Haemorrhagic enteritis of turkeys – current knowledge

Haemorrhagic enteritis virus (HEV), an adenovirus associated with acute haemorrhagic gastro-intestinal disease of 6–11-week old turkeys predominantly hampers both humoral and cellular immunity. Affected birds are more prone to secondary complications (e.g. colibacillosis and clostridiosis) and failure to mount an effective vaccine-induced immune response. HEV belongs to the new genus Siadenovirus. Feco-oral transmission is the main route of entry of the virus and it mainly colonizes bursa, intestine and spleen. Both naturally occurring virulent and avirulent strains of HEVs are serologically indistinguishable. Recent findings revealed that ORF1, E3 and fib genes are the key factors affecting virulence. The adoption of suitable diagnostic tools, proper vaccination and biosecurity measures have restrained the occurrence of disease epidemics. For diagnostic purposes, the best source of HEV is either intestinal contents or samples from spleen. For rapid detection highly sensitive and specific tests such as quantitative real-time PCR based on Taq man probe has been designed. Avirulent strains of HEV or MSDV can be effectively used as live vaccines. Novel vaccines include recombinant hexon protein-based subunit vaccines or recombinant virus-vectored vaccines using fowl poxvirus (FPV) expressing the native hexon of HEV. Notably, subunit vaccines and recombinant virus vectored vaccines altogether offer high protection against challenge or field viruses. Herein, we converse a comprehensive analysis of the HEV genetics, disease pathobiology, advancements in diagnosis and vaccination along with appropriate prevention and control strategies.     

Reduced efficacy of hemorrhagic enteritis virus vaccine in turkeys exposed to avian pneumovirus

Avian pneumovirus (APV) is an immunosuppressive respiratory pathogen of turkeys. We examined the effect of APV infection on the vaccine efficacy of hemorrhagic enteritis virus (HEV) vaccines. APV was inoculated in 2-wk-old turkeys. Two or four days later, an attenuated HEV vaccine (HEVp30) or marble spleen disease virus (MSDV) vaccine were administered. Virulent HEV challenge was given 19 days after HEV vaccination. APV exposure compromised the ability of HEVp30 and MSDV to protect turkeys against virulent HEV. The protective index values were as follows: MSDV (100%) versus APV + MSDV (0%) (P < 0.05); HEVp30 (60%) versus APV + HEVp30 (30%) (P < 0.05) (Experiment I) and HEVp30 (56%) versus APV + HEVp30 (20%) (P < 0.05) (Experiment II). These data indicated that APV reduced the efficacy of HEV vaccines in turkeys.     

Hemorrhagic enteritis of turkeys: influence of maternal antibody and age at exposure

The effect of maternal antibody (MAB) to hemorrhagic enteritis (HE) on the response of turkeys to infection with virulent and avirulent strains of HE virus (HEV) was examined. The influence of age at exposure and treatment with HEV antibody on development of clinical HE also was studied. MAB protected poults from clinical HE for up to 6 weeks of age. MAB also interfered with vaccination against the disease for at least 5 weeks after hatching, as indicated by absence of HEV antigen in spleens and by poor seroconversion at 6 days and at 3 weeks post-vaccination, respectively. The incidence of clinical HE in MAB-negative poults was significantly higher in poults inoculated with virus at 15 days of age or older than in poults inoculated at 1-13 days of age. Further, MAB-negative poults embryonally inoculated with virulent or avirulent strains of HEV did not develop disease; these poults developed antibody and resisted challenge with virulent virus at 6 weeks of age. Poults treated with HE antibody within 1 hour of challenge or at 1, 3, or 5 weeks before challenge with virulent virus were protected against lesions and mortality induced by HEV. These results suggest that MAB may influence susceptibility of turkeys to infection with HEV for at least 5 to 6 weeks after hatching, unlike the case with most other viral infections of poultry. The results confirm that early age resistance to clinical HE is independent of MAB and suggest that such resistance persists for up to 13 days of age. The data also suggest that turkeys lacking MAB can be immunized against HE by embryo vaccination.     

