Monograph

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definition: 

Non-pruriginous, exudative, pustular dermatitis common to several animal species. Transmissible but of a low degree of contagion, it is caused by a ubiquitous bacteria (Dermatophilus congolensis) which, in association with other risk factors, causes symptoms to appear.

Despite a low death rate, dermatophilosis leads to economic loss in terms of a fall in production, inability of pack animals to work and leather depreciation.

Situation in America: 

It is frequent in tropical and sub-tropical countries and especially where the Amblyomma variegatum tick, an aggravating factor of the disease, is present.

Declaration to the OIE by the following countries (2004): Canada, USA, Argentina, Brasil, Chile, Dominica, Guadeloupe, Martinique, St Kitts and Nevis, Trinidad and Tobago, Uruguay.

Susceptible species: 

Bovines, ovines, caprines, equids, camelids

Rarely: pigs, dogs, cats, man (minor zoonosis)

Wild species: mammals and reptiles

Goat dermatophilosis

Dermatophilosis lesions in a goat in Guadeloupe (N. Barré).

Receptivity factors 

In tropical regions: bovines are particularly susceptible. Local breeds are more resistant (eg: Creole bovines from Guadeloupe) than imported or improved breeds.  

In temperate regions: sheep and horses are susceptible.

Etiological agent: 

Dermatophilus congolensis is a polymorphic gram positive bacteria from the Actynomyces group. It exists in two forms:

• Filamentous (0.5 – 1 micrometre Ø): branching and multiplying in the living epidermis forming characteristic chains.

• Coccoid (1 micrometre Ø): mobile form (with flagella), responsible for infection.

The bacteria resists in the outside environment. Scabs falling to the ground remain infectious for several months.

Culture: by aerobiosis, optimum T° = 37°C, on blood or serum-enriched agar.

Methods of transmission
Source: 

Sick animals (scabs) and healthy carriers (germ surviving in hair follicles and in small, barely visible scabs). Soil and water contamination.

The disease is not transmitted by wildlife.

Transmission: 

Direct

Through contact with an infected animal (damp scabs)

Indirect

Through the outside environment (fallen scabs)

Method of contamination: 

Penetration in the skin via superficial scarring. Small lesions caused by biting arthropods (ticks, horseflies, glossina, etc.) or thorny vegetation.

Factors encouraging infection

Ticks: Adult Amblyomma variegatum aggravate dermatophilosis lesions even at a distance from their fixation point. The hypothesis exists that tick saliva possesses an immunomodulation mechanism.

NB: Dermatophilosis is not a disease transmitted by ticks. It is aggravated by their presence in animals.

Humidity and high temperatures: disease present during the wet season and regressing in the dry season.

• Alteration in the skin's protective role (softening)

• Increase in the number of biting insects

• Liberation of coccoïd forms, moisture enhances proliferation (diffusion of lesions on the animal).

Others

• Sources of cutaneous microlesions: arthropods, demodex, tick birds and thorny vegetation.

• Stress: more marked symptoms in suckling females.

Symptoms: 

Symptoms appear when both the bacteria and risk factors are present. 

Chronic form

This is the most frequent form with an incubation period of 2 to 4 weeks. Non-pruriginous lesions appear on the upper part of the body (neck, withers, back) followed by the lower part (ends of the legs). Atypical lesions may also appear (smooth areas: perineum, teats, scrotum).

Progressive weight loss, bacterial secondary infections. Death in 10 to 20% of cases, often through reactivation of intercurrent disease.

At the end of the wet season, the lesions disappear only to reappear the following season. 

 

Acute form

Less frequent. Exudative form, occurring in humid, tropical areas in predisposed subjects:

• imported susceptible breeds (taurines)

• young animals

• primipares

Significant erythema followed by hair loss, skin thickening - the skin forms pleats in which serous exudate accumulates. Rapid spreading of lesions to the whole body.

Overall health is affected: inappetence, weight loss, reduction in milk production.

In the absence of treatment, the animal dies within 10-12 days following general deterioration of health.

Lesions
Macroscopic lesions: 

Initial lesions: papular raising the fur + coagulated serous exudate. The fur takes on a "brush" like appearance and falls away easily. The number of lesions and their volume increase until they coalesce.  

Hyperkeratosis stage: formation of yellowish, thick and more or less flaky scabs.

 

Dermatophilosis lesions

Upper confluent lesions in bovine dermatophilosis (CIRAD EMVT)

Microscopic lesions: 

Dermatophilus congolensis multiplies in the corneal layer of the epidermis and in the hair follicles leading to cell degeneration.

Diagnostics
Clinical diagnosis: 

Any non-pruriginous, scabby lesion in the wet season is considered a warning sign.

Differential diagnosis: 

In bovines

• Non-pruriginous dermatoses: ringworm, chorioptic mange, photosensitivity, demodecosis.

• Pruriginous dermatoses: mange, phtiriasis.

• Other diseases with cutaneous symptoms: nodular dermatitis (sub-cutaneous lesions, oedema, hyperthermia) 

 

In ovines

Sheep pox, contagious ecthyma, mange. 

 

In equids

 Mange, phtiriasis, ringworm.

NB: All these diseases may coexist in a single animal or in a single herd.

Laboratory diagnosis: 

Based on scabs from recent lesions. Smears of exudate from beneath the scabs are taken as it is rich in Dermatophilus. If the lesions are old, rehydrate the scabs before taking a swab.

Microscopic examination of crushed scabs following bacterium staining. Observation of the filaments making up the chains of coccies arranged in 2, 4 or 8 parallel and sometimes branched rows, ("tracks"). This is the preferred test method.

Virological diagnosis: 

Bacterial isolation and identification

Difficult. On blood agar (5 – 10%) + bacterial inhibitors, following passage in rabbits or culture of humid scabs in CO2.

Serological diagnosis: 

ELISA test, immunofluorescence

Used during epidemiological investigations (detection of healthy carriers), enabling demonstration of sub-clinical infections.

Treatment: 

Antibiotherapy via general route

• Injectable Penicillin (75000 UI/kg) + dihydrostreptomycin (75 mg/kg).

• Oxytetracyclin (20 mg/kg), in a single intramuscular injection.

Above all, de-tick all animals!

Dermatophilosis pulverisation

Anti-tick pulverisation treatment (CIRAD EMVT)


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