Identification of an antigen from the sheep scab mite, Psoroptes ovis, homologous with house dust mite group I allergens

Plant and Arthropod IgE-Binding Papain-like Cysteine Proteases: Multiple Contributions to Allergenicity

Papain-like cysteine proteases are widespread and can be detected in all domains of life. They share structural and enzymatic properties with the group's namesake member, papain. They show a broad range of protein substrates and are involved in several biological processes. These proteases are widely exploited for food, pharmaceutical, chemical and cosmetic biotechnological applications. However, some of them are known to cause allergic reactions. In this context, the objective of this review is to report an overview of some general properties of papain-like cysteine proteases and to highlight their contributions to allergy reactions observed in humans. For instance, the literature shows that their proteolytic activity can cause an increase in tissue permeability, which favours the crossing of allergens through the skin, intestinal and respiratory barriers. The observation that allergy to PLCPs is mostly detected for inhaled proteins is in line with the reports describing mite homologs, such as Der p 1 and Der f 1, as major allergens showing a frequent correlation between sensitisation and clinical allergic reactions. In contrast, the plant food homologs are often digested in the gastrointestinal tract. Therefore, they only rarely can cause allergic reactions in humans. Accordingly, they are reported mainly as a cause of occupational diseases.

Proteases and pseudoproteases in parasitic arthropods of clinical importance

Parasitic arthropods feed on blood or skin tissue and share comparable repertoires of proteases involved in haematophagy, digestion, egg development and immunity. While proteolytically active proteases of multiple classes dominate, an increasing number of pseudoproteases have been discovered that have no proteolytic function but are pharmacologically active biomolecules, evolved to carry out alternative functions as regulatory, antihaemostatic, anti-inflammatory or immunomodulatory compounds. In this review, we provide an overview of proteases and pseudoproteases from clinically important arthropod parasites. Many of these act in central biological pathways of parasite survival and host-parasite interaction and may be potential targets for therapeutic interventions.

A genomic analysis and transcriptomic atlas of gene expression in Psoroptes ovis reveals feeding- and stage-specific patterns of allergen expression

Background

Psoroptic mange, caused by infestation with the ectoparasitic mite, Psoroptes ovis, is highly contagious, resulting in intense pruritus and represents a major welfare and economic concern for the livestock industry Worldwide. Control relies on injectable endectocides and organophosphate dips, but concerns over residues, environmental contamination, and the development of resistance threaten the sustainability of this approach, highlighting interest in alternative control methods. However, development of vaccines and identification of chemotherapeutic targets is hampered by the lack of P. ovis transcriptomic and genomic resources.

Results

Building on the recent publication of the P. ovis draft genome, here we present a genomic analysis and transcriptomic atlas of gene expression in P. ovis revealing feeding- and stage-specific patterns of gene expression, including novel multigene families and allergens. Network-based clustering revealed 14 gene clusters demonstrating either single- or multi-stage specific gene expression patterns, with 3075 female-specific, 890 male-specific and 112, 217 and 526 transcripts showing larval, protonymph and tritonymph specific-expression, respectively. Detailed analysis of P. ovis allergens revealed stage-specific patterns of allergen gene expression, many of which were also enriched in “fed” mites and tritonymphs, highlighting an important feeding-related allergenicity in this developmental stage. Pair-wise analysis of differential expression between life-cycle stages identified patterns of sex-biased gene expression and also identified novel P. ovis multigene families including known allergens and novel genes with high levels of stage-specific expression.

Conclusions

The genomic and transcriptomic atlas described here represents a unique resource for the acarid-research community, whilst the OrcAE platform makes this freely available, facilitating further community-led curation of the draft P. ovis genome.

Performance of the Psoroptes ovis antibody enzyme-linked immunosorbent assay in the face of low-level mite infestation

Psoroptes ovis mites, the causative agent of sheep scab, can severely compromise sheep welfare and production. However, in subclinical infections, mite detection is difficult increasing the risk of spread. A recent serodiagnostic test, based on detecting host antibodies to the P ovis allergen, Pso o 2, has made the detection of subclinical infection possible. The use of this test was demonstrated in subclinical situations, through an opportunistic observational study on an extensive hill farm and a lowland flock with recently introduced, quarantined livestock. Twelve animals were tested from each group. Breeding ewes and lambs on the hill farm had seroprevalences of 16 per cent (12.5-17.8 per cent) and 8.3 per cent (4.8-10.1 per cent), respectively. Quarantined store lambs had a seroprevalence of 16.7 per cent (13.2-18.5 per cent); no evidence of P ovis was found in quarantined replacement ewes. By detecting subclinical infection, this serological test could be a powerful tool in sheep scab control, for quarantine procedures, accreditation programmes, and possibly regional or national eradication protocols.

