Immune responses to Dermatophilus congolensis infections

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.

Molecular Characterization of Ticks and Tick-Borne Pathogens in Cattle from Khartoum State and East Darfur State, Sudan

Ticks transmit many pathogens with public health and veterinary importance. Despite the wide distribution of tick-borne pathogens in Sudan, the information on the tick–pathogen relationship needs to be updated, particularly using modern molecular techniques. This cross-sectional study, conducted between September and November 2019, used morphology, PCR, and sequencing to confirm the identity of adult cattle ticks (male and female; n = 536) from Khartoum State (n = 417) and East Darfur State (n = 119). Moreover, the presence of Theileria annulata, Babesia bigemina, B. bovis, Anaplasma marginale, and Ehrlichia ruminantium was detected and confirmed in each tick using species-specific PCR or nested PCR and sequencing. The most economically important tick genera, Rhipicephalus, Hyalomma, and Amblyomma, were prevalent in the study area, and 13 different tick species were identified. The most prevalent tick species were Rhipicephalusevertsi evertsi (34.3%) and Hyalomma anatolicum (57.3%) in Khartoum State, and Rhipicephalus annulatus (27%), Rhipicephalus decoloratus (25%), and Hyalomma rufipes (29%) in East Darfur State. We detected all five pathogens in both states. To the best of our knowledge, this is the first study to report the presence of E. ruminantium, its vector Amblyomma variegatum, and B. bovis in Khartoum State. Further, this is the first report on most tick and pathogen species identified in East Darfur State. Our findings indicate the migration of some tick and pathogen species beyond their distribution areas in the country, and this consideration is necessary to develop future control strategies.

An insight into the ecobiology, vector significance and control of Hyalomma ticks (Acari: Ixodidae): A review

Ticks (Acari:Ixodoidea) are important ectoparasites infesting livestock and human populations around the globe. Ticks can cause damage directly by affecting the site of infestation, or indirectly as vectors of a wide range of protozoa, bacteria and viruses which ultimately lead to lowered productivity of livestock populations. Hyalomma is a genus of hard ticks, having more than 30 species well-adapted to hot, humid and cold climates. Habitat diversity, vector ability, and emerging problem of acaricidal resistance in enzootic regions typify this genus in various countries around the world. This paper reviews the epidemiology, associated risk factors (temperature, climate, age, sex, breed etc.), vector role, vector-pathogen association, and reported control strategies of genus Hyalomma. The various proteins in saliva of Hyalomma secreted into the blood stream of host and the prolonged attachment are responsible for the successful engorgement of female ticks in spite of host immune defense system. The various immunological approaches that have been tried by researchers in order to cause tick rejection are also discussed. In addition, the novel biological control approaches involving the use of entomo-pathogenic nematodes and Bacillus thuringiensis (B. thuringiensis) serovar thuringiensis H14; an endotoxin, for their acaricidal effect on different species and life cycle stages of Hyalomma are also presented.

Incidence and biochemical parameters of dermatophilosis in Nigerian cattle breeds from livestock markets, Oyo state, Nigeria

Dermatophilosis is one of the major economically important diseases of cattle in Nigeria. Managing the condition has been very challenging and re-occurrence has been reported with moderate to high morbidity and mortality. The incidence and biochemical features of cattle with dermatophilosis was conducted in June to December 2016 across the four geopolitical zones of Oyo state, Nigeria. Clinical diagnosis were made based on appearance of dermatophilosis lesion, breeds morphologically characterized, ageing were based on rostral dentition and severity based on the extent and nature of the lesion. Biochemical analysis was based on standard procedure as prescribed by Fortress International. Fifty cattle were found to be infected with clinical dermatophilosis during the period of the study. Twenty four (48%) in Ibadan zone, 14 (28%) in Oyo/Ogbomosho, 8 (16%) in Oke-ogun and 4 (8%) in Ibarapa zone. Breeds distribution across the zones showed 28 (56%) White Fulani, 5 (10%) Sokoto Gudali, 3 (6%) Adamawa Gudali, 7 (14%) Red Bororo, 5 (10%) Cross breeds while the Kuri was 2 (4%). Regarding animal ages, less than 2 years old were 2 animals (4%) while the adults were 48 animals (96%) and they fall under the categories of 2-2½ years of age, 3-3½ years of age and those which are over 3½ years of age. The different levels of severity were categorized into mild (20 (40%)), moderate (23 (46%)) and severe (7 (14%)) .The best parameters were seen in White Fulani, while the least were seen in Adamawa Gudali. The age group in the category of 3 - 3½ years had most of the best serum values while the least values were seen in animals less than 2 years of age. Cattle exhibiting mild lesions had most of the best serum values and the least values were observed in cattle with severe lesions. No significant difference (p > 0.05) was observed in the mean values for the various parameters studied among the breeds, age, and severity of condition.

