IACUC Learning Module - Xenopus laevis | Previous  Next |
"Red leg" is the common name for a bacterial septicemia that causes high morbidity and mortality in frogs. Commonly implicated pathogens are Aeromonas hydrophila, Proteus hydrophilus and Pseudomonas hydrophilus. These are by no means the only organisms implicated in this disease. The causative agents are normal water flora that are opportunistic when animals have been stressed. By the time the frogs are clinically showing disease, their condition is usually terminal. Mortality can be particularly high in newly acquired or transported frogs. Sudden death is very common. In less acute cases, frogs are lethargic and refuse to eat. Congestion and petechial hemorrhages are often present on the legs and ventral surfaces. Ulcerations may be present on extremities. Infected Xenopus commonly show ascites, skin discolorations and heavy mucous secretions with minimal reddening on the legs and belly. Neurological signs, ocular lesions and generalized edema may be present.
Postmortem lesions of septicemia include congestion and hemorrhages. Diagnosis should be directed toward identification of the causative organism. Surface lesions may be cultured from live animals since there is a good correlation between surface and systemic pathogens. Cultures may also be taken from recently dead or euthanized animals; lymph fluid should be cultured.
It is highly recommended to run sensitivities on the cultured organisms to attain effective antibiotic therapy. Oral tetracycline was the standard recommendation for many years and it remains useful for sensitive organisms. Amikacin, gentamicin, carbenicillin or Baytril are effective against many enterobacteria and Pseudomonas. In aquatic species, the addition of salt to the water may increase survival when used with antibiotic therapy.
Chlamydia psittaci has been seen in Xenopus and other anurans. It produces signs similar to those of acute bacterial septicemia, but necrosis of the liver, spleen, kidneys, and heart is also present. Chlamydiosis may be differentiated from red leg histologically on the basis of typical intracytoplasmic inclusions. Successful therapy has not been reported, but doxycycline or tetracycline may be effective.
Tuberculosis occurs sporadically and can be caused by several soil or water-borne organisms such as Mycobacterium xenopi, Mycobacterium marinum, or Mycobacterium ranae. The organism probably gains entry through skin wounds and affects immunocompromised animals. Nodules and ulcers may be seen on the skin. Granulomas that develop within major visceral organs lead to progressive debilitation. The disease does not appear to be very contagious and is preventable with good hygiene.Fungal Diseases
Fungal infections are common in amphibians and are often secondary to stress, trauma or other immunosuppressive factors. Some fungal organisms are primary pathogens and others can be isolated from healthy animals without evident lesions. Chromomycosis is caused by various pigmented fungi, while phycomycosis results from infection with Mucor or other nonpigmented fungi. Granulomas or abscesses may be found in any organ. Skin ulcers and nodules are also common. Superficial lesions can be treated with topical antiseptics or fungicides. Systemic infections may require sulfadiazine or imidazoles, but the underlying cause of the disease must be considered. Infection tends to recur once therapy is completed.
In aquatic species, Saprolegnia and other aquatic fungal infections may develop on wounds or on bacterial ulcers.Parasites
Amphibians are hosts to a vast array of parasites which are not generally a problem unless stress from captivity and transport increase their pathogenicity.
Quarantine procedures should include routine antihelmintic therapy.
Nematodes such as Rhabdias (strongyloid lungworms) may cause pneumonia and failure to thrive. Adult worms are found in the lungs. Eggs and larvae are present in the gut in large numbers and can be found in the coelomic cavity and lymph spaces in severe infestations.
Capillaria infection in the skin of Xenopus will cause irritation, epidermal desquamation, and eventual death. Nematodes are 2-4 mm in length and can be found in skin scrapings or the sloughed epidermis.
Therapy of choice for Rhabdias is usually Ivermectin at a dose of 0.2 to 0.4 mg/kg administered orally or subcutaneously. This has proven to be effective and safe.
