|
|
Disease Origins
Dispatch
SARS-related Virus Predating
SARS Outbreak, Hong Kong
Bo Jian Zheng,* Yi Guan,* Ka Hing Wong,† Jie Zhou,* Kin Ling Wong,*
Betty Wan Y. Young,‡ Li Wei Lu,* and Shui Shan Lee*
*University of Hong Kong, Hong Kong Special Administration Region, People’s
Republic of China; †Department of Health, Hong Kong Special Administrative
Region, People’s Republic of China; and ‡Pamela Youde Nethersole Eastern
Hospital, Hospital Authority, Hong Kong Special Administrative Region,
People’s Republic of China
Suggested citation
for this article:
Zheng BJ, Guan Y, Wong KH, Zhou J, Wong KL, Young BWY, et al. SARS-related
virus predating SARS outbreak, Hong Kong. Emerg Infect Dis [serial online]
2004 Feb [date cited]. Available from: URL: http://www.cdc.gov/ncidod/EID/vol10no2/03-0533.htm
Using immunofluorescence
and neutralization assays, we detected antibodies to human severe acute
respiratory syndrome–associated coronavirus (SARS-CoV) and/or animal
SARS-CoV–like virus in 17 (1.8%) of 938 adults recruited in 2001. This
finding suggests that a small proportion of healthy persons in Hong
Kong had been exposed to SARS-related viruses at least 2 years before
the recent SARS outbreak.
A novel coronavirus has been identified as the cause of the 2003 global
outbreak of severe acute respiratory syndrome (SARS) (1–5).
Genetic analysis and epidemiologic studies suggest that SARS coronavirus
(CoV) was introduced into humans not long ago. Recently, SARS-CoV–like
viruses were isolated in Himalayan palm civets and racoon dogs in a retail
live animal market in Guangdong Province, southern China (6),
and some of the animals tested had antibodies to SARS-CoV–like virus.
Phylogenetic analysis showed that the SARS-CoV–like animal viruses were
closely related to the viruses found in humans. Serologic surveillance
demonstrated that, in the same market, approximately 40% of wild animal
traders and 20% of animal slaughterers had antibodies to SARS-CoV or SARS-CoV–like
animal virus, but none of them had had SARS-like symptoms in the past
6 months. These investigations raised questions about whether the presence
of the animal SARS-CoV–like virus in the market was an isolated event
or if this virus had been prevalent in the human population in southern
China before the SARS outbreak. A retrospective serologic study was conducted
to address these questions.
The Study
Serum samples collected in May 2001 from 938 healthy Chinese adults in
Hong Kong and 48 confirmed SARS patients diagnosed in February and March
2003 in Guangdong were studied. All serum samples were aliquoted and stored
at -20°C. The healthy adults were totally asymptomatic persons randomly
recruited after a telephone interview concerning hepatitis B virus. The
signs and symptoms of the SARS patients met the World Health Organization’s
definition for surveillance, and SARS-CoV infection had been confirmed
virologically.
All serum samples were heated at 56°C for 30 minutes. Specific antibodies
for SARS-CoV and SARS-CoV–like virus were tested by using immunofluorescence
(IF) assay at 1:10 dilution on FRhK-4 cells infected with either a human
SARS-CoV strain (GZ50) (5) or an animal SARS-CoV–like
virus (SZ16) (6), as reported (1). For
sera positive for anti–SARS-CoV or anti–SARS-CoV–like virus, the antibody
titer was further determined by serial titration. The IF-positive serum
samples were serially diluted from 1:20 to 1:640 and then mixed with 100
50% tissue culture infective dose (TCID)50 of the representative
human or animal virus strains for a serum neutralization assay. After
incubation for 1 hour at 37°C, the mixture was inoculated in triplicate
onto 96-well plates of FRhK-4 cell cultures. The results were determined
after 3-day incubation at 37°C.
