Trop. Anita. Hlth Prod. (1990) 22, 26%272

P R O T E C T I O N C O N F E R R E D BY V A C C I N A T I O N W I T H B L A C K S B U R G A N D K O M A R O V S T R A I N S O F

N E W C A S T L E D I S E A S E V I R U S A G A I N S T N E W C A S T L E D I S E A S E IN B A N G L A D E S H

i O SAIFUDDIN, T. I. M. F. R. CHOWDHURY, A. J. SARI~R and M. M. AMIN

Department of Microbiology and Hygiene, Bangladesh Agricultural Un~ersity, Mymensingh, Bangladesh

SUMMARY

An evaluation was undertaken of the efficacy of vaccination of day-old chicks with the Blacksburg (B1) strain o f Newcastle disease virus (NDV) followed at various times by vaccination with the Komarov (K) strain. Antibody was detected by the haemagglutination inhibition (HAI) test one week after vaccination with B1 and titres peaked at three weeks and had declined to undetectable levels by nine weeks.

After subsequent vaccination with K strain at five, seven or eight weeks o f age levels of HA1 antibody (titre 80 to 640) were detected after three weeks. Birds vaccinated at seven weeks were tested for antibody and resistance to challenge beyond 19 weeks o f age. In this group the H A l titres remained constant (80 to 640) up to 32 weeks o f age and then steadily declined to 10 to 20 at 44 weeks of age.

A linear relationship between HAI titre and virus neutralising index (VNI) was demonstrated with a range o f selected sera. Only birds with an HA1 titre o f 80 or greater resisted artificial challenge. It is recommended that, following B1 vaccincation at day-old and K vaccination at seven weeks old, revaccination with K strain should be performed at intervals of not more than seven months.

INTRODUCTION

Newcastle disease (ND) is a widely distributed and highly contagious viral disease of chickens. The disease is characterised by its sudden appearance and rapid spread within a flock resulting in high morbidity and mortality which can approach 100%. ND is a major constraint to the maintenance and development of the poultry industry in Bangladesh.

The control of ND relies on vaccination but no single vaccine or vaccination programme has proved ideal for all situations and all geographical areas (Lancaster, 1964). The virulence of the field virus and the level of maternal immunity of a flock are factors which affect the choice of vaccine strain. The age at which birds are to be vaccinated is also important (Allan, Lancaster and Toth, 1978; Lancaster, 1964). Allan et al. (1978) have suggested that a vaccination schedule should contain a lentogenic vaccine as the initial dose followed by a mesogerdc vaccine where the virulence of field virus requires the estabfishment of a high and consistent antibody response. Three lentogenic strains, F, B1 and La Sota and three mesogenic strains, K, Mukteswar (M) and Raokin, are used in live vaccines.

263

264 SAIFUDDIN, CHOWDHURY, SARKER AND AMIN

The inununogenicity of lentogenic strains parallels the degree of clinical reaction they induce in chickens (Allan et al., 1978). The F strain has the lowest virulence of the common lentogenic vaccine strains and the respiratory reaction in birds vaccinated with this strain is small (Allan e ta / . , 1978). The La Sota strain is the most virulent and often causes respiratory signs. Therefore it is usually administered as a booster dose following vaccination with the B1 or F strain (Al lan et al., 1978). Similarly, in comparative tests based on pathogenicity, the mesogenic K strain has been preferred to the M and Roakin strains (Allan et al., 1978) because it is less likely to produce disease.

The current vaccination programme against ND in Bangladesh includes eye-drop (ED) administration of live F strain vaccine to chicks below two months of age followed by intramuscular (i/m) injection of live M strain to growing and adult birds twice a year (Director of Livestock Services Report, see Arain, 1985). Since this protocol frequently fails to protect chickens against ND (Chowdhury, Sarker, Amin and Hossain, 1982; Amin, 1985) the present study was undertaken to evaluate the use of B1 and K strains in various vaccine protocols.