A robust method for the estimation and visualization of IgE cross-reactivity likelihood between allergens belonging to the same protein family

Among the vast number of identified protein families, allergens emanate from relatively few families which translates to only a small fraction of identified protein families. In allergy diagnostics and immunotherapy, interactions between immunoglobulin E and allergens are crucial because the formation of an allergen-antibody complex is necessary for triggering an allergic reaction. In allergic diseases, there is a phenomenon known as cross-reactivity. Cross-reactivity describes a situation where an individual has produced antibodies against a particular allergenic protein, but said antibodies fail to discriminate between the original sensitizer and other similar proteins that usually belong to the same family. To expound the concept of cross-reactivity, this study examines ten protein families that include allergens selected specifically for the analysis of cross-reactivity. The selected allergen families had at least 13 representative proteins, overall folds that differ significantly between families, and include relevant allergens with various potencies. The selected allergens were analyzed using information on sequence similarities and identities between members of the families as well as reports on clinically relevant cross-reactivities. Based on our analysis, we propose to introduce a new A-RISC index (Allergens’–Relative Identity, Similarity and Cross-reactivity) which describes homology between two allergens belonging to the same protein family and is used to predict the likelihood of cross-reactivity between them. Information on sequence similarities and identities, as well as on the values of the proposed A-RISC index is used to introduce four categories describing a risk of a cross-reactive reaction, namely: high, medium-high, medium-low and low. The proposed approach can facilitate analysis in component-resolved allergy diagnostics, generation of avoidance guidelines for allergic individuals, and help with the design of immunotherapy.

Phylogenetic relationships, stage-specific expression and localisation of a unique family of inactive cysteine proteases in Sarcoptes scabiei

Background

Scabies is worldwide one of the most common, yet neglected, parasitic skin infections, affecting a wide range of mammals including humans. Limited treatment options and evidence of emerging mite resistance against the currently used drugs drive our research to explore new therapeutic candidates. Previously, we discovered a multicopy family of genes encoding cysteine proteases with their catalytic sites inactivated by mutation (SMIPP-Cs). This protein family is unique in parasitic scabies mites and is absent in related non-burrowing mites. We postulated that the SMIPP-Cs have evolved as an adaptation to the parasitic lifestyle of the scabies mite. To formulate testable hypotheses for their functions and to propose possible strategies for translational research we investigated whether the SMIPP-Cs are common to all scabies mite varieties and where within the mite body as well as when throughout the parasitic life-cycle they are expressed.

Results

SMIPP-C sequences from human, pig and dog mites were analysed bioinformatically and the phylogenetic relationships between the SMIPP-C multi-copy gene families of human, pig and dog mites were established. Results suggest that amplification of the SMIPP-C genes occurred in a common ancestor and individual genes evolved independently in the different mite varieties. Recombinant human mite SMIPP-C proteins were produced and used for murine polyclonal antibody production. Immunohistology on skin sections from human patients localised the SMIPP-Cs in the mite gut and in mite faeces within in the epidermal skin burrows. SMIPP-C transcription into mRNA in different life stages was assessed in human and pig mites by reverse transcription followed by droplet digital PCR (ddPCR). High transcription levels of SMIPP-C genes were detected in the adult female life stage in comparison to all other life stages.

Conclusions

The fact that the SMIPP-Cs are unique to three Sarcoptes varieties, present in all burrowing life stages and highly expressed in the digestive system of the infective adult female life stage may highlight an essential role in parasitism. As they are excreted from the gut in scybala they presumably are able to interact or interfere with host proteins present in the epidermis.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2862-0) contains supplementary material, which is available to authorized users.

