Abstract

A male Amazon manatee (Trichechus inunguis) was kept for 17 years at Steinhart Aquarium. At death it was the only member of its species on public display in North America. Throughout life this animal was variously studied. Medical and husbandry problems deriving from an early harpoon wound were of interest. The general care and maintenance of the animal during the subsequent years provided valuable information. Death resulted from a long-standing pyogranulomatous pneumonia caused by mycobacteria. The responsible organism was Mycobacterium marinum which commonly causes tuberculosis in fish. Mycobacterium marinum generally produces only localized infections in homeotherms, being confined to the cooler parts of the body such as the extremities. This case history documents a clear-cut, lethal, systemic Mycobacterium marinum infection in an aquatic mammal.

Introduction

Manatees are aquatic, herbivorous mammals of the order Sirenia which are represented by two living genera, Trichechus and Dugong. There are three species of Trichechus remaining today, and of these, Trichechus inunguis is the species found only in the fresh waters of the Amazon basin. The Amazon manatee is smaller, darker and smoother skinned than its Florida relative (Trichechus manatus). It lacks fingernails on its flippers and never enters salt water. This species is difficult to study in the wild because it is elusive and lives in murky waters covered by mats of floating vegetation.

Life History

In August 1967 a Trustee of the California Academy of Sciences rescued a young male Amazon manatee from a fish market In Leticia Colombia. The animal was brought to Steinhart Aquarium. On arrival he weighed 29 kg and was estimated to be about 6 months old. At this time he had a partially healed harpoon wound in the left paralumbar region that was exuding pus. Radiology revealed osteolytic lesions in 3 lumbar vertebrae (L 3-L 5 ). Bacteriological cultures showed a mixed infection which was treated with appropriate antibiotics (1). The treatment was protracted and difficult, but after some months the wound healed, leaving a hard raised lump which persisted throughout the animal's life. Table 1 summarizes the treatment of the osteolytic lesions.

Table 1. Summarization of Harpoon Wound Treatment - (After Frye and Herald)

During treatment the animal ate well and was maintained on a diet consisting solely of iceberg lettuce. By the end of treatment his weight had increased to 66 kg, and he continued to eat prodigiously. He was fed lettuce at an average rate of 11 kg/day, but in unrestricted feeding the rate could be as high as 13.5 kg/ day. Table 2 suggests that the animal's diet was unremarkable in terms of the constituents of lettuce and the amount consumed.

Table 2. The Nutritional Value of Lettuce -From: The Analysis of nutrients In Food D. Osborn, & P. Voogt

Consider the energy requirement. Assume the manatee needs 1500 kcal/day. Consumption of 11.5 kg/day (25 lbs) required.

For instance, if available energy is considered, 11 kg/day of lettuce is equivalent to 1.430 kcals/day. This would be an adequate amount if the captive manatee was likened to a sedentary person. Similar equivalencies probably apply to the consumption of essential nutrients. It must be assumed that adequate amounts of vitamin B12 were produced by intestinal flora, and that sunlamps accounted for sufficient vitamin D. Nevertheless, the captive diet was always a cause for concern and constant efforts were made to improve it. For years, however, iceberg lettuce was the only food the manatee would accept.

Between 1972 and 1980 the manatee continued to eat well and seemed fit and healthy. From time to time various species of fish and turtles were introduced into his tank to relieve the monotony. He was brushed regularly. Gradually he was weaned from the exclusive iceberg lettuce diet, and Romaine lettuce, red and white cabbage and celery were accepted. Occasionally spinach was taken and, very rarely, an anchovy or whitebait. Divers spent time in the tank reporting on the animal's appearance, behavior, responsiveness and vocalizations.

In late 1980 the manatee's behavior began to change. A pattern developed in which periods of apparent good health alternated with extended periods of inactivity and anorexia. Invariably these bouts were accompanied by skin problems. Healthy black skin would slough off leaving irregular patchy grey areas about the head and back. The hair follicles in these areas appeared white and raised. The affected skin became dry and itchy and the animal actively sought brushing and constantly tried to scratch. Methylene blue and sodium chlorite were added to the tank to reduce algae and bacteria and alleviate itching {Sodium chlorite, at a concentration of 10-20 ppm is an effective algicide and bactericide. It is used in both marine and freshwater displays of aquatic animals.} Although these skin lesions persisted for long periods of time they did not ulcerate (2). Microbiological cultures of deep skin scrapings were negative for pathogenic fungi and specific bacterial pathogens. Initially, spontaneous remissions occurred, and the skin returned to its normal smooth, dark, oily texture. Concomitantly, the animal's appetite improved and approached normal. Gradually, however, through 1982 and 1983 the periods of ill health lengthened and intensified. Constipation often occurred and weight loss became increasingly evident. The skin had to be treated frequently with antipruritic agents. 15 mg of Dexamethasone (Schering Corp, Kenilworth, NJ) and 4 ml of vitamin B complex (Eli Lilly & Co, Indianapolis, Ind.) were given intramuscularly once a month. At first they evoked a positive response, increasing the manatee's appetite and producing remission. Eventually, however, this treatment became ineffective.

In the autumn of 1984 there was a further deterioration in the animal's condition, he became irritable and actively avoided brushing and diver contact, and no longer vocalized. The head, neck and chest became edematous and he had difficulty surfacing for air. However, breathing seemed normal and coughing or mucus in the mouth and nostrils were not observed. A blood sample drawn from the dorsal surface of a flipper showed a slightly elevated white cell count of 29.1 x 10³, as compared to the normal value of 8 to 10.6 x 10³. The manatee was treated vigorously with 150 mg/day of 5% furosemide (American Hoechst Corp, Somerville, NJ) and 12 x 10⁵U/day of penicillin (Wyeth Labs Inc. Philadelphia, PA). Despite treatment the manatee died in September 1984.