Reinfestation occurs rapidly so tank disinfection combined with repeated treatments may be necessary to achieve consistent negative fecal samples. Fenbendazole (30-50 mg/kg) appears effective for intestinal nematodes, but is not proven to work on lungworms. Levamisole has also been used in amphibians.Protozoans
Aquatic species are susceptible to infection by skin protozoa such as Trichodina, Costia, Oodinium, and Vorticella. Symptoms include skin irritation, cloudiness, and excessive mucous production.
Percutaneous salt therapy (2.5% for 2 hours) may be effective for some etiological agents. More resistant pathogens may require formalin or copper salts (used for fish). Therapy should be tried on a small number of animals.Noninfectious Diseases
Dehydration is the biggest enemy of amphibians. Care should be taken to avoid desiccation from heat sources, particularly lamps. The delicate nature of aquatic amphibian skin predisposes them to trauma of all kinds. Any wound or break in the skin becomes a port of entry for pathogenic organisms.
Chemicals can be very toxic to permeable, amphibian skin. Aquatic species are particularly sensitive to chlorine, ammonia, pesticides, disinfectants, and heavy metals. Careful attention should be given to water quality. High levels of ammonia cause lethargy, skin and gill sloughing, and death.
Gas bubble disease is seen in aquatic frogs when the water is supersaturated with air. Bubbles may be seen in the skin webs and foot webs, but death is usually from secondary septicemia. Careful monitoring of pumps and pressurized water systems should prevent this condition.
Neoplasia may occur sporadically, but there is no consistent pattern.
Rectal and cloacal prolapses are not uncommon and may be secondary to ascites. Prolapse replacement is simple to achieve, but many animals recover spontaneously.
Causes of death in many amphibians remain undetermined. Common histological findings include hepatitis, glomerulonephropathy, renal cysts, and skeletal myonecrosis. These may be related to chronic low grade toxicity or nutritional factors.Therapeutics
Very little pharmacological data exists although many drugs have been used on amphibians. The studies that have been done were usually on one species at one temperature.
Dose rates will vary with the temperature, renal function, hydration and ambient humidity.
For example, the rate of gentamicin excretion in leopard frogs depends upon whether the frogs were kept wet or dry and upon the environmental temperature.
As a general rule, dose rates for small mammals should be applied for warm, temperate and tropical species and aquatic species. Reptilian dose rates should be used for low temperatures and for poorly hydrated animals.
Percutaneous therapy is useful because of the permeability of amphibian skin. Anesthetics, antibiotics and antihelmintics can be effective when used in water or dropped onto the skin. In practical situations, considerable reductions in mortality/morbidity rates have been reached with conservative concentrations of antibiotics in the water. Percutaneous therapy is also useful for rehydration by misting animals or soaking them in water.
Crawshaw, Graham J.: "Amphibian Medicine". In Fowler M.(ed): Zoo and Wild Animal Medicine (ed3). Philadelphia, Saunders, 1993, p.131-139.
Mattison, Christopher: "Anura- Frogs and Toads". In Mattison C.(ed): The Care of Reptiles and Amphibians in Captivity (ed3). New York, Sterling Publishing Co., 1983, p.121-127.
Report of the Subcommittee on Amphibian Standards. ILAR, National Research council. National Academy of Sciences, Washington D.C., 1974.
Hitt, Nancy D.: "Basics of Amphibian Medicine". Presented at 44th Annual AALAS Meeting, Nashville, TN., Nov. 18, 1993.
Goodwin, Bradford S.: "Amphibians in the Lab". Presented at 44th Annual AALAS Meeting, Nashville, Tenn., Nov. 18, 1993.
Mulder, John B.: "Salamanders, Frogs and Turtles". Handout from Laboratory Animal Research Methods class, University of Arizona, 1990.
Etheridge, A.L. and Richter, S.M.A.: "Xenopus laevis: Rearing and Breeding of the African Clawed Frog". Booklet from Nasco, Fort Atkinson, Wi., 1978.
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