Rescue of infectious bursal disease virus from mosaic full-length clones composed of serotype I and II cDNA

Infectious Bursal Disease Virus (IBDV) is the causative agent of one of the most important and wide-spread infectious diseases among commercial chicken flocks. IBDV causes a depletion of B-lymphoid cells in the bursa of Fabricius, inducing immunosuppression, morbidity, or even acute mortality. Because currently used live IBDV vaccines are derivatives from field isolates no serologic discrimination between field isolates and live vaccines can be made. The recently developed reverse genetics techniques for IBDV allows one to generate genetically modified IBDVs which might have altered biological and antigenic properties. Here, we describe the rescue of mosaic serotype I IBDVs, of which the polyprotein encoding region was partly replaced by the corresponding region of a serotype II strain. A mosaic virus, containing the C-terminal part of serotype II VP3 showed only a slightly delayed release of progeny virus compared to unmodified serotype I virus, while maximum viral titers at 25 h post infection were equal. Since serotype specific epitope(s) are present in the C-terminal part of VP3, we were able to discriminate this rescued virus from serotype I and II IBDV strains. These findings make the use of a chimeric VP3 a promising approach to develop an IBDV marker vaccine.

Rescue of very virulent and mosaic infectious bursal disease virus from cloned cDNA: VP2 is not the sole determinant of the very virulent phenotype

Many recent outbreaks of infectious bursal disease in commercial chicken flocks worldwide are due to the spread of very virulent strains of infectious bursal disease virus (vvIBDV). The molecular determinants for the enhanced virulence of vvIBDV compared to classical IBDV are unknown. The lack of a reverse genetics system to rescue vvIBDV from its cloned cDNA hampers the identification and study of these determinants. In this report we describe, for the first time, the rescue of vvIBDV from its cloned cDNA. Two plasmids containing a T7 promoter and either the full-length A- or B-segment cDNA of vvIBDV (D6948) were cotransfected into QM5 cells expressing T7 polymerase. The presence of vvIBDV could be detected after passage of the transfection supernatant in either primary bursa cells (in vitro) or embryonated eggs (in vivo), but not QM5 cells. Rescued vvIBDV (rD6948) appeared to have the same virulence as the parental isolate, D6948. Segment-reassorted IBDV, in which one of the two genomic segments originated from cDNA of classical attenuated IBDV CEF94 and the other from D6948, could also be rescued by using this system. Segment-reassorted virus containing the A segment of the classical attenuated isolate (CEF94) and the B segment of the very virulent isolate (D6948) is not released until 15 h after an in vitro infection. This indicates a slightly retarded replication, as the first release of CEF94 is already found at 10 h after infection. Next to segment reassortants, we generated and analyzed mosaic IBDVs (mIBDVs). In these mIBDVs we replaced the region of CEF94 encoding one of the viral proteins (pVP2, VP3, or VP4) by the corresponding region of D6948. Analysis of these mIBDV isolates showed that tropism for non-B-lymphoid cells was exclusively determined by the viral capsid protein VP2. However, the very virulent phenotype was not solely determined by this protein, since mosaic virus containing VP2 of vvIBDV induced neither morbidity nor mortality in young chickens.

A comparative study of the pathogenesis of malabsorption syndrome in broilers

Five malabsorption syndrome (MAS) homogenates from The Netherlands and Germany were used to reproduce MAS in broilers. We studied the histopathology after inoculation of 1-day-old broiler chicks and the agents that might be involved. Generally, the MAS homogenates induced signs that differed in severity and pathobiology. We could distinguish and classify the inoculated groups best by histopathology: proventriculitis, lesions in the small intestines in combination with proventriculitis, or lesions of the small intestines only. Lesions in the small intestine had more impact on weight gain depression than lesions in the proventriculus. In three out of five inoculated groups, microscopic lesions of the pancreas were found. Reovirus was detected in the inoculated groups by virus isolation and seroconversion, and reoviral antigen was detected by immunohistochemistry of the small intestine. Also, enteroviruslike particles were detected in three of the five inoculated groups, although not in the most affected group. Additionally, bacteriophages and bacteria (hemolytic Escherichia coli, Pasteurella hemolytica, and Enterococcus durans) were isolated from inoculated chicks. The role these agents play in pathogenesis of MAS is still unsolved.