Evidence for bursal involvement in the pathogenesis of hemorrhagic enteritis of turkeys

The pathogenesis of hemorrhagic enteritis in turkey poults infected with hemorrhagic enteritis virus (HEV) at 3 days or at 2 or 5 weeks of age was compared with pathogenesis in poults that had been chemically bursectomized neonatally and exposed to cell-culture-propagated virus at 2 or 5 weeks of age. Conventional poults exposed to HEV at 2 or 5 weeks developed clinical disease, and mortality ranged from 38% to 100%. In addition to the splenic and intestinal lesions usually seen with HEV infection, the pancreas, bursa of Fabricius, and thymus were also affected. In contrast, although they were free from detectable maternal antibody, poults infected with HEV at 3 days of age failed to develop clinical disease or mortality; however, virus was demonstrated by histological and electron microscopic examinations in spleens of these poults. Neonatal chemical bursectomy completely prevented the clinical signs, gross lesions, and mortality induced by HEV in poults at 2 or 5 weeks of age. These findings strongly suggest that an intact bursa is necessary for HEV to induce disease in turkeys.     

Relationship between age of flock seroconversion to hemorrhagic enteritis virus and appearance of adenoviral inclusions in the spleen and renal tubule epithelia of turkeys.

A retrospective study was conducted to evaluate the temporal relationship between flock seroconversion to hemorrhagic enteritis virus (HEV) and the appearance of adenoviral inclusions in the spleen and renal tubular epithelium. The study was conducted on samples of turkey poults submitted to the Fresno Branch of the California Veterinary Diagnostic Laboratory System during May to December 1988. The study included 78 submissions (four to eight poults per submission) of ages ranging from 6 to 15 weeks. Sera were tested for antibodies to HEV using the agar gel immunodiffusion test. Spleen and kidney samples were examined by light microscopy for the presence of inclusions in the mononuclear phagocytes of the spleen or in the renal tubular epithelium of the kidney. Logistic regression statistical analysis was used to evaluate the association between the age of the bird and the likelihood of the presence of inclusions in the spleen and kidney, as well as the likelihood of seroconversion to HEV. A significant association (P less than 0.05) was found between the presence of splenic inclusion bodies and the age of the bird. The probability of splenic inclusions was higher in younger birds (6 weeks of age), and decreased as the birds became older, approaching zero at 11 weeks of age. The kidney inclusions were significantly associated with age. The probability of detecting the inclusions increased with age, reached a maximum at 10 weeks, and then declined, approaching zero by 14 weeks. However, the probability of seroconversion to HEV increased significantly with age up to 10 weeks and then remained positive throughout the remainder of the study period.     

Protection of turkeys from hemorrhagic enteritis with a recombinant fowl poxvirus expressing the native hexon of hemorrhagic enteritis virus.

Hemorrhagic enteritis (HE) is an economically important disease of turkeys caused by a type II aviadenovirus, hemorrhagic enteritis virus (HEV). The vaccines currently available to the commercial poultry producer are highly effective in preventing disease outbreaks; however, they are immunosuppressive. A recombinant fowl poxvirus (rFPV) expressing the native hexon of HEV has been shown to induce an anti-HEV humoral immune response in turkeys. In this study, the rFPV expressing the native hexon of HEV was compared with a commercial HEV vaccine (vxHEV) for its ability to protect turkeys from virulent HEV challenge. Complete protection from the enteritis of HE was achieved in experimental groups vaccinated with either the rFPV or the vxHEV. Lymphocyte stimulation was measured in turkeys inoculated with rFPV, vxHEV, or a sublethal dose of HEV or not inoculated. No statistically significant immunodepression was observed in turkeys receiving the rFPV.     

