Studies of infectious laryngotracheitis vaccines: immunity in layers

Ten-week-old layer chickens obtained from a commercial source were eye-drop vaccinated with chicken-embryo-origin (CEO) or tissue-culture-origin (TCO) vaccines for infectious laryngotracheitis (ILT). Controls were not vaccinated. Approximately one-third of the layers were challenged with virulent ILT virus at 21, 40, or 60 weeks of age. Serum samples taken from the layers before challenge were used in a virus neutralization (VN) test to determine vaccination titers at those three ages. Both vaccines induced low VN titers (geometric mean titer [GMT] less than 6). At 21 weeks of age, the titers produced by the two vaccines were not significantly different, but at 40 and 60 weeks of age the VN GMT of the CEO-vaccinated group was significantly greater than that of the TCO-vaccinated group. The VN GMTs did not drop over time in either group and actually rose between 21 and 60 weeks of age in the CEO group. Both vaccines protected layers against severe challenge with virulent ILT virus, neither being significantly better than the other under these experimental conditions. Unvaccinated sentinel chickens were maintained in contact with the vaccinated layers during three intervals between 1 day and 6 weeks post-vaccination. Diagnostic tests performed on the sentinels to detect lateral spread of vaccine virus from vaccinated to unvaccinated chickens showed scattered positive results.     

Effect of route of vaccination on the prevention of infectious laryngotracheitis in commercial egg-laying chickens

Commercial egg-laying chickens were vaccinated for infectious laryngotracheitis (ILT) with one of five commercially available vaccines (designated A, B, C, D, and E) on five separate farms by either eyedrop (e), spray (s), or double dose in the water (w) method. Groups were identified by the vaccine designation and the method of vaccination. Birds from the test groups were transferred to an isolation facility and challenged intratracheally 3 wk after vaccination. The remaining birds were given a second vaccination with the original chicken embryo origin vaccine by spray or a chicken embryo origin vaccine if the first vaccine was of tissue culture origin. After challenge, birds were monitored for clinical signs. Those surviving were euthanatized on day 6 postchallenge, and tissues and blood were collected for histopathology, virus isolation, and serology. On the basis of histopathology and enzyme-linked immunosorbent assay (ELISA) results, after one vaccination, all chickens given vaccines by eyedrop were provided better protection than nonvaccinated controls (CTLs). Birds in groups Bs and Ds had lower microscopic lesion scores whereas only birds given Bs had higher ELISA titers than CTLs. Birds in groups As and Cs and groups Bw birds taken from the rear of the barn (r) had microscopic lesion scores that were no different from those of CTLs. These same birds in addition to vaccine Ds had ELISA titers no different from those of CTLs. Of all vaccines, only A given by eyedrop or spray produced higher virus isolation titers than those of CTLs. The remainder of the vaccines produced virus isolation titers that were no different from those of CTLs. After two vaccinations, all groups had lower microscopic lesion scores than CTLs. Only Bw birds from the middle of the barn Bs, EeDs, and AsAs had virus isolation results that were higher than those of CTLs. Only groups BwrBs, CsCs, and DsDs had ELISA titers no different from those of controls. These results suggest that a priming vaccination followed by a booster dose offers better protection against ILT than a single vaccination alone. Vaccine application by eyedrop provides more uniform protection if only one vaccination is given, whereas spray vaccination may serve as an alternative method of vaccination for birds receiving two doses of vaccine.     

