Lumpy wool and pitted grains

Summary Dermatophilus congolensis is a bacterial infection of the skin that causes enlargement of the follicle mouths and pitting of the grain surface on leather, rendering it useless for the production of aniline or lightly finished leather. Despite being very easy to eradicate, animals are not normally treated for the disease. Consequently, damaged hides and skins are prevalent after a wet summer when climatic conditions are conducive to the spread of the disease. Dermatophilus congolensis infection Dermatophilus congolensis is bacterium of the Actinomycete family that causes a skin infection that can affect any animal, even humans on rare occasions. It occurs worldwide and is endemic in many tropical countries. In terms of importance to the leather industry, sheep, cattle and goats are most commonly infected. When cattle are affected, the disease is sometimes referred to as Streptothrichosis and in sheep it is often called 'lumpy wool' disease. The disease is sometimes incorrectly called Mycotic dermatitis; 'mycotic' implies that it is caused by a mould but it is in fact a bacterium that has a mould-like appearance. Dermatophilus congolensis has branching filaments that are up to 5 microns in diameter that eventually break up into small groups of cells. These cells then mature into spores. The spores can be transmitted from one animal to another either by direct contact or by biting insects, especially biting flies and ticks. They can remain dormant on the skin until such times as conditions become favourable for germination (warm and moist). The spores become mobile and migrate to susceptible areas on the skin where they germinate and penetrate the living parts of the epidermis with their hyphae. They are only able to do this where there is some damage to epidermal tissue in the form of an injury or a reduction in the protective lipid film within the epidermis by prolonged wetting of the skin. This is why the infection is more prevalent after a very wet summer. Any area of the skin can be affected and animals of any age or breed are equally susceptible. Damage to hides and skins In normal skin, the living layers of the epidermis gradually keratinise (cornify) and form layers of dead cells that gradually slough away. However, in skin infected with Dermatophilus congolensis, the invasion of the skin by the bacterial hyphae causes acute inflammation and small abscesses form in the epidermis that produce a layer of exudate. The epidermis below begins to grow rapidly and keratinise prematurely and forms a layer underneath the layer of exudates. This new epidermal layer is then invaded by the bacteria setting off the inflammatory process again. This cycle occurs repeatedly and eventually a thick scab forms that comprises of layers of dried or degenerating exudate and keratinised epidermis - hence the name 'lumpy wool' disease in sheep. On leather made from skin that has been infected by Dermatophilus congolensis the grain surface has a 'pock marked' appearance; the hair follicles are enlarged and the surface pitted (picture above left). Often the uptake of dye is different in the lesions resulting in a darker colour rendering it useless for aniline production. The pitted grain is not easily covered without heavy finishing or correcting the grain. In New Zealand a study2 conducted on lambs deliberately infected with Dermatophilus congolensis concluded that the disease was responsible for the defect known as 'white spot' that affects New Zealand lamb pelts. In this study it was found that it took at least six weeks after the infection had been cleared in the animal for the skin lesions to heal and the pelt to be free of the defect. Control of Dermatophilus congolensis Unfortunately for the leather industry, Dermatophilus congolensis infection is not considered to be of sufficient concern to warrant treatment other than for cosmetic purposes3. The infection is very easy to treat; usually a single antibiotic injection, eg penicillin, is sufficient. The control of the ectoparasites that transmit the disease is beneficial both in terms of breaking the cycle of infection and in the prevention of other damage likely to result in reduced leather quality.