999. We suggest that successive spillovers
of the pig population in the north can also explain the observed strain
differences between northern and southern isolates.
A model from experimental studies and active farm data demonstrate that
Nipah virus may have circulated repeatedly and become endemic within 1
or several large pig farms in Perak (J.R.C. Pulliam, unpub. data), which
is consistent with the occurrence of human cases in Perak before the 1998–1999
outbreak. Evolution of the virus population in pigs, followed by the reintroduction
in Perak of the original strain from bats and its subsequent southward
movement in infected pigs, would explain observed strain differences.
Models suggest that evolution of the virus within pig populations would
result in lower death rates but prolonged illness. Although the pig-adapted
virus strain may have circulated on both northern and southern farms,
sampling biases in favor of the more virulent strain would be expected
in areas of high death rates, which would explain the observed genetic
relationships between sequenced isolates.
We suggest that pigs be experimentally infected with the Perak strain
of Nipah virus to determine whether differences exist in illness and death
caused by this virus. Further sequencing of virus from archived pig samples
will clarify with greater confidence whether multiple strains circulated
in both regions.
Acknowledgments
This work is published
as part of a collaboration with the Australian Biosecurity Cooperative
Research Center for Emerging Infectious Disease.
Funding for the
Henipavirus Ecology Research Group is provided by a National Institutes
of Health/National Science Foundation 'Ecology of infectious diseases
award' (R01-TW05869) from the John E. Fogarty International Center.
Juliet R.C. Pulliam,*
Hume E. Field,† Kevin J. Olival,‡ and the Henipavirus Ecology Research
Group§1
*Princeton University, Princeton, New Jersey, USA; †Queensland
Department of Primary Industries, Brisbane, Queensland, Australia; ‡Columbia
University, New York, New York, USA; and §Consortium for Conservation
Medicine, New York, New York, USA
Suggested citation
for this article:
Pulliam JRC, Field HE, Olival KJ, and the Henipavirus Ecology Research
Group. Nipah virus strain variation [letter]. Emerg Infect Dis [serial
on the Internet]. 2005 Dec [date cited]. Available from http://www.cdc.gov/ncidod/EID/vol11no12/05-0220_05-1102.htm
Reference
- AbuBakar S, Chang LY, Ali AR, Sharifah SH, Yusoff
K, Zamrod Z. Isolation
and molecular identification of Nipah virus from pigs. Emerg Infect
Dis. 2004;10:2228–30.
1A full list of members of this group is
available at http://www.henipavirus.org
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In response: Pulliam et al. (1)
presented a model to help explain the observed Nipah virus (NV) strain
differences reported earlier by AbuBakar et al. (2).
The model is built around an assumption that NV was endemic in several
pig farms in the north of Malaysia and that a subsequent reintroduction
of the original NV caused the fatal encephalitis outbreak in 1998.
While the model is plausible, that NV infection was endemic among pigs
before the 1998 outbreak is difficult to imagine in the absence of verifiable
evidence. As with any virus that crosses species, NV would likely have
caused severe infection, and what happened in 1998 is a classic example.
Before NV could have evolved, become less virulent, and subsequently become
endemic, it would have been first introduced to pigs. This initial introduction
would have caused an outbreak, but no such outbreaks were reported before
1998. Furthermore, the life span of pigs reared in farms is relatively
short before they are sent to the markets, which limits the time in which
NV evolution could take place. Slaughtering these pigs would also have
caused infection among abattoir workers and pork handlers. At present,
the finding of 2 different NV strains from 2 different outbreak foci favors
the suggestion that 2 possibly overlapping NV outbreaks occurred in Malaysia
in 1998. Further investigation of NV archived materials would shed further
light into the possible origin of NV in the 1998 Malaysia outbreaks.
Sazaly AbuBakar*
*University Malaya, Kuala Lumpur, Malaysia
Suggested citation
for this article:
AbuBakar S. Nipah virus strain variation [Response]. Emerg Infect Dis
[serial on the Internet]. 2005 Dec [date cited]. Available from
http://www.cdc.gov/ncidod/EID/vol11no12/05-0220_05-1102.htm
References
- Pulliam JRC, Field HE, Olival KJ, and the Henipavirus
Ecology Research Group. Nipah virus strain variation. Emerg Infect Dis.
2005;11:1978–9.
- AbuBakar S, Chang L-Y, Mohd Ali AR, Sharifah SH, Yusoff K, Zamrod
Z. Isolation
and molecular identification of Nipah virus from pigs. Emerg Infect
Dis. 2004;10:2228–30.
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