MATERIALS AND METHODS

Virus strains The live lentogenic B1 and mesogenic g strains of vaccine v/rus were

produced in the Department of Microbiology and Hygiene, Bangladesh Agri- cultural University (BAU) using the standard method of chicken embryo propagation (Allan et al., 1978). The B1 and K strains were originally obtained from Laboratoria Dr de Zeener by Gist-Brocades uv Delft, Holland and Dr Alwar, Indian Veterinary Association, respectively. The virulent field isolate Faridpur-4 was used as the challenge virus.

Chickens and eggs One day-old to 44 week-old crossbred birds (White Leghorn × Rhode Island

Red) were used in this study. White Leghorn eggs, used for cultivation and titration of virus and for virus neutralisation (VN) tests were obtained from the BAU poultry farm. The parent flocks were vaccinated against ND using the F and M strains of NDV.

Isolation and propagation of virus Two hundred ~1 of a 10% tissue (trachea, lungs, brain) suspension in

phosphate buffered saline (PBS, pH 7-4) containing penicillin (1,000 units/ml) and streptomycin (1 mg/ml) were inoculated into the allantoic cavity (AC) of each of five embryonating eggs and incubation continued for six days. The aUantoic fluids (AF) were subjected to a slide haemagglutination (HA) test to determine the presence of virus while the embryos were examined for haemor- rhagic lesions on their body surfaces. The HA-positive AF were collected as virus isolate or virus steck. The virulent virus and vaccine viruses were similarly propagated and stocks prepared.

Serological tests The slide HA test was conducted by mixing two drops each of AF and 2%

chicken red blood cells (RBC) on a microscope slide. The clumping of RBC within one min was considered as a positive HA reaction. The microplate HA

PROTECTION AGAINST ND IN BANGLADESH 265

and standard HAI tests were carried out to determine the H A titres of vaccine viruses and HAI titres of heat-inactivated serum samples from vaccinated and control birds. The HA titre was expressed as the reciprocal of the highest dilution of virus in which a positive pattern of complete H A was observed; the HAI titre was expressed as the reciprocal of the highest dilution of serum inhibiting haemagglutination by four HA units of virus. Each test was simultaneously conducted with appropriate controls. For the microplate test a 0.5% RBC suspension was used.

A neutralisation test was conducted in embryonating eggs to determine the VNI of heat-inactivated serum samples possessing HAI titres of 20, 40, 80, 160, 320, 640 and 1,280 against homologous K strain virus following the method of Beard (1980). Five hundred /~1 of undiluted serum were mixed with an equal volume of each virus dilution (10 -z to 10 -9) and the mixture incubated for 1 h at 20"C to 25°C. Each of five embryonating eggs were inoculated with 200/Jl of each serum-virus mixture into the AC. Five eggs were also inoculated with 100/~1 of each virus dilution (10 -7 to 10-10). All the eggs were incubated for six days. The 50% embryo infective dose (EIDso) of the virus and serum-virus mixtures were calculated by the Reed and Muench (1938) method. The VNI of the serum was determined as the difference between the log,0 EIDso of virus control and lOgl0 EIDso of the serum-virus mixture.

Bird protection test The challenge dose was previously determined in 25 five to six months old

nonvaccinated crossbred birds. A 500 ~zl i/m inoculation of a 10 -5 dilution of stock virus was used and produced fatal infection in 100% of challenged birds within two weeks. The resistance of K-vaccinated birds was measured by challenging 12 randomly selected birds i/m at different time intervals after vaccination. Five Bl-vaccinated control birds were also challenged with each K-vaccinated group. The birds were observed for two weeks following inocula- tion. The titres of HA1 antibody in the sera of all control birds were measured before challenge. Attempts were made to isolate the NDV from trachea, lungs and brain of any birds which died after challenge.