Efficacy in turkeys of spray vaccination with a temperature-sensitive mutant of Bordetella avium (Art Vax)

Broad-breasted white turkeys were vaccinated with a temperature-sensitive mutant of Bordetella avium (Art Vax) at 2 and 15 days of age and challenged at 22 days of age by contact with infected birds. Necropsy was performed at 35 days of age. Two vaccination protocols (eyedrop/oral and spray cabinet/spray bottle) and two challenge isolates (Arkansas 105 and North Carolina [NC] isolates) were used. Neither the spray nor the eyedrop/oral methods of vaccination prevented infection of the anterior trachea with either of the virulent challenge strains. The spray and eyedrop/oral methods of vaccination were equally effective in reducing the severity of gross lesions in the trachea. The vaccine reduced the severity of gross lesions in the tracheas of turkeys challenged with the NC isolate to a level approximately equal to that observed in unchallenged vaccinated controls, but the vaccine only moderately reduced the severity of lesions in birds challenged with the 105 isolate.     

Characterization of infectious laryngotracheitis virus isolates: demonstration of viral subpopulations within vaccine preparations

Infectious laryngotracheitis (ILT) is a severe acute respiratory disease of chickens caused by ILT virus. To better understand the epidemiology of the disease, a polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) assay of the glycoprotein E gene has been developed and utilized to characterize vaccine strains and outbreak-related isolates. Enzymes EaeI and DdeI were used to differentiate the tissue culture origin (TCO) vaccine from chicken embryo origin (CEO) vaccines. Two RFLP patterns were observed with enzyme EaeI, one characteristic of the TCO vaccine and a second characteristic of all CEO vaccines. Three RFLP patterns were observed with enzyme DdeI. Patterns A and B were characterized as single patterns, whereas the type C pattern was a combination of patterns A and B. Analysis of vaccine strains showed the presence of patterns A and C. Pattern A was observed for the TCO vaccine and one CEO vaccine, whereas pattern C was observed for five of the six CEO vaccines analyzed. PCR-RFLP analysis of plaque-purified virus from pattern C CEO vaccine preparations demonstrated the presence of two populations (patterns A and B). Identification of molecularly different populations of viruses within currently used ILT vaccine is the first step to develop better molecular epidemiologic tools to track vaccine isolates in the field.     

Protection induced by infectious laryngotracheitis virus vaccines alone and combined with Newcastle disease virus and/or infectious bronchitis virus vaccines

Two types of live attenuated vaccines have been used worldwide for the control of infectious laryngotracheitis virus (ILTV): 1) chicken embryo origin (CEO) vaccines; and 2) tissue culture origin vaccines (TCO). However, the disease persists in spite of extensive use of vaccination, particularly in areas of intense broiler production. Among the factors that may influence the efficiency of ILTV live attenuated vaccines is a possible interference of Newcastle Disease virus (NDV) and infectious bronchitis virus (IBV) vaccines with the protection induced by ILTV vaccines. The protection induced by CEO and TCO vaccines was evaluated when administered at 14 days of age alone or in combination with the B1 type strain of NDV (B1) and/or the Arkansas (ARK) and Massachusetts (MASS) serotypes of IBV vaccines. Two weeks after vaccination (28 days of age), the chickens were challenged with a virulent ILTV field strain (63140 isolate, group V genotype). Protection was evaluated at 5 and 7 days postchallenge by scoring clinical signs and quantifying the challenge virus load in the trachea using real-time PCR (qPCR). In addition, the viral load of the vaccine viruses (ILTV, NDV, and IBV) was quantified 3 and 5 days postvaccination also using qPCR. The results of this study indicate that the NDV (B1) and IBV (ARK) vaccines and a multivalent vaccine constituted by NDV (B1) and IBV (ARK and MASS) did not interfere with the protection induced by the CEO ILTV vaccine. However, the NDV (BI) and the multivalent (B1/MASS/ARK) vaccines interfered with the protection induced by the TCO vaccine (P < 0.05). Either in combination or by themselves, the NDV and IBV vaccines decreased the tracheal replication of the TCO vaccine and the protection induced by this vaccine, since the ILTV-vaccinated and -challenged chickens displayed significantly more severe clinical signs and ILTV load (P < 0.05) than chickens vaccinated with the TCO vaccine alone. Although NDV and IBV challenges were not performed, the antibody responses elicited by NDV and/or the IBV vaccinations were significantly reduced (P < 0.05) when applied in combination with the CEO vaccine.     

