The dose response of cats to experimental infection with Feline Viral Rhinotracheitis virus

1. COMP.

PATH. l979.V0~.89.

THE DOSE INFECTION

179

RESPONSE OF CATS WITH FELINE VIRAL VIRUS

R. M. Department

of Veterinary

GASKELL

Medicine,

Uniuersity

TO EXPERIMENTAL RHINOTRACHEITIS

and R. C. of Bristol,

POKEY*

Langford,

Bristol

BS18 7DlJ,

CT.K.

INTRODUCTION

Experimental infection with Feline Viral Rhinotracheitis (FVR) virus has been studied by a number of workers including Crandell, Rehkemper, Niemann, Ganaway and Maurer (1961), Burki, Lindt and Freudiger (1964)) Povey (1970) and Hoover, Rohovsky and Griesemer (1970). Susceptible cats developed a severe upper respiratory tract syndrome characterized by pyrexia, sneezing, coughing, salivation, conjunctivitis and ocular and nasal discharges. However, some individual variation in response has been observed which Povey (1970) suggested may be due in part to variations in the infecting dose of virus. This study was undertaken to investigate the clinical and virological effects of varying dose levels of FVR virus in experimental infection of cats. MATERIALS

AND

METHODS

li’rolog. A local isolate of FVR virus (B927 strain) purified by 3 successive terminal dilutions was used at its fourth passage level in primary or low passage feline embryo cells. Cell culture methods and techniques of viral isolation have been described previously (Povey and Johnson, 1969a; LYardley, Gaskell and Povey, 1974). Stock virus at a titre of 10’ CCID,, (median cell culture infective doses) per ml was diluted in phosphate-buffered saline to contain the required dosage levels immediately before infection. Serology. The constant virus-varying serum technique was used (Povey and Johnson, 1969a, b). Experimental animals. Eighteen conventionally reared domestic-shorthaired cats agf-d 4 to 6 months were used. They were shown to be free of FVR and calicivirus by swab culture, and serum samples obtained from all cats 3 days before inoculation were free of FVR antibody. Inoculation procedure. Cats were held in a supine position and inoculated by slow instillation of 0.5 ml of the required dose through the broken-off hub of a 26 gauge net-dle into each nostril. Cats were maintained in dorsal recumbency for about a

minute following the inoculation to ensure maximal retention. Groups of cats were infected

with

doses of 107, 105, 103, and

lo1 CCID,,

of FVR

virus.

As cats in the

group infected with lo1 CCID,, of virus failed to show any clinical signs, detectable virus excretion or serological responseto FVR virus following infection, they were rcinoculated, 27 days after the original infections, with lo2 CCID,, of FVR virus. This second inoculation

was intended

* Present address: Department Guelph, Guelph, Ontario, Canada 0021-9975/79/020179+

13 YO2.00/0

to confirm

of’ Clinical N16 2Wl.

the failure

Studies.

Ontario c

1979

of infection \‘eterinary

Academic

with College,

Press

Inc.

10L CCID,, University

(London)

ot Limited

180

R.

M.

GASKELL

AND

R.

C.

POVEY

of virus by demonstraling the continuing susceptibility of‘ the cams 10 PVK, and al\o to provide information on another dose Ic’vcl, 10” C:C:Il[),,. ~111 cats wcrc observed daily following inoculation for clinical signx which WYI individually scored from 1 (mild) to 4 (severe). Daily ore-pharyngeal and occasional conjunctival swab samples were collected at weekly intervals fi-om all cats. One ~‘211 from each of the groups infected with lOi, 10: and IO3 WID,, was killed on postinfection day (PID) 6 for another study.

RESULTS

All cats an upper junctivitis, panied by

which received dose levels of lo2 to lo7 CCID,, inclusive developed respiratory syndrome characterized by pyrexia, sneezing, conocular and nasal discharges and dyspnoea, and generally accomdepression and loss of appetite. Ocular and nasal discharges were TABLE

THE

ONSET*

AND

SEVERlTYt OF CLINICAL CATS: INFECTED WITH

ME Clinical

sign

Onset

Severity

I

SIGNS AND 10’ CCID6”

DURATION OF FVR

cat codes PE onset Sever+

OF VIRAL VIRUS

EXCRETION

Croup mean clinical score qf seuerily

4SP &verily

Onsel

IN

-__ Anorexia Sneezing Conjunctivitus Ocular discharge Nasal discharge Tongue ulcer Dyspnoea Salivation Fever (>39*5 “cl)