Experimental design Seven hundred day-old chicks having had standard ED B1 vaccination were

used. Five hundred and sixty five birds were divided into four groups A, B, C and D containing 55, 360, 85 and 65 birds respectively for different trials of K vaccine. The remaining 135 birds were used to study the development and persistence of the HA1 antibody response to the B1 vaccine administered at one day of age. The serum samples of 10 randomly selected birds were tested weekly for HAI antibody to B1 virus starting from one week until nine weeks of age. The birds in groups A, B and C were revaccinated with a standard dose (1 ml containing 105"a3 EIDso) of K vaccine i/m at five, seven and eight weeks of age, respectively. From the third week after K vaccination the serum HAI titres of six randomly selected birds were measured each fortnight up to 16, 44 and 19 weeks of age in groups A, B and C, respectively. Birds in group D were not revaccinated and served as controls during experimental challenge. Following K vaccination 12 randomly selected birds were challenged i/m with virulent virus after five, seven, nine, i1, 25, 27, 29 and 31 weeks in group B and after nine and 11 weeks in group C. Birds

266 SAIFUDDIN, CHOWDHURY, SARKER AND AMIN

I . . . . . . I '

A B 55 360

Vaccinated with K at the age of

I ..... 1 5W 7W

700 day-old chickens

" - - - - - - - - - - - - - - - -vacc ina t ed with BI

I I C D 85 65

Not vaccinated [ with K

8W

HAl titres from 6 birds per group from 3 weeks after vaccination until the age of

I . . . . . . . . . I 16W 44W

I 19W

Challenge i/m of 12 birds per group with virulent virus at the age of 5 birds challenged ] I per occasion

12W - - C 14W - - O 16W 17W N 18W 19W T 32W - - R 34W m 0 36W - - L 38W - - S

FIG. 1. Flow diagram of experimental design.

I

135

I Scra from 10 birds tested weekly for HAI antibodies from I to 9 weeks.

in group A were not challenged with virulent virus. The experimental design is summarised in Fig. 1.

RESULTS

Persistence of HA1 antibody after vaccination with B1 One week after B1 vaccination the serum HAI titres of the 10 selected birds

ranged from 40 to 160 with an arithmetic mean of 80 (Fig. 2). Titres increased to 80 to 640 (arithmetic mean 352) after three weeks and then decreased. After nine weeks, six of 10 birds had serum HAI titres of five to 10 while HAI antibody was not detected in the remaining four birds.

HA1 antibody response following K vaccination When group A birds were vaccinated with K strain at five weeks of age their

HAI titres ranged from 40 to 160 (arithmetic mean 68), rose to 80 to 640 (arithmetic mean 413) by three weeks post-vaccination and remained fairly constant up to 11 weeks post-vaccination (16 weeks of age) when the final measurements were made (Fig. 2).

When group B birds were vaccinated with K strain at seven weeks of age their

PROTECTION AGAINST ND IN BANGLADESH 267

1 2 8 0 "

8 4 0 '

3 2 0 '

UJ n- _ l e o . I,-,

• 1- 8 0 . =i

n -

4 0 "

20"

10-

./i

..i

_/

i . :'~-I

I I

!_1 \

e- . . . . . . . . e B 1 vaccination only

............ • +,,6 G r o u p A

0 , O G r o u p B

o . . - - - - o G r o u p C

\

• , • • • , , , , , , , , , . , • • , , , , , J , , , . , , • f , • . . . . , , , • • , , , , , , , 0 , • • •

1 3 5 7 9 11 13 15 17 19 21 2 3 2 5 27 29 3 1 3 3 3 5 3 7 3 9 4 1 4 3 45 t A A AGE OF B IRDS ( W E E K S )

FrO. 2. Development and persistence of serum HAI antibody titres following different vaccination procedures. Arithmetic mean of HAI titres with range of variations are shown by different signs and vertical fines respectively. Arrows show when birds were vaccinated with K vaccine following B1 vaccination at one day old.

HAI titres ranged from 20 to 40 (arithmetic mean 30), increased to 80 to 640 (arithmetic mean 333) by three weeks and persisted up to 17 weeks post- vaccination. The titres increased to 160 to 1,280 (arithmetic mean 666) by the nineteenth week post-vaccination and from the twenty-fifth week declined to 10 to 20 (arithmetic mean 13) by the thirty-seventh week after K vaccination (44 weeks of age) (Fig. 2).