A recombinant subunit vaccine for the control of ovine psoroptic mange (sheep scab)

Sheep scab, caused by infestation with the mite Psoroptes ovis, is highly contagious, causing intense pruritus and represents a major welfare and economic concern. Disease control strategies rely upon chemotherapy, however, sustainability is questionable due to issues of chemical residues, eco-toxicity and acaricide resistance. Control by vaccination is supported by demonstration of protective immunity in sheep previously infested with P. ovis. We identified vaccine candidates for P. ovis based on: (1) antigens selected by their interaction with host signalling pathways and the host immune-response; and (2) those shown to be either immunogenic or involved in mite feeding. This resulted in the development and validation, in repeated immunisation and challenge trials, of a seven recombinant protein sub-unit cocktail vaccine. Sheep were inoculated on three occasions, 2 weeks apart, along with QuilA adjuvant. Vaccination resulted in highly significant reductions in both lesion size (up to 63%) and mite numbers (up to 56%) following challenge. Mean lesion size in vaccinates was significantly smaller than controls from 1 week post infestation (wpi) until the end of the experiment at 6 wpi. All antigens elicited serum IgG responses following immunisation and prior to infestation, whereas controls did not produce antigen-specific IgG during the pre-infestation period. Vaccinated animals showed an amnestic response, with levels of antigen-specific IgG against muGST, Pso o 1 and Pso o 2 increasing following infestation. This vaccine represents the greatest reduction in lesion size to date with a sheep scab vaccine, providing encouragement for future production of a commercially-viable means of immunoprophylaxis.

Recent developments in the diagnosis of ectoparasite infections and disease through a better understanding of parasite biology and host responses

Some conventional methods of diagnosis of ectoparasite infections can have low sensitivity and/or specificity. In addition, early infestations, sub-clinical and carrier hosts often go un-diagnosed, allowing infestations to spread. This review focuses on the important ectoparasites of human, livestock and companion animals for which improved diagnostic tools are either already in use, or in development. These advances in diagnostic technologies have resulted in improved treatment, control and preventative strategies for many ectoparasitic diseases. Immunodiagnostic methods have had a large impact, with the emergence of highly sensitive and specific enzyme-linked immunosorbent assays (ELISAs) for sarcoptic and psoroptic mange, with further improved tests in development. In the present review, the advantages and limitations of such tests are discussed and the potential for future development explored. The increasing use of molecular tools, for example, PCR and other molecular methods, has improved our understanding of the epidemiology of ectoparasitic diseases, with practical consequences for community-based control programmes. Recently, the identification of specific signalling pathways during the host response to ectoparasites has led to the identification of disease biomarkers which, along with new technologies, such as multiplexed assays and microfluidic platforms, could lead to more cost-effective, rapid and accurate diagnosis of infectious diseases.

Generation, analysis and functional annotation of expressed sequence tags from the ectoparasitic mite Psoroptes ovis

Background

Sheep scab is caused by Psoroptes ovis and is arguably the most important ectoparasitic disease affecting sheep in the UK. The disease is highly contagious and causes and considerable pruritis and irritation and is therefore a major welfare concern. Current methods of treatment are unsustainable and in order to elucidate novel methods of disease control a more comprehensive understanding of the parasite is required. To date, no full genomic DNA sequence or large scale transcript datasets are available and prior to this study only 484 P. ovis expressed sequence tags (ESTs) were accessible in public databases.

Results

In order to further expand upon the transcriptomic coverage of P. ovis thus facilitating novel insights into the mite biology we undertook a larger scale EST approach, incorporating newly generated and previously described P. ovis transcript data and representing the largest collection of P. ovis ESTs to date. We sequenced 1,574 ESTs and assembled these along with 484 previously generated P. ovis ESTs, which resulted in the identification of 1,545 unique P. ovis sequences. BLASTX searches identified 961 ESTs with significant hits (E-value < 1E-04) and 584 novel P. ovis ESTs. Gene Ontology (GO) analysis allowed the functional annotation of 880 ESTs and included predictions of signal peptide and transmembrane domains; allowing the identification of potential P. ovis excreted/secreted factors, and mapping of metabolic pathways.

Conclusions

This dataset currently represents the largest collection of P. ovis ESTs, all of which are publicly available in the GenBank EST database (dbEST) (accession numbers FR748230 - FR749648). Functional analysis of this dataset identified important homologues, including house dust mite allergens and tick salivary factors. These findings offer new insights into the underlying biology of P. ovis, facilitating further investigations into mite biology and the identification of novel methods of intervention.

