An Epidemiologic Survey to Determine Factors Associated with Corneal and Lenticular Lesions in Captive Harbor Seals and California Sea Lions
J. Lawrence Dunn1; Neal A. Overstrom1; David J. St.
Alibis1; Donald A. Abt2
A number of authors have speculated on the cause(s) of the higher
prevalence of ocular disease in some populations of captive pinnipeds as compared with wild
conspecifics. Rigorous research to verify these speculations and identify the etiologies of
ocular lesions has not been conducted. Despite this lack of directed studies, most authors
commenting on these conditions have suggested that ocular disorders in captive pinnipeds are
environmentally induced. Salinity, water quality, light intensity, nutrition, and spatial
characteristics have all been suggested to influence the ocular health of captive pinnipeds, even
though in some cases similar or even harsher conditions may be encountered in the wild without
apparent effect. Traumatic and infectious causes are frequently cited as the origin of ocular
lesions in wild seals and sea lions, and yet similar opportunities for corneal damage or
infection exist for captive specimens.
An epidemiologic questionnaire designed to identify specific risk factors was
created with a view toward improving the ocular health of pinnipeds held in captivity. The
questionnaire was intended to characterize the captive environment (water type and treatment,
holding facilities), husbandry (stocking density, nutrition) and medical care (procedures and
medications) for each of the species studied. The harbor seal, Phoca vitulina and the
California sea lion Zalophus califomianus, were selected because they are maintained in
greater numbers and in more diverse holding facilities than other pinniped species. The survey
yielded more than 8000 data points characterizing the holding facilities and animal management
programs of 20 of the 40 institutions which received the survey. These data define the captive
environment for 107 harbor seals held in 26 separate systems, and 193 California sea lions in 49
systems. Many of the facilities, and sometimes different systems within a facility, represented
distinct sets of environmental conditions making it difficult to combine data into larger sets
for more robust statistical analysis. Subsequent analysis and statistical comparisons focused on
specific variables considered to have greatest importance, based on the clinical and husbandry
experience of the investigators. The prevalence of current ocular conditions for each species was
analyzed in relation to the following variables:
Water
- Fresh versus Salt
Pool
location - indoor versus outdoor versus mixed in/out
Pool
capacity in gallons
Pool
capacity as a function of stocking densities
Water
quality control - turbidity, flocculation, open versus closed systems, rates of replacement,
chlorine disinfection, ozone disinfection, animal stocking densities.
The most commonly reported condition in Phoca vitulum was
cataracts, visible as a clouding of one or both lenses. In Zalophus californianus, corneal
lesions predominated. These were recognizable as one of three separate entities occurring in one
or both corneas: corneal edema which was manifested as a diffuse, often transitory cloudiness;
corneal ulceration or erosion, a discontinuity of the corneal surface sometimes extending deep
into the corneal stroma; and corneal scarring which frequently can be a permanent sequel to
severe ulcerations or the result of major trauma.
While segregation of the data sets along these parameters allowed calculation
of ocular disease prevalence rates, serious concerns arose over the possibility that certain
ocular diseases were underreported. Of particular concern is the completeness of the diagnostic
information, especially as this applies to the full recognition of cataracts. Our concern was
generated by the apparently low rates of ocular disease reported by some institutions in contrast
to the widely held perception among health care workers in the marine mammal community that
cataracts are a common wo and aquarium-wide problem. Because they are frequently examined
outside, the pupils of pinnipeds may be highly constricted. The combination of constricted
pupils, certain light conditions, and the presence of accompanying corneal lesions can make
cataracts very difficult to detect, thereby complicating accurate diagnosis. Some of the
retrospective ocular disease data was found to be suspect to the point where further analysis was
not justified. Existing record systems at many of the respondent institutions apparently are not
designed for the ready and reliable extraction of such retrospective data.
Histopathologic examination of eleven eyes substantiated antemortem diagnoses
of several ocular disorders, but revealed no features which were pathognomonic of the exact
etiology of these disorders.
The epidemiologic analysis of survey responses demonstrated a significant
statistical association (p<.01) between the maintenance of harbor seals in fresh water holding
facilities and an increased risk of cataract development. The reported overall prevalence of
cataracts in this species was 24% with approximately a three times higher prevalence in fresh
water holding facilities than in salt water. No such association was demonstrated for similarly
held California sea lions. No association was demonstrated between fresh water maintenance and
corneal edema in harbor seals whereas a very strong association (p<.00001) exists for
California sea lions maintained in fresh water.
Suspected sampling bias concerns relating to the fresh water-cataract
association in harbor seals were confirmed by eliminating data from one responding institution.
Recalculation following this trial deletion resulted in the loss of the statistically significant
association.
We will continue to attempt to increase our sample size and to improve on the
accuracy of previously submitted data with the hope of garnering sufficient additional data to
refine these and any subsequently developed associations between ocular disease and the captive
environment. The assistance of health care and husbandry personnel at a greater percentage of
facilities holding harbor seals and California sea lions will be required if we are to
successfully complete our task.