Differentiation of vaccine strains and Georgia field isolates of infectious laryngotracheitis virus by their restriction endonuclease fragment patterns

Seven restriction endonucleases (REs) were used to cleave the DNA from seven vaccine strains of infectious laryngotracheitis (ILT) virus and from six Georgia field isolates of ILT virus. After electrophoresis of the resulting RE fragments, the patterns were compared in order to differentiate strains of ILT virus. The six chicken-embryo-origin (CEO) vaccines were identical with each RE, but the tissue-culture-origin (TCO) vaccine strain differed from the CEO vaccines using five of the REs. Four of the six field isolates were identical by each RE, but two field isolates differed from each other and from the four identical field isolates on the basis of patterns produced by some but not all of the REs. The four identical field isolates could not be differentiated from the CEO vaccine strains by any RE, but the other two field isolates were not identical to either strain of vaccine virus. This work demonstrates that differentiable strains of ILT virus exist in the United States and that viruses other than vaccine viruses are involved in field outbreaks of ILT.     

Virulence of five live vaccines against avian infectious laryngotracheitis and their immunogenicity and spread after eyedrop or spray application

Five live virus vaccines against avian infectious laryngotracheitis were studied with regard to safety, immunogenicity and route of administration. Significant differences in virulence between the vaccine strains were found. Reduced virulence was accompanied by a reduction of immunogenicity and capacity to spread. After eyedrop application, a low virulent vaccine induced 90-100% flock immunity for the first 10 weeks after vaccination (PV), followed by a slow decline to 50% at 31 weeks PV, whereas flock immunity induced with the more virulent types remained at about 90% till the end of the experiments (24 and 48 weeks PV). Aerosol vaccination induced 70-100% flock immunity but vaccine reactions were severe. Application of vaccine in a coarse spray did not result in adverse vaccine reactions but induced a maximal protection rate of only 50%. Microneutralisation titres provided a useful indicator of immunity from the onset of immunity until immunity started to decline. A vaccine virus carrier state was demonstrated by means of sentinel birds.     

Efficacy of swine influenza A virus vaccines against an H3N2 virus variant

We compared the efficacy of 3 commercial vaccines against swine influenza A virus (SIV) and an experimental homologous vaccine in young pigs that were subsequently challenged with a variant H3N2 SIV, A/Swine/Colorado/00294/2004, selected from a repository of serologically and genetically characterized H3N2 SIV isolates obtained from recent cases of swine respiratory disease. The experimental vaccine was prepared from the challenge virus. Four groups of 8 pigs each were vaccinated intramuscularly at both 4 and 6 wk of age with commercial or homologous vaccine. Two weeks after the 2nd vaccination, those 32 pigs and 8 nonvaccinated pigs were inoculated with the challenge virus by the deep intranasal route. Another 4 pigs served as nonvaccinated, nonchallenged controls. The serum antibody responses differed markedly between groups. After the 1st vaccination, the recipients of the homologous vaccine had hemagglutination inhibition (HI) titers of 1:640 to 1:2560 against the challenge (homologous) virus. In contrast, even after 2nd vaccination, the commercial-vaccine recipients had low titers or no detectable antibody against the challenge (heterologous) virus. After the 2nd vaccination, all the groups had high titers of antibody to the reference H3N2 virus A/Swine/Texas/4199-2/98. Vaccination reduced clinical signs and lung lesion scores; however, virus was isolated 1 to 5 d after challenge from the nasal swabs of most of the pigs vaccinated with a commercial product but from none of the pigs vaccinated with the experimental product. The efficacy of the commercial vaccines may need to be improved to provide sufficient protection against emerging H3N2 variants.     