When group C birds were vaccinated with K strain at eight weeks of age their HAI titres ranged from <5 to 20, increased to 80 to 640 (arithmetic mean 253) by three weeks post-vaccination and most remained constant about that level up to the final measurement at the eleventh week (19 weeks of age). Two birds however, one at the seventh and one at the ninth week post-vaccination, showed HAI titres of 40 (Fig. 2).

Figure 3 illustrates that there is a positive correlation between loglo HAI values and VNI values of serum samples from K vaccinated birds. The regression equation is given by loglo HAI = 1.18 + 0-29 VNI.

Resistance to challenge with Faridpur-4 virus All 50 control birds (group D) died within two weeks after challenge with

Faridpur-4 virus. Birds in group B resisted challenge up to 27 weeks post- vaccination with K strain virus but deaths occurred in those challenged at 29 and 31 weeks post-vaccination (Table I). Birds from group C were only challenged at nine and 11 weeks after K vaccination. Deaths occurred in the group challenged at nine weeks but all birds survived challenge at 11 weeks. All birds with HAI

268 SAIFUDDIN, CHOW'DHURY, SARKER AND AMIN

I t l

' r

o

4 '

3 -

2 -

1 -

o ~ ~ ~ ~ ~ ~ VNl VALUES

FIG. 3. Relationship between VNI and logao HAI values of serum samples from birds vaccinated with K strain of NDV.

TABLE I

Protection of birds against intramuscular challenge with Faridpur-4 strain of NDV at various 6rues after vaccination of day-old chicks with Blacksburg swain followed by Komarov strain of NDV at seven and

eight weeks of age

Weeks after % protection against death K vaccination (age

of birds, weeks) Group B a Group C a Age matched control 2

5 (12) 100 - - 0 7 (14) 100 - - 0 9 (B-16, C-17) 100 83-3 0

11 (B-18, C-19) 100 100 0 25 (32) 100 - - 0 27 (34) 100 - - 0 29 (36) 66.7 - - o 31 (38) 50 - - o

12 birds challenged at each time. 25 birds challenged at each time.

not done.

PROTECTION AGAINST ND IN BANGLADESH 269

TABLE II

Relationship between HA1 antibody and protection of Komarov strain vaccinated birds against intramuscular

challenge with Faridpur.4 strain of NDV

HAI antibody No. of birds % protection titre challenged against death

640 10 100 320 16 100 160 15 I00 80 13 100 40 5 0 20and<20 41 t 0

1 Includes 40 control birds vaccinated with B1 only.

titres of 80 or greater resisted challenge while those with lower titres died (Table II).

DISCUSSION

Moderate levels (40 to 160) of HAI antibody were attained by seven days after B1 vaccination. This was consistent with previous studies which have demonstrated the production of HAI and neutralising antibody within four to eight days after NDV infection or immunisation (Anon, 1971; Hanson, 1980). The maximum HAI antibody titres (80 to 640) were found three weeks after B1 vaccination. These were higher than those demonstrated by Satyanarayana, Madhava Krishna Reddy, Madhusudhana Swamy and Akbar (1977) where only 20% of birds showed titres of 80 by three weeks after vaccination with F strain at three days of age. This difference in results could be due to the use of a different strain of virus (Anon, 1971; Hanson, 1980). Spradbrow, Samuel and Ibrahim (1988) reported higher HAI titres with higher doses of V4 strain of vaccine. They also found that the HAI responses peaked at about three weeks after first administration of V4 vaccine which was consistent with the present report. The highest level in our study (80 to 640) declined to <5 to 10 by the end of the ninth week. Kumar, Narayana Rao, Pankajam and Kannan Babu (1981) recorded the highest HAI titre at the fifth week followed by a decline to undetectable levels at the tenth week following primary F vaccination. This was similar to the pattern of responses seen in this study. As maternal antibody has no inhibitory effect on the immune response to primary vaccination (Satyanarayana et al., 1977; Chow- dhury, Chowdhury, Sarker and Amin, 1981) the risk of NDV infection during the brooder period can be avoided by vaccinating the chicks with the B1 strain at one day of age.

