Studies on skin diseases of crocodiles

Buenviaje, Gilbert (2000) Studies on skin diseases of crocodiles. PhD thesis, James Cook University.

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This study was conducted to determine the occurrence of skin diseases in farmed crocodiles in Australia. Six farms, three in Queensland and three in the Northern Territory were visited during the period May to August 1996. Samples of skin with lesions were obtained and data on morbidity and mortality, possible aetiological agents, and other likely predisposing factors were collected. The approximate number of crocodiles on these farms varied from 800 to 3,000. Furthermore, pathology case records of histopathological slides of crocodile skin lesions on file from 1989 to 1995 were included. All data gathered on recent (1996) cases, especially the gross and microscopic findings were combined for analysis with the data on retrospectively examined cases of skin diseases on file in the Australian Institute of Tropical Veterinary and Animal Science, James Cook University of North Queensland.

Two hundred and three lesions (119 were from retrospectively examined cases and 84 were recent) from 180, mostly young crocodiles were examined. The skin lesions were obtained from crocodiles on nine farms, from a group of experimental animals and from one adult found dead in the wild. Necropsies were performed either on the crocodile farms, at James Cook University, or at the Department of Primary Industries and Fisheries - Berrimah Veterinary Laboratory, Northern Territory. The histopathological and bacteriological examination of the 84 recent cases was carried out at the Australian Institute of Tropical Veterinary and Animal Science. Samples of skin with poxvirus lesions were examined with an electron microscope.

Five specific skin diseases in uncomplicated form including dermatophilosis, mycotic dermatitis, poxvirus infection, probable mycobacterial dermatitis and capillariasis were identified. Dermatophilosis was the most prevalent skin disease and was also frequently diagnosed, being present in 62 of 66 (94%) in mixed or dual infections. The lesions were discrete focal, 1 to 4 mm diameter 'brown spots' of variable prominence, predominantly on the ventral abdomen but also elsewhere in the body. The histopathological examination confirmed the presence of filamentous organisms mostly in debris that had accumulated on the ulcerated or eroded epidermis but also elsewhere in the subcutis, and in severe cases in the muscular layer.

Duplicate samples of skin confirmed histologically as dermatophilosis were homogenised for bacterial culture. Dermarophilus sp was positively identified initially based on the cultural characteristics which included haemolysis, pitting into the medium and white to grey colonies. The organisms were filamentous and branching, Gram-positive, non-acid fast, catalase positive and oxidase negative. Other biochemical tests also supported the identity of the organism as closely resembling Dermatophilus congolensis.

Several transmission experiments were carried out. In a pilot study on transmission of dermatophilosis, two isolates of Dermatophilus sp (strains TVS 96-366-5A and TVS 96-490-7B), both from cases of dermatophilosis in farmed crocodiles were used. Within several days after inoculation, the hatchlings developed typical 'brown spot' lesions not only at inoculation sites but also other locations both in infected and in·contact control animals. Histopathological examination revealed changes characteristic of dermatophilosis. Dermatophilus sp confirmed as TVS 96-490-7B but not strain TVS 96-367-5A on the morphological, cultural and biochemical characterisation was isolated from 'brown spot' skin lesions on several occasions after infection.

In the second transmission experiment, the protocol was changed to evaluate possible control and treatment procedures. Although occasional minute skin lesions developed in crocodiles, the lesions quickly regressed so effective transmission was not achieved. A third transmission experiment was conducted using facilities and procedures as in the pilot study but with control and inoculated hatchlings maintained in separate tanks in separate buildings 60 metres apart. Both inoculated and in-contact controls in the principal group developed 'brown spot' lesions. Again only strain TVS 96-490-7B was isolated from both the principal and control groups. A few hatchlings in the control group however became infected with either 'brown spot' disease or poxvirus after 20 days post inoculation.

The Dermatophilus sp isolated from spontaneous outbreaks and recovered isolates from several transmission experiments was compared with Dermatophilus congolensis type strain (ATCC 14637) and Dermatophilus chelonae (DCH 2) by morphological, cultural and biochemical characterisation. In addition, more advanced comparison at the molecular level using 16SrDNA sequence and ribotyping were also used. The results showed that the isolate used for inoculation and the recovered isolates were identical, thus confirming its role in causing 'brown spot' disease. Overall, both the phenotypic and genotypic differences were sufficient to suggest that the crocodile isolate is a distinct species of Dermatophilus, thus a new species of Dermatophilus - Dermatophilus crocodyli sp nov. is proposed.

Following successful transmission of 'brown spot' disease and identification of the aetiological agent - Dermatophilus crocodyli, treatment and control studies were carried out. An initial in vitro study on the antibacterial properties of copper sulphate, salt and formalin showed that copper sulphate was effective against Dermatophilus sp. This was followed by experimental transmission of hatchlings as in the pilot experiment but the inoculated animals were kept in separate pens, one group was placed in pen with flowing water and the other group was placed in pen with static water to compare the development of 'brown spot' lesions. This experiment showed the flowing water had no effect on the control of 'brown spot' disease. In the second experiment, all infected animals were treated with either formalin or copper sulphate. Hatchlings treated with formalin developed severe lesions compared with those treated with copper sulphate. All infected hatchlings were treated with copper sulphate using three protocols. Of the three protocols, it was found that immersion of infected hatchlings for 15 minutes in medicated water containing 1 ppm copper sulphate was most effective.

Studies on poxvirus were carried out to include the gross and histopathological characterisation of the disease, electron microscopy of poxvirus and transmission of poxvirus. The gross and microscopic appearance of the skin lesions infected with poxvirus was consistent in all 11 hatchlings. Marked circumscribed grey-white lesions up to 3 mm diameter were present on the lower limbs, foot pads, tail and back, whereas the lesions on the neck, chest and abdomen were diffuse, irregular and translucent. Electron microscopy revealed the presence of 'dumb bell' shaped virus typical of poxvirus. Attempts to grow the virus in cell culture, chicken chorioallantois and crocodile embryos were unsuccessful. A transmission experiment was attempted but failed to produce lesions.

This study concludes that 'brown spot' disease is the most prevalent and probably the most important skin disease in farmed crocodiles in Australia. A newly proposed species - Dermatophilus crocodyfi sp nov. was identified as the aetiological agent of 'brown spot' disease. Copper sulphate at 1 ppm concentration was proven to be an effective treatment for 'brown spot' disease.

Item ID: 27713
Item Type: Thesis (PhD)
Keywords: farmed crocodiles; skin diseases; brown spot disease aetology; effective treatment; Dermatophilus crocodyli; poxvirus; copper sulphate
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 4: Buenviaje, G.N., Ladds, P.W. and Martin, Y. (1998) Pathology of skin diseases in crocodiles. Australian Veterinary Journal, 76(5). pp.357-363.

Chapter 5: Buenviaje, G.N., Hirst, R.G., Ladds, P.W. and Millan, J.M. (1997) Isolation of Dermatophilus sp from skin lesions in farmed saltwater crocodiles (Crocodylus porosus). Australian Veterinary Journal, 75(5). pp.365-367.

Chapter 6: Buenviaje, G.N., Ladds, P.W., Hirst, R.G., Summers, P.M. and Millan, J.M. (1998) Attempted transmission of dermatophilosis in saltwater crocodiles (Crocodylus porosus). Australian Veterinary Journal, 76(7). pp.495-496.

Date Deposited: 26 Jun 2013 01:28
FoR Codes: 06 BIOLOGICAL SCIENCES > 0605 Microbiology > 060502 Infectious Agents @ 80%
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