6 7 6

4 1:

11 7 9

4 12 3

g 6

65 10

l”;.:, 7.7

6

8

9

10

6

17

11.7

7

21

9

37

7

37

31.7

ii

11 -

s -

3

5

Total clinical score

-

9

6

6

g:; 2.0

11

2

3.0

-i

93

79

3.5

5-12

29 2

73

Mean daily clinical score Virus isolation$

:; -

3.8

5-13

4.4

Group mean total score

81.7

Group mean daily score

3.9

3-15

* Onset of sign, days post-infection (infection day 0). t Cumulative clinical score for sign over 21 days daily scoring of 1 (mild) $ Cats were reinoculated after failure to infect with 10’ CUD,,. $ Days post-infection.

+ 4 (severe).

initially serous, but in the majority of cats, within 3 or 4 days, these discharges became muco-purulent or purulent. At this stage, crusting of the discharges occurred around the eyes and nose, particularly in the more severely affected animals. In the earlier stages of the disease, salivation was sometimes a feature,

DOSE

RESPONSE

OF

CATS

FVR

TO

181

VIRUS

and in 3 animals tongue ulcers approximately 1 mm in diameter were seen, but persisted only for 1 to 2 days. By PID 21, the main clinical signs had resolved in the majority of cats, but some persistence of mild clinical signs did occur in most until PID 30, and in 8 cats beyond PID 30. The summated clinical sign scores for each cat together with the incubation periods are shown in Tables 1 to 4. TABLE2 THE

ONSET*

AND

SEVERITYt OF CLINICAL CATS INFECTED WlTH

SIGNS AND

DURATION

lo3

OF

CCID~~

FVR

OF VIRAL WRUS

EXCRETION

IN

10 Onset

Anorexia Sneezing Conjunctivitis OcLllar discharge

NiWd ~lischarge D);SplOCl Salivation Fever : > 39.5 “C)

5

3

1 4 14

10 7 7

I 8 1.j

5

3

NA N.4

7

28

2

NA

32 13 8

;

NA NX x.4

24.0 11.7 3.7

3

4

NA

.i.?J

22

7

23

:

24 15

9 11 9

16 ;

z 8

7

3

4

10

Total c:linical scow

74

Mean daily clinical score Virus isolation:

10 5 8

6

3

71

3.5

4-11

Seuericv

* Onset of sign, days post-infection (infection day 0). t Cumulative clinical score for sign over 2 1 days daily : Days post-infection. 5 Killed day 6. NA, Not assessed.

-

108

3.4

5-12

NA

NA

5.1

4-18

scoring

NA

Group mean total 84.4 score Group mean daily

4.0

l-6$

of 1 (mild)

+ 4 (severe,!.

A dose level of lo1 CCID,, failed to infect, as evidenced by absence of clinical signs, detectable virus excretion, or serological response over a 26 day observation period. When re-inoculated with lo2 CCID of virus, all 3 cats developed an upper respiratory syndrome similar to that described above (Table 1). FVR virus was recovered from oro-pharyngeal swabs and usually conjunctival swabs, of all cats showing clinical signs of upper respiratory infection. Viral excretion patterns for individual animals are recorded in Tables 1 to 4. Geometric mean titres of virus shed by 3 cats (12, 13, 6) were 102*7 CCID,, per ml of oro-pharyngeal sample (range loo*’ to 104*7).

sign

isolation:

4

7

3

3

2-1 I

11 9

6

3.7

78

18

ii

9

Severi&

i

6

Onset

Onset

9 Seuer$y

* Onset of sign, days post-infection (infection day 0). ? Cumulative clinical score for sign over 2 I days daily : Days post-infection. $ Killed day 6. NA. not assessed.

Virus

Mean daily clinical score

Total clinical score

Anorexia Sneezing Conjunctivitis Ocular discharge Nasal discharge Tongue ulcers Dyspnoea Salivation Fever (>39.5 Y:)

Clinical

I

I-13

scoring

T

6

Onset

: 2

.i 4

i? 5

-+ 4 (severe;.