Between the third and the eleventh week after K vaccination, although a similar range (80 to 640) of HAI titres was obtained in three groups, the mean titres in groups A and B were higher than those in group C. It is also apparent that the mean HAI response in group A was higher than that in group B. The higher HAI titres in groups A and B at the time of K vaccination might have influenced the secondary antibody response and the comparatively lower immune response in group C may be related to the very low persistent HAI titres (<5 to 20) following primary vaccination. Kumar et aL (1981) administered K vaccine to

270 SAIFUDDIN, CHOWDH-URY, SARKER AND AMIN

two groups of birds at six and eight weeks after primary F vaccination when persistent mean HAl titres were 53 and 26, respectively. At 14 days after K vaccination the mean HAI titre recorded was higher in the former group (368) than in the latter (243). It was concluded that the residual immunity from primary vaccination doesno t interfere with the secondary K vaccination, rather birds having higher HAI titres following primary vaccination produced higher HAI antibody after secondary vaccination. Crowther (1964) and Kumar et al. (1981) indicated that birds showing a good primary immune response generally produce a higher secondary response. Revaccination of birds with K vaccine has been suggested at the sixth week following primary F vaccination (Crowther, 1964). The patterns of secondary HAI responses in this report are consistent with those of Kumar et al. (1981) and Crowther (1964). However, from this investigation it may be suggested that the sixth to seventh week following primary B1 vaccination would be the safe and suitable period to revaccinate birds with K vaccine.

Different individual HAI responses in the same group of vaccinated birds were observed following B1 and K vaccination. This may have been due to variable maternal or residual immunity, individual variation in immune respon- siveness to viral antigen and the possible impairment of the immune response to vaccination due to other infectious or nutritional conditions (Toth and Markovits, 1964; Lancaster, 1966; Ahraed, Roda, Abbasi and E1-Sisi, 1967). It has been suggested that there might be a genetic inability of some birds to react to NDV (Herbert, 1970; Haplin, 1978).

The HAI titre is usually not regarded as being as good an indicator of protection as the VNI for NDV (Hanson, 1964). Some investigators, however, prefer the HAl[ test to the VN test (Stone, Boney and Coria, 1975; Allan et al., 1978; Kuhavanta, 1978). In the present study a significant and linear correlation was found between the HAl titre and VNI in vaccinated birds (Fig. 2). Crowther (1964) also demonstrated a direct correlation between HAI and VNI in K vaccinated birds but suggested that the VN test was a more reliable indicator of resistance to challenge particularly in the later stages when birds may still resist challenge yet have no detectable HAI antibody. Although the development of HA/and VN antibodies follow a similar course, VN antibody persists longer than HA/antibody (Beard and Hanson, 1984). It has been emphasised that the HAI response cannot be compared directly with the immune status as measured by challenge with virulent NDV (Hamann, 1958; Levine and Fabricant, 1952; Raggi and Lee, 1962). This is due to the fact that the HAI test measures only the humeral antibody which inhibits the agglutination of chicken RBC by NDV whereas the protection of birds against NDV involves both humeral and cellular immunity and other nonspecific immunologic factors or defense mechanisms (Lain and Hao, 1987). Furthermore, in all these reports antibody activity in serum was measured whereas the content of antibody in local secretions, particularly respiratory secretions, may be of more signLficance in protection against field challenge (Laln and Hao, 1987). An apparent role of local immunity in protection of the respiratory tract from NDV infection independent of circulatory antibodies has been proposed (Beard and Easterday, 1967; Holmes, 1979).