Protective efficacy of commercial inactivated Newcastle disease virus vaccines in chickens against a recent Korean epizootic strain

Despite the intensive vaccination policy that has been put in place to control Newcastle disease virus (NDV), the recent emergence of NDV genotype VII strains in Korea has led to significant economic losses in the poultry industry. We assessed the ability of inactivated, oil-emulsion vaccines derived from La Sota or Ulster 2C NDV strains to protect chickens from challenge with Kr-005/00, which is a recently isolated Korean epizootic genotype VII strain. Six-week-old SPF chickens were vaccinated once and challenged three weeks later via the eye drop/intranasal route. All vaccinated birds were fully protected from disease, regardless of the vaccine strains used. All vaccinated and challenged groups showed significant sero-conversion 14 days after challenge. However, some vaccinated birds, despite being protected from disease, shed the challenge virus from their oro-pharynx and cloaca, albeit at significantly lower titers than the unvaccinated challenged control birds. The virological, serological, and epidemiological significance of our observations with regard to NDV disease eradication is discussed.     

Protection of chickens after live and inactivated virus vaccination against challenge with nephropathogenic infectious bronchitis virus PA/Wolgemuth/98

Protection provided by live and inactivated virus vaccination against challenge with the virulent nephropathogenic infectious bronchitis virus (NIBV) strain PA/Wolgemuth/98 was assessed. Vaccinations with combinations of live attenuated strains Massachusetts (Mass) + Connecticut (Conn) or Mass + Arkansas (Ark) were given by eyedrop to 2-wk-old specific-pathogen-free leghorn chickens. After live infectious bronchitis virus (IBV) vaccination, some chickens at 6 wk of age received an injection of either an oil emulsion vaccine containing inactivated IBV strains Mass + Ark or an autogenous vaccine prepared from NIBV PA/Wolgemuth/98. Challenge with PA/Wolgemuth/98 was given via eyedrop at 10 wk of age. Serum IBV enzyme-linked immunosorbent assay antibody geometric mean titers (GMTs) after vaccination with the combinations of live attenuated strains were low, ranging from 184 to 1,354, prior to NIBV challenge at 10 wk of age. Both inactivated vaccines induced an anamnestic response of similar magnitudes with serum GMTs of 6,232-12,241. Assessment of protection following NIBV challenge was based on several criteria virus reisolation from trachea and kidney and renal microscopic pathology and IBV-specific antigen immunohistochemistry (IHC). Live attenuated virus vaccination alone with combinations of strains Mass + Conn or Mass + Ark did not protect the respiratory tract and kidney of chickens after PA/Wolgemuth/98 challenge. Chickens given a live combination vaccination of Mass + Conn and boosted with an inactivated Mass + Ark vaccine were also susceptible to NIBV challenge on the basis of virus isolation from trachea and kidney butshowed protection on the basis of renal microscopic pathology and IHC. Live IBV-primed chickens vaccinated with an autogenous inactivated PA/Wolgemuth/98 vaccine had the highest protection against homologous virulent NIBV challenge on the basis of virus isolation.     

Increased virulence of modified-live infectious laryngotracheitis vaccine virus following bird-to-bird passage.

Modified-live (ML) infectious laryngotracheitis (ILT) vaccine viruses, both tissue-culture-origin (TCO) and chicken-embryo-origin (CEO), were passaged 20 times in specific-pathogen-free chickens. After serial bird-to-bird passage, increased virulence was observed for CEO virus but not TCO virus. Increased mortality and increased severity and duration of respiratory disease were observed in chickens inoculated with chicken-passaged CEO viruses; only mild respiratory disease (no mortality) occurred in chickens inoculated with chicken-passaged TCO viruses. These findings suggest that ML ILT vaccine viruses may increase in virulence after bird-to-bird passage.     

Virulence of five live vaccines against Avian Infectious Laryngotracheitis and their immunogenicity and spread after eyedrop or spray application

Summary

Five live virus vaccines against avian infectious laryngotracheitis were studied with regard to safety, immunogenicity and route of administration. Significant differences in virulence between the vaccine strains were found. Reduced virulence was accompanied by a reduction of immunogenicity and capacity to spread.