N.-l

NX

N.4 NA NA

NA NA NA NA NA

N.4

Cat codes 5 Onset Sever+

of 1 (milcl)

4.5

95

5

20

13 Serleri&

43 2

5 4 ‘4

5

Onset

.-

156

7.4

,;.; 4

4.5 37

1:

15.5

PI Seueri!y

5 3 4

:i 4 4

i

Onset

“-25

4.8

100

1

13 3

15 18.5 30

12.5 7

1s Seuerify Onset

3-!3

EE Ser’erity

DOSE

RESPONSE

OF

CATS

TO

FVR

183

VIRUS

Doxe Response Figure 1 shows that an increase in the infecting viral dose was significantl) correlated with a decrease in the length of the incubation period before onset of la) pyrexia (P < 0*05), (b) other clinical signs (P < 0.001) and (c) viral excretion (P < OeOl). More detailed analysis of the data from (b) showed that an increase in the infecting dose was significantly correlated with a decrease in the length of the incubation period before onset of anorexia (P ( 0.05). TAB1.E THE

ONSET*

AND

SEVERITY? OF CLINICAL CATS INFECTED WITH

Anorexia Sneezing gc;m;ctivitis

DURATION OF Fvl<

OF VIRAL VIRUS

EXCRETION

IN

‘6 3 3 3

C)

34 23

31 28

4

.i

Total clinical score

8

5

107

123

Mean daily clinical score

Yirus isolation$

SIGNS AND 10’ CCIDSO

35 11

discharge Nas;11 discharge Dyspnora Salivation Fewr

(; *39.5

4

5.1

5.9

2-2611

l-7

* Onset of sign, days post-infection (infection day 0). t Cumulative clinical score for sign over 21 days daily z Days post-infection. $ Killed day 6. 1: Subsequent virus isolation PID 31. N.\, Not assessed.

4.2

2-25

x.4

Group mean dailv stork

5-O

2--65

scoring

of 1 (mild)

-

4 (srverr).

sneezing (P < OaOl), conjunctivitis (P < O-01) (P < 0*05), ocular discharge nasal discharge (P < O.OOl), dyspnoea (P < O*OOl), and hypersalivation (P < O-05). All correlation coefficients and regression lines for statistically significant dose/response results are recorded in Table 5. By dividing total clinical scores for each cat by the number of days on which detailed observations were made (21), a mean daily clinical score was calculated. These scores are plotted against infecting dose of virus in Fig. 2(a).

184

K.

WI. GASKELL

AND

K.

C. POVEY

Although much individual variation was noted there appears to be a dirrc,r relationship between an increase in the infecting dose and the severity, of tht, resultant syndrome. This could not be proved statistically significant hoc~r~~r.

12

(Q) l

PSO.05

t 0

8 t

l

1

0 Infecting Fig.

I

I

I

2

I

4 virus

1. The effect of ari increase in infecting viral infected cats before onset of (a) pyrexia,

I

6 dose

loglo

1

I

8 CCIDeo

dose on the length of the incubation (b) other clinical signs and (c) virus

periods excretion.

in FVK-

When the effect of increasing virus dose on the severity of the various individual clinical signs was examined, no statistically significant relationships were found, except for the effect of increasing dose on the severity of dyspnoea [Fig. 2(b)] (P < 0.01).

DOSE

RESPONSE

OF

CATS

TABLE CORRELATION

Defiendant

COEFFICIENTS,

onset of pyrexia Onset of any other clinical sign except pyrexia Onset of virus excretion Onset of anorexia Onset of sneezing Onset of conjunctivitis Onset of ocular discharge Onset of nasal discharge Onset of dyspnoea Onset of hypersalivation Total score of severity of dyspnoea Duration of virus excretion Mean daily score of severity of all clinical signst

‘* Intlependent

variable

Degrees oj freedom

-0.5851

5

.Sigm$canc~e

16

P ‘_

SERUM

g-8

LINES

Standard error

Intercept

7.77

-0.79

0.27

7.22 505 10.26 7.56 7.91 7.90 10.33 12.99 10.12

0.73 0.70 1.51 0.93 1.28 0.96 0.71 0.86 1.64

.-0.72 - 0.54 -0.91 ~.-0.57 -0.58 -0.71 - 1 I5 .- 1 .io - I 1i

O.l,i 0.1-l 0.30 0.19 0.26 0.20 0.l.i O- 18 0.J.i

16 i”, 16 14 16 16 16 7

P P P P P P P P P

0.7137

13

P ‘. 0.01

24Ll

4.24

3.29

0.4583

13

P*

., .60

4.40

1.73

0.6155

13

P < 0.05

3.13

0.52

0.10

was log,,

(virus

< ‘* -c Q ** G < -2 .<

NEUTRALIZING

10-12

ANTIBODY

Post-infection 16-20

0.001 0.01 0.05 0.01 0.0.5 0.01 0.001 0.001 oa5

0.1

dose) except

for t where

it was duration

TITRES

INFECTION

48

53

59

12 8 12

NT NT NT NT NT

12 16 12

Y-1’ N’I ITI 3’1 N’I

16 12

2: 16 4

3!j. 24 4 6

XT16 24

1;

