Mycobacterium ulcerans infection: causes, symptoms, diagnosis, treatment, and prognosis

Mycobacterium ulcerans infection is found all over the world, mainly in tropical rain forests. The natural host and human transmission modes of this bacterium are still unclear, and insects may be the vector of transmission. Skin lesions occur mostly in the forearms and calves.


Causes

Mycobacterium ulcerans is a slow-growing mycobacterium. The optimal growth temperature is 24 - 31 °C, and colonies are formed after 3 - 5 weeks of culture. Negative nitrate test, positive nicotinic acid test, positive catalase test, and positive neutral red test are present. Guinea-pig inoculation test is insensitive, but mouse footpad inoculation is effective.


Signs and Symptoms

Skin lesions occur mostly in the forearms and calves. An initial, solid, painless, subcutaneous nodule evolves into a necrotizing ulcer after rupture, with perforating edges, with raised peripheral skin, with infiltration and pigmentation. The surface is dry, and the base of the ulcer is an adhesive gray pseudomembrane. The ulcer is usually superficial, but can be down to the periosteum. Systemic symptoms, local lymph nodes enlargement, and fever are absent. Generally, the skin lesion is solitary, but satellite lesions can also be seen.


Histopathology

In the early stage, subcutaneous adipose tissue necrosis, hypochromatosis, fibrin deposition, fine calcium deposition in the necrotic area, and increased reticular fibers are present. There are bacillus colonies in the center of the lesion, and large colonies can be seen without the aid of instruments in acid-fast staining. The bacillus count is sharply reduced on the non-necrotic site. Significant inflammatory reaction is absent, and congestion and cell infiltration at the necrotic site and periphery are absent.

In the middle stage, dermal collagen fiber degeneration, edema around the sweat glands, inflammatory cells infiltration around the small blood vessels, epidermal degeneration, formation of the ulcer are visible.

In the late stage, giant cells and foam cells in the lesion, subepidermal zonate lymphocytes infiltration, and tuberculoid granulation tissue in the upper necrotic tissue are present.


Diagnosis

Clinical diagnosis depends upon the clinical manifestations such as a single nodule or ulcer, absent local lymph nodes enlargement and systemic symptoms, and a large number of acid-fast bacilli in the ulcer smear or tissue section. Definitive diagnosis requires bacterial culture and animal inoculation.


Treatment

Systemic clofazimine is effective. The small nodule can be excised, and the large nodule may require skin grafting after resection. Secondary infections should be actively prevented.


Prognosis

The disease can heal spontaneously in few months, but can last for several years in very few patients. The longer the duration is, the larger the skin lesion is. Scar contracture can result in severe deformity.