Med. Microbiol Immunol. 162,217--226 (1976) 9 by Springer-Verlag 1976
The Hamster as an Experimental Animal for the Study of Influenza I. T h e Role of A n t i b o d y in P r o t e c t i o n R. Jennings, M. D. Denton, and C. W. P o t t e r Academic Division of Pathology, Department of Virology, University of Sheffield Medical School, Sheffield Received April 14, 1976 Abstract. Hamsters were used to examine the role of serum antibody in protection against influenza virus infection. :Following intranasal instillation, influenza viruses replicated well in these animals, and high, reproducible amounts of virus could be subsequently recovered from nasal washings and lung suspensions. A specific serum antibody response to the infecting virus was also observed; but no local antibody production was detected. The passive transfer of serum antibody gave some measurable protection, against homologous influenza virus challenge, to recipient hamsters. However, evidence that protection can occur in the absence of detectable serum antibody in individual hamsters, is also presented.
Introduction Several animal species, including mice (Fazekas de St. Groth and Donnelley, 1950), ferrets (Potter et al., 1972) and guinea pigs (Wetherbee, 1973) have been used as experimental animals to s t u d y influenza virus infection, and the mechanisms of i m m u n i t y against this disease. P r o b a b l y the experimental animal m o s t suited for the s t u d y of h u m a n influcnza is the ferret, since this animal suffers a clinical response to influenza virus infection similar to t h a t seen in m a n (Potter et al., 1972), and develops demonstrable humoral and cellular i m m u n e responses following infection or immunization with influenza viruses or their antigens. However, ferrets are expensive to maintain, and have a cellular immune response which is distinct from ttlat of m a n (Potter et al., 1974). I n addition, the problems of handling large n u m b e r s of ferrets make these animals unsuitable for extensive studies of influenza viruses or vaccines. F o r these reasons, an alternative animal model for the s t u d y o f influenza wou]d be of considerable va~ue. The ability of influenza viruses to replicate in hamsters has been k n o w n for some time (Taylor and Parodi, 1942), and this species has recently been used to s t u d y the i m m u n e response to influenza virus vaccines (Jennings et a]., 1974). I n this paper, we report the replication of influenza virus in the h a m s t e r a n d the role of the humoral i m m u n e response of the animal in protection against influenza virus infection.
Materials and Methods Hamsters Young, adult randomiy-bred Syrian hamsters aged 6 to 12 weeks and weighing 60--80 g, were obtained from a colony at the University of Sheffield, or from accredited, commercial dealers.
218
R. Jennings et al.
Viruses and Vaccines Influenza viruses A/FM/1/47 (H1N1), A/England/42/72 (H3N2), A / P o r t Chalmers/1/73 (H3N2) a n d A/Scotland/840/74 (H3N2), were kindly supplied b y Dr. G. C. Schild, National I n s t i t u t e of Medical Research, Mill Hill, London. The viruses were grown b y allantoic inoculation of 10-day-old fertile eggs. After 48 h incubation a t 33 ~ C, allantoic fluids were collected a n d stored a t --80 ~ C. Virus identity was confirmed b y cross-HI tests using monospecific sera. I n a c t i v a t e d A / P o r t Chalmers/73 whole influenza virus vaccine was kindly provided b y Dr. I. Furminger, E v a n s Biologicals Ltd., Speke, Liverpool. This material contained 13260 international units (i.u.) per ml. An inactivated, monovalent influenza A/FM/1/47 (HIN1) vaccine, contained 3840 i.u./ml was prepared in this laboratory, as described previously (Jennings et al., 1974). B o t h the above vaccines were diluted in phosphate-buffered saline (PBS) and inoculated intramuscularly into hamsters in 0.5 ml volumes.
Hamster Nasal Washings and Lung Suspensions Nasal washings for virus isolation were collected from hamsters 3 days following infection or challenge, b y dropping 1 ml of PBS containing 2~ (v/v} bovine serum albumen into the hamster nostrils. By holding the skin a t the back of tile neck tightly while instilling the PBS, the m o u t h of the animal remained open, a n d the washings were collected using a pasteur pipette from the buccal cavity after drainage t h r o u g h the nasal passages. Lungs were obtained from hamsters killed b y cervical dislocation. The lungs were ground with carborundum powder in PBS to give 40~ (w/v) suspensions, centrifuged a t 2500 g for 10--15 min and the s u p e r n a t a n t fluids collected. Both nasal washings a n d lung suspensions were stored a t - - 8 0 ~ prior to assay for virus. Nasal washings for a n t i b o d y studies were collected as described above, b u t were subsequently pooled a n d concentrated 10-fold b y dialysis against 3 0 % carbowax. The concentrated nasal washings were stored a t --20 ~ C prior to testing. The 50~ hamster infectious dose (HIDs0) for a given influenza virus was determined b y virus recovery from lung suspensions obtained from hamsters infected with varying doses of virus 3 days previously. The endpoints were calculated using the method of Reed a n d Mueneh
(1938).