NT 64 24 12 NT NT N-I 16 --6

FOLLOWING

day (infection PID 01 25 J30-34 A42

4

24

4 8

32 12

6 6 6 6 4

4 4 6 (i

12

I’

6

12

16 8 4 6

PI:*

hII:*

mm-~,no titre at dilution of 1 in 4. NT, not tested. * Titres following infection with lo2 CUD,,

virus

excretion.

FVR

OF CATS

-

1;

of virus

6

4

1: 4SP*

Stands/d en,,,

Slofw

1.32

0.05

4 15 PI EE 2

FOR STATISTICALLY

-0.7625 - 0.6825 -0.6917 -0.5939 -0.5128 - 0.6644 - 0.8899 -0-9036 -0.7778

TABLE RECIPROCAL

185

VIRGS

SIGNIFICANCE LEVELS, AND REGRESSION SIGNIFICANT RESULTS OF DOSE/RESPONSE

Correlation coeficient

mriable*

FVK

TO

PID

0.

Nk NT NT

--8 NI NT NT

N’I XI’ N’l N-l NT NT

186

K. 3%. GASKELI,

AKD

K. C. I’OVEY

Figure 3 demonstrates some indication of an effect i\P 0. I,! ol’ an itr(~rc;t~(~ in infecting viral dose on the duration of the subsequent \+ral cxc.r.c*tiotr. However this was not significant at Y -2 0.05. Figure 4 demonstrates a significant (P < 0.05) relationship bet\\ecn tlrt. duration of viral excretion and the severity of the resultant syndrome ~,exprc~t~i as mean daily clinical score over 21 days observation) recorded in FVKinfected cats (regardless of dosage group).

I

01

I

I

I

(b) 0 P s 0.01

0

2468 Infecting

Fig. 2. The effect of an increase dyspnoea in FVR-infected

in infecting cats (over

virus

dose

loglo CCIDeo

viral dose on the severity of (a) all clinical 21 day observation period).

signs and (b)

Serology

Serology showed a generally slow response in terms of serum neutralizing antibody (Table 6). By PID 16 to 20, 6 of 15 (40 per cent) of cats tested had detectable antibody; this proportion increased to 73 per cent by PID 30 to 34. Only 2 cats (other than following the IO1 dose) did not have detectable serum neutralizing antibody at any time. Titres were in all cases fairly low, the reciprocal titres ranging from 4 to 64 (mean 12).

DOSE

RESPONSE

OF

CATS

TO

FVR

187

VIRUS

0 30 P so.1 t l

0

l

0

I I

I 2

I 3

Infecting

Fig.

3. The effect of an increase in infecting period in FVR-infected cats.

virus

viral

I 4 dose

I 6

I 5 log,0

I 7

CClDsD

dose on the duration

of the subsequent

virus

excretion

P IO.05

I

I

I

1

I

I

I

5

IO

15

20

25

30

35

Duration

of

virus

excretion

Fig. 4. The relationship between the duration of viral excretion FVR-infected cats (over 2 1 day observation period).

(days)

and the severity

of clinical

signs ir

188

R.

M.

GASKELL

AND

R.

C.