After eyedrop application, a low virulent vaccine induced 90‐ 100% flock immunity for the first 10 weeks after vaccination (PV), followed by a slow decline to 50% at 31 weeks PV, whereas flock immunity induced with the more virulent types remained at about 90% till the end of the experiments (24 and 48 weeks PV).

Aerosol vaccination induced 70 ‐ 100% flock immunity but vaccine reactions were severe. Application of vaccine in a coarse spray did not result in adverse vaccine reactions but induced a maximal protection rate of only 50%.

Microneutralisation titres provided a useful indicator of immunity from the onset of immunity until immunity started to decline. A vaccine virus carrier state was demonstrated by means of sentinel birds.     

Vaccine genotype and route of administration affect pseudorabies field virus latency load after challenge

The influence of vaccine genotype and route of administration on the efficacy of pseudorabies virus (PRV) vaccines against virulent PRV challenge was evaluated in a controlled experiment using five genotypically distinct modified live vaccines (MLVs) for PRV. Several of these MLVs share deletions in specific genes, however, each has its deletion in a different locus within that gene. Pigs were vaccinated with each vaccine, either via the intramuscular or intranasal route, and subsequently challenged with a highly virulent PRV field strain. During a 2-week period following challenge with virulent PRV, each of the vaccine strains used in this study was evaluated for its effectiveness in the reduction of clinical signs, prevention of growth retardation and virulent virus shedding. One month after challenge, tissues were collected and analyzed for virulent PRV latency load by a recently developed method for the electrochemiluminescent quantitation of latent herpesvirus DNA in animal tissues after PCR amplification. It was determined that all vaccination protocols provided protection against clinical signs resulting from field virus challenge and reduced both field virus shedding and latency load after field virus challenge. Our results indicated that vaccine efficacy was significantly influenced by the modified live vaccine strain and route of administration. Compared to unvaccinated pigs, vaccination reduced field virus latency load in trigeminal ganglia, but significant differences were found between vaccines and routes of administration. We conclude that vaccine genotype plays a role in the effectiveness of PRV MLVs.     

Functional analysis of antibody responses of feedlot cattle to bovine respiratory syncytial virus following vaccination with mixed vaccines.

The antibody response of cattle to bovine respiratory syncytial virus (BRSV) immunization was investigated using 4 different commercially available mixed vaccines. Forty, 5-6 month old, beef calves, randomly assigned to groups of 10, were vaccinated on day 0 and 21 with 1 of 3 inactivated vaccines, (3 groups), or a modified live virus (MLV) vaccine. BRSV-specific antibody responses were measured prior to vaccination and on day 35 by using an enzyme linked immunosorbent assay (ELISA), virus neutralization assay (VN), a fusion inhibition assay (FI); and responses were also measured for their ability to facilitate antibody dependent, complement mediated cytotoxicity (ADCMC) of BRSV infected cells. Sera from day 35 were, in addition, analyzed by use of an IgG1, IgG2 isotype specific ELISA. All vaccines induced significant increases in BRSV specific IgG antibody as measured by ELISA, but only one inactivated and the MLV vaccine induced significant increases in VN titers. Fusion inhibiting antibody titers were low or undetected in calves vaccinated with the inactivated vaccines. Vaccination with modified live virus induced significantly higher titers of fusion inhibiting antibodies, which are considered to be most highly correlated with protection. The VN to ELISA and FI to ELISA ratio of the calves that received MLV vaccine were significantly greater than the calves receiving the 3 inactivated vaccines. Vaccination with MLV induced the highest IgG2/IgG1 ratio. This difference was small, and only significant relative to 2 of the inactivated vaccine groups, which were not significantly different from each other. The higher proportion of IgG2 isotype in the MLV sera was not associated with lower ADCMC, a function not attributed to this isotype. The VN and FI titers, but not the ELISA value of the sera, were most predictive of ADCMC. The inactivation processes apparently alter epitopes and affect the induction of functional antibodies.     