Effect of field beans and supplemental methionine on growth of turkey poults

Male poults housed at 10 and 15 birds/pen and female poults housed at 15 birds/ pen were fed, from 6 to 10 weeks, on diets containing o, 15, 30 and 30% field beans (Vicia faba L.). The difference between the last two diets was in the level of methio‐nine; one contained 1.6 and the other 1.9%. From 10 to 16 weeks of age birds were fed on diets containing 0 or 20% field beans. At 10 weeks birds given diets with 30% beans were significantly lighter than other poults. At 16 weeks only the poults previously given a diet with 1.9% methionine were significantly lighter than control birds. Males given 0.37 m² of floor space were significantly heavier at 16 weeks than those given 0.25 m².     

Protective efficacy of H5 inactivated vaccines in meat turkey poults after challenge with Egyptian variant highly pathogenic avian influenza H5N1 virus

In contrast to chickens, there is a paucity of information on the potency of H5 vaccines to protect turkeys against the highly pathogenic avian influenza (HPAI) H5N1 virus infections. In this study, 4 groups, 10 turkey poults each, were vaccinated at seven days old with one of H5N2 or H5N1 commercial vaccines or one of two prepared H5N1 vaccines from a local Egyptian variant HPAI H5N1 (EGYvar/H5N1) strain. At 35 days age, all vaccinated and 10 non vaccinated birds were challenged intranasal with 10(6) EID(50)/0.1 ml of EGYvar/H5N1. All vaccines used in this study were immunogenic in turkeys. There was no cross reaction between the commercial vaccines and the Egyptian variant H5N1 antigen as obtained by the hemagglutination inhibition test. Birds vaccinated with H5N2 vaccine were died, while other H5N1 vaccinated groups have had 20-40% mortality. The highest virus excretion was found in non-vaccinated infected and H5N2 vaccinated birds. Eleven peculiar amino acid substitutions in H5 protein of the variant strain were existed neither in the vaccine strains nor in the earliest H5N1 virus introduced into Egypt in 2006. In conclusion, single vaccination at seven days old is inadequate for protection of meat turkeys against variant HPAI H5N1 challenge and multi-dose vaccination at older age is recommended. For the foreseeable future, continuous evaluation of the current vaccines in H5N1 endemic countries in the face of virus evolution is a paramount challenge to mitigate the socio-economic impact of the virus.     

Assessment of 2 Salmonella enterica serovar Typhimurium-based vaccines against necrotic enteritis in reducing colonization of chickens by Salmonella serovars of different serogroups

This study assessed the protective efficacy of oral vaccination with 2 experimental attenuated Salmonella Typhimurium-vectored vaccines for necrotic enteritis in protecting chickens against intestinal colonization by common serovars of Salmonella belonging to the 4 major serogroups affecting chickens. Birds were vaccinated orally with 1 × 10⁸ colony-forming units (CFU) of 1 of the vaccine strains χ9241 and χ9352, which express a plasmid-encoded partial recombinant hypothetical protein gene (tHP) of Clostridium perfringens, at days 1 and 7 of age, and then were challenged at 14 d of age with 10⁶ CFU of Salmonella serovars Anatum, Enteritidis, Heidelberg, Kentucky, or Typhimurium (representative serovars of serogroups B, C, D, and E). Birds were necropsied at 4 wk of age, and samples were collected to determine reduction in tissue and intestinal colonization. The chickens vaccinated with χ9241-tHP showed reduced colonization by Salmonella Enteritidis (serogroup D) and by Salmonella Heidelberg and Salmonella Typhimurium (serogroup B) compared with the control birds. No reduction in colonization was observed in the chickens vaccinated with χ9352-tHP. There was an association between the efficacy of these vaccine strains in protecting against necrotic enteritis, assessed on an earlier occasion, and their efficacy in protecting against Salmonella colonization. Thus, the choice of an attenuated Salmonella Typhimurium vaccine vector for delivery of heterologous antigens to chickens should be based partly on the vaccine’s value in protecting against colonization by serovars within serogroups B and D. Such vectors would have the additional benefit of reducing colonization of important Salmonella serovars.     