Phylogenetic analysis of Dermatophilus congolensis isolated from naturally infected cattle in Abeokuta and Ilorin, Nigeria

Dermatophilus congolensis, the aetiological agent of dermatophilosis, is a pleomorphic, Gram‐positive actinomycete, which infects animals and humans. Often, there is a wrong diagnosis of the infection in animals because of the close resemblance of the organism with other members of the family Actinomycetaceae. In this study, molecular tools were applied to suspected isolates of D. congolensis obtained from naturally infected cattle in Nigeria for confirmation of dermatophilosis. DNA extraction from 54 suspected pure colonies of D. congolensis was carried out using the QIAamp^®DNA Mini extraction kit. PCR targeted at the 16S rRNA gene was employed for the confirmation of D. congolensis using 5′‐ACATGCAAGTCGAACGATGA‐3′ and 5′‐ACGCTCGCACCCTACGTATT‐3′ as forward and reverse primers, respectively. Positive amplicons were then sequenced directly using Big Dye Terminator Cycle Sequencing Kit with the forward primers and AmpliTaq‐FS DNA Polymerase. Nucleotide sequences were aligned using bioedit (Ibis Biosciences Carlsbad, CA USA) and the phylogenetic analysis was carried out using mega 5.2 (Center for Evolutionary Medicine and Informatics, The Biodesign Institute, Tempe, Arizona, USA) software programme. The aligned nucleotide sequences of 10 positive D. congolensis isolates had between 94% to 99% homology with the sequences of D. congolensis satellite DNA in GenBank. This result also revealed that the sequenced D. congolensis are of different strains. Phylogenetic analysis revealed that D. congolensis, though closely related to Nocardia brasiliensis (NR 074743.01) and Streptomyces sp. (JN 400114.1), belongs to different genus. In conclusion, molecular tools employed in the study were able to confirm the identity of the test organisms as D. congolensis. It can also be concluded that two strains of D. congolensis obtained from the study can still be accommodated within the previously listed strains available in GenBank while the remaining eight may be different strains of D. congolensis not yet listed in GenBank.

Isolation and characterization of flagellar filament from zoospores of Dermatophilus congolensis

Highly motile zoospores from Dermatophilus congolensis bovine isolates from clinical dermatophilosis in Japan were obtained by culturing at 27°C in an ambient atmosphere on heart infusion agar supplemented with 5% defibrinated sheep blood for 72h or in heart infusion broth for 48h with gentle shaking. After vigorous mechanical agitation of the zoospore suspension, the flagellar filaments detached from motile zoospores and were isolated in the clear gelatinous part of the final pellet by differential centrifugation. Typical morphology of a flagellar filament, with a width of approximately 15nm, was observed in the isolated flagellar filament by electron microscopy. A single major protein (flagellin) band with an apparent molecular mass of 35kDa was detected in the flagellar filament of D. congolensis strain AM-1 and that of 33kDa was detected in strain IT-2 by SDS-PAGE. In immunoblot analysis of whole-cell proteins from seven isolates of D. congolensis, antiserum to strain AM-1 zoospores reacted with the 35-kDa antigen band of strain AM-1, but not with any antigen band of other strains in a similar molecular mass range. In contrast, antiserum to strain IT-2 zoospores reacted with antigen bands at 33kDa from six strains, except strain AM-1. Similar strain-specific reactions of these anti-zoospore sera with isolated flagellar filaments from strains AM-1 and IT-2 were confirmed by immunoblot, indicating the presence of antigenic variations of flagellins of D. congolensis zoospores.