Virus Titrations These were carried out using the allantois-on-shell method described b y Fazekas de St. Groth et al. (1958). I n some experiments, h a m s t e r nasal washings or lung suspensions were tested for t h e presence or absence of virus by inoculating 0.1 ml of undiluted specimen into two to three, 10-day-old fertile eggs; the allantoic fluids were tested for virus b y haemagglutination after 48 h incubation a t 33 ~ C.
Serological Tests Haemagglutination inhibition (HI) tests on serum specimens were carried out as described previously (Jennings et al., 1974), using t h e microtitre m e t h o d (Sever, 1962). E i g h t haemagglutinating units (HAU) of virus were used a n d the a n t i b o d y titre was expressed as the highest serum dilution causing a 500/0 reduction in virus haemagglutination. I n tests for nasal H I a n t i b o d y 4 H A U of virus were employed. Tests for neutralizing a n t i b o d y in concentrated h a m s t e r nasal washings were carried out according to the m e t h o d of Fazekas de St. G r o t h et al. (1958).
Passive Serum Trans/ers Passive transfer experiments were carried out using serum obtained b y retro-orbital bleeding of groups of normal hamsters or h'om hamsters infected 3 - - 4 weeks previously with 104.0 to 105.o EIDs0 of influenza A/FM/1/47 virus. The serum pools from infected hamsters h a d H I antibody titrcs of 320--640 per 0.2 ml against influenza A/FM1 virus. U n d i l u t e d sera or dilutions in P B S were inoculated intraperitoneally into hamsters in volumes of 1.O ml. Twentyfour hours after serum transfer, the animals were bled, a n d the titres of serum H I a n t i b o d y were determined.
Influenza in the Hamster. I. Antibody and Protection
219
Results
Virus Isolation ]rom Hamster Lungs and Nasal Washings G r o u p s o f h a m s t e r s were l i g h t l y a n a e s t h e t i s e d w i t h e t h e r a n d i n o c u l a t e d i n t r a n a s a l l y w i t h 105.0 EIDs0 of u n a d a p t e d A / F M / I / 4 7 influenza virus in a n 0.2 ml v o l u m e o f P B S given dropwise. A t 3, 5, 7 a n d 10 d a y s post-infection, n a s a l washings were collected a n d pooled. I n a d d i t i o n , p o o l e d lung suspensions were p r e p a r e d f r o m groups o f four h a m s t e r s a t similar i n t e r v a l s o f t i m e . T h e results o f v i r u s t i t r a t i o n s carried o u t on these specimens are shown in Table 1. Virus was r e c o v e r e d from b o t h nasal washings a n d lung suspensions a t 3 a n d 5 d a y s following infection, b u t n o t a t 7 or i 0 d a y s . The virus t i t r e s in b o t h t y p e s o f s p e c i m e n were similar, a n d t h e g r e a t e s t a m o u n t s o f virus were f o u n d a t 3 d a y s , p o s t infection. I n a f u r t h e r e x p e r i m e n t , g r o u p s o f h a m s t e r s were a n a e s t h e t i s e d a n d i n o c u l a t e d i n t r a n a s a l l y with 0.2 ml of v a r y i n g doses of u n a d a p t e d influenza virus A/Eng/42/72. N a s a l washings a n d lung suspensions were collected 3 d a y s a f t e r infection, a n d t e s t e d i n d i v i d u a l l y for virus. The results are shown in T a b l e 2. The t i t r e s of v i r u s in lung suspensions a n d in nasal washings from i n d i v i d u a l a n i m a l s i n o c u l a t e d w i t h t h e s a m e dose o f v i r u s were v a r i a b l e , b u t the differences were n o t more t h a n 2.0 logl0 a n d were u s u a l l y less t h a n this. F u r t h e r m o r e for e v e r y h a m s t e r where v i r u s was recovered f r o m n a s a l washings, v i r u s was also r e c o v e r e d from lung
Table 1. Recovery of influenza A/FM/1/47 virus from hamster lungs and nasal washings at various times following infection Time post-infection (days)
3 5 7 10
No. of hamsters tested 4 4 4 4
Virus recovery (logx0/ml) from : Nasal Washings 3.16 2.50 < 1.00 < 1.00
Lung Suspensions 2.94 2.27 < 1.00 < 1.00
Table 2. Replication of A/England/42/72 influenza virus in hamsters as reflected in virus concentrations from nasal washings or lung suspensions Hamster No.