I’OVEY

DISCUSSION

The characteristic upper respiratory syndrome described hcrc closc~l~ resembles the experimental infections described by Crandrll et al. ,,l!ltjl . Biirki et al. (1964), and Povey (1970). Coughing, however, was not observcstl. although it was a fairly constant feature of these previously described infect ic;lls. HorvAth, Bartha, Papp and Juh6sz (1965) stated that coughing was an infrequent finding in the natural disease. Tongue ulceration, though usually rnorc extensive than that seen here, has been seen in both the natural disease (Karpas and Routledge, 1968; Povey and Johnson, 1971; Kahn and Hoover, 1976) and in the experimental disease (Karpas and Routledge, 1968; Po\:ey, 1970) but it is not the regular feature that it is in feline calicivirus infection. X \.ariablr mortality, ranging from 0 to 33 per cent has been observed following esperimental infection with FVR (Crandell et al., 1961; HorvAth et al., 1965 ; Bartholomew and Gillespie, 1968; Hoover et al., 1970). Lack of mortality in the present work may have been due in part to the selection of 3 of the more severely affected animals for killing on PID 6. The individual variations seen in the syndrome have been noted previously (Crandell et al., 1961; Povey, 1970; Scott, 1975), and the results of the present study demonstrate that the infecting dose is a significant factor in determining the length of the clinical and virological incubation periods. In addition, the infecting dose may have some influence on the duration of the subsequent viral excretion and the intensity and duration of the clinical signs, particularly dyspnoea. These studies also showed a direct relationship between the duration of viral excretion and the severity of the resultant syndrome. Differences between dosage groups may perhaps have been more marked had each cat been housed individually and hence not exposed additionally to contact infections from within the group. The failure of the lo1 CCID,, dosage level to infect implies not only that the tissue culture system in use was highly sensitive: but suggests that non-specific immune mechanisms were successful in protecting the host. The continued susceptibility of the lo1 CCID,, cats to FVR uxs effectively demonstrated by their successful infection with IO2 CCID,,. The apparent lack of the anamnestic serological response (Walton and Gillespie, 1970) following this second exposure to virus also suggests that previous infection had not been established. The overall results of the dose/response experiments confirm the work of Povey (1970) w h o observed longer incubation periods and a generally milder syndrome in cats inoculated intranasally with 10ze7 CCID,, compared with 1Oj CCID,, ofvirus. Brehaut, Jones, McEwan and Miles (1969) used 3 different dose levels with 4 kittens but made no detailed comparisons of the resultant syndrome. Most other workers have used a constant infecting dose level of usually undiluted tissue culture harvests (Crandell et al., 1961; Btirki et al., 1964; Bartholomew and Gillespie, 1968). In FVR infections resulting from natural challenge of apparently susceptible cats, the resultant syndrome has varied from a sub-clinical immunizing syndrome to the characteristicall> severe syndrome usually seen with FVR (Povey and Johnson, 1967 ; Bartholomew and Gillespie, 1968). Bartholomew and Gillespie also observed a

DOSE

RESPONSE

OF

CATS

TO

FVR

VIRUS

189

slightly longer incubation period in natural infections. Results of the present study would suggest that infecting dose may account for some of the variations seen in the natural disease. Other factors have been suggested that may influence the response of cats to FVR. These include biological variations in the virulence of FVR virus strains (Horvath et al., 1965; McKercher, 1973; Bittle and Ruhic, 1974) and the route of inoculation (Povey, 1970). Host differences, such as differences in age or genotype may be important and in some other herpesvirus infections hod! temperature has been shown to play a role (Schmidt and Rasmussen, 1960; Carmichael, Barnes and Percy, 1969). Temperature-sensitive mutants of FVR virus are the basis of intranasal vaccines (Slater and York, 1975; Davis and Beckenhauer, 1976). Intercurrent infection with feline leukaemia virus may predispose to a more severe syndrome because of its immunosuppressive activity (Mackey, 1975). Although the presence of microbial flora is not essential to the pathogenesis of FVR (Hoover et al., 1970), it is probable that some individual variation may be related to differences in bacterial flora (Crandell et al., 1961 ; Povey, 1969, 1970). Both the initial development and the low magnitude of the initial serological response following exposure to FVR virus confirm the findings of other workers, including Crandell et al. (1961) and Walton and Gillespie (1970). Walton and Gillespie (1970) attributed the continual rise in the proportion of animals with titres between PID 21 and 42 to the re-exposure of their animals to virus 011 PID 21. However, it has recently been shown that over 80 per cent of cats recovered from FVR remain persistently infected with the virus (Gaskell and Povey, 1977). This persistent virus, periodically replicating, and sometimes being shed in saliva and other secretions, could explain the late rises in antibody seen in some cats in the present study though they may be due to change in antibody type. SUMMARY

The clinical and virological aspects of the response of cats to intranasal infections with various doses of Feline Viral Rhinotracheitis (FVR) were studied. Cats inoculated with doses of 102, 103, 105, or IO’ CCID,, of virus developed a characteristic upper respiratory syndrome whereas a dose of lo1 CCID,, failed to establish an infection. Although individual variation in the response was quite marked, it was shown that an increase in the infecting viral dose was correlated with a decrease in the length of the incubation period before onset of pyrexia, other clinical signs and viral excretion. Although increasing viral dose appeared to be directly related to the severity of the clinical signs, this relationship was only found to be statistically significant for dyspnoea. There was some indication of an effect of an increase in infecting viral dose on the duration of the subsequent viral excretion. A significant relationship was found between the duration of viral excretion and the severit) of the resultant syndrome. The serological responses of the cats to FVR were generally of low magnitude. The proportion of infected animals with neutralizing antibody titres rose from 6 of 15 (40 per cent) by PID 16 to 20, to 11 of 15 (73 per cent) by PID 30 to 34.