The nipple: a simple intersection of mammary gland and integument, but focal point of organ function

Having glands that secrete milk to nourish neonatal offspring characterizes all mammals. We provide a brief overview of the development and anatomy of nipples and mammary glands in monotremes, marsupials, and marine mammals, and focus on the nipples and mammary glands in terrestrial eutherian species. We first classify eutherians into three groups: the altricial, precocial, and arboreal types based on their rearing system. We then summarize the physiology of lactation and the cell biology of nipples with specific focus on comparing these in the mouse, cow, and human, which represent the three different groups. Finally we propose that the nipple is an example of specialized epidermis. As specialized epidermis, it is dependent the underlying stroma for development and maintenance in adult life. The development of the nipple and signaling pathways that regulate its formation are described.

Tick infestations in extensively grazed cattle and efficacy trial of high-cis cypermethrin pour-on preparation for control of ticks in Mvomero district in Tanzania

BACKGROUND

This study aimed at determining the extent of tick infestations in extensively grazed cattle and assess the efficacy of Ecotix® acaricide (2.5% high cis cypermethrin) in Mvomero district in Tanzania. A total of 1200 Tanzanian short horn Zebu (TSHZ) from two farms in two villages were qualitatively assessed for tick infestations and 40 animals (grouped in 10s from each farm) were separated in their herds and quantitatively examined to establish the tick load. The animals were grouped in treatment regime groups (TxRG 1, 2, 3, and 4), TxRG 1 being the control group was treated with water. Ecotix® was applied on day 0 for TxRG 2, days 0 and 7 for TxRG 3 and days 0, 7 and 14 for TxRG 4 and tick load was monitored for 28 days.

RESULTS

All the animals examined were infested with ticks. The identified ticks were Rhipicephalus appendiculatus (55%), Amblyomma variegatum (18%), R. (Boophilus) microplus (12.9%), R. (B) decoloratus (7.1%), R. evertsi evertsi (4.4%) and R. composites (2.6%). The overall mean (mean±SEM) tick density on day zero was 63±30 ticks per animal (ranging from 20-160). The mean tick density on day zero was 44.6±25.4, 74.6±30.3, 55.0±26.2 and 77±33.5 for groups one, two, three and four respectively. Post-treatment quantitative assessment of tick burden revealed that the TxRG 1 animals maintained a tick load throughout the study period. A significant decrease in tick load in animals in groups TxRG 2, 3 and 4 (P=0.0001) with increasing frequency of Ecotix® application was recorded. There was however no significant difference in tick reduction between TxRG 3 and 4 (P=0.0986). Thus TxRG 3 would be sufficient for the monthly tick treatment and with this regime, the farmer would save up to 2.5 USD per animal during six months of dry season.

CONCLUSIONS

The study revealed a high tick infestation among the TSHZ kept in extensive grazing systems in Mvomero district and that when treated with Ecotix® as pour-on preparation using a two application regime per month, the tick control strategy was effective.

In vitro and in vivo efficacies of ivermectin and cypermethrin against the cattle tick Hyalomma anatolicum anatolicum (Acari: Ixodidae)

This study investigated the comparative efficacy of ivermectin and cypermethrin pour-on, for the treatment of Hyalomma anatolicum (a.) anatolicum infestations in bovines. For examining acaricidal efficacy, 480 ticks were exposed in vitro to graded doses of both the acaricides and in vivo efficacy was examined in 360 tick-infested bovines treated at the recommended doses of ivermectin (IVM) and cypermethrin (CYM) pour-on. The comparative quantitative assessment of tick burden was done on days 0, 5, 10, 15, and 20 after treatment using "finger counting." The results of the tick survival assay indicated both compounds were effective in vitro against H. a. anatolicum. The arc transformed mean surviving ticks, 24 h post immersion, was 2.66 and zero in groups treated with the highest dilutions of IVM and CYM, respectively. At 15 days post-treatment, the CYM pour-on showed a higher in vivo efficacy (no surviving ticks) compared to IVM (mean of 20 surviving ticks). A single dose of CYM and IVM was found effective for 20 and 15 days post-treatment, respectively. Additionally, a questionnaire was used to gather information from 30 small holder dairy farms on the farmer's approach toward the control of ticks. The majority (90%) of respondents were using acaricides incorrectly along with poor husbandry practices on their farms. Overuse of IVM in the tested area of Pakistan may be the reason the IVM is not as effective as expected. These results provide useful tools for the decision making in tick control, as well as providing the basis for testing the findings on provincial and national levels in future studies.