Seventeen (1.8%) archived samples from healthy adults showed IF antibodies
against the human virus, animal virus, or both (titer range 1:20 to 1:1,280)
and were confirmed by serum neutralization assay. An additional six samples
were IF-antibody positive at a 1:10 dilution to either animal or human
viruses, but they were negative in neutralization assay and were treated
as negative. The positive rate was highest in the group ages 51 to 60
years and appeared to be more prevalent in female (13/561, 2.3%) than
male patients (4/377, 1.1%) (Table). Of the 17 seropositive
serum samples, 10 were from housewives, retired, or unemployed persons;
6 were from clerks, unskilled workers, or students; and one was from a
professional (Table). Most of the seropositive persons
(13/17) had a higher IF or neutralization antibody titer to the animal
virus than the human virus (Figure). By contrast,
the control group, comprising convalescent-phase sera from 48 confirmed
SARS patients recruited from hospitals in Guangdong, all showed positive
antibody results for both human SARS-CoV and animal SARS-CoV–like viruses,
but they invariably exhibited higher IF and neutralization antibody levels
against the human virus than the animal virus (Figure).
Conclusions
While the exposure history and symptoms of study participants were unavailable
for assessment, our results suggest that a small portion of Hong Kong
adults had acquired a SARS-CoV–related virus infection at least 2 years
before the 2003 SARS outbreak. Cross-reactivity of the antibody to human
SARS-CoV and the animal SARS-CoV–like virus must have occurred, in view
of the marked similarity between the two viruses. Recently, we reported
that the very similar sequences differed only by 60 to 80 nt, including
an additional 29 nt in the animal virus (6). We speculate
that the viruses that affected the 17 healthy persons >2 years ago
were antigenically closer to the recently isolated animal SARS-CoV–like
virus than human SARS-CoV, but interspecies transmission from animal to
human was probably inefficient as the viruses might not have adapted in
the new host. This hypothesis would explain why only a few persons became
infected and why they were likely to be asymptomatic. Avian influenza
is another example of a virus appearing first in animals before causing
a human disease. While approximately 3%–10% of healthy persons who were
in close contact with farm or market chicken or fowls showed positive
antibody to avian influenza viruses at the time of the H5N1 outbreak in
humans in 1997, none of them had symptoms of influenza (7).
Although human SARS-CoV and animal SARS-CoV–like viruses are related
to the three families of coronaviruses that cause respiratory and gastrointestinal
diseases in animals, phylogenetic analysis has shown that they are different
enough to make up their own, fourth group. The number of members in this
new group is not clear. Important factors in the emergence of novel infectious
diseases from animal sources include extensive exposure and rapid virus
evolution (8), which facilitate human-to-human transmission.
The growth of the demand for wildlife in markets in Guangdong in the past
15 years has provided an ideal platform to facilitate interspecies virus
transmission from animals to humans. Such factors could even directly
trigger a zoonotic disease outbreak. Our observations distinguished two
distinct serologic patterns. The high ratio of antibodies to the animal
virus compared to the relatively low ratio of antibodies to the human
virus in a small proportion of healthy adults >2 years ago signifies
the circulation of a SARS-CoV–like virus and its ineffective propagation
in the human population. Following rapid virus evolution and in the presence
of an unknown trigger, the novel SARS-CoV may have effectively adapted
to the human host, as illustrated by a second pattern characterized by
a higher human-to-animal virus antibody titer in infected persons. Although
this pilot study was limited by an unstandardized design of sample collection,
our preliminary findings suggest that the occurrence of SARS might not
be due to an isolated cross-species transmission event, but rather to
the rapid evolution of a related virus that has taken root in the human
population. This implies an expected pattern of potential SARS recurrence.
Measuring the prevalence of the two antibodies in different species of
animals and persons who had close contact with the animals is important
to improve our understanding of SARS-CoV transmission dynamics.
Acknowledgments
We acknowledge the
excellent technical assistance of Xiu Ying Zhao, Hai Ng, and the nursing
team of the Integrated Treatment Centre.