Extent and duration of virulent virus excretion upon challenge of pigs vaccinated with different glycoprotein-deleted Aujeszky's disease vaccines

Different deleted Aujeszky's disease vaccines were compared for their ability to induce an immunity which suppresses virus excretion optimally upon infection. Groups of pigs were vaccinated once with attenuated deleted Aujeszky's disease vaccine (gI, gX or gp63 negative), suspended in phosphate buffered saline. Two additional groups were vaccinated with a gI deleted vaccine virus suspended in an oil-in-water emulsion. Other groups were vaccinated twice with gI deleted inactivated vaccines. The three control groups included were: pigs immune after infection, unvaccinated pigs and pigs receiving vaccine without known deletion in the envelope. Experimental challenge took place 3 or 4 weeks after the only or the last vaccination. The number of excreting pigs, the duration of excretion and the virus titers excreted, were determined for all the groups. All the pigs vaccinated with glycoprotein deletion vaccines suspended in phosphate buffered saline, excreted virus for 2 to 6 days after challenge. A 100 to 1000 fold reduction in excreted virus titers was obtained in vaccinated pigs compared to unvaccinated ones. Some vaccines suppressed virus excretion better than others, but no correlation could be made between the type of deletion (gI, gX or gp63) and the degree of reduction in virus excretion. Similar results were obtained with two applications of inactivated vaccines. The lowest number of excreting pigs, the lowest duration of excretion and the lowest titers were obtained in groups vaccinated with the attenuated vaccine suspended in an oil-in-water emulsion. No vaccine suppressed virus excretion totally.     

Avian influenza mucosal vaccination in chickens with replication-defective recombinant adenovirus vaccine

We evaluated protection conferred by mucosal vaccination with replication-competent adenovirus-free recombinant adenovirus expressing a codon-optimized avian influenza (AI) H5 gene from A/turkey/WI/68 (AdTW68.H5ck). Commercial, layer-type chicken groups were either singly vaccinated ocularly at 5 days of age, singly vaccinated via spray at 5 days of age, or ocularly primed at 5 days and ocularly boosted at 15 days of age. Only chickens primed and boosted via the ocular route developed AI systemic antibodies with maximum hemagglutination inhibition mean titers of 3.9 log2 at 32 days of age. In contrast, single vaccination via the ocular or spray routes maintained an antibody status similar to unvaccinated controls. All chickens (16/16) subjected to ocular priming and boosting with AdTW68.H5ck survived challenge with highly pathogenic AI virus A/chicken/Queretaro/14588-19/95 (H5N2). Single ocular vaccination resulted in 63% (10/16) of birds surviving the challenge followed by a 44% (7/16) survival of single-sprayed vaccinated birds. Birds vaccinated twice via the ocular route also showed significantly lower (P < 0.05) AI virus RNA concentrations in oropharyngeal swabs compared to unvaccinated-challenged controls.     

Safety and efficacy of the cell-associated vaccine prepared by an attenuated infectious laryngotracheitis virus.

The safety and efficacy of the cell-associated (C-A) vaccine prepared by chicken embryo fibroblast (CEF) cells infected with the tissue-culture-modified strain of infectious laryngotracheitis (ILT) virus were studied in chickens. Over seventy percent of chickens inoculated with the C-A vaccine by the subcutaneous (S.C.) or intramuscular (I.M.) route at 1 day of age was protected against challenge with a virulent strain of ILT virus without any clinical signs. Chickens vaccinated with the C-A vaccine at 1 day of age acquired immunity within 6 days after vaccination, and the protection rate maintained more than 60% until 10 weeks post-vaccination. The C-A vaccine was invariably effective for chickens at various age. There was no evidence that the development of immunity was hindered by further vaccination with Newcastle disease and infectious bronchitis combined live vaccine. In addition, the C-A vaccine was safe when chickens were inoculated with 10 doses. In the field trials of the C-A vaccine, no adverse reaction was observed, and over 65% of vaccinated chickens was protected against the challenge of the virulent ILT virus at 8 weeks after vaccination.     