Inoculum Dose
Virus recovery (lOgl0/ml) at 3 days post infection from Nasal washings
Lung suspensions
1 2 3
104 EIDso
2.30 3.74 2.74
2.30 3.52 3.46
4 5 6
102 EID~
3.30 2.30 4.30
6.30 4.96 5.47
7 8
1 EIDa0
5.30 < 1.00
6.30 < 1.00
< 1.00
< 1.00
9
220
R. Jennings et al.
suspensions at 3 days post-infection, but virus was not isolated front either nasal washings or lung suspensions of two of three hamsters receiving the highest virus dilution. The results indicate t h a t the infectious dose of virus for hamsters was approximately 1 to 100 EIDs0.
Serum Antibody Response o/ Hamsters to Influenza A/Port Chalmers/1/73 Virus In/ection Samples of blood were obtained from hamsters, and groups of three of these animals were then infected intranasally with various dilutions of influenza A/Port Chalmers/1/73 virus in an 0.2 ml volume given dropwise. Three days later, nasal washings were collected from individual hamsters and tested for virus. Three weeks after virus infection, a second blood sample was obtained from each animal, and all sera tested for H I antibodies to homologous virus, and to the heterologous influenza viruses A/England/42/72 and A/Scotland/840/74. The results are shown in Table 3. Virus was recovcred from all hamsters infected with 104.~ or 103.~ EID~0 of the A/Port Chalmers/73 virus. The serum H I antibody response was measured in animals inoculated with these virus doses, and homologous H I antibody titres were found to range from 240 to 640. In addition, although serum H I antibody to A/England/72 and A/Scotland/74 were also seen in these animals, the titres were considerably lower than those against the homologous A/Port Chalmers/73 virus. Serum H I antibody responses to A/Port Chalmers/73 and A/Scotland/74 were also observed in hamsters receiving lower doses of A/Port Chalmers/73 virus, but only one of these animals had detectable antibody to A/England/42/72 influenza virus. One hamster only showed a serum H I antibody response where no detectable virus was recovered from the nasal washings collected 3 days after virus inoculation (Table 3). Based on virus recovery and the serum antibody response, the 50O/o infecting dose of A/Port Chalmers/73 virus for hamsters was 101.~ ~.~ EIDs0, and this result was similar to t h a t found for influenza virus A/England/72 (Table 2).
Absence o/ Detectable Nasal Antibody in Hamsters Following Influenza Virus In/eetion or Immunization Groups of hamsters were bled, then infected with 100 hamster infectious doses (HIDs0) of influenza A/Port Chalmers/73 virus, or immunized with 2500 i.u. of inactivated A/Port Chalmers/73 virus vaccine. At 5, 7, 9, 12, 14 and 21 days following infection or immunization, nasal washings were collected from each animal, pooled, concentrated 10-fold, and tested for H I and neutralizing antibody to A/Port Chalmcrs/73 virus. No H I or neutralizing antibodies were detected in a n y of the samples from either infected or immunized animals at a n y time. However, serum specimens collected 21 days following infection or immunization contained high titres of H I antibody to A/Port Chalmcrs/73 virus.