Efficient rescue of infectious bursal disease virus from cloned cDNA: evidence for involvement of the 3'-terminal sequence in genome replication

To study the mechanism of replication of infectious bursal disease virus (IBDV), and to determine factors on the IBDV RNA which are involved in viral replication, we used cloned full-length cDNA of both the A- and B-segments to generate infectious IBDV. Infectious IBDV was rescued from plasmids that contained full-length IBDV cDNA behind a T7 promoter, by transfecting these plasmids into cells which were infected with a recombinant Fowlpox virus that expressed T7 RNA polymerase. By using the cDNA transfection system we evaluated the effect of the length of the 3' terminus of the A-segment plus strand of IBDV. Although wild-type IBDV predominantly contains four cytosines at the 3' terminus, no difference in virus yield was found when virus was rescued from cDNAs containing three to six adjacent cytosines. When the 3' terminus was shorter than three cytosines the efficiency to generate infectious IBDV from cDNA was reduced, but IBDV could still be recovered reproducibly. The rescued viruses from cDNAs containing 3'-terminal deletions appeared to have a restored 3'-terminal sequence. The missing nucleotides are probably restored by using complementary bases of a stem-loop structure as template.

Histological and bacteriological evaluation of digital dermatitis in cattle, with special reference to spirochaetes and Campylobacter faecalis

Tissue samples from the feet of slaughtered cattle exhibiting different stages of digital dermatitis were sectioned and stained with haematoxylin and eosin and silver staining techniques. Three morphological variations of spirochaetes were observed, whereas control samples from feet which were macroscopically negative for digital dermatitis were also negative for spirochaetes. In an immunofluorescence test, Campylobacter faecalis was found to be abundant on superficial wound smears from the classical ulceration of digital dermatitis.

Swine dysentery: more unknown than known

Swine dysentery (SD) is an economically important disease. It is caused by the spirochete Serpulina hyodysenteriae. In order to minimize the economic damage of SD, several methods to control this disease are recommended. Whereas hygienic measures and use of antimicrobials are actually practised for prevention, detection and exclusion of carriers of S. hyodysenteriae and vaccination against the disease still needs further attention. The last two methods require reliable and sensitive diagnostic tests and understanding of the pathogenesis of and immune development against SD. At present the detection of all individual carriers of S. hyodysenteriae is not yet assured, since the tests for screening individual animals are not satisfactorily evaluated as far as sensitivity and/or specificity are concerned. Studies on the pathogenesis of SD have been performed to develop a vaccine. Besides hemolysin/cytotoxin production, no other virulence factors have been identified with certainty for S. hyodysenteriae. Recently however, further indications for a role of motility in the pathogenesis of SD have been obtained. In this manuscript we summarize the most relevant recent findings.

Reduced virulence of Serpulina hyodysenteriae hemolysin-negative mutants in pigs and their potential to protect pigs against challenge with a virulent strain

The role of the Serpulina hyodysenteriae hemolysin encoded by the tlyA gene in the pathogenesis of swine dysentery (SD) was studied. tlyA mutants of two S. hyodysenteriae strains (B204 and C5) were tested for virulence in pigs. None of the animals developed SD. However, after infection with wild-type strain B204 or C5, the incidence of SD was 100 or 60%, respectively. Thus, the tlyA-encoded hemolysin of S. hyodysenteriae is an important virulence factor in SD. The potential of these mutants to protect pigs against challenge with a virulent S. hyodysenteriae strain was also studied. After challenge with wild-type strain B204, 50% of pigs previously inoculated with the B204 tlyA mutant were protected, whereas all control pigs contracted SD. None of the pigs previously inoculated with the C5 tlyA mutant developed SD upon challenge with wild-type strain B204, whereas 40% of the control pigs developed SD in this experiment. Thus, previous colonization with S. hyodysenteriae tlyA mutants in pigs provides partial protection against challenge with a virulent S. hyodysenteriae strain.

Characterization of three putative Serpulina hyodysenteriae hemolysins

Serpulina hyodysenteriae hemolysin is though to be an important virulence factor in swine dysentery. One gene, tlyA, previously called tly, encoding a hemolysin in S. hyodysenteriae strain B204 has been characterized (Muir et al. Infect Immun 1992; 60: 529-35). Two other genes of S. hyodysenteria strain B204, designated tlyB and tlyC, encoding hemolytic proteins in Escherichia coli strain DH5 alpha were cloned and sequenced. The tlyB and tlyC genes, when expressed in E. coli, encode heat-labile, protease-sensitive proteins which exhibit both hemolytic and cytotoxic activity in vitro. The calculated molecular weights of the tlyB and tlyC gene products are 93.3 kDa and 30.8 kDa, respectively. The tlyB gene product has sequence homology with the Clp proteins, whereas for the tlyC-encoded protein no homology with other protein sequences was observed. Southern hybridization showed that the tlyB and tlyC genes are present in a single copy on the chromosome of S. hyodysenteriae.