Characterization of infectious laryngotracheitis virus (ILTV) isolates from commercial poultry by polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP)

Infectious laryngotracheitis (ILT) is a highly contagious, acute respiratory disease of chickens, of worldwide distribution, that affects growth and egg production and leads to significant economic losses during periodic outbreaks of the disease. Live attenuated vaccines (chicken embryo origin [CEO] and tissue-culture origin [TCO]) have been widely used to control the disease in the United States. It is believed that most of the outbreaks in the United States are caused by vaccine-related isolates that persist in the field and spill over into na?ve poultry populations. The objective of this study was to utilize the previously developed polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) analysis to genotype recent ILT virus (ILTV) isolates from commercial poultry. Forty-six samples were collected during January 2006 to April 2007 from five poultry production regions of the United States and were characterized within PCR-RFLP groups III-VI. Sixty-three percent of the samples analyzed were categorized as closely related to the vaccine strains (groups III-V), whereas 33% were categorized as group VI viruses that differed in six and nine PCR-RFLP patterns from the CEO and TCO vaccines; a mixture of group IV and V viruses was detected in two samples (4%). In general, groups V and VI were the most prevalent viruses, found in 52% and 33% of the samples tested respectively. Both types of viruses were detected in vaccinated and nonvaccinated flocks. Although genetically different, both viruses produced severe disease in the field.     

Replication and transmission of live attenuated infectious laryngotracheitis virus (ILTV) vaccines

The aim of this study was to evaluate the replication of live attenuated infectious laryngotracheitis virus vaccines in selected tissues and their ability to transmit to contact-exposed birds. Four-week-old specific-pathogen-free chickens were eye drop-inoculated with tissue culture origin (TCO) and chicken embryo origin (CEO) vaccines. Contact-exposed chickens were housed in direct contact with eye drop-inoculated chickens from the first day postinoculation. Virus isolation and real-time polymerase chain reaction were used to detect the presence of live virus and viral DNA, respectively, in the trachea, trigeminal ganglia, eye conjunctiva, cecal tonsils, and cloaca from eye drop-inoculated and contact-exposed birds at days 2, 4, 5 to 10, 14, 18, 21, 24, and 28 postinoculation. No differences were observed in the ability of the TCO and CEO vaccines to replicate in the examined tissues. Both vaccines presented a localized replication in the eye conjunctiva and the trachea. Both vaccines were capable of transmitting to contact-exposed birds, attaining peaks of viral DNA as elevated as those observed in inoculated birds. The CEO vaccine replicated faster and reached higher viral genome copy number than the TCO vaccine in the conjunctiva and trachea of eye drop-inoculated and contact-exposed birds. The viral DNA from both vaccines migrated to the trigeminal ganglia during early stages of infection. Although the CEO and TCO vaccines were not recovered from the cecal tonsils and the cloaca, low levels of viral DNA were detected at these sites during the peak of viral replication in the upper respiratory tract.     

Newcastle disease vaccination of broilers with high- and low-titered commercial vaccines

The virus titers of seven commercial B1 strain Newcastle disease vaccines were evaluated. A 2 log difference in virus content was found between the vaccine with the highest titer (10(8.8) EID 50/ml) and the one with the lowest titer (10(6.8) EID 50/ml). Broiler chickens were vaccinated with the high- and low-titered vaccines to compare hemagglutination-inhibition (HI) antibody and challenge responses. The effect of vaccination at different ages on the HI titers was also examined. There were no significant differences between vaccine groups in HI antibody response or resistance to challenge. However, the high-titered vaccine may provide a margin of safety with the currently used methods of mass vaccination.