Introduction
Hydration status is a measure of interstitital fluid content (Hackett 2009). Clinical evaluation of hydration status can be challenging in the avian patient.
Dehydration
Physical examination parameters used to assess hydration are essentially the same in birds and mammals.
- Reduced elasticity of the skin or decreased skin turgor is seen with dehydration however normal bird skin is relatively inelastic and checking skin turgor is not as straightforward as in mammals. To check skin turgor in birds, tent the skin on the on the dorsal surface of the foot or lift the upper eyelid. In patients with severe dehydration, the skin will not fall back into place. Skin turgor is also more difficult to assess in cachectic or obese patients.
- Oral mucous membranes should be moist, however oral mucosa will appear dry or tacky with severe dehydration. Thick, stringy mucus may also be visible within the oropharynx.
- Sunken eyes can also be observed with profound (10%) dehydration.
- Since daily changes in body weight reflect fluid balance, weight loss reflects dehydration. Daily body weights using a reliable gram scale are the most objective way to monitor hydration status (Fig 1).
Figure 1. The most objective way to monitor hydration is with daily body weights. Click on image to enlarge.
- Birds have the ability to absorb significant amounts of ureteral water in the colon and ceca, therefore dehydration can lead to changes in the appearance of the droppings. Of course the normal appearance of the droppings will vary with the species. From relatively dry in the xerophilic budgerigar parakeet to wet and voluminous in nectar-eaters like lories and lorikeets.
Perfusion parameters such as turgor and filling of the basilic vein (normally fills in less than 1 second), mucous membrane color, capillary refill time and heart rate are a measure of intravascular volume (Fig 2). Vascular perfusion is a related but separate issue to interstitial fluid and hydration. Therefore these parameters should be gauged separately.
Figure 2. Use turgor and filling of the basilic vein (arrow) to evaluate vascular perfusion. Image provided by Dr. Lauren Powers. Click on image to enlarge.
Because dehydration can be challenging to identify on physical exam in the bird, laboratory testing is an invaluable way to confirm suspicions regarding your patient’s hydration status.
- Hemoconcentration or an increase in packed cell volume and total protein levels is consistent with dehydration.
- Elevations in blood urea nitrogen (BUN) levels are also a sensitive indicator of dehydration. Although BUN has little value in diagnosing renal disease in most birds, up to 99% of BUN undergoes tubular reabsorption in the dehydrated bird. Therefore levels typically rise from negligible to 3-6 mg/dL. Note: A significant differential for elevated BUN is the postprandial period in healthy carnivores such as raptors and penguins.
- Uric acid excretion is largely independent of urine flow and therefore is unaffected by moderate changes in glomerular filtration (72). Elevations in uric acid (up to 20 mg/dL) may be seen with severe dehydration (23, 73, 74).
- Urine specific gravity (USG) is not very useful in birds, because the bird’s ability to concentrate urine is limited. Urine specific gravity can range from 1.005 to 1.020 g/mL but is highly variable among species.
- Plasma osmolality can also help to confirm dehydration. Comparison of osmolality values in mammals and values reported in birds suggests that plasma osmolality is slightly higher in parrots than in mammals (Beaufrère 2011).
| Box 1. Plasma osmolality levels calculated by using the robust method (Beaufrère 2011) | |||
|---|---|---|---|
| Species | Osmolality (mOsm/kg) | ||
| Range | Mean | SD | |
| African grey parrot | 228-324 | 306 | 7 |
| Hispaniolan Amazon parrot | 308-345 | 327 | 7 |
| Red-fronted macaw | 223-369 | 304 | 18 |
Fluid replacement
When isotonic fluids are required, fluids with an osmolarity close to 300-320 mOsm/L, such as Normosol-R, Plasmalyte-R, Plasmalyte-A and NaCl 0.9%, can be recommended for parrots requiring fluid replacement (Beaufrère 2011).
Summary
Although the parameters used to assess hydration are essentially the same in birds and mammals, clinical evaluation of hydration status can be challenging in the avian patient due to anatomical and physiological differences. Normal bird skin is relatively inelastic. Tent the skin on the dorsal surface of the foot or the upper eyelid to check skin turgor. Since birds have the ability to absorb significant amounts of ureteral water in the colon and ceca, the appearance of the droppings can also change with dehydration. Use the minimum database (hematocrit, urea nitrogen, plasma osmolality) to confirm dehydration in the clinical patient.