Pathology

At necropsy the skin was dry with patchy grey areas. Edema was seen throughout the subcutaneous tissues. The hard lump in the left paralumbar region consisted of thickened skin with proliferation of fibrous connective tissue. The 3 lumbar vertebrae in this region (L 3- L5) were badly damaged and deformed, and showed osteolytic lesions. The lungs were completely covered by tubercles. There appeared to be little functional tissue left.

Tubercles were also found attached to the caudal walls of the pleural cavities. Tubercles ranged in size from miliary to lesions up to 7 cm in diameter. On incision the tubercles were found to be firm and filled with caseous material. The testes were also filled with caseous necrotic material. The heart had an enlarged left ventricle, hypertrophle blood vessels and pale flabby musculature. The liver was firm and without grossly visible lesions. However, an incision through a lobe revealed cut surfaces that were discolored and mottled with areas of brown pigmentation. Except for a mild gastritis of the intestinal tract, the remaining organs appeared normal.

Histological examination showed heavy infiltration of all tissues with copious amounts of edematous fluid. Cirrhosis, chronic passive congestion and fatty changes were seen in the liver. The parenchyma of the testes was replaced completely by caseous necrotic substance, surrounded by a fibrous connective tissue capsule. Langhans' giant cells were visible near the edges of the lung tubercle sections. In lesions of the testes and lungs acid-fast organisms were seen. Although all organs were examined microscopically, only the lungs and testes had caseation necrosis.

The immediate cause of death was cardiac failure associated with long-standing pyogranulomatous pneumonia due to a microbacterial infection. The responsible organism, isolated from carefully prepared lung lesions, was Mycobacterium marinum. This was the only organism isolated (Mycobacterium marinum was isolated and identified by the Microbiology Lab. of the San Francisco Dept. of Publ. Health. The results were confirmed by the State of California Dept. of Publ. Health.)

Discussion

Mycobacterium marinum is a common cause of piscine tuberculosis, occurring in both salt and freshwater fish, where it produces a chronic wasting disease characterized by cutaneous and systemic granulomatous lesions (3). The disease is thought to spread from fish to fish by ingestion of infective material. Unlike lesions in higher vertebrates, in which mycobacterial organisms are few, mycobacteria in fish are easily seen in tissue sections and are usually present in large numbers. Langhans' giant cells are not easily observed and whether they are ever present is disputed.

Mycobacterium marinum is capable of infecting warm blooded vertebrates and in man causes chronic skin infections (4). It has generally been accepted, however, that the organism is not responsible for systemic infections in homeotherms because it does not normally grow at or near 37°C. Its optimal growth range is 30- 0 C, which usually confines it to the cooler parts of the homeothermic body, namely the skin of the outer extremities. In man, fish tank finger and swimming pool granuloma are well known infections caused by this organism. Infection is believed to be via an abrasion, puncture wound or laceration, in association with an aquarium or aquatic environment. Systemic Mycobacterium marinum in a homeotherm was first reported in 1983 (5). In that case a European hedgehog (Erinaceus europaeus) was kept in an old fish tank in a pet store. It was suggested that the infection resulted from airborne organisms. The concentration of M. marinum in the manatee's lungs suggest that here too, the infecting bacteria may have been airborne. Because the organism was also found in the testes, migration from the lungs may have occurred. However, M. marinum was not isolated from the skin indicating that these lesions had an independent origin. Most probably they reflected nutritional deficiencies as suggested by the correlation between skin repair and improved diet.

The death of this manatee indicates that Mycobacterium marinum can be systemic in warm-blooded vertebrates, and suggests that the organism is a potential hazard to captive homeotherms held in or near aquatic environments where fish are or have been kept.

Its food consists exclusively of aquatic plants on which it browses from beneath the surface. Although now legally protected, it is heavily poached as a source of food. This, combined with loss of habitat, has placed the animal in danger of extinction. Little is known of its biology and it is rare in captivity.

Acknowledgements

The author thanks Dr. Stewart Madin for helpful advice throughout this study. Aline Hunter expertly prepared the tissue sections. Dr. Arthur Back (San Francisco Dept. Publ. Health) first identified the causative organism. Mr. Frank Glennon provided useful observations on the husbandry of the animal.

References

1.  Frye, F.L. and Herald E.S. Osteomyelitis in a manatee. JAVMA 155: 1073-1076 (1969).

2.  Boever, W.J., Thoen, C.O., Ia, A. and Wallach, J.D. Mycobacterium chelonei infection in a Natterer's manatee. JAVMA 169: 927-929 (1967).

3.  Wolke, R.E. and Stroud, R.K. Piscine mycobacteriosis. in: Proc. Symp. Microbacterial Infections of Zoo Animals. Washington D.C. Smithsonian Inst. Press. 269-275 (1978).

4.  Yeager. H. Other mycobacterium species. in: Principals and Practice of Infectious Diseases. Eds. Mandell, G.L., Douglas, R.G., and Bennett, J.E. John Wiley and Sons. 1953-1962. (1979).

5.  Tappe, J.P Weitzman, I., Lui, S., Dotensek, E.P. and Karp, D. Systemic Mycobacterium marinum infection in a European hedgehog. JAVMA 183: 1280-1281. (1983).