Serum and skin surface antibody responses in merino sheep given three successive inoculations with Dermatophilus congolensis

Dermatophilosis (lumpy wool) in sheep: a review of pathogenesis, aetiology, resistance and vaccines

Lumpy wool (dermatophilosis) develops following prolonged wetting of sheep when bacterial proliferation in wool and on skin induce an exudative dermatitis, causing a superficial skin lesion and damage to wool follicles and fibres. The incidence of dermatophilosis is strongly dependent on wet and warm weather and, hence, infection is sporadic. While older animals are less at risk than are lambs, it is unclear whether this reflects naturally acquired immune resistance or the maturation of skin and wool fibres. Dermatophilosis directly causes wool production losses and it also is a risk factor for blowfly strike, which has a substantial economic impact and increasing challenges associated with current control procedures. This review assessed research on the bacterial causes of lumpy wool, the characteristics of the resulting immune defence reactions in sheep, current control strategies, and limitations of previous attempts to control lumpy wool by sheep vaccination.

Fleece rot and dermatophilosis (lumpy wool) in sheep: opportunities and challenges for new vaccines

During prolonged wetting of the fleece, proliferation of bacterial flora often dominated by Pseudomonas aeruginosa or Dermatophilus congolensis can induce dermatitis and fleece damage termed fleece rot and dermatophilosis respectively, which predispose sheep to blowfly strike. A large research effort in the 1980s and 1990s on vaccines to control fleece rot and dermatophilosis met with limited success. This review examines theoretical and technological advances in microbial ecology, pathogenesis, immunology, vaccine development and the characterisation of microbial virulence factors that create new opportunities for development of vaccines against these diseases. Genomic technologies have now created new opportunities for examining microbial dynamics and pathogen virulence in dermatitis. An effective vaccine requires the combination of appropriate antigens with an adjuvant that elicits a protective immune response that ideally provides long-lasting protection in the field. A clinical goal informed by epidemiological, economic and animal welfare values is needed as a measure of vaccine efficacy. Due to dependence of fleece rot and dermatophilosis on sporadic wet conditions for their expression, vaccine development would be expedited by in vitro correlates of immune protection. The efficacy of vaccines is influenced by genetic and phenotypic characteristics of the animal. Advances in understanding vaccine responsiveness, immune defence in skin and immune competence in sheep should also inform any renewed efforts to develop new fleece rot and dermatophilosis vaccines. The commercial imperatives for new vaccines are likely to continue to increase as the animal welfare expectations of society intensify and reliance on pharmacotherapeutics decrease due to chemical resistance, market pressures and societal influences. Vaccines should be considered part of an integrated disease control strategy, in combination with genetic selection for general immune competence and resistance to specific diseases, as well as management practices that minimise stress and opportunities for disease transmission. The strategy could help preserve the efficacy of pharmacotherapeutics as tactical interventions to alleviate compromised welfare when adverse environmental conditions lead to a break down in integrated strategic disease control. P. aeruginosa and D. congolensis are formidable pathogens and development of effective vaccines remains a substantial challenge.

Fleece rot and dermatophilosis in sheep

Fleece rot and dermatophilosis reduce health and production of sheep and predispose them to blow fly strike. This paper reviews aetiology, prevalence, pathogenesis, resistance, attempts to develop vaccines and prospects for new control strategies to these important skin diseases. Although the severity of fleece rot is associated with the abundance of Pseudomonas aeruginosa on skin, microbial ecology studies are providing new insights into the contribution of other bacteria to the disease. Wool traits and body conformation traits that predispose sheep to fleece rot and dermatophilosis are heritable and have been used as indirect selection criteria for resistance for many years. Selection against BoLA-DRB3-DQB class II haplotype in cattle can substantially reduce the prevalence of dermatophilosis and holds promise for identification of gene markers for resistance to these bacterial diseases in sheep. Immune responses in skin and systemic antibody responses to bacterial antigens are acquired through natural infection and contribute to resistance; however, prototype antibacterial vaccines have to date failed to provide protection against the diversity of isolates of Dermatophilus congolensis and Pseudomonas species present in the field. Opportunities for future control through breeding for resistance, vaccines and non-vaccine strategies for controlling the microbial ecology of fleece are discussed. In combination, control strategies need to reduce the risk of transmission, minimise exposure of animals to stressors that enhance the risk of infection, and enhance resistance though genetics or vaccines.