E//ect o/Passively Trans/erred Antibody on Subsequent In/ection o] Hamsters with Influenza A/FM/1/47 Virus To determine the level of circulating H I antibody t h a t would protect hamsters against subsequent homologous influenza virus challenge, groups of hamsters were
Influenza in the Hamster. I. Antibody and Protection
221
Table 3. Serum HI antibody response of hamsters following A/Port Ctmlmers/1/73 virus infection Hamster Inoculum No. Dose (EIDso)
Virus recovery
Serum HI antibody respong~ to A/England/ 42/72
A/Port Chalmers/1/73
A/Scotland/840/74
1 2 3
104.0
-~q-
< 10--15
The Hamster as an Experimental Animal for the Study of Influenza I. T h e Role of A n t i b o d y in P r o t e c t i o n R. Jennings, M. D. Denton, and C. W. P o t t e r Academic Division of Pathology, Department of Virology, University of Sheffield Medical School, Sheffield Received April 14, 1976 Abstract. Hamsters were used to examine the role of serum antibody in protection against influenza virus infection. :Following intranasal instillation, influenza viruses replicated well in these animals, and high, reproducible amounts of virus could be subsequently recovered from nasal washings and lung suspensions. A specific serum antibody response to the infecting virus was also observed; but no local antibody production was detected. The passive transfer of serum antibody gave some measurable protection, against homologous influenza virus challenge, to recipient hamsters. However, evidence that protection can occur in the absence of detectable serum antibody in individual hamsters, is also presented.
Introduction Several animal species, including mice (Fazekas de St. Groth and Donnelley, 1950), ferrets (Potter et al., 1972) and guinea pigs (Wetherbee, 1973) have been used as experimental animals to s t u d y influenza virus infection, and the mechanisms of i m m u n i t y against this disease. P r o b a b l y the experimental animal m o s t suited for the s t u d y of h u m a n influcnza is the ferret, since this animal suffers a clinical response to influenza virus infection similar to t h a t seen in m a n (Potter et al., 1972), and develops demonstrable humoral and cellular i m m u n e responses following infection or immunization with influenza viruses or their antigens. However, ferrets are expensive to maintain, and have a cellular immune response which is distinct from ttlat of m a n (Potter et al., 1974). I n addition, the problems of handling large n u m b e r s of ferrets make these animals unsuitable for extensive studies of influenza viruses or vaccines. F o r these reasons, an alternative animal model for the s t u d y o f influenza wou]d be of considerable va~ue. The ability of influenza viruses to replicate in hamsters has been k n o w n for some time (Taylor and Parodi, 1942), and this species has recently been used to s t u d y the i m m u n e response to influenza virus vaccines (Jennings et a]., 1974). I n this paper, we report the replication of influenza virus in the h a m s t e r a n d the role of the humoral i m m u n e response of the animal in protection against influenza virus infection.
Materials and Methods Hamsters Young, adult randomiy-bred Syrian hamsters aged 6 to 12 weeks and weighing 60--80 g, were obtained from a colony at the University of Sheffield, or from accredited, commercial dealers.
218
R. Jennings et al.
Viruses and Vaccines Influenza viruses A/FM/1/47 (H1N1), A/England/42/72 (H3N2), A / P o r t Chalmers/1/73 (H3N2) a n d A/Scotland/840/74 (H3N2), were kindly supplied b y Dr. G. C. Schild, National I n s t i t u t e of Medical Research, Mill Hill, London. The viruses were grown b y allantoic inoculation of 10-day-old fertile eggs. After 48 h incubation a t 33 ~ C, allantoic fluids were collected a n d stored a t --80 ~ C. Virus identity was confirmed b y cross-HI tests using monospecific sera. I n a c t i v a t e d A / P o r t Chalmers/73 whole influenza virus vaccine was kindly provided b y Dr. I. Furminger, E v a n s Biologicals Ltd., Speke, Liverpool. This material contained 13260 international units (i.u.) per ml. An inactivated, monovalent influenza A/FM/1/47 (HIN1) vaccine, contained 3840 i.u./ml was prepared in this laboratory, as described previously (Jennings et al., 1974). B o t h the above vaccines were diluted in phosphate-buffered saline (PBS) and inoculated intramuscularly into hamsters in 0.5 ml volumes.
Hamster Nasal Washings and Lung Suspensions Nasal washings for virus isolation were collected from hamsters 3 days following infection or challenge, b y dropping 1 ml of PBS containing 2~ (v/v} bovine serum albumen into the hamster nostrils. By holding the skin a t the back of tile neck tightly while instilling the PBS, the m o u t h of the animal remained open, a n d the washings were collected using a pasteur pipette from the buccal cavity after drainage t h r o u g h the nasal passages. Lungs were obtained from hamsters killed b y cervical dislocation. The lungs were ground with carborundum powder in PBS to give 40~ (w/v) suspensions, centrifuged a t 2500 g for 10--15 min and the s u p e r n a t a n t fluids collected. Both nasal washings a n d lung suspensions were stored a t - - 8 0 ~ prior to assay for virus. Nasal washings for a n t i b o d y studies were collected as described above, b u t were subsequently pooled a n d concentrated 10-fold b y dialysis against 3 0 % carbowax. The concentrated nasal washings were stored a t --20 ~ C prior to testing. The 50~ hamster infectious dose (HIDs0) for a given influenza virus was determined b y virus recovery from lung suspensions obtained from hamsters infected with varying doses of virus 3 days previously. The endpoints were calculated using the method of Reed a n d Mueneh
(1938).
