Plasma leakage in skin of sheep selected for resistance or susceptibility to fleece rot and fly strike

Control of the sheep blowfly in Australia and New Zealand--are we there yet?

The last 50 years of research into infections in Australia and New Zealand caused by larvae of the sheep blowfly, Lucilia cuprina, have significantly advanced our understanding of this blowfly and its primary host, the sheep. However, apart from some highly effective drugs it could be argued that no new control methodologies have resulted. This review addresses the major areas of sheep blowfly research over this period describing the significant outcomes and analyses, and what is still required to produce new commercial control technologies. The use of drugs against this fly species has been very successful but resistance has developed to almost all current compounds. Integrated pest management is becoming basic to control, especially in the absence of mulesing, and has clearly benefited from computer-aided technologies. Biological control has more challenges but natural and perhaps transformed biopesticides offer possibilities for the future. Experimental vaccines have been developed but require further analysis of antigens and formulations to boost protection. Genetic technologies may provide potential for long-term control through more rapid indirect selection of sheep less prone to flystrike. Finally in the future, genetic analysis of the fly may allow suppression and perhaps eradication of blowfly populations or identification of new and more viable targets for drug and vaccine intervention. Clearly all these areas of research offer potential new controls but commercial development is perhaps inhibited by the success of current chemical insecticides and certainly requires a significant additional injection of resources.

A genomics-informed, SNP association study reveals FBLN1 and FABP4 as contributing to resistance to fleece rot in Australian Merino sheep

BACKGROUND

Fleece rot (FR) and body-strike of Merino sheep by the sheep blowfly Lucilia cuprina are major problems for the Australian wool industry, causing significant losses as a result of increased management costs coupled with reduced wool productivity and quality. In addition to direct effects on fleece quality, fleece rot is a major predisposing factor to blowfly strike on the body of sheep. In order to investigate the genetic drivers of resistance to fleece rot, we constructed a combined ovine-bovine cDNA microarray of almost 12,000 probes including 6,125 skin expressed sequence tags and 5,760 anonymous clones obtained from skin subtracted libraries derived from fleece rot resistant and susceptible animals. This microarray platform was used to profile the gene expression changes between skin samples of six resistant and six susceptible animals taken immediately before, during and after FR induction. Mixed-model equations were employed to normalize the data and 155 genes were found to be differentially expressed (DE). Ten DE genes were selected for validation using real-time PCR on independent skin samples. The genomic regions of a further 5 DE genes were surveyed to identify single nucleotide polymorphisms (SNP) that were genotyped across three populations for their associations with fleece rot resistance.

RESULTS

The majority of the DE genes originated from the fleece rot subtracted libraries and over-representing gene ontology terms included defense response to bacterium and epidermis development, indicating a role of these processes in modulating the sheep's response to fleece rot. We focused on genes that contribute to the physical barrier function of skin, including keratins, collagens, fibulin and lipid proteins, to identify SNPs that were associated to fleece rot scores.

CONCLUSIONS

We identified FBLN1 (fibulin) and FABP4 (fatty acid binding protein 4) as key factors in sheep's resistance to fleece rot. Validation of these markers in other populations could lead to vital tests for marker assisted selection that will ultimately increase the natural fleece rot resistance of Merino sheep.

Relationships between faecal dry matter, worm burdens and inflammatory mediators and cells in parasite-resistant Merino rams

Immune-mediated scouring due to ingested parasite larvae is a major concern for sheep producers in Mediterranean climates. We investigated immune-mediated scouring in parasite-resistant Merino sheep in Australia. Forty-adult, parasite-resistant Merino rams were judged to be either susceptible or non-susceptible to immune-mediated scouring on the basis of dag scores taken under field conditions. We hypothesised that the susceptible rams would have lower faecal dry matter during larval challenge than non-susceptible rams and that, at post-mortem examination, inflammatory mediators and granulocytes would be negatively correlated with both faecal dry matter and worm numbers. In pens, the rams received a dose of 500 Teladorsagia circumcincta L(3) and 500 Trichostrongylus colubriformis L(3) each day for 6 weeks before euthanasia. Ten rams acted as unchallenged controls. Challenging sheep with larvae reduced faecal dry matter at 2, 3 and 4 weeks after challenge began and the greatest reductions were with the sheep susceptible to scouring. The sheep showed good resistance to the parasite challenge as evidenced by low faecal worm egg counts and low total worm counts at post-mortem, with the numbers of T. colubriformis particularly low. Sheep with low faecal dry matter had significantly higher numbers of eosinophils in small intestine tissue. Sheep with low total worm counts had significantly higher levels of bradykinin in abomasum mucus. Sheep with more granulocytes in tissue and inflammatory mediators in mucus tended to have fewer numbers of T. circumcincta but there was little relationship with numbers of T. colubriformis. Our results show that dag scores are correlated to a reduction in faecal dry matter, which can be attributed to the challenge with infective parasite larvae. Inflammation during worm infection is associated with rejection of the worm challenge and may result in more fluid faeces and consequently diarrhoea. Therefore, sheep breeders should focus on breeding for both low worm egg counts and also low dag scores.

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.

16S rRNA gene microbial analysis of the skin of fleece rot resistant and susceptible sheep

Fleece rot is a bacterial dermatitis that follows prolonged wetting of the sheep’s skin, and a major pre-disposing condition to body strike in the Australian Merino. Several studies have examined bacterial load of the fleece in relation to fleece rot using traditional culture-based techniques focussing on only a few bacterial species. We examined the natural bacterial diversity of the healthy sheep skin and changes that occurred in fleece-rot resistant and susceptible animals during fleece rot development. Presented is a preliminary molecular genetic analysis of the bacterial ecology of the sheep skin. Eight 16S rRNA gene libraries were constructed from susceptible and resistant sheep both before and after onset of the disease following induction by simulated rainfall. Approximately 75% of the sequences obtained in this study have not been previously identified in fleece-rot studies. Four operational taxonomic units (OTU; groups of >97% sequence similarity) of major interest were present on susceptible animals and absent from resistant animals. Data on these OTU expand current knowledge of bacteria involved in inflammation and wounding of sheep skin tissue, and provide direction for future research that may lead to new treatment options for fleece rot and body strike.