Vaccines against Aujeszky's disease: Evaluation of their efficacy under standardized laboratory conditions

Summary

A standardized test was developed to compare the efficacy of Aujeszky's disease virus (ADV) vaccines under laboratory conditions. Per test 3 groups of 6 to 8 sero‐negative pigs were used. The first vaccination was done at 10 weeks of age. One group was vaccinated once, another was vaccinated twice and the 3rd served as control. Pigs were challenge exposed to the virulent NIA‐3 strain of ADV 12 weeks after the first vaccination. Apart from mortality, average periods of growth arrest, fever and virus shedding after challenge were used as parameters to evaluate vaccine efficacy.

Two inactivated and 4 attenuated vaccines were tested. Two attenuated vaccine viruses were excreted after vaccination. Despite maximal standardization, a considerable variation still existed between the experiments in mortality and growth arrest periods of control pigs after challenge. However, the controls were always more severely affected than the vaccinated pigs. All vaccines except one were effective in preventing death after challenge, but none conferred complete protection. Most vaccinated pigs still lost weight, developed fever and shed virus after challenge. Revaccination after 3 or 4 weeks had little effect, particularly with the attenuated vaccines. The results of the present study indicate that 2 of the attenuated vaccines conferred the best protection, I attenuated vaccine appeared to be as effective as the 2 inactivated ones, and the 4th attenuated vaccine was least effective.     

THE SEROLOGICAL RESPONSE OF CHICKENS TO OIL EMULSION VACCINES CONTAINING THE V4 STRAIN OF NEWCASTLE DISEASE VIRUS

SUMMARY Experiments were conducted with vaccines containing the V₄ strain of Newcastle disease virus (NDV). Both living aqueous vaccines and vaccines consisting of virus incorporated in an oil emulsion were used. The calculated dose of virus contained in the oil emulsion vaccine was 10^8,7 50% embryo infectious doses (EID₅₀) per bird dose. Haemagglutinin inhibition (HI) antibody levels of 8 are presumed protective. One-day-old chicks with low levels of maternal antibody were vaccinated intraocularly with 10^6,3EID₅₀ of live vaccine, and concurrently with oil emulsion vaccine. Presumed protective levels of antibody were present at two weeks post vaccination and were maintained for at least seven weeks longer. When adult birds 15 weeks old with no previous exposure to NDV were vaccinated intraocularly with 10^6,7EID₅₀ per bird, protective levels of antibody were produced within a week. Unvaccinated birds put in contact with the vaccinated birds produced similar antibody levels within 14 days. Revaccination with oil emulsion vaccine after antibody levels had fallen resulted in a rapid response with high levels of antibody. When antibody-free adult commercial birds with an unknown history of exposure to NDV were vaccinated intramuscularly with oil emulsion vaccine, high antibody levels were produced for at least 21 weeks. Concurrent intraocular inoculation with 10^7,0EID₅₀ live virus did not enhance the response. Natural infection of unvaccinated birds occurred during the experiment. This was detected by the presence of HI antibody levels of short duration. When antibody-free commercial birds were inoculated intramuscularly with oil emulsion vaccine containing 10^6,0, 10^7,0, or 10^8,0EID₅₀ per bird dose, 100% of birds inoculated with the highest dose produced presumed protective levels of antibody within two weeks, as compared with a 5-week delay when using the 10^7,0EID₅₀ per bird dose.     

