Paul T. Cardeilhac1, DVM, PhD; D.K. Peters
Management problems or disease outbreaks on alligator farms usually require treatment of the entire colony. Stress is the most common management problem and often leads to an increased incidence of bacterial infections (Hatchling Alligator Syndrome), nutritional disease, runts and stunted animals. Methods had to be developed to evaluate performance of the colony in order to determine efficacy of therapeutic agents used to control these problems. The methods had to be simple but give an accurate evaluation of performance. Poor performance was defined as the incidence of runts, stunted animals and deaths. Evaluations were based on growth rate and body condition. Programs were designed to use the measurement of length and weight for a determination of growth rate, body condition and performance. Transformations of the measurements were made so that the final values had a range of 0 to 200 with values of 80 to 120 indicating well managed hatchlings. For example, runts would have a growth rate of less than 50 with a body condition greater than 50 while stunted animals have values of less than 50 for both growth rate and body condition. Stunted animals were identified by performance but a more detailed examination required diagnostic microbiology, clinical values, organ weights and necropsy. These techniques have been used to evaluate antibiotics and to identify those useful as management tools.
Alligator production has become an important aquaculture enterprise in Florida. Intensive culture of the animals has resulted in the emergence of production problems not previously known. A major problem has been bacterial infections in hatchling alligators following stress. Stress can be identified by serum glucose and cortisol levels or more easily by food consumption and performance. It is caused by factors such as the following: handling; temperature fluctuations; overcrowding; pens containing both large and small hatchlings (improper pen sizing); nutritional stress; noise; birds; visitors and other disturbances. The bacterial infections that follow stress are caused by opportunistic organisms that are normally present but are not considered virulent or highly invasive. A number or organisms have been isolated from animals with these infections and the problem has been called Hatchling Alligator Syndrome (HAS). It has been shown that antibiotic treatments will reduce the incidence of runts, stunted animals and deaths caused by HAS (1,2,3). Procedures that may be used for selecting a potentially good antibiotic, determining an effective dose and evaluating animal safety are under investigation (1,2,3).
Hatchling Growth Performance Criteria
The growth rate of hatchling alligators is determined in inches per day over a defined period of time, usually 90 days (15 September to 15 December). Most farmers have inch rulers and can accurately report the length of their animals in inches. The daily growth rate in inches x 1000 produces a value between 0 and 200 which is easily interpreted. The value can be converted to centimeters by multiplying by 2.54 producing a range of 0 to 508.
A condition factor is used which relates body weight to length. Weight gains for growing animals are related to length by a cubic function. Animals of different ages but having the same growth rate will show significant difference in weight gain. More accurate comparisons of performance of groups of animals of similar but slightly different ages are made by growth rate and condition rather than growth rate and weight gain. The condition factor used by our laboratory is 1000 x the cube root of weight in grams divided by the length in inches. Three hundred (300) is subtracted from the factor to give an integer value between 0 and 200. A condition factor of this magnitude is more easily interpreted.
A single performance (overall growth) criteria was needed that considered both growth rate (length) and gain (weight). The growth rate and condition factor were of approximately the same magnitude although growth rate was found to have significantly more variance. The average value for growth rate plus condition factor was termed performance or growth rate plus condition divided by two. The performance value can range from 0 to 200 but is approximately 80 to 120 for hatchlings maintained under good conditions of husbandry.
Hatchlings with a condition factor greater than 50 but a growth rate 20% below the mean value established for the colony are considered runts.
Runts with a condition factor less than 50 are considered stunted.
Whole blood and tissue sections or swabs from lungs, liver and kidney are placed in enriched culture medium at each necropsy. Bacteria recovered are identified and their susceptibility to the following antibiotics determined (disc concentration in mcg): Tetracyclines (30); Ampicillin (10); Chloramphenicol (30); Erythromycin (15); Kanamycin (30); Gentamicin (10); Polymyxin-B (300); Trimethoprim and Sulfamethoxazole (1.25 & 23.75); Vancomycin (30); Virginiamycin (5).
Erythrocyte Values - hematocrit, RBC count, hemoglobin and mean corpuscular volume
White Blood Cell Counts - white blood cell counts and differentials
Electrolytes - sodium, potassium, chloride, inorganic phosphate and calcium.
Enzyme Activities - alkaline phosphatase, SGPT, SGOT.
Organic Constituents (low molecular weight) uric acid, glucose, total bilirubin and creatinine.
Proteins - total protein, albumin, globulin, and A/G.
Gross lesions were recorded when observed during necropsy. An examination of the gastrointestinal tract was made for the presence of parasites or lesions caused by parasites. Tissue from the lungs, liver and kidneys were routinely saved for microscopic examination.
Results and Discussion
Bacterial infections have been reported to be the major cause of disease (90.5%) in captive alligators (4). Hatchling alligators which survive 8 months are more likely to survive until they reach commercial size (4 ft.). The most critical period is the first 90 days and for this reason the test animals were treated continuously for up to 90 days.
Treatments were evaluated by their ability to reduce the number of animals suffering from HAS. The presence of HAS in the colony was determined by performance criteria. HAS produces runts, stunted animals, bacteremias and deaths. An effective antibiotic treatment regimen should improve the performance level of animals under stress. Runting may be a hereditary trait but stunted animals are often suffering from HAS and the opportunistic organisms may be isolated from blood and tissues. Stunted animals suffering from HAS often have changes in clinical values and lesions in affected organs.
Two antibiotics were chosen to study their effect on the control of HAS. Oxytetracycline (OTC) was chosen to primarily control gram negative and Virginiamycin to control gram positive organisms. These antibiotics were chosen because of their long history of use, safety, cost and availability (1). Dose titrations were performed by determining a minimum level that would significantly improve performance. Higher doses do not further improve performance.
Studies indicated that both antibiotics improved performance in hatchlings. OTC improved performance level of the entire treated groups over controls by 65% and 27% respectively in two trials involving 696 hatchlings. Virginiamycin was particularly effective in improving performance of stressed animals. Treating the bottom 33% (in level of performance) improved their performance level by 91% over controls.
The use of objective criteria to determine growth rate, condition, runts, stunted animals and animals suffering with HAS has allowed a more accurate evaluation of antibiotics and husbandry procedures.
1. Cardeilhac, PT; DK Peters. 1988. Management problems of alligator production. J. Vet. and Human Tox. (In Press).
2. Cardeilhac, PT; BR Collins; F Ruisi; S. Sundogg; R. Larsen; T Lane. 1986. Preliminary studies on the use of oxytetracycline to control hatchling alligator syndrome (HAS). Proc. Int. Assoc. Aquatic An. Med. 1(3): 21-27.
3. Cardeilhac, PT; DK Peters. 1987. Estimation of an effective oral dose of virginiamycin to control hatchling alligator syndrome (HAS). 18th Proc. Int. Assoc. Aquatic An. Med. (In Press).
4. Shotts, EB 1983. Bacterial diseases of alligators: an overview. Proc. 1st Alligator Production Conference, University of Florida pp 36-41.