At the twenty-ninth and thirty-first week 33-3% and 50% of birds, respec- tively, in group B had HAI titres of 40 or below and failed to resist the challenge with virulent Faridpur-4 strain of NDV (Table I). As protection is inadequate agaivst challenge from seven months onward after K vaccination, birds should be

PROTECTION AGAINST ND IN BANGLADESH 271

revaccinated with the K strain at intervals of less than seven months. Since this study was completed, poultry flocks which have adopted a vaccination schedule of day-old ED administration of B1 strain followed by i/m inoculation of K strain at seven weeks and revaccination with K strain at six monthly intervals, have not experienced clinical outbreaks of ND (unpub.).

ACKNOWLEDGEMENTS

The authors are most grateful to Drs C. R. Wilks and K. M. Moriarty for critical review of the manuscript. The study was supported by research grants from UNICEF.

Accepted for publication April 1989

REFERENCES

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AJ.L~, W. H., LAnCASTm~, J. E. & TO'm, B. (1978). Newcastle disease vaccines: Their production and use. Food and Agriculture Organization of the United Nations, Rome, pp 15-17, 80-108.

AMId, M. M. (1985). Asian Livestock, 10, 8-11. ANON (1971). Methods for examining poultry biologies and for identifying and quantifying avian

pathogens, National Academy of Sciences, Washington DC, pp 82. BEARD, C. W. (1980). In: Isolation and Idtattilication of Avian Pathogens, 2rid edn: (Eds S. B.

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BEARD, C. W. & EASTERDAY, B. C. (1967). Journal oflnfectious Diseases, 117, 55-70. BEARD, C. W. & HANSON, R. P. (1984). In: Diseases of]Poultry, 8th edn. (Eds M. S. Hofstad, etal.),

Iowa State University Press, Ames, Iowa, pp 452-470. CHOWDHURY, M. S. I., CHOWDHURY, T. I. M. F. R., SARKER, A. J. & AMItq, M. M. (1981).

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Research Report: Studies on Newcastle disease in Bangladesh, pp 12-20. CROWTrlER, R. W. (1964). Journal of Comparative Pathology, 74, 151-162. HAMAI~, I. (1958). Monatshefle far Veterinarmedizin, 13, 99-102. HANSON, R. P. (1964). Newcastle disease vinm: an evolving pathogen, University of Wisconsin Press,

Madison, USA, pp 1-352. HANSON, R. P. (1980). In: Isolation and Identiflcmtioa of Avian Pathogem, 2nd edn.(Eds S. B.

Hitchner et al.), American Association of Avian Pathologists, Creative Printing Company, Inc., Endwell, New York 13760, pp 63-66.

HAPLY, B. (1978). Patterns of .&nimat D ~ , English Language Book Society and Bailliere Tindall, London, pp 151-157.

I-IEP~r~T, W. J. (1970). Veterimh~ I m m a m ~ , Blackwell Scientific Publications, London, pp 13-24.

H o ~ , H. C. (1979). Journal of Comparative Pathology, 89, 11-19. KUI-mVArCrA, S. (1978). Asian Livestock, 3, 4-7. KUMAR, K., NARAYANA RAO, C. V., PANKAJAM, B. & KANNAN BABU, N. (1981). Indian Veterinary

Journal, 58, 593-598. LAM, K. M. & HAO, Q. (1987). Veterinary Microbiology, 15, 41-48. L A N C ~ , J. E. (1964). Veterinary Bulletin, 34, 57-76. LANCASTER, J. E. (1966). Newcastle Disease: a review of the literature published between 1926 and

1964, Canada Department of Agriculture, Ottawa, Monograph No. 3. 1 2 v ~ , P. P. & FABRICAUT, J. (1952). Cornell Veterinarian, 42, 449-457. RAC-GI, L. G. & LEE, G. G. (1962). Avian Diseases, 6, 297-301. READ, L. J. & MuEt,~"rt, H. (1938). American Journal of Hygiene, 27, 493-497. SATYANARAYANA, A., MADHAVA KRISHNA REDDY, A., MADHUSUDHANA SWAMY, D. & AKBAR, S.