Immunomodular effect of fusion gene DNA vaccine of avian metapneumoviruses

The objective of this work was to develop and evaluate the immunomodular effect of a DNA vaccine based on the fusion (F) gene of avian metapneumoviruses (aMPV) and to study its protection against field virus challenge, as this will help to better control the disease in turkeys. In this study, the F protein of the Egyptian isolate (Giza-turkey rhinotracheitis-4) of the B-subtype of aMPV isolated in 2009 was expressed from a DNA plasmid in Vero cells. After 1 i.m. injection of turkey poults with this plasmid, the antibody response was detected by ELISA. The turkey poults inoculated with locally prepared DNA aMPV vaccine had highly significant phagocytic activity, as measured by phagocytic percent and index of macrophage activation, in comparison to those inoculated with inactivated and live attenuated vaccines and with the noninfected control group. Intratracheal challenge of turkey poults at 21 d postvaccination by a dose of 100 uL of field Egyptian Giza-turkey rhinotracheitis-4 virus of a titer 6 log₁₀ tissue culture infective dose 50 resulted in 100% protection in poults that received locally prepared DNA aMPV vaccine, whereas those that received commercial aMPV vaccines experienced 80 and 90% protection; typical clinical signs of aMPV infection were seen in control nonvaccinated poults. Therefore, a high success rate was noted when using F gene DNA plasmid vaccine by the induction of a potent immunomodular effect for both cell-mediated and humoral immune response. The use of the F gene DNA plasmid vaccine developed in this study provided 100% protection in vaccinated poults, which can help in controlling aMPV infections in turkeys.     

Protection of laying hens against infectious bronchitis with inactivated emulsion vaccines

Commercially-reared laying chickens were challenged at 31 weeks of age with a virulent infectious bronchitis (IB) virus. They showed a sharp drop in egg production, despite having been vaccinated at four and eight weeks old with live attenuated IB vaccines to a recommended schedule. In contrast, similar birds that had been further immunised at point-of-lay with inactivated oil emulsion IB vaccine, or with a combined IB/Newcastle disease (ND) emulsion vaccine, showed no detectable fall in egg production after the same challenge. Unvaccinated susceptible specific pathogen-free birds challenged at the same time stopped laying almost completely. In the birds revaccinated with emulsion vaccine, measurement of haemagglutination inhibition antibody levels to IB showed their geometric mean titres to be raised from less than 5 log2 at the time of vaccination to over 10 log2 four weeks later. Their antibody levels did not rise further followining the IB challenge whereas in the birds that had not been revaccinated antibody rises to nearly 10 log2 were detected after the same challenge. For pullets vaccinated earlier with live IB vaccine, revaccination with inactivated IB or IB/ND oil emulsion vaccine at point-of-lay provides a safe and effective way of protecting their egg production against IB infection.     

Mucosal vaccination with formalin-inactivated avian metapneumovirus subtype C does not protect turkeys following intranasal challenge

Studies were performed to determine if mucosal vaccination with inactivated avian metapneumovirus (aMPV) subtype C protected turkey poults from clinical disease and virus replication following mucosal challenge. Decreases in clinical disease were not observed in vaccinated groups, and the vaccine failed to inhibit virus replication in the tracheas of 96% of vaccinated birds. Histopathologically, enhancement of pulmonary lesions following virus challenge was associated with birds receiving the inactivated aMPV vaccine compared to unvaccinated birds. As determined by an enzyme-linked immunosorbent assay (ELISA), all virus-challenged groups increased serum immunoglobulin (Ig) G and IgA antibody production against the virus following challenge; however, the unvaccinated aMPV-challenged group displayed the highest increases in virus-neutralizing antibody. On the basis of these results it is concluded that intranasal vaccination with inactivated aMPV does not induce protective immunity, reduce virus shedding, or result in decreased histopathologic lesions.     

Field vaccination against hemorrhagic enteritis of turkeys by a cell-culture live-virus vaccine

A cell-culture-propagated (CC) live-virus hemorrhagic enteritis (HE) vaccine was evaluated for efficacy and safety in two field trials conducted in North Carolina (NC) and Minnesota (MN). At 4 or 5 1/2 weeks of age, 9,839 poults in NC and 15,857 poults in MN were vaccinated with a CC HE vaccine administered via the drinking water. A comparable number of poults were maintained as unvaccinated controls. Vaccinated and unvaccinated poults were compared for seroconversion, response to laboratory challenge with a virulent HE virus at 3 weeks postvaccination, livability, percentage graded A, and average weight at marketing. In both trials, vaccination with the CC HE vaccine resulted in immunity against HE as indicated by seroconversion and by resistance to HE lesions following laboratory challenge with virulent HE virus. Compared with unvaccinated groups, vaccinated groups had a significantly higher percentage of turkeys graded A in the NC trial and in two of three flocks in the MN trial (P less than 0.005). Further, in the NC trial, livability was significantly higher (P less than 0.005) in vaccinated turkeys than in unvaccinated turkeys. These data indicate that the CC HE vaccine is efficacious and safe to use in the field.     