A critical analysis of disease-associated DNA polymorphisms in the genes of cattle, goat, sheep, and pig

Genetic variations through their effects on gene expression and protein function underlie disease susceptibility in farm animal species. The variations are in the form of single nucleotide polymorphisms, deletions/insertions of nucleotides or whole genes, gene or whole chromosomal rearrangements, gene duplications, and copy number polymorphisms or variants. They exert varying degrees of effects on gene action, such as substitution of an amino acid for another, shift in reading frame and premature termination of translation, and complete deletion of entire exon(s) or gene(s) in diseased individuals. These factors influence gene function by affecting mRNA splicing pattern or by altering/eliminating protein function. Elucidating the genetic bases of diseases under the control of many genes is very challenging, and it is compounded by several factors, including host x pathogen x environment interactions. In this review, the genetic variations that underlie several diseases of livestock (under monogenic and polygenic control) are analyzed. Also, factors hampering research efforts toward identification of genetic influences on animal disease identification and control are highlighted. A better understanding of the factors analyzed could be better harnessed to effectively identify and control, genetically, livestock diseases. Finally, genetic control of animal diseases can reduce the costs associated with diseases, improve animal welfare, and provide healthy animal products to consumers, and should be given more attention.

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.

Renitelo cattle dermatophilosis and PCR-RFLP analysis of MHC gene

Renitelo breed is a cattle breed created at Kianjasoa station (Madagascar) by a triple crossing Malagasy Zebu x Limousine x Afrikander. This breed besides many valuable advantages, such as rapid growth and drought power, presents a huge disadvantage which is sensitivity to skin disease, dermatophilosis, previously known as streptotrichosis. This disease caused by Dermatophilus congolensis is one of the major threats for the population of Renitelo cattle. An allele of MHC gene has been shown to be dramatically associated to hypersensitivity to the disease in other cattle breed. To bring further information to tick borne disease clinical survey, mainly dermatophilosis, we wanted to verify if such allele could be found in this breed. Renitelo cattle included in this study were chosen for the presence of dermatophilosis lesions in more or less severe form (N = 17). These animals were blood sampled and a genetic analysis on the MHC gene BoLA-DRB3 was performed, by PCR amplification using BOD 31 & BOD 32 primers. Amplified products were analyzed by RFLP using enzymes. Restriction band profiles were characterized according to previously defined patterns. Three cows out of the 17 cattle analyzed for MHC gene presented the hypersensitive allele FDA. Two out of the three hypersensitive cows were pure breed while one was half breed. All the cows presented dermatophilosis lesions at least during rainy season but one of them particularly suffered from severe lesions covering all its body and died of the illness. This study shows that hypersensitivity allele found in other bovine breeds can be found in Renitelo breed. This result seemed to suggest that this characterization could be utilized in breeding program for this breed.

Pustular dermatitis caused by Dermatophilus congolensis

We describe a case of pustular dermatitis in a 15-year-old girl who had just returned from horseback riding camp. Based on gram staining, colony characteristics, biochemical reactions, and whole-cell fatty acid analysis, the causative agent was identified as Dermatophilus congolensis. The literature contains few reports of human infection with this organism.