Peripheral blood lymphocyte subsets in fleece rot-resistant and -susceptible sheep

OBJECTIVE

To compare haematological values and lymphocyte phenotypes in the peripheral blood of fleece rot-resistant and -susceptible sheep.

PROCEDURE

Experiments were conducted on 2- and 3-year-old Merino rams, flock 1 (17 rams) and flock 2 (32 rams), respectively. Within each flock, individual rams were classified as fleece rot-resistant or -susceptible, based on established criteria. Total and differential white cell counts, and indirect fluorescent antibody tests specific for B cells and T cells were performed on all sheep. The concentration of various subsets of circulating lymphocytes was then determined in each sheep.

RESULTS

There were no significant differences between fleece rot-resistant and -susceptible sheep from either flock in the mean total or differential white cell counts. However, fleece rot-resistant rams in flock 1 did have a significantly higher concentration of circulating SBU-T1+ cells than fleece rot-susceptible rams from the same flock. No such difference was noted in the rams from flock 2. While all rams in flock 1 were free of clinical fleece rot, 24 rams in flock 2 (comprising all 17 fleece rot-susceptible and 7 of 15 fleece rot-resistant animals) had clinical signs of the disease. Fleece rot-free rams in this flock (irrespective of their classification as fleece rot-resistant or -susceptible) had significantly higher concentrations of circulating SBU-T1+ cells compared with fleece rot-affected animals. They also had significantly higher concentrations of circulating B cells, and total lymphocytes.

CONCLUSIONS

An examination of peripheral blood lymphocyte subsets in fleece rot-resistant and -susceptible sheep revealed a possible association between resistance to fleece rot and the concentration of circulating SBU-T1+ cells.

Control of blowfly strike in sheep: current strategies and future prospects

Blowfly strike is a cutaneous myiasis in sheep caused by infestations of larvae principally from the family Calliphoridae, particularly the species Lucilia cuprina and Lucilia sericata. These larval infestations cause considerable economic losses to the wool industry. Established control methods have served the industry well in the past, but there are growing deficiencies with these methods. In particular, there is widespread resistance to organophosphorus insecticides and potential difficulties associated with the presence of chemical residues derived from insecticides in wool and waste products which must be disposed of by the industry. There is also growing opposition to the radical surgical procedures used to decrease the susceptibility of sheep to blowfly strike. Consequently, there is a need for the development of alternative control measures. This review examines critically the present control methods and discusses the range of options available for the development of new control strategies. Many of the latter involve novel approaches which will strongly complement current control measures.

Cellular inflammatory responses in skin of sheep selected for resistance or susceptibility to fleece rot and fly strike

Leucocyte populations were examined in normal and inflamed skin of sheep bred for resistance (R) or susceptibility (S) to bacterial fleece rot and the common sequela, body strike caused by the dipteran parasite Lucilia cuprina. No differences between R and S lines were found in numbers of neutrophils accumulating in acute inflammatory lesions induced by activated complement, leukotriene B4, interleukin (IL)-1 beta, tumour necrosis factor-alpha, IL-8 or endotoxin from Pseudomonas aeruginosa. T19+ (alpha gamma delta T cell subset) lymphocytes and eosinophils were more prevalent in skin of sheep from the S line whereas IgE+ cells were more prevalent in skin of sheep from the R line. In an unrelated population of sheep, animals with low fleece rot scores had more intense neutrophil migration into inflammatory lesions induced by all the mediators examined than did animals with high fleece rot scores. IgE+ cells were more prevalent in animals with low fleece rot scores, although in contrast to R and S lines, T19+ cells tended to be elevated in this group of animals. The results suggest that defence mechanisms associated with IgE+ cells in skin may play an important role in resistance to fleece rot and fly strike.

The effect of immune and inflammatory mediators on growth of Lucilia cuprina larvae in vitro

Immune and inflammatory responses occurring during dermal infestation by larvae of Lucilia cuprina can retard larval growth and development. This study examined the effect of 4 classes of humoral inflammatory mediators on larval growth in an in vitro assay. Mediators of plasma leakage (histamine, bradykinin, platelet-activating factor and serotonin), leucocyte chemotactic agonists (activated complement, leukotriene B4 and interleukin-8), effector molecules of immune responses (interleukin-1 beta, tumour necrosis factor-alpha and interferon-gamma) and endotoxin all failed to inhibit larval growth. In contrast, immunoglobulins isolated from immune serum caused marked retardation of larval growth. The results suggest that humoral mediators of inflammatory and immune responses do not play a role in immune defence against Lucilia cuprina.

High and low responsiveness to vaccines in farm animals

Low responsiveness in some farm animals is emerging as a problem in the application of newly developed vaccines which operate at skin surfaces and mucous membranes. Breeding for resistance to specific diseases seems to be associated with breeding for specific immune responsiveness in farm animals and very likely this involves selection for major histocompatibility complex (MHC) haplotype. However, other factors contribute to low responsiveness and these include poor nutrition, sire effects, antigenic competition and inadequate effector mechanisms. The future of newly developed vaccines will rest on the solution to the low responder problem, and once solved, the application of these vaccines will be fully utilized for disease control in farm animals.