Fleece rot in sheep: a review of pathogenesis, aetiology, resistance and vaccines

Abstract

Fleece rot develops following prolonged wetting of sheep when bacterial proliferation in wool and on skin induces exudation of serum proteins onto the skin surface and causes damage to wool follicles and fibres. These processes create an attractive environment for blowflies to lay eggs, leading to body strike. Current reliance on insecticides for prevention and treatment of fly strike is being increasingly challenged by development of insecticide resistance. This review examines the large body of past research on the bacterial causes of fleece rot, the genetics of sheep susceptibility and resistance, the characteristics of the resulting immune defence reactions, and attempts to control fleece rot by vaccination. The high dependence on weather conditions for expression of fleece rot hampers studies on the disease. Normal skin and wool are populated by a dynamic microbial community. During adverse environmental conditions, natural resistance to fleece rot associated with physical characteristics of wool and skin can be overwhelmed and a complex mix of bacteria flourishes. Prolonged hydration alone, and in combination with bacterial exoproducts, induces dermatitis and exudation of immunoglobulins and other serum proteins onto the skin surface. Pathogens do not usually penetrate the epidermis. Nonetheless, during prolonged skin hydration, sheep can become sensitised to fleece rot pathogens and produce antibodies. Antibody titres rise late within a typical (3 week) case of fleece rot. High naturally acquired antibody titres may contribute to resistance to fleece rot. In contrast to some evidence for a protective role of antibody, there is little evidence for innate or adaptive cellular immune responses contributing to protection against fleece rot pathogens. Previous attempts to develop vaccines have met with mixed success. Nonetheless, there remain prospects for development of a new vaccine to control fleece rot. Further knowledge on the microbial ecology of normal and wet skin would assist this endeavour and may help identify other control strategies.

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

“…Populations of different species of bacteria at different abundances are present in fleece rot lesions and dermatophilosis (for reviews, see Colditz et al 2021;Tellam et al 2021). Most past investigations of the bacterial diversity and dynamics in fleece rot and dermatophilosis were technically limited by modern standards.…”

“…A high priority for future research is to use modern and unbiased microbiome analyses (Qin et al 2010;Denman et al 2015;Forbes et al 2017) to detect and quantify all bacterial species on normal skin and fleece, and the bacterial dynamics that lead to the expression of fleece rot and dermatophilosis. Previous research based largely on aerobic culture methods led to the conclusion that within the mixed flora of normal skin and fleece rot lesions, Pseudomonas aeruginosa plays a dominant role in the expression of severe fleece rot lesions and in attraction of blowflies causing body strike, as part of a disease complex (Colditz et al 2021). In contrast, dermatophilosis is considered to result from a pure overgrowth of Dermatophilus congolensis.…”

“…The bacterium P. aeruginosa is thought to be the primary species that initiates and sustains fleece rot, although contributions from other pseudomonad species and other bacterial genera may also be possible (reviewed by Colditz et al (2021)). At present, the incidence and severity of fleece rot caused by these bacterial species are minimised by managing environmental risk factors, such as, for example, exposure to warm and wet weather, and the timing of shearing, and by matching sheep genotype to the production environment.…”

“…Development of an effective vaccine requires identification of (1) relevant antigens, (2) a protective immune defence mechanism, (3) a clinical measure of vaccine efficacy, and (4) a strategy for vaccine use that is practical for producers while addressing epidemiological characteristics of the disease in the field. Most research on vaccines for fleece rot and dermatophilosis was undertaken about three decades ago (for reviews, see Colditz et al 2021;Tellam et al 2021). Technological changes over the past 15 years in understanding and exploiting biology have been revolutionary and continue at a rapid pace.…”

“…Two accompanying reviews examine past research on the pathogenesis, aetiology, host defence and previous vaccination trials to protect sheep from fleece rot and dermatophilosis (Colditz et al 2021;Tellam et al 2021). From this background, the current review examines how recent techno logical developments can renew prospects for effective vaccines to fleece rot and dermatophilosis through (1) more rapid and comprehensive identification of microbial popula tions and their abundances in complex mixtures, (2) measures of the genetic diversity of individual species of bacteria involved in initiating and sustaining fleece rot or dermatophilosis, (3) a means to monitor antibiotic resistance in bacteria, (4) more efficient identification and isolation of candidate vaccine antigens, particularly molecules secreted from bacteria to enhance their infectivity and pathogenicity in the natural state of infection (as opposed to secreted proteins identified from laboratory culture of bacteria), and novel undetected antigens discovered using bacterial genomic sequence analyses, (5) improved vaccine formulations to enhance vaccine efficacy and duration of protection, (6) more efficient means of manufacturing antigens and (7) more efficient quality control of vaccine manufacture.…”