Genetic variation and responses to vaccines

Disease is a major source of economic loss to the livestock industry. Understanding the role of genetic factors in immune responsiveness and disease resistance should provide new approaches to the control of disease through development of safe synthetic subunit vaccines and breeding for disease resistance. The major histocompatibility complex (MHC) has been an important candidate locus for immune responsiveness studies. However, it is clear that other loci play an important role. Identifying these and quantifying the relative importance of MHC and non-MHC genes should result in new insights into host–pathogen interactions, and information that can be exploited by vaccine designers. The rapidly increasing information available about the bovine genome and the identification of polymorphisms in immune-related genes will offer potential candidates that control immune responses to vaccines. The bovine MHC,BoLA, encodes two distinct isotypes of class II molecules, DR and DQ, and in about half the common haplotypes theDQgenes are duplicated and expressed. DQ molecules are composed of two polymorphic chains whereas DR consists of one polymorphic and one non-polymorphic chain. Although, it is clear that MHC polymorphism is related to immune responsiveness, it is less clear how different allelic and locus products influence the outcome of an immune response in terms of generating protective immunity in outbred animals. A peptide derived from foot-and-mouth disease virus (FMDV) was used as a probe for BoLA class II function. Both DR and DQ are involved in antigen presentation. In an analysis of T-cell clones specific for the peptide, distinct biases to particular restriction elements were observed. In addition inter-haplotype pairings of DQA and DQB molecules produced functional molecules, which greatly increases the numbers of possible restriction elements, compared with the number of genes, particularly in cattle with duplicatedDQgenes. In a vaccine trial with several peptides derived from FMDV,BoLAclass IIDRB3polymorphisms were correlated with both protection and non-protection. Although variation in immune responsiveness to the FMDV peptide between different individuals is partly explainable byBoLAclass II alleles, other genetic factors play an important role. In a quantitative trait locus project, employing a second-generation cross between Charolais and Holstein cattle, significant sire and breed effects were also observed in T-cell, cytokine and antibody responses to the FMDV peptide. These results suggest that both MHC and non-MHC genes play a role in regulating bovine immune traits of relevance to vaccine design. Identifying these genes and quantifying their relative contributions is the subject of further studies.

Abomasal nematode community in an alpine chamois (Rupicapra r. rupicapra) population before and after a die-off

Abomasa of 185 chamois shot during 5 consecutive hunting seasons were collected as part of a health monitoring program in an alpine area of Italy and examined for nematodes. The data were obtained during both the preceding period and that following a severe die-off caused by a pneumonia outbreak. Prevalence, mean abundance, mean intensity, and Thul Importance index were consistently high, in particular for Haemonchus contortus, having a low host specificity and high pathogenic potential. Species typical of cervids were also consistently detected. The abomasal nematode community showed an isolationist structure, suggesting its composition was primarily determined by external factors such as interspecific interaction among host species and environmental conditions. The effect of different factors (host sex, sampling site, and time) on nematode counts and aggregation were analyzed and discussed considering the peculiarities of the study site and the chamois population crash. In the light of parallel results for health monitoring, abomasal parasitism could represent a predisposing factor for the observed die-off.

The global importance of ticks

Ticks and tick-borne diseases affect animal and human health worldwide and are the cause of significant economic losses. Approximately 10% of the currently known 867 tick species act as vectors of a broad range of pathogens of domestic animals and humans and are also responsible for damage directly due to their feeding behaviour. The most important tick species and the effects they cause are listed. The impact on the global economy is considered to be high and although some estimates are given, there is a lack of reliable data. The impact of ticks and tick-borne diseases on animal production and public health and their control are discussed.

Evaluation of randomly amplified polymorphic DNA and pulsed field gel electrophoresis techniques for molecular typing of Dermatophilus congolensis

This study aimed to evaluate molecular typing methods useful for standardization of strains in experimental work on dermatophilosis. Fifty Dermatophilus congolensis isolates, collected from sheep, cattle, horse and a deer, were analyzed by randomly amplified polymorphic DNA (RAPD) method using twenty-one different primers, and the results were compared with those obtained by typing with a pulsed field gel electrophoresis (PFGE) method using the restriction digest enzyme Sse8387I. The typeability, reproducibility and discriminatory power of RAPD and Sse8387I-PFGE typing were calculated. Both typing methods were highly reproducible. Of the two techniques, Sse8387I-PFGE was the least discriminating (Dice Index (DI), 0.663) and could not distinguish between epidemiologically related isolates, whereas RAPD showed an excellent discriminatory power (DI, 0.7694-0.9722). Overall, the degree of correlation between RAPD and PFGE typing was significantly high (r, 0.8822). We conclude that the DNA profiles generated by either RAPD or PFGE can be used to differentiate epidemiologically unrelated isolates. The results of this study strongly suggest that at least two independent primers are used for RAPD typing in order to improve its discriminatory power, and that PFGE is used for confirmation of RAPD results.

Characterisation of an extracellular serine protease gene (nasp gene) from Dermatophilus congolensis

A partial amino acid sequence of a serine protease from Dermatophilus congolensis allowed the design of oligonucleotide primers that were complemented with additional ones from previously published partial sequences of the gene encoding the enzyme. The polymerase chain reaction (PCR), using combinations of specific and degenerate oligonucleotide primers, allowed the amplification of a 1738-bp internal fragment of the gene, which was finally characterised by inverse PCR as the first full-length sequenced serine protease gene (nasp) from Dermatophilus congolensis. The deduced amino acid sequence of this enzyme, probably involved in the pathogenesis of dermatophilosis, links it to the subtilisin family of proteases.

