On 28 February 2003, the highly pathogenic avian influenza
A virus subtype H7N7 (HPAI A/H7N7) was isolated for the first time
in the Netherlands from poultry on a farm, identifying the start of
a large epizootic that also affected Germany and Belgium. In the Netherlands,
infected poultry on 255 farms were culled, as well as poultry on 1094
surrounding farms, resulting in the killing of more than 30 million
chickens . Hygienic measures, and application of personal protective
equipment and antiviral prophylaxis were advised. The following weeks,
A/H7N7 was diagnosed by RT-PCR in 89 humans, of whom 78 had conjunctivitis.
A Dutch veterinarian reported having conjunctivitis, which developed
one day after he had visited an affected farm, and he died a week later
from respiratory distress . Three of the 89 cases were household
contacts of A/H7N7 confirmed cases and had no known exposure to A/H7N7
infected poultry. This strongly suggested human-to-human transmission,
either direct or indirect. All three patients had conjunctivitis, and
one also had influenza-like illness (ILI).
Influenza in humans and HPAI is caused by influenza
A virus, belonging to the family Orthomyxoviridae. All currently
known influenza A virus subtypes have been found to circulate in waterfowl
[3,4]. Avian influenza viruses have been known to infect humans, but
transmission between humans has so far only occurred sporadically [5,6,7].
Influenza A/H7N7 in humans was first reported in 1959 . In January
2004, human cases of influenza A/H5N1 related to an outbreak of avian
influenza A/H5N1 were identified in Vietnam and Thailand  and in
September 2004, probable human-to-human transmission was reported in
a family cluster in Thailand .
Simultaneous infection of a susceptible host with
a human and an animal influenza A virus could lead to re-assortment
of genetic material and consequently cause the generation of a virus
subtype capable of replicating and spreading between humans and with
surface proteins that are novel for the human population (antigenic
shift). Such strains could cause a major influenza pandemic
In order to measure secondary transmission of avian
influenza A/H7N7 in household members, to identify risk factors for
transmission, and to describe the clinical course of illness, we conducted
a retrospective cohort study among household members of infected poultry
Patients who were A/H7N7 confirmed index cases were contacted by telephone
for recruitment of their household members in the study. People living
on poultry farms or those who kept poultry in their gardens (backyards)
were excluded from the study.
An A/H7N7 confirmed index case was a person who
had conjunctivitis and/or ILI, who had been exposed to influenza
A/H7N7 infected poultry since 28 February 2003 in the Netherlands,
and who had positive influenza A/H7N7 laboratory results by PCR and/or
Conjunctivitis - a possible case
of A/H7N7 conjunctivitis was a household member with no known exposure
to poultry and with two or more of the following symptoms since 28
February 2003: red eyes, tearful eyes, itching eyes, painful eyes,
burning eyes, purulent fluid in eyes, or sensitivity to light. A confirmed
case of A/H7N7 conjunctivitis was a possible case of A/H7N7 conjunctivitis
with positive influenza A/H7N7 laboratory results by PCR and/or virus-isolation.
Influenza-like illness - a possible
case of A/H7N7 influenza was a household member with no known exposure
to poultry and with fever (if measured, then = 38.5°C), and at
least one of the following symptoms since 28 February 2003: cough,
rinorrhoea, sore throat, myalgia, or headache. A confirmed case of
A/H7N7 influenza was a possible case with positive influenza A/H7N7
laboratory results by PCR and/or virus-isolation.
Seropositive –a serology confirmed
case of A/H7N7 infection (symptomatic or asymptomatic) was a household
member who had an antibody titre of 1:10 or higher for influenza A/H7N7
by haemagglutination assay .
Information on demographics, occupation, smoking, medical history, pets,
contact with A/H7N7 confirmed index cases (including hygienic measures
by index cases and contacts), exposure to A/H7N7-infected poultry,
influenza vaccination status, and symptoms since 1 March 2003 were
collected using a standardised, self-administered, postal questionnaire.
All participants were asked to provide single serum samples, at least
3 weeks after diagnosis of the primary A/H7N7 case in their household,
to ascertain (sub)clinical infection with influenza A/H7N7. Sera were
tested in a modified haemagglutination inhibition as described in detail
by Meijer et al .
Ethical clearance for the study was obtained from the Dutch Medical Ethics
Data were analysed with STATA 8.0. For multivariate analysis of significant
or biologically plausible variables in univariate analysis we preferred
binomial to logistic regression because of high prevalence of positive
A/H7N7 serology in household members in this cohort study, which calls
for adjusted risk ratio’s rather than odd rations. Fisher’s
exact test was used to calculate significance.
Description of study participants
Of 86 households of A/H7N7 infected poultry workers, 63 (73.3%) households
agreed to participate and 14 declined. Nine poultry workers could not
be reached, of which four were immigrant workers that had returned
to their home country Poland. Of the 200 household members in the 63
participating households, 104 (52%) completed and returned the questionnaire.
Of these 104, 42 were excluded, as they had either been exposed to H7N7-infected
poultry, or were family members who were not living at the same address
as the index case. A total of 62 household members of 25 A/H7N7 confirmed
index cases were included in the study, with one single A/H7N7 confirmed
index case in each of these households.
