Fluid Administration in Amphibians

Introduction

Most amphibians do not drink water (Fig 1). Fluid instead diffuses across semipermeable skin, and sometimes gills, directly from water or moist substrates (Pessier 2009). Excess fluid is excreted primarily by the kidneys, while conserving electrolyte levels. In some amphibians, skin is also involved in osmoregulation and respiration.

Figure 1. Most amphibians do not drink water,instead fluid diffuses directly across the semipermeable skin. Photo credit: Keith Robinson via Flickr Creative Commons. Click image to enlarge.

Skin disease or critical illness in amphibians leads to massive electrolyte and fluid disturbances (Fig 2). For instance, plasma of the normal aquatic amphibian is hyperosmolar to freshwater. Fluids are easily absorbed in excess of requirements, and large volumes of urine are produced to compensate for this. Disease commonly leads to fluid retention and loss of electrolytes.

Figure 2. Skin disease can lead to massive osmotic losses and potentially fatal electrolyte imbalances in amphibians. Photo credit: Brian Gratwicke via Flickr Creative Commons. Click image to enlarge.

Fluid selection

Dehydrated terrestrial amphibians generally benefit from isotonic to hypotonic fluids, however standard mammalian fluids are hypertonic (Clayton 2007, Mylniczenko 2009). There are a number of isotonic solutions available for use in amphibians, and no one fluid is considered ideal. One of the easiest fluids to make is amphibian Ringer’s solution (Box 1).

Clayton has also recommends “reptile Ringer’s solution” or Jarchow’s modification which consists of two parts 2.5% dextrose in 0.45% saline and one part lactated Ringer’s solution.
Less than ideal alternatives for use in critically ill amphibians include:

• 9:1 ratio of 0.9% sodium chloride and sterile water (Mylnczenko 2009)
• Three parts 5% dextrose with one part 0.9% sodium chloride for intracoelomic fluid administration (Mylnczenko 2009)
• Normosol-R and Plasma-lyte A can also be used although they are slightly hypertonic to normal amphibian plasma (Wright 2001).
• Other simple hypertonic solutions include 0.9% saline, hypertonic amphibian Ringer’s solution, or isotonic amphibian Ringer’s solution dissolved in 900 ml or less of distilled water (Wright 2006).
• Dextrose (5% or higher) or hetastarch solution may also be used as a short-term hypertonic bath (Wright 2006).

Hypertonic solutions are a more appropriate choice when fluid overload is observed in the form of edema or ascites. Hypertonic fluids theoretically reduce this fluid burden, although this seems to be most helpful as a prophylactic measure. Once fluid overload develops, correction is difficult (Clayton 2007, Wright 2001).

Fluid resuscitation

Clayton has described a shock dose bolus of isotonic fluids given at 5-10 ml/kg intraosseously or intravenously. This dose is repeated as needed. The use of colloids has not been investigated in amphibians.

Dehydration

Replacement fluids are indicated for ongoing loss caused by vomiting, diarrhea, or diuresis and for dehydration. Dehydration is a common problem in sick amphibians because the rate of evaporative water loss through the skin is greater in amphibians than in terrestrial vertebrates (Box 2). According to Wright (2006), the initial fluid volume provided should not exceed 25 ml/kg. Also avoid potassium-containing fluids initially.

*Excessive tackiness of the slime coat may be the only sign of mild dehydration in some amphibians (Wright 2006).

Routes of administration

The most common route of fluid administration in amphibians is topical or transdermal. Injection into the lymph sacs and oral fluids are also appropriate for mild to moderate dehydration. Fluids are administered intracoelomically, intraosseously (IO), or in rare instances intravenously (IV), for moderate to severe dehydration.

Topical

Topical fluid administration should always be the first step in supportive care of the amphibian. Transdermal fluids are less stressful than other routes, and since this method utilizes normal physiologic processes there is less risk of fluid overload (Clayton 2007). Transdermal fluid support is even a viable option in species such as toads. Warty anurans utilize dark, vascular patches on their ventral pelvic region that allow fluid uptake (Wright 2001, Pessier 2009).

To provide transdermal fluid therapy, place the amphibian in a shallow layer of well-oxygenated, chlorine-free water at the preferred optimum temperature zone of the amphibian. Many temperate zone amphibians prefer 65-72°F (18-22°C) while many tropical species are housed at 70-85°F (21-29°C) (Wright 2006).

Soaking schedules range from 60 minutes once or twice daily to 24 hours (Clayton 2007). If a hypertonic solution is used, maintain amphibians in the bath for no more than 4 hours and monitor your patient carefully. Remove the amphibian if it appears distressed or when it reaches 25% of its normal weight (Wright 2001). If the amphibian is extremely weak, ensure the nares and mouth cannot fall underwater as aspiration can occur. A parenteral route of fluid administration may be more appropriate in these debilitated patients.

Subcutaneous

Subcutaneous injections in amphibians are actually injections into the extensive lymphatic system, which is most accessible dorsally, along the lateral body wall, and in the thigh area of anurans. The volume of lymph sac injections is usually 10-20 ml/kg/day (Clayton 2007).

Oral fluids

Oral fluids may be given to amphibians, but this is not a primary route of delivery since the transdermal method is so useful and easy. Passing a tube is a straightforward process since the esophagus is short and straight and the mouth is relatively wide (Clayton). Anurans also tend to swallow whatever is introduced into the caudal oral cavity or cranial esophagus. The glottis should also close spontaneously so aspiration is not a concern unless the amphibian is severely ill (Clayton 2007).

Intracoelomic

Intracoelomic is a potential route for fluid administration in the amphibian with moderate to severe dehydration, and a good choice for critically ill amphibians too weak to sit up in a soaking bath. Fluids are rapidly absorbed by the coelomic membrane and visceral serosa.

1. Position the amphibians in dorsal recumbency so the viscera fall away from the body wall.
2. To prevent injection of fluids or drugs into the lungs and air sacs, insert the needle in the caudal third of the coelom. Avoid the ventral midline to prevent laceration of the ventral abdominal vein.
3. Insert the needle parallel to the body wall.
4. Always aspirate the needle first to make sure the needle did not enter the respiratory tract.
5. Infuse up to 10-20 ml/kg/day.

Intravenous

Intravenous catheters are difficult to place and poorly tolerated in amphibians (de la Navarre 2006). Bolus fluids may be delivered into the ventral tail vein of caudatans or the ventral abdominal vein of anurans with a butterfly catheter.

Intraosseous

Intraosseous fluids are good option for critically ill anurans. The catheter is placed normograde into the tibiafibula or retrograde into the femur.

Patient monitoring

Weigh patients at least twice daily and monitor the animal closely for evidence of overhydration. Edema and ascites are important signs of fluid overload in amphibians.

References