Transcriptomic analysis of the temporal host response to skin infestation with the ectoparasitic mite Psoroptes ovis

Background

Infestation of ovine skin with the ectoparasitic mite Psoroptes ovis results in a rapid cutaneous immune response, leading to the crusted skin lesions characteristic of sheep scab. Little is known regarding the mechanisms by which such a profound inflammatory response is instigated and to identify novel vaccine and drug targets a better understanding of the host-parasite relationship is essential. The main objective of this study was to perform a combined network and pathway analysis of the in vivo skin response to infestation with P. ovis to gain a clearer understanding of the mechanisms and signalling pathways involved.

Results

Infestation with P. ovis resulted in differential expression of 1,552 genes over a 24 hour time course. Clustering by peak gene expression enabled classification of genes into temporally related groupings. Network and pathway analysis of clusters identified key signalling pathways involved in the host response to infestation. The analysis implicated a number of genes with roles in allergy and inflammation, including pro-inflammatory cytokines (IL1A, IL1B, IL6, IL8 and TNF) and factors involved in immune cell activation and recruitment (SELE, SELL, SELP, ICAM1, CSF2, CSF3, CCL2 and CXCL2). The analysis also highlighted the influence of the transcription factors NF-kB and AP-1 in the early pro-inflammatory response, and demonstrated a bias towards a Th2 type immune response.

Conclusions

This study has provided novel insights into the signalling mechanisms leading to the development of a pro-inflammatory response in sheep scab, whilst providing crucial information regarding the nature of mite factors that may trigger this response. It has enabled the elucidation of the temporal patterns by which the immune system is regulated following exposure to P. ovis, providing novel insights into the mechanisms underlying lesion development. This study has improved our existing knowledge of the host response to P. ovis, including the identification of key parallels between sheep scab and other inflammatory skin disorders and the identification of potential targets for disease control.

Identification of an abundant allergen from the sheep louse, Bovicola ovis

Infestation of sheep with the louse Bovicola ovis is common worldwide and leads to an allergic dermatitis referred to as 'scatter cockle'. IgE from an infested lamb was used in immunoaffinity chromatography to purify allergens from crude preparations of whole B. ovis and its faeces. SDS-PAGE of the affinity-purified eluates from both preparations showed a dominant band with M(r) of 28.5 kDa. Spleen cells from a mouse immunised with B. ovis faecal antigens were used to produce hybridomas which were screened by ELISA to identify those producing monoclonal antibodies (mAb) to the allergens purified by IgE immunoaffinity chromatography. Western blotting demonstrated that all of the mAbs examined recognised the 28.5 kDa allergen. The allergen, purified using immunoaffinity columns constructed with one of the specific mAbs, was shown to cause immediate and late-phase responses on intradermal skin testing in B. ovis-infested but not in naïve lambs. Levels of serum IgE specific for the purified allergen were significantly higher in infested than in naïve lambs (P < or = 0.0025). N-terminal and internal amino acid (aa) sequences obtained from the purified 28.5 kDa allergen were used to design primers to amplify a partial cDNA probe from B. ovis cDNA by PCR. The amplified probe was radiolabeled and used to screen a B. ovis cDNA library. The complete nucleotide sequence of the allergen was determined from the sequences of the positive clones from the library. The full-length cDNA encodes a 255 aa protein including a secretory leader sequence of 26 aas and a mature protein of 229 aas. The encoded protein showed strong homology to several hypothetical proteins of unknown function from diverse species and weak homology with lipid-binding proteins of Xenopus tropicalis and Galleria mellonella. This is the first allergen to be identified from a louse and it has been designated Bov o 1 in accordance with the criteria of the World Health Organization/International Union of Immunological Societies Allergen Nomenclature Subcommittee.

Ovine IgE and its role in immunological protection and disease

The importance of internal and external parasites in limiting productivity and compromising the welfare of sheep has provided the impetus for extensive research on ovine IgE with the objectives of better understanding protective immunological responses and developing novel methods of control; particularly vaccination. The molecular structures of ovine IgE and its high affinity receptor have been determined and the former information has assisted the development of monoclonal antibodies (mAb) to ovine IgE by 2 of 3 groups who have produced these reagents. The availability of these mAbs has enabled the description of IgE responses following infections with a wide variety of parasites in sheep and in an ovine model of atopic asthma. While IgE responses are consistently associated with parasitic diseases of sheep, it has not been proven that this antibody isotype is involved in protection. The foundation of present knowledge and reagents, together with new emerging technologies, should allow the role of IgE in parasitic diseases of sheep to be determined.