Production of antibodies to recombinant antigens from Lucilia cuprina following cutaneous immunisation of sheep

Immunological control of cutaneous myiasis of sheep caused by Lucilia cuprina larvae has been an elusive goal. Antibody to antigens derived from the peritrophic membrane can stunt or kill larvae in a dose dependent fashion. Thus efficacy of vaccines employing these antigens may be limited by the amount of antibody in skin available for ingestion by larvae. The potential for elevating antibody concentrations in skin by intradermal immunisation with the recombinant peritrophic membrane antigens peritrophin-44, peritrophin-48 and peritrophin-95 was therefore examined. Using within-animal comparisons, specific antibody was significantly higher in skin transudates from locally immunised sites than from adjacent adjuvant control sites. It was concluded that cutaneous immunisation may assist immunological control of blowfly larvae.

Effect on systemic antibody concentrations of topical application of choleratoxin to skin of sheep

OBJECTIVE

To examine the ability of a vaccine formulation combining choleratoxin with an experimental antigen to induce a systemic antibody response when applied topically on unbroken skin of sheep.

DESIGN

Seven treatment groups of five adult sheep received systemic or topical priming followed 4 weeks later by systemic or topical boosting with choleratoxin and/or bovine serum albumin. Topical vaccines were administered to clipped skin on the ventral abdomen for 2 h. Booster immunisations were repeated 8 weeks after initial boosting. Serum antibody titres to choleratoxin and bovine serum albumin were determined by ELISA.

RESULTS

An antibody response to choleratoxin was observed in serum, but no antibody response to bovine serum albumin was detected.

CONCLUSION

Transdermal delivery may be feasible for livestock vaccines, however, further work is necessary to develop formulations that induce protective immunity by this route.

Immune responses to Dermatophilus congolensis infections

Complex mechanisms underly the establishment of dermatophilosis, an exudative and proliferative skin disease of ruminants. This multicomponent system involves the bacterium Dermatophilus congolensis, transmission by various routes including flies, host genetic factors and immunosuppression by Amblyomma variegatum ticks. Here, Nick Ambrose and colleagues summarize recent evidence for an association between A. variegatum and severe chronic dermatophilosis in cattle. Breed-based differences in resistance to dermatophilosis are probably related to immunity to ticks or resistance to the immunosuppressive effects of ticks. Immunity to dermatophilosis might involve non-classic responses mediated by CD1 antigen presentation and gammadelta T cells. Progress towards vaccination is further complicated by strain-specific acquired immunity to D. congolensis.

Do sheep regulate the size of their mallophagan louse populations?

Alternatives to chemicals for controlling parasites are required to minimise problems from resistance, residues in animal products and occupational exposure. Utilisation of host response to parasites through selection of resistant types or vaccination is an appealing option. To date most studies have been with haematophagous or invasive parasites which directly contact elements of the host immune system. Sheep lice (Bovicola ovis) feed superficially on the skin of sheep ingesting lipid, scurf, bacteria and loose stratum corneum squames. Evidence is presented that despite their surface feeding habit Bovicola ovis stimulate an immune response in sheep and that this response may play a part in regulating the size of louse populations.

Electrophoretic and antigenic characterisation of Dermatophilus congolensis extracellular products

Dermatophilus congolensis is the causative agent of bovine dermatophilosis and lumpy wool in sheep. Two field isolates of D. congolensis, one each from a cow in Ghana and a sheep in Scotland, were cultured for 24-72 h in a synthetic medium based on RPMI-1640. Culture filtrates were examined by SDS-PAGE and considered to contain extracellular products released by growing hyphae and filaments. Electrophoretic profiles of culture filtrates of the two isolates contained common bands and bands that were unique to each isolate. The composition of extracellular products altered with increasing culture periods indicating that specific products were released at different stages of growth. Culture filtrate prepared in the presence of serine protease and metalloprotease inhibitors contained more and better defined bands than that prepared without protease inhibitors indicating the presence of proteases in culture filtrates. Western blot analysis of extracellular products using a panel of sera showed that the two isolates from different host species and distant geographical locations contained cross-reactive antigens. Natural and experimental infections stimulated antibody responses to antigens in culture filtrates, sera from animals that were disease free but in-contact with dermatophilosis-infected animals also contained antibodies to extracellular antigens. The antigens recognised by most sera had molecular weights of 200 kDa in the bovine isolate, 170 kDa in the ovine isolate and 67, 27 and 52-55 kDa in both isolates. The number of antigenic bands of both isolates was positively correlated with the intensity of challenge and the severity of infection: antibodies in sera from disease-free cattle in Ghana recognised more antigens than sera from disease-free sheep in Scotland and more antigens were recognised by sera from chronically-infected Ghanaian cattle than by sera from experimentally-infected calves and sheep. The latter developed antibodies to antigens of 27 and 24 kDa during the course of infection. The electrophoretic profiles of extracellular products of D. congolensis are less complex than those of other structures of the bacterium yet they exhibit differences between the two isolates. Extracellular products contain antigens recognised by sera from naturally exposed and experimentally-infected animals that may be involved in immunity to D. congolensis or immunopathogenesis of dermatophilosis.