Effect of dietary fatty acids on humoral immune response of Turkeys

1. This study examined the effect of increasing amounts of dietary polyunsaturated fatty acids on the fatty acid composition in serum and antibody production following a standard vaccination programme in growing turkeys. Turkey poults were fed on 5 diets containing 75g/kg added fat made up of different proportions of palm and soyabean oils, and were vaccinated against Newcastle disease, infectious bronchitis and necrotic enteritis according to a standard vaccination programme. Blood samples were taken before and one week after each vaccination.

2. Fatty acid composition in serum reflected the composition of the diets although arachidonic acid concentration was not changed by dietary fatty acid content. Growth, erythrocyte and leukocyte parameters were not affected by the respective diets.

3. Specific antibody production was related quadratically to serum linoleic and total n‐6 polyunsaturated fatty acid concentrations. No correlation was found with linolenic or arachidonic acids.

4. It is concluded that dietary fatty acid composition can augment the specific anti‐vaccine immune response in turkey poults.     

Effect of Endobronchial Challenge with Actinobacillus pleuropneumoniae Serotype 9 of Pigs Vaccinated with a Vaccine Containing Apx Toxins and Transferrin‐binding Proteins

The efficacy of a subunit vaccine containing the Apx toxins of Actinobacillus pleuropneumoniae and transferrin‐binding proteins was determined. Ten pigs were vaccinated twice with the vaccine. Eight control animals were injected twice with a saline solution. Three weeks after the second vaccination, all pigs were endobronchially inoculated with 10^6.5 colony‐forming units (CFU) of an A. pleuropneumoniae serotype 9 strain. In the vaccine group, none of the pigs died after inoculation. Only one pig of the control group survived challenge. Surviving pigs were killed at 7 days after challenge. The mean percentage of affected lung tissue was 64% in the control group and 17% in the vaccine group. Actinobacillus pleuroípneumoniae was isolated from the lungs of all animals. The mean bacterial titres of the caudal lung lobes were 5.0 × 10⁸ CFU/g in the control group and 3.0 × 10⁶ CFU/g in the vaccine group. It was concluded that the vaccine induced partial protection against severe challenge.     

Effect of IBV-H120 vaccination in broilers on colibacillosis susceptibility after infection with a virulent Massachusetts-type IBV strain

Vaccination against infectious bronchitis (IB) is aimed to protect against clinical IB. The question is, however, whether vaccinated birds are also protected against predisposure for colibacillosis after a subsequent IBV infection. We examined this research question in four experiments. One-day-old commercial broilers, housed in isolators, were vaccinated with IB vaccine strain H120 by coarse spray or ocularly. Twenty-eight days after vaccination, broilers were challenged with the virulent IBV strain M41. Five days later, broilers were inoculated with Escherichia coli strain 506. Body weight uniformity, severity of E. coli airsacculitis, and systemic E. coli infection at 7 days following E. coli inoculation were used as parameters for colibacillosis. IBV vaccination reduced both the number of broilers with E. coli airsacculitis as well as the severity of airsacculitis significantly after challenge with IBV-M41 and E. coli 506. However, in spray-vaccinated groups, no significant reduction of the number of birds with systemic colibacillosis or the severity of this infection was obtained, and body weight uniformity was not significantly improved compared with nonvaccinated, IBV-M41, and E. coli 506-challenged groups. Eye-drop vaccination resulted in conflicting results.     

