Transmission of Influenza Virus Between Cocaged Guinea Pigs. Our
hypothesis that influenza virus would be transmitted from
infected to uninfected guinea pigs was initially tested under
conditions where direct contact between the two animals occurred.
Specifically, three guinea pigs were infected intranasally
with 102 pfu of Pan99 virus and, at 24 h p.i., one uninfected
animal was added to the cage of each infected animal. Nasal
wash samples were collected every 2 days, beginning from day
2 p.i., until shedding ceased. Titers in the nasal passages of
inoculated animals reached 5 106 pfuml on day 2 p.i. and
dropped to undetectable levels by day 8 p.i. (Fig. 2). At day 1 p.c.
(day 2 p.i.), low titers of virus were already detected in the nasal
washings of contact guinea pigs. Levels of virus shed by these
animals increased up to 106 pfuml on day 3 p.c. (Fig. 2),
indicating that productive replication was occurring. Thus, the
contact guinea pigs acquired influenza virus infections from their cagemates, demonstrating that guinea pig-to-guinea pig
transmission of influenza virus had occurred.
Transmission of Influenza Virus by Means of Droplet Spread. We next
tested whether contact between guinea pigs was required for
influenza virus transmission to occur or whether droplet andor
aerosol transmission was also possible. We assessed transmission
between immediately adjacent cages and between cages separated
by a 91-cm space. Two pairs of guinea pigs were inoculated
intranasally with 103 pfu of Pan99 virus, and, at 24 h p.i., a cage
containing infected animals was placed either next to or 91 cm
away from a cage containing two uninfected guinea pigs, as
illustrated in Fig. 3A. The infection status of each animal was
then determined by nasal washing at 48-h intervals. The results, shown in Fig. 3B, indicate that one sentinel animal in each cage
acquired influenza virus infection between days 2 and 4 p.i. (days
1 and 3 p.c.). The second sentinel in each cage became infected
between days 4 and 6 p.i., most likely from their respective
cagemates. The results shown are representative of two similar
experiments; however, transmission was not observed when the
relative positions of the infected and uninfected animals were
reversed, suggesting that spread depended on the direction of
airflow in the room.