A comparison of inflammatory exudates released from myiasis wounds on sheep bred for resistance or susceptibility to Lucilia cuprina

Sheep bred for resistance (R) or susceptibility (S) to fleece rot and myiasis (blowfly strike) were experimentally infected with L. cuprina larvae. Exudates released from the wound site were collected during the infection at 6, 12, 18 and 24 h. The exudates were separated using two-dimensional gel electrophoresis, and proteins were silver stained and identified by immunoblotting with specific antibody and by their isoelectric points and molecular weights. Comparisons of exudate composition were made over time and between R and S sheep. Between 6 and 12 h post larval implantation the exudate was rich in IgG and fibrinogen, which is before extensive tissue damage and suggests that the exudate is not simply tissue haemorrhage but the result of an inflammatory response by the sheep to L. cuprina. The exudate grew in complexity between 12 and 18 h and contained a maximum of 74 distinct peptide spots by 24 h. Exudate from wounds on resistant sheep contained many more peptides in the first 12 h of infection, suggesting a more rapid inflammatory response. The source of proteins from the exudate remains speculative; it appears to be composed of many acute-phase proteins, large amounts of immunoglobulin G and proportionally low levels of serum albumin. Exudate composition is likely to be influenced by the local synthesis of acute-phase proteins and perhaps immunoglobulins, selective transport to the infection site and also enzymic degradation by L. cuprina larval enzymes. The more rapid exudation of acute-phase and serum proteins at infection sites on R sheep may allow the inhibition of the establishment of fleece rot bacteria or L. cuprina larvae under natural challenge.

Variation in cultural, morphological, biochemical properties and infectivity of Australian isolates of Dermatophilus congolensis

Recent vaccination studies with Dermatophilus congolensis showed that variation of challenge strains had a considerable influence on protection afforded by the vaccines. In this study cultural, morphological and biochemical properties of 30 D. congolensis isolates from throughout Australian were investigated. The infective dose required to produce lesions of equivalent severity by these isolates for sheep, rabbits and guinea pigs was also examined and the isolates were grouped into four clusters of similar infectivity ranking. Analysis of the relationship between cultural, morphological and biochemical characteristics and infectivity rankings of clusters was undertaken to determine if certain properties were linked to infectivity. Considerable variability was found in haemolytic activity on blood agar, mucoid nature of colonies, motility, flagella density and polarity, capsule width, restriction enzyme profiles of bacterial DNA, protein electropherotype, carbohydrate content, and enzymic activity against proteins, maltose, chondroitin-4-sulphate, phospholipids and lipids. Of these properties haemolytic activity and enzyme activity against casein, chondroitin-4-sulphate and lipids showed some link with infectivity ranking for these isolates.

Uptake and fate of specific antibody in feeding larvae of the sheep blowfly, Lucilia cuprina

The quantity of specific antibody ingested by larvae of Lucilia cuprina and its fate after ingestion were studied in larvae grown on sheep and on an artificial diet. Larvae grown to late first or early second instar on sheep vaccinated with horse myoglobin contained 66% less specific antibody detected by enzyme linked immunosorbent assay than larvae grown to a similar stage on an artificial diet containing 75% serum from the same sheep. A similar result was obtained when larvae were grown to mid-third instar. Larvae grown on sheep to first or second instar contained approximately the same quantity of specific antibody per unit weight of larvae as those grown to third instar. Larvae grown on diet to third instar contained 22% less specific antibody per unit weight than those grown to first or second instar. In larvae grown on diet to late third instar, ingested diet retained 91 +/- 12% of its original specific antibody activity in the crop, 50 +/- 11% in the anterior midgut, 8 +/- 2% in the posterior midgut and 13 +/- 6% in the hindgut. The mean concentration of total immunoglobulin detectable in the haemolymph of individual third instar larvae grown on diet was 1.7 +/- 2.8 micrograms/ml. Assays of specific antibody in the haemolymph of similarly reared larvae indicated that all or most of this immunoglobulin remained functional. The implications of the quantities and distribution of ingested functional antibody found in feeding larvae of L.cuprina are discussed in relation to the possibility of vaccinating sheep against these larvae and the selection of likely internal targets as sources of potential protective antigens.