Oral vaccination of brushtail possums with BCG: Investigation into factors that may influence vaccine efficacy and determination of duration of protection

AIMS: To determine factors that may influence the efficacy of an oral pelleted vaccine containing Mycobacterium bovis bacille Calmette-Guérin (BCG) to induce protection of brushtail possums against tuberculosis. To determine the duration of protective immunity following oral administration of BCG.

METHODS: In Study 1, a group of possums (n=7) was immunised by feeding 10 pellets containing dead Pasteur BCG, followed 15 weeks later with a single pellet of live Pasteur BCG. At that time, four other groups of possums (n=7 per group) were given a single pellet of live Pasteur BCG orally, a single pellet of live Danish BCG orally, 10 pellets of live Pasteur BCG orally, or a subcutaneous injection of live Pasteur BCG. For the oral pelleted vaccines, BCG was formulated into a lipid matrix, and each pellet contained approximately 10⁷ colony forming units (cfu) of BCG, while the vaccine injected subcutaneously contained 10⁶ cfu of BCG. A sixth, non-vaccinated, group (n=7) served as a control. All possums were challenged by the aerosol route with a low dose of virulent M. bovis 7 weeks after vaccination, and killed 7–8 weeks after challenge. Protection against challenge with M. bovis was assessed from pathological and bacteriological findings.

In Study 2, lipid-formulated live Danish BCG was administered orally to three groups of possums (10–11 per group), and these possums were challenged with virulent M. bovis 8, 29 or 54 weeks later. The possums were killed 7 weeks after challenge, to assess protection in comparison to a non-vaccinated group.

RESULTS: The results from Study 1 showed that vaccine efficacy was not adversely affected by feeding dead BCG prior to live BCG. Feeding 10 vaccine pellets induced a level of protection similar to feeding a single pellet. Protection was similar when feeding possums a single pellet containing the Pasteur or Danish strains of BCG. All vaccinated groups had significantly reduced pathological changes or bacterial counts when compared to the non-vaccinated group. In Study 2, oral administration of Danish BCG induced protection against challenge with M. bovis, which persisted for at least 54 weeks after vaccination. Some protection was observed in possums challenged 54 weeks after vaccination, but this protection was significantly less than that observed in groups vaccinated 29 or 8 weeks prior to challenge. There was a strong relationship between the proportion of animals producing positive lymphocyte proliferation responses to M. bovis antigens and protection against challenge with M. bovis.

CONCLUSIONS: Factors considered potentially capable of interfering with vaccination, including feeding dead BCG to possums prior to feeding live BCG, feeding multiple doses of BCG at one time, and changing strains of BCG, were shown not to interfere with the acquisition of protective immune responses in possums. Protection against tuberculosis was undiminished up to 29 weeks after vaccination with BCG administered orally. It is concluded that vaccination of possums by feeding pellets containing BCG is a robust and efficient approach to enhance the resistance of these animals to tuberculosis.     

Influence of infectious bronchitis strains and vaccines on the incidence of Mycoplasma synoviae airsacculitis

A model system was used to study infectious bronchitis virus (IBV) and Mycoplasma synoviae (MS) interaction. The system involved exposure of chickens to IBV, followed by exposure to MS 2-5 days later. The chickens were subjected to a cold environment (10 +/- 2 C) for 3 weeks starting one day before MS exposure. Under these conditions, differences in the capacity of various strains of IBV to exacerbate MS airsacculitis was demonstrated. Exposure to IBV field isolates generally resulted in more air-sac lesions than did higher-egg-passaged laboratory strains and vaccine strains. Use of lower-egg-passaged vaccines resulted in a higher incidence of airsacculitis than did higher-egg-passaged vaccines. When chickens were IBV-vaccinated before being used in the model system, the incidence of airsacculitis was lowered, even though the chickens became infected by the challenge virus. Vaccination of MS-free chickens with IBV had no effect on airsacculitis incidence when MS exposure occurred after the vaccine reaction was past.