190

R.

M.

GASKELL

AND

12. C.

POVEY

The authors gratefully acknowledge the technical assistance of Mr C:. J. Halt and Mrs S. Wiltshire, the financial support of one of the authors (R. M. G., by th(, Wellcome Trust, and the assistance given by Pedigree Petfoods Ltd. ‘l’hc aurhor~ would also like to thank Mr J. Robinson of the Meat Research Institute., LarlFford for statistical analysis, and Professor C. S. G. Grunsell for his support and for f’arllltic>\. REFERENCES

Bartholomew, P. T., and Gillespie, J. H. (1968). F e 1ine viruses. I. Characterization of four isolates and their effect on young kittens. Cornell Veterinarian, 58, 248--265. Bittle, J. L., and Rubic, W. J. (1974). Studies of feline viral rhinotracheitis vaccinr. Veterinary Medicine and Small Animal Clinician, 69, 1503-l 505. Brehaut, L., Jones, R. H., McEwan, P. J., and Miles, J. .4. R. (1969). Viruses associated with feline respiratory disease in Dunedin. New Zealand Veterinq Journal, 17, 82-86. Biirki, F., Lindt, S., and Freudiger, U. (1964). Enzootischer, virusbedingter Katzenschnupfen in einem Tierheim. 2. Mitteilung: virologischer und experimentellcI Teil. zentralblatt fir Veteriniirimedizin, llB, 110-l 18. Carmichael, L. E., Barnes, F. D., and Percy, D. H. (1969). Temperature as a factot in resistance of young puppies to canine herpesvirus. Journal of Infectious Disease. 120, 669-678. Crandell, R. A., Rehkemper, J. A., Niemann, W. H., Ganaway, J. R., and Maurcr, F. D. (1961). E x p erimental feline viral rhinotracheitis. Journal of American Veterinary Medical Association, 138, 191-197. Davis, E. V., and Beckenhauer, W. H. (1976). Studies and efficacy of an intranasal feline rhinotracheitis-calicivirus vaccine. Veterinary Medicine and Small Animal Clinician, 71, 1405-I 409. Gaskell, R. M., and Povey, R. C. ( 1977). E x p erimental induction of feline viral rhinotracheitis virus re-excretion in FVR-recovered cats. Veterinary Record, 100, 128-133. Hoover, E. A., Rohovsky, M. W., and Griesemer, R. A. (1970). Experimental feline viral rhinotracheitis in the germ-free cat. American Journal qf Pathology, 58, 269-282. Horvith, Z., Bartha, A., Papp, L., and Juh&, M. (1965). On feline rhinotracheitis. Acta veterinaria Acadamie Scientiarium Hungaricae, 15, 415-420. Kahn, D. E., and Hoover, E. A. (1976). Infectious respiratory diseases of cats. Veterinary Clinics of JVorth America, 6, 399-413. Karpas, A., and Routledge, J. K. (1968). F e1ine herpes virus: isolations and expcrimental studies. zentralblatt fur Veteriniirmedizin, 15B, 559-606. McKercher, D. G. (1973). Viruses of other vertebrates. In The Herpesviruses, A. S. Kaplan, Ed. Academic Press, New York and London. Mackey, L. (1975). Feline leukaemia virus and its clinical effects in cats. Veterinql: Record, 96, 5-l 1. Povey, R. C. (1969). Viral respiratory disease. Veterinary Record, 84, 335-338. Povey, R. C. (1970). Studies on viral induced respiratory disease in cats. Ph.D. Thesis, University of Bristol. Povey, R. C:., and Johnson, R. H. (1967). Further observations on feline viral rhinotracheitis. Veterinary Record, 81, 686-689. Povey, R. C., and Johnson, R. H. (1969a). A standardized serum neutralization test for feline viral rhinotracheitis. I. Virus assay. Journal of Comparative Pathology, 79, 379-385. Povey, R. C., and Johnson, R. H. (1969b). A standardized serum neutralization test for feline viral rhinotracheitis. II. The virus-serum system. Journal of Comparative Pathology, 79, 387-392.

DOSE

RESPONSE

OF

CATS

TO

FVR

VIRUS

191

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57S586. [Received for publicatiotz,

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