AvGI, an index of genes transcribed in the salivary glands of the ixodid tick Amblyomma variegatum

Random clones from a cDNA library made from mRNA purified from dissected salivary glands of feeding female Amblyomma variegatum ticks were subjected to single pass sequence analysis. A total of 3992 sequences with an average read length of 580 nucleotides have been used to construct a gene index called AvGI that consists of 2109 non-redundant sequences. A provisional gene identity has been assigned to 39% of the database entries by sequence similarity searches against a non-redundant amino acid database and a protein database that has been assigned gene ontology terms. Homologs of genes encoding basic cellular functions including previously characterised enzyme activities, such as stearoyl CoA saturase and protein phosphatase, of ixodid tick salivary glands were found. Several families of abundant cDNA sequences that may code for protein components of tick cement and A. variegatum proteins which may contribute to anti-haemostatic and anti-inflammatory responses, and, one with potential immunosuppressive activity, were also identified. Interference with the function of such proteins might disrupt the life cycle of A. variegatum and help to control this ectoparasite or to reduce its ability to transmit disease causing organisms. AvGI represents an electronic knowledge base, which can be used to launch investigations of the biology of the salivary glands of this tick species. The database may be accessed via the World Wide Web at http://www.tigr.org/tdb/tgi.shtml.

Molecular immunogenetics in susceptibility to bovine dermatophilosis: a candidate gene approach and a concrete field application

To identify molecular genetic markers of resistance or susceptibility to dermatophilosis in cattle, we used a functional candidate gene approach to analyze the DNA polymorphisms of targeted genes encoding molecules implicated in known mechanisms of both nonspecific and specific immune responses existing in the pathogen/host interface mechanisms. The most significant results were obtained within the Major Histocompatibility Complex (MHC) where the BoLA-DRB3 and DQB genes encode molecules involved in the antigen presentation to T cell receptors. A unique BoLA class II haplotype, made up of one DRB3 exon 2 allele and one DQB allele, highly correlates with the susceptibility character (P < 0.001). This haplotype marker of susceptibility was also found and validated in other bovine populations. A eugenic marker-assisted selection was developed in the field by eliminating only the animals having this haplotype. The disease prevalence was thereby reduced from 0.76 to 0.02 over 5 years. A crossbreeding plan is in progress to study the genetic transmission of the genotypic and phenotypic characters of susceptibility to dermatophilosis. In conclusion, we discuss several hypotheses at the molecular and cellular levels to better define the exact role of the MHC molecules in disease control and to answer the question: How is MHC diversity selectively maintained by natural selection imposed by pathogens?

A simple random amplified polymorphic DNA genotyping method for field isolates of Dermatophilus congolensis

Dermatophilus congolensis is the pathogenic actinomycete that causes dermatophilosis in cattle, lumpy wool in sheep and rain scald in horses. Phenotypic variation between isolates has previously been described, but its genetic basis, extent and importance have not been investigated. Standard DNA extraction methods are not always successful for D. congolensis due to its complex life cycle, one stage of which is encapsulated. Here we describe the development of rapid and reliable DNA extraction and random amplified polymorphic DNA (RAPD) methods that can be used for genotyping D. congolensis field isolates. Our results suggest that genotypic variation between isolates correlates with host species. Several DNA extraction methods and RAPD protocols were compared. An extraction method based on incubation of the bacterium in lysozyme, sodium dodecyl sulphate (SDS) and proteinase K treatments and phenolic extraction yielded high-quality DNA, which was used to optimize RAPD-polymerase chain reaction (PCR) protocols for two random primers. An alternative rapid, non-phenolic extraction method based on proteinase K treatment and thermal shock was selected for routine RAPD typing of isolates. DNA extracted from reference strains from cattle, sheep and horse using either method gave reproducible banding patterns with different DNA batches and different thermal cyclers. The rapid DNA extraction method and RAPD-PCR were applied to 38 D. congolensis field isolates. The band patterns of the field and type isolates correlated with host species but not with geographical location.