Development of a single dilution ELISA to detect antibody to Dermatophilus congolensis in goat and cattle sera

A solid phase immunosorbent assay to detect antibodies to Dermatophilus congolensis in ruminant sera was developed to be used as a single dilution ELISA in large epidemiological surveys. Optimal conditions for the test are described. The use of blocking proteins to reduce non specific binding was necessary. Non fat dry cow milk and fetal calf serum were the only two efficient blocking agents out of six tested. Comparison of 4 antigenic fractions obtained after sonication and differential centrifugations of D. congolensis cultures showed that cell-wall (CW) or membrane (M) enriched preparations were more specific than a crude extract (CR) or a soluble (S) antigen. Whole spores and filaments performed poorly as antigens. The best sensitivity and specificity of the ELISA were obtained when the cut-off point of positivity was fixed at mean absorbance of negative sera + 2.58 sd. The specificity was then 97.6% either with M or CR antigen. The sensitivity was improved from 93.4% with CR to 98.2% with M antigen. Threshold values for a positive test varied between the 3 geographical areas tested. CW and M were also the most efficient antigens for discerning between serotypes of D. congolensis. The precision of the test was evaluated with CR antigen and expressed in residual expressed in residual coefficient of variation (CV). The precision was CV = 5.1% when each serum was titrated in duplicate and the antibody levels were expressed in absorbances. The expression of antibody levels in arbitrary standard units estimated from calibration curves reduced the precision (CV = 13.8%). Several methods were tested to decrease between plate variability but these did not greatly improve the reproducibility since it was shown that the main source of variation was within the plate.

Intradermal and percutaneous transudation of IgG1 and transferrin in sheep

The leakage of [125I]-IgG1 into skin sites following injection of mediators of enhanced vascular permeability and during induction of transudates on the skin surface under negative pressure was examined to determine whether IgG is selectively transported into cutaneous transudates. 111In-transferrin was employed as a marker of plasma leakage unaided by selective transport. The leakage of IgG1 into interstitial spaces of untreated skin, into inflammatory transudates and into transudate fluid drawn to the skin surface under vacuum occurred at a lower rate than did leakage of transferrin. No evidence was found in favour of a selective transport mechanism to aid transport of IgG1 into extravascular skin compartments.

Relationship between chronic ovine dermatophilosis and levels of T6 lymphocyte antigen staining in peripheral blood mononuclear cells

Quantification of a T6-lymphocyte antigen in peripheral blood mononuclear cells of sheep was used to select 15 from 48 one year old Merino ewes not previously exposed to Dermatophilus congolensis infection. These sheep were compared in response to challenge with D. congolensis zoospores and levels of T-6 lymphocyte antigen in peripheral blood mononuclear cells with 15 Merino ewes of similar age and strain from a different site that had been treated and recovered from chronic dermatophilosis. The T-6 lymphocyte antigen levels were significantly lower in the chronic dermatophilosis sheep and they developed significantly more severe lesions than the selected, previously unexposed sheep despite the former sheep having high serum antibody levels to D. congolensis. Measurement of the fleece characteristics, wax and suint concentration showed no differences between the groups that might have explained the considerable differences in their susceptibility to dermatophilosis.

Strain variation in Dermatophilus congolensis demonstrated by cross-protection studies

Cross-protection studies were conducted with vaccines prepared from two isolates of Dermatophilus congolensis (designated strain 1 and strain 2). The vaccines were prepared as either heat-inactivated, washed, formalized filamentous phase bacterium, mixed with alum as an adjuvant, and inoculated intramuscularly (type A vaccine) or sedimented live filaments inoculated intradermally (type B vaccine). The vaccinated sheep were challenged with D. congolensis zoospores of one or other strain. Challenge sites were observed for the presence and severity of lesions. Serum antibody levels to D. congolensis were monitored after vaccination and challenge. Type A and B vaccines from both strains produced some reduction in the severity of lesions when sheep were challenged with strain 1 but not with strain 2. Unvaccinated control sheep developed more severe and persistent lesions when challenged with strain 2 than controls challenged with strain 1. Serum antibody levels to the type B vaccine prepared from strain 1 were significantly higher (P less than 0.05) than antibody levels to type B vaccine from strain 2. These findings showed there was significant variation in virulence and antigenicity between these two isolates of D. congolensis.