Virus Titrations These were carried out using the allantois-on-shell method described b y Fazekas de St. Groth et al. (1958). I n some experiments, h a m s t e r nasal washings or lung suspensions were tested for t h e presence or absence of virus by inoculating 0.1 ml of undiluted specimen into two to three, 10-day-old fertile eggs; the allantoic fluids were tested for virus b y haemagglutination after 48 h incubation a t 33 ~ C.
Serological Tests Haemagglutination inhibition (HI) tests on serum specimens were carried out as described previously (Jennings et al., 1974), using t h e microtitre m e t h o d (Sever, 1962). E i g h t haemagglutinating units (HAU) of virus were used a n d the a n t i b o d y titre was expressed as the highest serum dilution causing a 500/0 reduction in virus haemagglutination. I n tests for nasal H I a n t i b o d y 4 H A U of virus were employed. Tests for neutralizing a n t i b o d y in concentrated h a m s t e r nasal washings were carried out according to the m e t h o d of Fazekas de St. G r o t h et al. (1958).
Passive Serum Trans/ers Passive transfer experiments were carried out using serum obtained b y retro-orbital bleeding of groups of normal hamsters or h'om hamsters infected 3 - - 4 weeks previously with 104.0 to 105.o EIDs0 of influenza A/FM/1/47 virus. The serum pools from infected hamsters h a d H I antibody titrcs of 320--640 per 0.2 ml against influenza A/FM1 virus. U n d i l u t e d sera or dilutions in P B S were inoculated intraperitoneally into hamsters in volumes of 1.O ml. Twentyfour hours after serum transfer, the animals were bled, a n d the titres of serum H I a n t i b o d y were determined.
Influenza in the Hamster. I. Antibody and Protection
219
Results
Virus Isolation ]rom Hamster Lungs and Nasal Washings G r o u p s o f h a m s t e r s were l i g h t l y a n a e s t h e t i s e d w i t h e t h e r a n d i n o c u l a t e d i n t r a n a s a l l y w i t h 105.0 EIDs0 of u n a d a p t e d A / F M / I / 4 7 influenza virus in a n 0.2 ml v o l u m e o f P B S given dropwise. A t 3, 5, 7 a n d 10 d a y s post-infection, n a s a l washings were collected a n d pooled. I n a d d i t i o n , p o o l e d lung suspensions were p r e p a r e d f r o m groups o f four h a m s t e r s a t similar i n t e r v a l s o f t i m e . T h e results o f v i r u s t i t r a t i o n s carried o u t on these specimens are shown in Table 1. Virus was r e c o v e r e d from b o t h nasal washings a n d lung suspensions a t 3 a n d 5 d a y s following infection, b u t n o t a t 7 or i 0 d a y s . The virus t i t r e s in b o t h t y p e s o f s p e c i m e n were similar, a n d t h e g r e a t e s t a m o u n t s o f virus were f o u n d a t 3 d a y s , p o s t infection. I n a f u r t h e r e x p e r i m e n t , g r o u p s o f h a m s t e r s were a n a e s t h e t i s e d a n d i n o c u l a t e d i n t r a n a s a l l y with 0.2 ml of v a r y i n g doses of u n a d a p t e d influenza virus A/Eng/42/72. N a s a l washings a n d lung suspensions were collected 3 d a y s a f t e r infection, a n d t e s t e d i n d i v i d u a l l y for virus. The results are shown in T a b l e 2. The t i t r e s of v i r u s in lung suspensions a n d in nasal washings from i n d i v i d u a l a n i m a l s i n o c u l a t e d w i t h t h e s a m e dose o f v i r u s were v a r i a b l e , b u t the differences were n o t more t h a n 2.0 logl0 a n d were u s u a l l y less t h a n this. F u r t h e r m o r e for e v e r y h a m s t e r where v i r u s was recovered f r o m n a s a l washings, v i r u s was also r e c o v e r e d from lung
Table 1. Recovery of influenza A/FM/1/47 virus from hamster lungs and nasal washings at various times following infection Time post-infection (days)
3 5 7 10
No. of hamsters tested 4 4 4 4
Virus recovery (logx0/ml) from : Nasal Washings 3.16 2.50 < 1.00 < 1.00
Lung Suspensions 2.94 2.27 < 1.00 < 1.00
Table 2. Replication of A/England/42/72 influenza virus in hamsters as reflected in virus concentrations from nasal washings or lung suspensions Hamster No.