The male:female ratio was 2:3. Mean age was 27.3 years, ranging from
0 to 61 years. The mean household size was 3.5 people (range 2 – 8).
Eight people (12.9%) reported health complaints. Two met the case definition
of conjunctivitis only, four met the case definition of ILI only and
two met both case definitions. In table 1, the risk factors for conjunctivitis
among household members are summarised. Attack rates were higher in
those who had allergies in their medical history than in those who
did not (RR = 10.3, 95% confidence interval 1.2 – 91.0).
In total, 56 of the 62 people in the cohort agreed to provide blood samples,
of which 33 (58.9%) had detectable antibodies against H7. Five of eight
household members with health complaints were serologically tested;
four (80.0%) had detectable antibodies against H7, of which two had
conjunctivitis only with onset two to six days after onset of symptoms
in the index case, and two had conjunctivitis as well as symptoms of
ILI with onset unknown or 5 days after onset of symptoms in the index
case. Out of 24 households serologically tested, 15 (62.5%) had one
or more household contacts with detectable H7 antibodies [TABLE 2].
A/H7 seroprevalence in household members was higher
among those who had pet birds (e.g., canary) kept indoors at home and
among those having any other indoor pets in their homes (e.g., cat,
dog, hamster) than among those who did not [TABLE 3]. Furthermore,
seroprevalence was higher among those who frequently used cloth handkerchiefs
than among those who did not. Conversely, those who used paper handkerchief
had a lower seroprevalence of H7 antibodies than those who did not.
Seroprevalence was higher among those who had at least two toilets
in their homes, than among those who had only one toilet. At household
level, seroprevalence was higher among the 17 households that had two
or more toilets in the home than among the 7 households with only one
toilet at home (RR = 2.7, 95% confidence interval 0.8-8.9, p = .061).
Family members of index patients who had their first poultry exposure
on or after 5 March 2003 had lower seroprevalence, showing borderline
significance, than household members of index cases with first poultry
contact before 5 March.
Two (3.2%) of 62 persons received the 2002-2003 influenza vaccination.
It was not possible to develop a stable model of significant
and biologically plausible risk factors in univariate analysis for
The HI assay had a sensitivity of 85% and a specificity
of 100% at a cut-off HI titre of =10. HI antibodies against influenza
A/H7, A/H1, and A/H3 were not cross-reactive with the heterologeous
virus. None of the human sera tested showed neutralisation of the A/H7N7
virus in the microneutralisation assay.
We describe the occurrence of infection with avian
influenza A virus subtype H7N7 in household contacts of human A/H7N7
confirmed index cases, in the absence of contact with infected poultry.
Thirty three of 56 household members (58.9%) had an A/H7N7 infection
confirmed by RT-PCR or serology, four of 62 household members (6.5%)
met the possible case definition of conjunctivitis and all four cases
(100%) had positive H7 serology.
The authors assume that the presence of H7-antibodies
is indicative of a past AI A/H7N7 infection. This is supported by the
results of another study in which the prophylactic use of oseltamivir
was found to significantly reduce the seroprevalence of H7 antibodies
in professionals exposed to infected poultry using the same serological
test . In that study, a significant association was found between
the presence of H7 antibodies and the occurrence of eye symptoms, after
correcting for prophylactic use of oseltamivir.
When using the adjusted HI assay, but not when using
the microneutralisation assay, we detected a measurable antibody response
in a high proportion of sera from persons exposed to laboratory-confirmed
A/H7N7 infected persons. Evidence that these antibodies are real comes
from three observations. First, any cross reaction of the A/H7 specific
HI-assay with antibodies against A/H1 or A/H3 viruses would have been
detected in the sera from persons recently vaccinated with the seasonal
human influenza vaccine, but no reaction (0%) in the A/H7 HI assay
was found. Second, as the sera of the recently vaccinated persons were
collected in autumn 2002, just before the H7 epizootic started, the
anti-H7 antibodies in the household contacts can not be explained as
being the result of previous circulation of A/H7 virus. Third, none
of the samples collected in autumn 2002 from 100 recently vaccinated
persons had reactivity with the adjusted H7 assay . This suggests
that our results cannot be explained by aspecific reactivity of the
Our results suggest that during the outbreak of avian
influenza A virus, subtype H7N7, household members of poultry workers
were at increased risk of avian influenza either by direct (person
to person) or by indirect (fomite) transmission. Previous observations
of influenza transmission within households had shown secondary attack
rates among household members of influenza cases in the same high range
as observed in our study . These high secondary attack rates are
in contrast with findings for subtype A/H9N2 and A/H5N1, where no to
limited secondary transmission was observed among healthcare workers
and household contacts of cases [5,6,7,14,15]. However, we used a method
for the detection of antibodies against the H7 virus which has a high
analytical sensitivity. Detailed studies to analyse person to person
transmission of H5 and H9 with the same methodology are sparse. Interestingly,
for H9, a recent publication showed that in 44.6% of suspected cases
of H9N2 infection and in 33.5% of the general population in Shantou
city in China, antibody titres against H9 could be detected . This
observation suggests that secondary transmission of H9 viruses may
be more common than has previously been assumed. In addition, the primary
site of infection, the conjunctiva for H7 virus and the airway epithelium
for H5 and H9 virus, and the possible difference in virus receptor
expression on the conjunctiva and the airway epithelium together with
the difference in affinity of the respective viruses for these receptors,
may also account for the observed differences.
