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Bacterial Disease - Dermatophilosis

Synonyms:

Dematophilus infection, cutaneous streptothricosis, Lumpy wool, strawberry foot rot.

Introduction

This infection of the epidermis, which is seen worldwide but more prevalent in the tropics, is also erroneously called mycotic dermatitis. The lesions are characterized by exudative dermatitis with scab formation.

Etiology, Transmission and Epidemiology

Dermatophilus congolensis is a gram-positive, non-acid-fast, facultative anaerobic bacterium.

Factors such as prolonged wetting by rain, high humidity, high temperature, and various ectoparasites that reduce or permeate the natural barriers of the integument influence the development, prevalence, seasonal incidence, and transmission of dermatophilosis.

Biting insects particularly flies and ticks, that act as mechanical vectors.

Infection can be spread by shearing, dipping or introducing an infected animal into a herd or flock.

Dermatophilosis is contagious only in that any reduction in systemic or local skin resistance favors establishment of infection and subsequent disease.

Pathogenesis

To establish infection, the infective zoospores must reach a skin site where the normal protective barriers are reduced or deficient.

The respiratory efflux of low concentrations of carbon dioxide from the skin attracts the motile zoospores to susceptible areas on the skin surface.

In acute infections, the filamentous invasion of e epidermis ceases in 2-3 wk, and the lesions heal spontaneously.

In chronic infections, the affected hair follicles and scabs are sites from which intermittent invasions of noninfected hair follicles and epidermis occur.

The invaded epithelium cornifies and separates in the form of a scab. In wet scabs, moisture enhances the proliferation and release of zoospores from hyphae.

Lesions

In cattle, the lesions are observed in three stages:

  • » Hairs matted together as "paintbrush" lesions
  • » Crust or scab formation as the initial lesions
  • » Accumulations of cutaneous keratinized material forming "wart-like" lesions that are 0.5-2 cm in diameter. Typical lesions consist of circular, dome shaped scabs 2-8 nm in diameter.

Lesions initiated by biting flies are found primarily on the back, whereas lesions induced by ticks are primarily on the head, ears, axillae, groin, and scrotum.

Chronic lumpy wool infections are characterized by pyramid-shaped masses of scab material bound to wool fibers.

The crusts are primarily on the dorsal areas of the body and prevent the shearing of sheep; spiny plants often predispose to lesions on the lips, legs, and feet. Strawberry foot rot is a proliferative dermatitis affecting the skin from the coronet to the carpus or hook.

Histopathological examination of the lesions reveals the characteristic branching hyphae with multidimensional septations, coccoidal cells and zoospores in the epidermis.

The organisms are usually abundant in active lesions but can be sparse or absent in chronic lesions.

Diagnosis

Presumptive diagnosis depends largely on the appearance of lesions in clinically diseased animals and demonstration of D.congolensis in stained smears or histologic sections from scabs.

A definitive diagnosis is made by culture and identification.

An indirect fluorescent antibody technique and a single dilution ELISA test have been developed for large serologic and epidemiology surveys.

Differential diagnosis

Dermatomycoses in most species, warts and lumpy skin disease in cattle, contagious ecthyma and ulcerative dermatosis in sheep, and dermatophytosis and immune-mediated scaling diseases of horses.

Treatment

Organisms are susceptible to wide range of antimicrobials- erythromycin, spiramycin, penicillin G, ampicillin, chloramphenicol, streptomycin, amoxicillin, tetracyclines and novobiocin.

Usually, chronic infections can be rapidly and effectively cured with a single IM injection of procaine penicillin (22,000 IU/kg) and streptomycin (22mg/kg). If this fails, the penicillin-streptomycin combination can be administered for 5 days, or a single injection of long-acting oxytetracycline (20mg/kg) can be substituted.

Control

Isolating clinically affected animals, culling affected animals, and controlling ectoparasites are methods used to break the infective cycle.

External treatment with disinfectants that contain a cresol or copper salt base can decrease the spread of infection if applied at times when transmission is likely.

Insecticides applied externally are frequently used to control biting insects.


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