Eukaryotic expression of recombinant Pso o 1, an allergen fromPsoroptes ovis, and its localization in the mite

A cDNA encoding the immunogen Pso o 1 fromPsoroptes oviswas obtained by polymerase chain reaction (PCR) amplification. The amplicon contained the entire coding sequence for the prepro-enzyme in an open reading frame (ORF) of 966 bp. This gene encoded a predicted protein of 322 amino acids (aa) with 64% aa identity (80% similarity) to the major house dust mite faecal allergen Der f 1. The pro-enzyme form of Pso o 1 was expressed as a recombinant protein in thePichia pastoris-eukaryotic expression system. Maturation of the recombinant pro-enzyme by autocatalytic activation was not observed, and such maturation could not be achieved using a number of techniques known to activate recombinant Der p 1 and Der f 1 expressed in the same system. Serum raised against recombinant Pso o 1 cross-reacted with mature Der p 1 and allowed Pso o 1 to be immunolocalized to the gut ofP. ovis.

Progress and opportunities in the development of vaccines against mites, fleas and myiasis-causing flies of veterinary importance

Despite the potential benefits offered by vaccination against ectoparasites, there have been few commercial successes with this strategy, in spite of sustained efforts using increasingly sophisticated techniques. This review outlines the progress and challenges offered by recent research into vaccination against some of the major ectoparasites of veterinary importance, and provides an insight into the opportunities arising from our increased understanding of the immunology of host-parasite relationships and the potential for exploitation of this knowledge and that arising from new genomic data provided by expressed sequence tag projects.

Seafood allergy: lessons from clinical symptoms, immunological mechanisms and molecular biology

Food allergy consists of a wide range of disorders that result from adverse immune responses to dietary antigens. Manifestations of allergic response includes acute, potentially fatal anaphylactic reactions and a variety of chronic diseases that mainly affect the gastrointestinal tract, skin, and respiratory tract. Tools for clinical diagnosis and management, which have not changed much in the past two decades, include the clinical history, tests for specific IgE antibody to suspected foods, elimination diets, oral food challenges, and provision of medications such as epinephrine for emergency treatment. On the other hand, recent immunological and molecular biological research have enhanced our understanding of the mechanisms of these disorders and revealed the identities of many food allergens. Here, we will discuss seafood allergies with respect to the clinical manifestations, diagnosis, immunological mechanisms, and molecular biology of seafood allergens. Furthermore, potential applications and future directions in the clinical management of seafood allergies are discussed.

Identification of tropomyosin, paramyosin and apolipophorin/vitellogenin as three major allergens of the sheep scab mite, Psoroptes ovis

Analysis by one-dimensional (1-D) SDS-PAGE/Western blotting of whole mite extract (larval and adult stages) with sheep sera taken 0-84 days after infection with the sheep scab mite, Psoroptes ovis revealed a progressive IgE antibody response, with a dominant high molecular weight allergen (MW 180 kDa) detected early during infection. Further analysis by two-dimensional (2-D) SDS-PAGE/Western blotting with post-infection sera, revealed a more complex picture with numerous (> 20) IgE reactive spots. Trypsin digest and Maldi-ToF analyses of these spots identified two house dust mite allergen homologues, namely tropomyosin (Der p 10) and paramyosin (Der p 11), and analysis of a third spot indicated an apolipoprotein-like IgE reactive protein (Der p 14). Further 1-D and 2-D SDS/Western blotting, with specific antibodies to the house dust mite allergens Der p 10, 11, and to the IgE reactive peptide of the high molecular weight house dust mite allergen, Der p 14 (vitellogenin/apolipophorin), provided firm evidence for the presence of these three allergens in extracts of the Psoroptes mite. These studies show for the first time that homologues of the house dust mite 10, 11 and 14 group allergens are represented as immunodominant allergens of the sheep scab mite, and may represent important vaccine candidates.