The effect of malnutrition on vaccination against Dermatophilus congolensis infection in ruminants

Vaccination against Dermatophilus congolensis was carried out in groups of lambs raised on optimal or energy deficient diets. The groups differed significantly in weight, body condition score and plasma total protein and albumin. All animals were then challenged with D. congolensis in a dose response infection model. The vaccine was effective in the well nourished animals, reducing the number of affected lambs in the vaccinated group and the severity of the lesions and increasing the minimum dose required to cause infection. In contrast, all of the vaccinated energy-deficient lambs developed lesions. There was some evidence of vaccine effect in these animals but this was not as marked as that seen in the well nourished lambs. The malnourished lambs, vaccinated and non-vaccinated, took longer to heal than the well nourished groups. Resistance to challenge was not associated with serum antibodies or skin test reactivity to D. congolensis antigens.

Evaluation of vaccines against ovine dermatophilosis

Intradermal vaccination of live crude filaments (vaccine A) was compared with a vaccine (vaccine B) consisting of a 45 kD zoospore protein and mucoid material coating filaments in its ability to protect sheep from experimental Dermatophilus congolensis infection. Fourteen and 21 days after challenge, vaccine A sheep had fewer lesions (P less than 0.001) than the vaccine B sheep. The lesions on the vaccine A sheep were also less severe 14 and 21 days after challenge (P less than 0.05, P less than 0.01 respectively). In a second study, vaccine A was assessed for its ability to protect against natural challenge. Ten weeks after contact with sheep with active and generalised dermatophilosis no difference was found between the number of lesions present on the vaccine A and unvaccinated sheep and no differences were found in the number of sheep in each group with active lesions.

Inflammatory cell and immune function in Merino sheep with chronic dermatophilosis

Components of inflammatory and immunological responses were compared in 17 Merino sheep with chronic dermatophilosis (Group 1) and 15 Merino sheep that had recovered from the disease (Group 2). The functions studied included: (i) total and differential white cell counts; (ii) phagocytic function and intracellular killing by neutrophils; (iii) humoral immune response to T-dependent and T-independent antigens and to Dermatophilus congolensis. (iv) lymphocyte blastogenic responses to phytohaemagglutinin; (v) bovine serum albumen and D. congolensis antigens; (vi) quantification of T-lymphocyte subsets in skin lesions resulting after re-infection with D. congolensis zoospores. After all lesions were treated and the sheep were shorn, both groups of sheep were re-infected with D. congolensis. Both groups had similar infection rate, severity of lesions and rate of resolution after re-infection. The Group 2 sheep had significantly higher primary and secondary antibody responses to killed Brucella abortus cells than Group 1 sheep, but Group 1 sheep had higher levels of specific D. congolensis antibody throughout the trial. Neutrophils from Group 1 sheep showed a higher phagocytic rate for D. congolensis zoospores than Group 2 sheep when the zoospores were opsonised by sera from the Group 1 sheep, but there was no difference in their ability to kill ingested zoospores. Although there were some differences between the groups in the proportion of lymphocytes in lesions that reacted with monoclonal antibodies to T4, T8 and T19-19 lymphocyte markers at various times after re-infection, the sheep in Group 2 consistently had higher levels of lymphocytes reacting to a monoclonal antibody for the T6 lymphocyte antigen in skin biopsies collected 9, 15 and 21 days post-inoculation (p.i.) than did sheep in Group 1. Group 2 sheep also had higher levels of epidermal cells with immunohistochemical properties of Langerhans cells at lesion sites 15 and 21 days p.i.

Vaccination against ovine dermatophilosis

Zoospore, filamentous and soluble antigens were prepared from Dermatophilus congolensis and examined for their ability to protect sheep from challenge with D. congolensis zoospores. In 1 experiment, sheep were vaccinated with Antigens A, B and C. The number of sheep protected in the group vaccinated with Antigen B was greater (P less than 0.05) than that in the unvaccinated group after challenge. The group vaccinated with Antigen B had a higher antibody response (P less than 0.05) to Antigen B than to Antigen A or C. In a second experiment, 2 groups of sheep were vaccinated with Antigen B. All sheep in this study developed lesions after challenge, but those on the vaccinated sheep were less severe (P less than 0.05) than those on the unvaccinated sheep. The antibody response to Antigen A, 28 days after vaccination, was higher (P less than 0.05) than the response to Antigen B.