Important Clinical Aspects of Amphibian Physiology

THERMAL HOMEOSTASIS Amphibians are poikilothermic ectotherms. Their core body temperatures are determined by ambient environmental temperatures, and an amphibian thermoregulates by moving through the mosaic of environmental temperatures to find those appropriate for its current physiological needs. The preferred operating temperature zones (POTZ) vary tremendously between species, between geographic variants of the same species, and even between different physiological states of an individual (eg, full stomach, empty stomach, fighting infection) and its larval and adult life stages. If an amphibian cannot maintain its body temperatures appropriately it will fail to thrive. Captive amphibians maintained at temperatures below their POTZs typically show inappetence, lethargy, abdominal bloating from decomposition of ingesta, poor growth rate, and infections associated with immunosuppression. If captive amphibians are maintained at temperatures above their POTZs, they show signs such as agitation, excessive movement, changes in skin color, inappetence, weight loss despite good appetite, seizures, infections associated with immunosuppression, and sudden death. An appropriately constructed vivarium should provide a mosaic of temperatures so that the inhabitants can thermoregulate. (The thermal mosaic should be integrated with other aspects of the environment, such as moisture, illumination, substrate, so that the amphibian is not forced to satisfy one physiological need at the expense of another.) Amphibians will often use behavioral fever when fighting infection, moving to stay within temperatures at or exceeding the upper limit of their POTZs for longer periods of time than healthy amphibians would. Fever's potential impact on disease in amphibians must be accommodated in the treatment plan. Recent work with chytridiomycosis has demonstrated that fever alone can successfully eliminate this fatal fungal infection. Providing supplemental heat sources is recommended for the treatment of many infectious diseases. Conversely, sometimes cooling is an appropriate choice for an amphibian showing signs of infectious disease. Since bacteria have a growth curve that is temperature-dependent, the doubling time of a pathogenic bacterium in an amphibian patient may be significantly delayed by cooling the amphibian. This inhibits the spread of bacteria while allowing antibiotics to disperse through the amphibian. The goal of treatment is for the antibiotic to reach tissue levels that achieve or exceed MIC for the pathogen before the pathogen overwhelms the patient. Caution must be exercised before adding cooling to an amphibian’s treatment plan since some pathogens, such as saprolegniasis, actually cause more significant lesions at cooler temperatures.