[Latency of Brucella abortus causes problems in oriented control: a review]

This review on Brucella abortus in cattle covers the pathogenesis, the epidemiology and the diagnostics of brucellosis. Emphasis is given to the presence of latent infections in young stock. Calves infected by B. abortus in utero or after ingestion of infected milk may acquire a persistent infection. These animals might present a significant problem in brucellosis control and eradication schemes, since they are difficult to detect by the usual serological tests as they remain negative until near the first calving or abortion. The diagnostics must be improved: (new) tests need to be made more sensitive and herds and/or animals should be tested more frequently after introduction of cattle into a herd. Moreover more attention should be paid to cases of abortion. It is also suggested that if the slaughter of infected herds is limited to adult animals, the heifer calves could be a source of infection to the restocked herd.

The role of hemolysin(s) in the pathogenesis of Serpulina hyodysenteriae

Serpulina (Treponema) hyodysenteriae, an anaerobic beta hemolytic spirochaete, is the etiologic agent of swine dysentery. Not much is known at present about the virulence factors of S. hyodysenteriae. However, the hemolysin production of this bacterium is generally accepted to be a virulence factor. To study the exact role of hemolysin production in the pathogenesis of swine dysentery, the gene encoding a hemolysin, tly, was cloned and its nucleotide sequence determined. After inactivation of this gene, the virulence of a tly-minus mutant in mice was tested. The mutant had reduced hemolysis indicating that the tly-encoded hemolysin was not the only hemolysin produced by S. hyodysenteriae. Mice infected with the tly-minus mutant had fewer cecal lesions than mice infected with the wild-type S. hyodysenteriae. It was concluded that the tly-encoded hemolysin might be an important virulence factor, but not the only one. Since it was demonstrated that spirochaetes can be transformed through electroporation, this has made a genetic approach to elucidate the pathogenesis of spirochaetal infections possible.

Characterization of Dutch porcine Serpulina (Treponema) isolates by restriction endonuclease analysis and DNA hybridization

Genomes of 55 Dutch porcine Serpulina (Treponema) hyodysenteriae and non-pathogenic Serpulina isolates were characterized by restriction endonuclease analysis (REA) and DNA hybridization. The Dutch porcine isolates were compared with American Type Culture Collection (ATCC) strains of S. hyodysenteriae and S. innocens and isolates of S. hyodysenteriae with known serotypes (reference strains). REA of the Dutch S. hyodysenteriae isolates resulted in two main patterns, while the non-pathogenic isolates had many distinct REA patterns, all different from the S. hyodysenteriae strains. The S. hyodysenteriae reference strains all had distinct REA patterns, different from the Dutch strains. Upon Southern hybridization with a S. hyodysenteriae DNA fragment encoding a flagellar protein, all S. hyodysenteriae strains could be divided in two groups. The non-pathogenic Serpulina strains had many distinct hybridization patterns and hybridized less intensely. Upon hybridization with a S. hyodysenteriae DNA fragment encoding a haemolysin, DNA of all S. hyodysenteriae strains reacted in the same band. DNA of non-pathogenic Dutch Serpulina strains and S. innocens did not hybridize. It was concluded that there are two main genotypes of S. hyodysenteriae in the Netherlands. This could be of importance for recombinant DNA vaccine development.

Prevalence of trypanosomiasis in cattle in south-west Zambia

A trypanosomiasis survey was conducted in South-West Zambia. From a total of 3,346 cattle sampled 342 cattle showed a positive trypanosomiasis parasitaemia. During the survey trypanosome species and PCV values were also recorded. With simple statistical analysis populations with higher and lower prevalence rates were differentiated. The results indicated that the Kwando River Basin Tsetse Fly Belt and the Kafue River Basin Tsetse Fly Belt infested a larger area than originally assumed and that a link-up between both belts occurred or will occur in the near future.

Quantitative correlation of parasitological and serological techniques for the diagnosis of Trypanosoma congolense infection in cattle

A comparison was made between serological and parasitological techniques for the diagnosis of bovine trypanosomiasis in Zambia. Overall sero-prevalence rates as determined by IFAT and ELISA were respectively 2.7-fold and 2.9-fold greater then the percentage of samples found positive with the dark ground/phase contrast buffy coat technique (DG). The results obtained by the two serological techniques were found to be closely correlated (94.2%) agreement) and titres obtained by ELISA tended to be slightly higher than those obtained by IFAT. Linear regression analysis of the results obtained by the IFAT and DG techniques revealed a highly significant correlation. This finding would permit the use of only one of the techniques in an epidemiological survey and to extrapolate the results from the regression line.

Cross immunity between strains of Cowdria ruminantium

Cross immunity tests with strains of Cowdria ruminantium from South Africa, the Sudan, São Tomé and Nigeria failed to demonstrate antigenic differences. The antibiotic ampicillin showed no activity against heartwater.