Practical Approaches for Studying Sea Turtle Health and Disease
389
A critical component of population health is the overall abundance and
age–stage structure of the population. This is information that population ecologists
and conservation biologists need to determine whether there is adequate recruitment
to the population and whether the population is stable, increasing, or declining.
The population sampling methods and life history models that are needed for
population assessment are beyond the focus of this chapter. Suffice it to say,
however, that individual health and health risk assessments must be integrated into
these studies to evaluate the true impact of disease on populations. The marine
environment and life history of sea turtles make population assessment especially
complex and difficult to monitor. Loss of individuals from the population may not
be appreciated until there is sufficient decline to affect sample estimates. Increased
mortality may be seen as increased numbers of stranded turtles, but one can only
speculate on the true impact on the population unless monitoring can be performed
in relatively confined areas.
15.2.3 C APTIVE VS . F REE -R ANGING T URTLES
The range of health problems that will be encountered in captive animals can differ
greatly from those encountered in free-ranging animals. The clinical manifestations,
magnitude, and severity of any particular health problem may also vary markedly
between captive and wild animals. Both situations, however, have a role in turtle
health and disease studies.
Compared to the free-ranging condition, captivity presents relatively confined
living space and artificially high animal densities that, even with the best husbandry
programs, will enhance the transmission of contagious infectious agents, in a densitydependent process. The confined living quarters can accumulate high levels of
environmentally persistent parasites and pathogens as well. Confinement and crowding also contribute to stress, which can alter a turtle’s resistance to disease. Captivity
may also bring together animals from different parts of the world or species that
may never come together in the wild. Where the animal husbandry program is
suboptimal, poor nutrition, poor water quality, and poor sanitation and infection
control procedures multiply the risks of transmission and disease.
Disease in all animals can exist in a subclinical state. That is, although an animal
might appear to be healthy, a significant problem may be ongoing internally. Sea
turtles with chronic illness that would probably die in the wild may live for extended
periods in captivity. Thus, captivity provides a favorable environment for subclinical
diseases (undetected in apparently healthy animals) to manifest themselves clinically
(sick animals), for latent infections to recrudesce, and for otherwise innocuous
opportunistic agents to cause disease. It is not surprising that many of the known
sea turtle diseases and infectious agents were first observed and in some cases only
observed in outbreaks among captive animals (Herbst and Jacobson, 1995). Examples include gray-patch disease (Rebell et al., 1975), lung–eye–trachea (LET) disease
(Jacobson et al., 1986), and chlamydiosis (Homer et al., 1994).
Although the unnatural conditions of captivity can result in disease syndromes
that are unlikely to be seen in the wild (e.g., growth anomalies resulting from
imbalanced nutrition [George, 1997]) and therefore of limited interest to students
1123 book.book Page 389 Monday, November 11, 2002 11:11 AM
389
A critical component of population health is the overall abundance and
age–stage structure of the population. This is information that population ecologists
and conservation biologists need to determine whether there is adequate recruitment
to the population and whether the population is stable, increasing, or declining.
The population sampling methods and life history models that are needed for
population assessment are beyond the focus of this chapter. Suffice it to say,
however, that individual health and health risk assessments must be integrated into
these studies to evaluate the true impact of disease on populations. The marine
environment and life history of sea turtles make population assessment especially
complex and difficult to monitor. Loss of individuals from the population may not
be appreciated until there is sufficient decline to affect sample estimates. Increased
mortality may be seen as increased numbers of stranded turtles, but one can only
speculate on the true impact on the population unless monitoring can be performed
in relatively confined areas.
15.2.3 C APTIVE VS . F REE -R ANGING T URTLES
The range of health problems that will be encountered in captive animals can differ
greatly from those encountered in free-ranging animals. The clinical manifestations,
magnitude, and severity of any particular health problem may also vary markedly
between captive and wild animals. Both situations, however, have a role in turtle
health and disease studies.
Compared to the free-ranging condition, captivity presents relatively confined
living space and artificially high animal densities that, even with the best husbandry
programs, will enhance the transmission of contagious infectious agents, in a densitydependent process. The confined living quarters can accumulate high levels of
environmentally persistent parasites and pathogens as well. Confinement and crowding also contribute to stress, which can alter a turtle’s resistance to disease. Captivity
may also bring together animals from different parts of the world or species that
may never come together in the wild. Where the animal husbandry program is
suboptimal, poor nutrition, poor water quality, and poor sanitation and infection
control procedures multiply the risks of transmission and disease.
Disease in all animals can exist in a subclinical state. That is, although an animal
might appear to be healthy, a significant problem may be ongoing internally. Sea
turtles with chronic illness that would probably die in the wild may live for extended
periods in captivity. Thus, captivity provides a favorable environment for subclinical
diseases (undetected in apparently healthy animals) to manifest themselves clinically
(sick animals), for latent infections to recrudesce, and for otherwise innocuous
opportunistic agents to cause disease. It is not surprising that many of the known
sea turtle diseases and infectious agents were first observed and in some cases only
observed in outbreaks among captive animals (Herbst and Jacobson, 1995). Examples include gray-patch disease (Rebell et al., 1975), lung–eye–trachea (LET) disease
(Jacobson et al., 1986), and chlamydiosis (Homer et al., 1994).
Although the unnatural conditions of captivity can result in disease syndromes
that are unlikely to be seen in the wild (e.g., growth anomalies resulting from
imbalanced nutrition [George, 1997]) and therefore of limited interest to students
1123 book.book Page 389 Monday, November 11, 2002 11:11 AM