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Epidemiology of black rhinoceroses in the United States. Patricia M. Dennis, DVM, PhD, Dipl. ACZM Ohio State University Dept. of Veterinary Preventive Medicine Cleveland Metroparks Zoo Dept. of Conservation and Science. Perceived problems in captivity. Unusual disease syndromes

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epidemiology of black rhinoceroses in the united states

Epidemiology of black rhinoceroses in the United States

Patricia M. Dennis, DVM, PhD, Dipl. ACZM

Ohio State University

Dept. of Veterinary Preventive Medicine

Cleveland Metroparks Zoo

Dept. of Conservation and Science

perceived problems in captivity
Perceived problems in captivity
  • Unusual disease syndromes
    • Hemolytic anemia
    • Leukoencephalomalacia
    • Superficial necrolytic dermatitis
    • Idiopathic hemorrhagic vasculopathy
    • Toxic hepatopathy
  • Skewed natal sex ratio
  • What is the health and reproductive status of the captive population?
  • Is there a skewing of the natal sex ratio?
  • What are the current health problems of the captive population?
the survey
The survey
  • Survey of AZA accredited black rhino holding institutions in the United States
  • 40 of 43 institutions participated (93.0%)
  • 296 of 334 animals included in the survey (88.9%)
  • On-site collection of all medical, husbandry and nutrition data
demographics of survey population
Demographics of survey population
  • Two subspecies
    • Diceros bicornis michaeli 240 animals
    • Diceros bicornis minor 51 animals
  • Wild-born animals 97
  • Captive-born animals 199
  • Living 105
  • Dead 165
reproductive characteristics of survey population
Reproductive characteristics of survey population
  • Of those animals that have died:
  • Wild-born animals
    • 43% (35/81) died without reproducing
    • 22% (8/35) were less than 6 years of age
  • Captive-born animals
    • 73% (61/84) died without reproducing
    • 77% (47/51) were less than 6 years of age
  • Of the entire survey population, 35% (96/270) died without reproducing
  • Of the captive-born animals, 26 were stillbirthsNo difference in the mean age of dams having stillbirths vs. live calves
hemolytic anemia
Hemolytic anemia
  • Cases peaked between 1976-1980
  • Early cases associated with acute hemolysis and death
  • More recent cases either survived or were secondary to other diseases
  • Coincide with vitamin E supplementation and leptospirosis vaccination
idiopathic hemorrhagic vasculopathy syndrome
Idiopathic hemorrhagic vasculopathy syndrome
  • Anemia, swelling of limbs, subcutaneous pooling of blood
  • 20 animals in survey
  • Cases identified prior to 1995
  • Cases occurred in FL, IL, CA, CO, OK
toxic hepatopathy
“Toxic” hepatopathy
  • Hepatocellular cholestasis
  • 16 affected animals identified
  • 5 reported previously had association with creosote exposure
  • 11 cases with no known creosote exposure
ihvs and hepatopathy
IHVS and hepatopathy
  • 6 cases identified with clinical signs and lesions consistent with IHVS and hepatocellular cholestasis
  • Suggest possibility of similar / same pathophysiology of disease?
ectopic mineralization
Ectopic mineralization
  • Inappropriate biomineralization of soft tissues
  • Can occur due to systemic mineral imbalance or with tissue trauma, inflammation or disease
  • Number of cases appear to be increasing over time
  • Was there a management change in the early 1990s that may have influenced the disease syndromes?
  • Is the pathophysiology of IHVS and hepatocellular cholestasis the same?
  • Does systemic mineral imbalance play a role in the pathophysiology of some of these diseases?
skewed natal sex ratio
Skewed Natal Sex Ratio
  • 2001 International Studbook for the African Black Rhinoceros
  • 62 females in U.S. with calf of known gender
  • 34 wild-born dams
    • 133 calves – 78 male / 55 female
  • 28 captive-born dams
    • 64 calves – 34 male / 30 female
wild born vs captive born dams
Wild-born vs captive-born dams
  • Wild-born dams produced twice as many calves as captive-born dams
  • Only 50% of captive-born dams produced more than 1 calf vs 82% for wild-born
  • 32% of captive-born dams produced 3 or more calves vs 50% of wild-born
variables considered
Variables considered
  • Outcome variable = male calf
  • Independent variables:
  • Age of dam (Young, prime breeding age, aging)
  • Time in captivity
  • Subspecies of dam
  • Housing (number of institutions) of dam
  • Calves born on or before 1990, or after
  • Initial stratification based on birth status of dam (wild-born vs captive-born)
  • Separate models for wild-born vs captive
  • No variables significantly associated with outcome variable for captive-born dams
results wild born dams
Results wild-born dams
  • Calves 5x more likely to be male if dam in captivity >12 years
  • Calves 4x more likely to be male if dam in captivity 7-12 years
  • Dams between 12-19 years of age ¼x as likely to have male calf than dams < 12 years of age
why no associations for captive born dams
Why no associations for captive-born dams?
  • Natal sex ratio is balanced
  • More captive-born dams are in the age range favoring female calves
  • Not enough power in the study to detect an imbalance in the sex ratio
  • Why have wild-born dams produced twice as many calves as captive-born dams?
  • What factor(s) of captivity favor male calf production by wild-born dams?
  • As the captive-born dam population ages, will there be a skewing of the natal sex ratio?
methods survival analysis
Methods – Survival Analysis
  • Survey conducted on-site
  • Information collected on individual animals from entrance into captivity until death or time of survey visit
  • Information gathered on:
    • History, preventive medicine, anesthesia
    • Clinical illness, necropsy
statistical analysis
Statistical analysis
  • Cox proportional hazards model
  • Dependent variable = age at time of death or censoring (survey visit date)
  • Time dependent variables created for:
    • Clinical signs of disease
    • Movement to different institution
  • Fixed variables for necropsy findings
  • Time-dependent factors associated with decreased survival
    • Skin lesions, hypercalcemia, dental calculus (3x), jaundice (75x), neurologic signs, IHVS (4x)
  • Significant fixed covariates
    • No offspring, muscle lesions (5x), eastern subspecies, housed at multiple institutions
variables significant only at univariate level



Tail necrosis


Limb swelling


Hepatic lesions

Cardiac lesions

Birth year





Variables significant only at univariate level
limitations of the study
Limitations of the study
  • Information lacking on historic nutrition and husbandry practices
  • Incomplete records for many animals
  • Wide variation in record keeping over time and among institutions
  • Wide variation in diagnostic approach
future research
Future research
  • Identify risk factors associated with dental calculus, hypercalcemia, skin lesions, neurologic signs, jaundice, and IHVS to ameliorate their effects on survival time
  • Identify factors influencing skewed sex ratio in wild-born dams
  • International Rhino Foundation Department of Preventive Medicine

The Ohio State University for funding

  • All of the zoological institutions, primarily the veterinary, husbandry and nutrition staff participating in this study