Although sharing bath towels and washcloths, and using
cloth handkerchiefs seemed to increase the risk of clinical conjunctivitis,
none of these observations was statistically significant, presumably
due to lack of study power. However, it seems plausible to assume that
patients with a viral conjunctivitis are more likely to expose household
members to virus when sharing towels and washcloths or using cloth
handkerchiefs. This is supported by our observation of higher seroprevalence
among people using cloth handkerchiefs and of lower seroprevalence
among those using paper (disposable) handkerchiefs, all of which were
statistically significant. Studies on transmission of other viral conjunctivitis
within households identified crowding and high numbers of persons per
bathroom as risk factors [17, 18, 19, 20,21].
Seroprevalence was significantly higher among those
who had at least two toilets in their homes than among those who had
only one toilet. We have no explanation for this result. Hygienic measures,
such as using soap for handwashing and good hygiene by the index case,
associated with seropositivity were of borderline significance. Although
we observed higher seroprevalence in those household members who had
pet birds kept indoors at home, this cannot account for all seropositive
secondary cases, as only 7 of all 33 cases had indoor birds at home.
However, this finding raises the question of whether indoor pet birds
could play a role in the household transmission of avian influenza
virus, especially since six of seven cases with pet birds in the home
were part of the same household. It is conceivable that these animals
could serve as an amplifier for multiplication and shedding of the
virus in the home environment. This deserves further attention in future
outbreaks, for example, by monitoring and screening pet birds in the
homes of poultry workers.
It was not possible to perform binomial regression
for the outcome of A/H7N7 infection, presumably due to low numbers
in the cohort.
If the detection of H7 antibodies is indicative for
human (subclinical) influenza A/H7N7 infection, then the secondary
spread of A/H7N7 to household contacts is on an unexpectedly large
scale. Although the pathogenity of the A/H7N7 virus seemed to be low,
the high transmissibility is directly related to an increased risk
for double infection and reassortment. Current outbreak control measures
did not take transmission to household contacts into account. This
also raises the question of whether or not subclinical A/H7N7 cases
can transmit the virus efficiently to other close contacts, which would
imply that outbreak control strategy for A/H7N7 should be thoroughly
revised. Consideration may be given to early isolation of cases and
quarantine of contacts. Prophylactic treatment with oseltamivir should
be considered for all household contacts of poultry workers during
outbreaks of avian influenza, although its role must be further assessed
in order to determine the risk of developing antiviral resistance.
Moreover, in order to assess the role of fomites in secondary transmission
of the A/H7N7 virus, further studies of contacts outside the household
should be performed, as well as investigations to obtain background
information on the spread of A/H7N7 in the general population of the
The study had the following limitations. Non-response
was high and may be associated with rates of illness (selection bias),
but we see no reason why it would have differed between exposed and
non-exposed members of the cohort, therefore not biasing the estimate
of the risk ratios. However, selection bias is not likely to play a
major role with respect to seroprevalence, since most household members
with detectable antibodies were asymptomatic.
In conclusion, our study suggests that human-to-human transmission of
HPAI A/H7N7 can occur within household contacts in the absence of contact
with infected poultry. Monitoring of clinical symptoms alone in household
contacts of confirmed A/H7N7 cases underestimates the extent of human-to-human
spread. In addition, our results suggest that cloth handkerchiefs, having
indoor pet birds at home or having at least two toilets at home could
be risk factors for household transmission A/H7N7 .
Taking all the results together, we recommend that during an outbreak
of avian influenza: 1) Household members should be encouraged to use
paper handkerchiefs instead of cloth handkerchiefs; 2) Household members
of poultry workers exposed to A/H7N7 should be advised on enhanced general
hygiene measures; 3) In the case that oseltamivir prophylaxis is offered
to exposed poultry workers in future A/H7N7 epizootics, this should also
be considered for household members of A/H7N7 cases; 4) Indoor pet birds
of poultry workers should be screened and monitored during future outbreaks
of avian influenza, in order to determine the role of indoor birds in
household transmission of the virus; and 5) Further seroprevalence studies
among contacts of asymptomatic persons with positive H7 serology should
be conducted in order to assess the risk of person to person transmission,
and consequently the potential for a new pandemic strain, in the absence
We thank Alain Moren, James Stuart and Katrine Borgen,
European Programme for Intervention Epidemiology Training (EPIET),
for review of the manuscript; Susan van den Hof, RIVM, and Gabrielle
Breugelmans, EPIET fellow, for their help in the statistical analysis;
and Jim van Steenbergen, and Jean-François Aguilera for their
support for this study. G. Natrop, Municipal Health Service ‘Gelderland
Midden’, Arnhem, the Netherlands, for facilitating the sampletaking
and diagnostics in the study population.