Inoculum Dose
Virus recovery (lOgl0/ml) at 3 days post infection from Nasal washings
Lung suspensions
1 2 3
104 EIDso
2.30 3.74 2.74
2.30 3.52 3.46
4 5 6
102 EID~
3.30 2.30 4.30
6.30 4.96 5.47
7 8
1 EIDa0
5.30 < 1.00
6.30 < 1.00
< 1.00
< 1.00
9
220
R. Jennings et al.
suspensions at 3 days post-infection, but virus was not isolated front either nasal washings or lung suspensions of two of three hamsters receiving the highest virus dilution. The results indicate t h a t the infectious dose of virus for hamsters was approximately 1 to 100 EIDs0.
Serum Antibody Response o/ Hamsters to Influenza A/Port Chalmers/1/73 Virus In/ection Samples of blood were obtained from hamsters, and groups of three of these animals were then infected intranasally with various dilutions of influenza A/Port Chalmers/1/73 virus in an 0.2 ml volume given dropwise. Three days later, nasal washings were collected from individual hamsters and tested for virus. Three weeks after virus infection, a second blood sample was obtained from each animal, and all sera tested for H I antibodies to homologous virus, and to the heterologous influenza viruses A/England/42/72 and A/Scotland/840/74. The results are shown in Table 3. Virus was recovcred from all hamsters infected with 104.~ or 103.~ EID~0 of the A/Port Chalmers/73 virus. The serum H I antibody response was measured in animals inoculated with these virus doses, and homologous H I antibody titres were found to range from 240 to 640. In addition, although serum H I antibody to A/England/72 and A/Scotland/74 were also seen in these animals, the titres were considerably lower than those against the homologous A/Port Chalmers/73 virus. Serum H I antibody responses to A/Port Chalmers/73 and A/Scotland/74 were also observed in hamsters receiving lower doses of A/Port Chalmers/73 virus, but only one of these animals had detectable antibody to A/England/42/72 influenza virus. One hamster only showed a serum H I antibody response where no detectable virus was recovered from the nasal washings collected 3 days after virus inoculation (Table 3). Based on virus recovery and the serum antibody response, the 50O/o infecting dose of A/Port Chalmers/73 virus for hamsters was 101.~ ~.~ EIDs0, and this result was similar to t h a t found for influenza virus A/England/72 (Table 2).
Absence o/ Detectable Nasal Antibody in Hamsters Following Influenza Virus In/eetion or Immunization Groups of hamsters were bled, then infected with 100 hamster infectious doses (HIDs0) of influenza A/Port Chalmers/73 virus, or immunized with 2500 i.u. of inactivated A/Port Chalmers/73 virus vaccine. At 5, 7, 9, 12, 14 and 21 days following infection or immunization, nasal washings were collected from each animal, pooled, concentrated 10-fold, and tested for H I and neutralizing antibody to A/Port Chalmcrs/73 virus. No H I or neutralizing antibodies were detected in a n y of the samples from either infected or immunized animals at a n y time. However, serum specimens collected 21 days following infection or immunization contained high titres of H I antibody to A/Port Chalmcrs/73 virus.
E//ect o/Passively Trans/erred Antibody on Subsequent In/ection o] Hamsters with Influenza A/FM/1/47 Virus To determine the level of circulating H I antibody t h a t would protect hamsters against subsequent homologous influenza virus challenge, groups of hamsters were
Influenza in the Hamster. I. Antibody and Protection
221
Table 3. Serum HI antibody response of hamsters following A/Port Ctmlmers/1/73 virus infection Hamster Inoculum No. Dose (EIDso)
Virus recovery
Serum HI antibody respong~ to A/England/ 42/72
A/Port Chalmers/1/73
A/Scotland/840/74
1 2 3
104.0
-~q-
< 10--15
