Actinobacillus lignieresii infections


Introduction and aetiology

Actinobacillosis is caused by Actinobacillus lignieresii and is an ubiquitous, but usually sporadic, disease of cattle, sheep and goats. The disease is generally chronic and is characterized by pyogranulomatous inflammation, particularly of the skin and other soft tissues of the head and mouth, and of the regional lymph nodes. Infection in the tongue of cattle, a common site in this species, induces a marked fibroplastic response resulting in its common name, ‘wooden tongue’. The bacterium occasionally causes abscesses or granulomas in the teats and udders of sows.10 In horses, lower airway disease and abcesses have also been associated with A. lignieresii and related bacteria infections,9, 15 but these are rare.

The cultural, morphological and other characteristics of A. lignieresii are outlined in the introduction to Actinobacillus spp. infections.

Epidemiology, pathogenesis, clinical signs and pathology

Actinobacillus lignieresii occurs as a commensal of the oral mucosae of ruminants in particular. Disease may follow injury to the mucous membranes in cattle, sheep and goats. Such injuries might be caused by coarse feeds, grass awns, or teeth abrasions or eruptions. Injuries to the skin, such as those inflicted by thorns and tick bites, may also lead to disease. In cattle the disease tends to occur sporadically, with only single cases being encountered, but in sheep it may occur as outbreaks involving several or many animals in a flock are involved. Because of the differences in prehension of feed between cattle and sheep (cattle use their tongues, while sheep use their lips and generally feed with their heads closer to the ground), the lesions in cattle occur most frequently in the tissues of the tongue. The site of entry of the organisms is often the lingual groove because of injuries to the tongue epithelium that have followed the entrapment there of sharp objects, such as the awns of grasses. Lesions in sheep, on the other hand, tend to occur initially in the cutaneous and subcutaneous tissues of the lips and cheeks, with multiple cases occurring in sheep as a result of the nature of the feed.13 In one outbreak, sheep that were fed the leaves of prickly pears (Opuntia sp.) developed lesions in the mouth, pharynx, and forestomachs.12

In cattle, sheep and goats, pyogranulomas caused by A. lignieresii most commonly in the soft tissues of the head and oral cavity and less commonly in other sites such as the forestomachs, lungs, uterus 5 and skin. Lesions spread by extension, as well as by the lymphatic system to regional lymph nodes which are frequently affected and become enlarged and firm. The lesions are intitially hard, circumscribed nodules, varying in size from a few to several millimetres in diameter. Some eventually become soft and fluctuate, and discharge their contents through the skin or mucous membrane, leaving deep ulcers or fistulae (Figure 158.1 and Figure 158.2). These lesions may expand and extend into surrounding tissues. When the tongue is involved, part of the organ becomes indurated, enlarged and immobile. This interferes with mastication and deglutition, and results in salivation and loss of weight. In some, the surface of the tongue becomes unevenly elevated. In severe cases the enlarged tongue may protrude from the mouth. Similarly, when the skin and subcutaneous tissues of the lips, cheeks and submandibular region of the head of sheep are affected, the thickened indurated lesions interfere progressively with prehension and mastication of feed, and animals gradually starve to death.

Abscesses and granulomas may occur in the teats and udders of sows as a result of small wounds inflicted by the sharp teeth of suckling pigs.

Other bacteria such as Trueperella (Corynebacterium) pyogenes, Streptococcus spp. and Pseudomonas aeruginosa are commonly found together with A. lignieresii in lesions, and it is likely that they gain entry to tissues in the same way.

The lesion is pyogranulomatous and contains barely visible colonies of the organism. In some lesions, or parts of lesions, the granulation tissue is hard and firm and somewhat resembles a fibroma,4 the colonies standing out as yellowish- white specks which are referred to as ‘sulphur granules’;in others the affected tissue has a nodular appearance and has more numerous granules dispersed in the fibromatous connective tissue. Lesions may coalesce to form large nodules which may be of a softer consistency and show evidence, on cut surface, of suppuration and consequently abscessation. The pus is thick, mucoid, greenish-yellow and odourless, and contains numerous granules which have the appearance of grains of sand. Ulcers and discharging fistulae and sinuses are sometimes formed when lesions break out into the lumen of hollow organs or to the externum. Frank abscessation does not often occur in tongue lesions.4 Affected lymph nodes may contain only a few nodules or they may be considerably enlarged and oedematous. In long- standing cases the nodes are fibrous and nodular, the nodules containing greenish-yellow pus and characteristic granules.4 Affected nodes, if superficially located, may adhere to the skin and some may form sinuses and discharge pus. Histologically, the sulphur granules...

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