Early Innate and Longer-term Adaptive Cutaneous Immunoinflammatory Responses during Primary Infestation with the Sheep Scab Mite, Psoroptes ovis

Clinical observation has indicated that Psoroptes ovis mites provoke cutaneous inflammation within hours of experimental infestation, but the nature of this reaction has not been described. After infestation of naive sheep with ovigerous P. ovis mites, significant influxes of eosinophils (P<0.004) and neutrophils (P<0.001) were detected within 24 h. A significant (P<0.001) increase in mast cell numbers was observed by 96 h post-infestation. In addition, marked degenerative and proliferative epidermal lesions were evident 24 and 96 h, respectively, after infestation. The influence of the later, adaptive response on the cellular infiltrate at the advancing margin of the lesion and the original site of infestation was also monitored. Mast cell numbers were greatest at 21 days while recruitment of eosinophils and neutrophils was maximal 63 days after infestation. Lesional severity was particularly pronounced from 42 to 63 days after infestation, but significant resolution had occurred by 84 days. Pathological changes at the advancing margin of the lesion were more severe than at the initial site of infestation, and this was reflected by the numbers of mites present. These data suggest that P. ovis elicits an early innate cutaneous response that is subsequently augmented by the development of an adaptive immune response, the intensity of which corresponds to the local population density of mites.

A physiological and biochemical model for digestion in the ectoparasitic mite, Psoroptes ovis (Acari: Psoroptidae)

Mites are an important group of arthropod pests affecting crops, animals and humans. Despite this, detailed physiological studies on these organisms remain sparse due largely to their small size. Unifying models are required to draw together the diverse information from studies on different groups and species. This paper describes a model for digestion in the parasitic mite, Psoroptes ovis, the causative agent of psoroptic mange or sheep scab disease. The limited information about this species is supplemented with data from other acarines, especially house dust mites and ticks. We review the range of enzymes and allergens found in mites and consider their possible roles in digestion in mites, generally and in particular, P. ovis. Histological studies, enzyme biochemistry and molecular biology and experimental evidence suggest that P. ovis utilises a digestive system reliant upon acid peptidases functioning in a largely intracellular environment. The actions of the digestive enzymes are supplemented by the involvement of bacteria as potential direct and indirect sources of nutrition. It is possible that some extra-corporeal digestion also takes place. The interaction of bacteria and digestive enzymes on the skin surface of the sheep may be responsible for the excessive pathological reactions evident in clinical sheep scab.

Temporal pattern of isotype-specific antibody responses in primary and challenge infestations of sheep with Psoroptes ovis--the sheep scab mite

In sheep Psoroptes ovis provokes an allergic dermatitis with significant P. ovis antigen-specific IgE responses. The kinetics of the IgE response to primary and challenge infestations of P. ovis were reported earlier [Parasite Immunol. 22 (2000) 407]. The present study examines IgG, IgM and IgA responses to primary and challenge infestations of P. ovis and the profile of antigens/allergens reacting with IgG and IgE antibodies. Antigen-specific enzyme-linked immunosorbent assays (ELISAs) demonstrate that primary infestations elicited significant increases in levels of IgG and IgM but not IgA antibodies. IgG and IgM responses to primary and challenge infestations were not significantly different. Western blots of reduced P. ovis proteins indicate that IgG antibodies reacted with five major antigens following primary infestation and only three of these after challenge infestation. IgE antibodies bound to three major and five minor allergens after primary infestation and two additional minor allergens after challenge infestation. Immunodominant antigens >100 and <15 kDa and allergens >100 kDa were most consistent in stimulating substantial IgG and IgE antibody responses, respectively. These antigens/allergens may be exploited in immunodiagnosis and modulation of the host immune response.

Cutaneous hypersensitivity reactions to Psoroptes ovis and Der p 1 in sheep previously infested with P. ovis--the sheep scab mite

We have previously shown that infestation with Psoroptes ovis induces an IgE response and intense tissue eosinophilia, typical of a Type I hypersensitivity response [Parasite Immunol. 22 (2000) 407]. Intradermal tests (IDSTs) suggest that there are also delayed and Arthus-type responses to this parasite. In order to study the nature of ovine cutaneous reactions to P. ovis, naïve controls and experimentally infested sheep (n = 5) were challenged intradermally with mite antigen. Challenge elicited immediate (P < 0.001) and delayed (P < 0.005) wheal reactions in sensitised sheep. At 6 (P < 0.02) and 30 h (P < 0.001) the predominant infiltrating cells were eosinophils. To explore the role of circulating antibodies, naïve sheep (n = 5) were subjected to Prausnitz-Kustner (PK) tests. These elicited immediate (P < 0.02) but not delayed wheal reactions. At 6 h eosinophils (P < 0.001) dominated the infiltrate. These results suggest that P. ovis allergens provoke an IgE-dependent immediate and late phase response and a cell-mediated eosinophil-rich delayed-type hypersensitivity response (ER-DTH).