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272 SAIFUDDIN, CHOWDHURY, SARKER AND AMIN

STONE, H. D., BONEY, JL W. Ao & COP.JA, M. F. (1975). Avian Diseases, 19, 651-656. TOTH, B. & MARKOVITS, P. (1964). Acta Veterinaria Hungarica, 14, 339-345.

PROTECTION CONFEREE CONTP..E LA MALADIE DE NEWCASTLE A U BANGLADESH PAR LA VACCINATION A L 'AIDE DES SOUCHES BLACKSBURG ET KOMAROV DE

VIRUS DE LA MALADIE DE NEWCASTLE

R~ma~,---Une evaluation de l'effu:adt~ de la vaccination de ponssins d'un jour ~t l'aide des souches Blackburg ( B I ) du virus de la maladie de Newcastle, suivie a cliff, rents moments par la vaccination ~t l'aide de la souche Komarov (K), a 6t~ entreprise. Les anticorps furent d~t6ct~s par inhibition de l'hemagglutination (HAI), one semaine apr~s la vaccination avec la souche B1 et le titre atteignit un p ic~ 3 semaines pour d~.~:liner jusqu'a des uiveaux ind~tectables ~t neuf semaines. A la suite de vaccinations cons~,cutives avec la souche K ~ cinq, sept ou huit semaines, les niveaux d'anticorps HAI furent d6tect~s apr~s trois semaines. Les oiseaux vaccin~ a sept semaines furent examines pour les anticorps et la r~sistance t one ~preuve au del~ de 19 semaines d'~tge, Dans ce groupe les titres HAI sont rest~s constants (80 a 640) jusqu'a l ' tge 32 semaines, puis ont diminu~ r6guli~rement jusqu'a 10 i 20 a l'age de 44 semaines.

Une relation lin~aire entre le titre HAI et l'index de neutralisation du virus (VNI) a ~t~ d~montr~e sur tme gamme de s6rums ¢hoisis. Seuls les oiseaux ayant un titre HAI de 80 ou plus ont r~sist6 l'6preuve artifideUe. On recommande que, apr~s une vaccination BI a un jour, et une vaccination K ~t l'~tge de sept semaines, une revaccination K soit r~alis~e a des intervalies n'exc~dant pas sept mois.

PROTECCION CONFERIDA MEDIANTE VACUNACION CON LAS CEPAS BLACKSBURG KOMAROV DEL VIRUS DE NEWCASTLE EN BANGLADESH

R e s ~ a u ~ llev6 a cabo una evaluaci6n de la eficada de la vacunaci6n de pollitos de un dla, con la cepa Blacksburg (B1) del virus de la enfermedad de Newcastle, seguida de varias aplicaciones vacunale~ peri6dicas con la cepa Komarov (K). Se detectaron anticuerpos mediante ia prueba de inhibici6n de la hemaglutinaci6n, una semana despu~s de la vacunaci6n con B1, los t~tulos alcanzando el pico alas tres semanas y declinaron a niveles no detectables alas nueve semanas.

Despu(:s de aplicaciones vacunales peri6dicas con la cepa K, a tas cinco, siete u ocho semanas de edad, se detectaron anticuerpos mediante la prueba de inhibici6n de la hemaglutinaci6n (t~tulos 80 a 640), despu~s de tres semanas. Las ayes vacunadas a las siete semanas, recibieron una descarga y se les hizo prueba de anticuerpos, despu~ de las 19 semanas de edad. En este grupo los ~tulos de anticuerpos permanecieron constantes (80 to 640) hasta las 32 semanas de edad y de~u~s declinaron lentamente a I0 6 20 alas 44 semanas de edad.

Se demostr6 una relaci6n lineal entre los titulos haUados y el lndice viral neutralizante, en un tango de sueros seleccionados. Solamente las ayes con titulos de 80 o m ~ altos resistieron la descarga artifi~al. Se recomienda que, seguidamente a la vacunaci6n con B1 a un dia de edad y con K a las siete semanas, las revacunaciones con ia cepa K debeda hacerse con intervalos de no m~s de siete moses.