This project was
supported by research funding from Research Grants Council-SARS grants,
Hong Kong, and SARS Research Grant, the University of Hong Kong.
Dr. Zheng has been
an assistant professor in the Department of Microbiology, the University
of Hong Kong, since 1997. His research field is immunotherapy for chronic
viral infections and related cancers, with a focus on chronic hepatitis
B infection, HIV/AIDS, and nasopharyngeal carcinoma.
References
- Peiris JSM, Lai ST, Poon LL, Guan Y, Yam LY, Lim W,
et al. Coronavirus
as a possible cause of severe acute respiratory syndrome. Lancet
2003;361:1319–25.
- Poutanen SM, Low DE, Henry B, Finkelstein S, Rose D, Green K, et al.
Identification
of severe acute respiratory syndrome in Canada. N Engl J Med 2003;348:1995–2005.
- Ksiazek TG, Erdman D, Goldsmith C, Zaki SR, Peret T, Emery S, et al.
A
novel coronavirus associated with severe acute respiratory syndrome.
N Engl J Med 2003;348:1953–66.
- Fouchier RA, Kuiken T, Schutten M, van Amerongen G, van Doornum GJ,
van den Hoogen BG, et al. Aetiology:
Koch’s postulates fulfilled for SARS virus. Nature 2003;423:240.
- Zhong NS, Zheng BJ, Li YM, Poon LLM, Xie ZH, Li PH, et al. Epidemiological
and aetiological studies of patients with severe acute respiratory syndrome
(SARS) from Guangdong in February 2003. Lancet 2003;362:1353–8.
- Guan Y, Zheng BJ, He YQ, Liu XL, Zhuang ZX, Cheung CL, et al. Isolation
and characterization of viruses related to the SARS coronavirus from
animals in southern China. Science 2003;302:276–8.
- Bridges CB, Lim W, Hu-Primmer J, Sims L, Fukuda K, Mak KH, et al.
Risk
of influenza A (H5N1) infection among poultry workers, Hong Kong, 1997–1998.
J Infect Dis 2002;185:1005–10.
- Holland JJ, de la Torre JC, Clarke DK, Duarte E. Quantitation
of relative fitness and great adaptability of clonal populations of
RNA viruses. J Virol 1991:65:2960–7.
| Table.
Distribution of age, gender, and occupation of SARS-CoV–seropositive
adults recruited in 2001a |
|
| Age (y) |
No. of positive/total (%)
|
No. of positive/total in males (%)
|
No. of positive/total in females (%)
|
|
Occupation groupsb
|
No. of positive/total (%)
|
|
|
|
17–30
|
2/162 (1.2)
|
0/73 (0)
|
2/89 (2.2)
|
1
|
10/367 (2.7)
|
|
31–40
|
3/236 (1.3)
|
0/93 (0)
|
3/143 (2.1)
|
2
|
5/235 (2.1)
|
|
41–50
|
6/283 (2.1)
|
1/100 (1.0)
|
5/183 (2.7)
|
3
|
2/221 (0.9)
|
|
51–60
|
4/150 (2.7)
|
3/57 (5.3)
|
1/93 (1.1)
|
4
|
0/110 (0)
|
|
>60
|
2/107 (1.9)
|
0/55 (0)
|
2/52 (3.8)
|
5
|
0/5 (0)
|
|
Total
|
17/938 (1.8)
|
4/378 (1.1)
|
13/560 (2.3)
|
|
17/938 (1.8)
|
|
| aSARS-CoV, severe
acute respiratory syndrome–associated coronavirus. |
| bGroup 1: Housewives
(235), retired persons (96), and unemployed persons (36); Group 2:
clerks (141), students (40), and associate professionals (54); group
3: service workers (47), craft-related workers (41), machine operators
(56), and unskilled workers (77); group 4: managers and administrators
professionals (33), professionals (35), civil servants (9), and sales
persons (33); group 5: undefined. |
|
