MAY 20, 2015
A
northern Gulf of Mexico (GoM) cetacean unusual mortality event (UME) involving
primarily bottlenose dolphins (Tursiops truncatus) in Louisiana, Mississippi,
and Alabama began in February 2010 and continued into 2014.
Overlapping in time
and space with this UME was the Deepwater Horizon (DWH) oil spill, which was
proposed as a contributing cause of adrenal disease, lung disease, and poor
health in live dolphins examined during 2011 in Barataria Bay, Louisiana.
To
assess potential contributing factors and causes of deaths for stranded UME
dolphins from June 2010 through December 2012, lung and adrenal gland tissues
were histologically evaluated from 46 fresh dead non-perinatal carcasses that
stranded in Louisiana (including 22 from Barataria Bay), Mississippi, and
Alabama.
UME dolphins were tested for evidence of biotoxicosis, morbillivirus
infection, and brucellosis. Results were compared to up to 106 fresh dead
stranded dolphins from outside the UME area or prior to the DWH spill.
UME
dolphins were more likely to have primary bacterial pneumonia (22% compared to
2% in non-UME dolphins, P = .003) and thin adrenal cortices (33% compared to 7%
in non-UME dolphins, P = .003). In 70% of UME dolphins with primary bacterial
pneumonia, the condition either caused or contributed significantly to death.
Brucellosis and morbillivirus infections were detected in 7% and 11% of UME
dolphins, respectively, and biotoxin levels were low or below the detection
limit, indicating that these were not primary causes of the current UME. The
rare, life-threatening, and chronic adrenal gland and lung diseases identified
in stranded UME dolphins are consistent with exposure to petroleum compounds as
seen in other mammals.
Exposure of dolphins to elevated petroleum compounds
present in coastal GoM waters during and after the DWH oil spill is proposed as
a cause of adrenal and lung disease and as a contributor to increased dolphin
deaths.
Introduction
A large, multi-year
cetacean unusual mortality event (UME) has been ongoing in the northern Gulf of
Mexico (GoM) since February 2010, continuing into 2014 [1].
This event has involved predominantly (87%) common bottlenose dolphins (Tursiops
truncatus) (hereafter referred to as ‘dolphins’) stranded in Louisiana,
Mississippi, and Alabama [2].
The UME coincided with the Deepwater Horizon (DWH) oil spill, the largest
marine-based spill in U.S. history [3].
During and following the DWH oil spill, significantly elevated polycyclic
aromatic hydrocarbon (PAH) levels attributed to this spill were detected in
coastal GoM waters, including Louisiana, Mississippi, and Alabama [4].
These locations coincided with the states most impacted by the ongoing UME
since the DWH oil spill [2].
Dolphin strandings, however, were elevated during March and April before the
spill, necessitating an investigative approach including numerous potential
causes [1,2,5].
Combined oil exposure, an unusually cold winter during 2011, and fresh water
infusions have been proposed as potential causes contributing to this UME [6].
Barataria Bay, Louisiana
was one of the heaviest oiled coastal areas from the DWH oil spill, including
visualized oiling from the spill encompassing 40 km and 366,000 m2 of Barataria
Bay’s shoreline lasting in decreasing amounts for at least 2 years [7–10].
The presence of increased coastal PAH levels associated with the DWH oil spill,
especially near Grand Isle, Louisiana in Barataria Bay have been confirmed [4].
Further, within the time period of January 2010 to June 2013, the longest
lasting cluster of dolphin strandings throughout the northern GoM was in
Barataria Bay (August 2010 through 2011) [2].
During the DWH oil spill and response period, numerous dolphins, including
dolphins in Barataria Bay, were observed swimming through visibly oiled waters
and feeding in areas of surface, subsurface, and sediment oiling [11].
Due to the extensive
oiling in Barataria Bay, health assessments were conducted on live dolphins in
this area during the summer of 2011 [11].
Barataria Bay dolphins had a high prevalence of moderate to severe lung disease
and blood value changes indicative of hypoadrenocorticism; specific blood
changes included low serum cortisol, aldosterone, and glucose, and high
neutrophil counts [11].
Nearly half (48%) of Barataria Bay dolphins were given a guarded to grave
prognosis for long-term survival [11].
The DWH oil spill was proposed as a contributor to adrenal gland and lung
disease in live Barataria Bay dolphins.
Previous to the ongoing
event, there have been ten dolphin GoM UMEs since 1991, as well as one large
die-off during 1990 that occurred before the UME declaration process [1,
12–15].
The majority (82%) of previous dolphin GoM events had brevetoxicosis or
morbillivirus as confirmed or suspected causes [1].
While brevetoxicosis events do not leave a histologic signature in affected
dolphins, brevetoxicosis-related events are often associated with known algal
blooms and deaths that appear to be acute in otherwise healthy-looking dolphins
[15].
In prior events classified as brevetoxicosis-related, 50% or more sampled
dolphins were positive for brevetoxin with most at high concentrations [15].
Similarly, past UMEs that have been attributed to morbillivirus involved
successful detection of morbillivirus in greater than 60% of dolphins tested. [13,16].
There is evidence that Brucella, which is commonly found in marine mammals
worldwide, can cause disease in cetaceans, including bottlenose dolphins [17–21].
As such, there was a need to evaluate all of these potentially important
diseases as playing contributing or leading roles in the ongoing UME.
To assess contributing
factors and causes of deaths for stranded UME dolphins following the DWH oil
spill, tissues were histologically evaluated from 46 carcasses that stranded in
Louisiana, Mississippi, and Alabama, including 22 from Barataria Bay, from June
2010 through December 2012.
Perinatal dolphins, stranded dolphins that were
less than 115 cm in body length that likely died during late-term pregnancy or
shortly after birth, were excluded from this study. On the basis of the live
dolphin health assessment findings from Barataria Bay, this study included a
focused evaluation of adrenal, lung, and liver lesions with the expectation
that if stranded dolphins had been impacted by the DWH oil, they would have
lesions consistent with the clinical evidence indicating lung disease and
hypoadrenocorticism found in live dolphins.
Other potential causes of and
contributors to dolphin deaths were investigated, including the presence of
histologic lesions and diagnostic test results consistent with brevetoxicosis,
morbillivirus infections, and brucellosis. Results were compared to a reference
group of fresh dead dolphins from North Carolina, South Carolina, Texas, and
the Gulf coast of Florida that stranded prior to or remote from the UME and DWH
oil spill timeframes and geographic location.
Discussion
To our knowledge, adrenal
cortical atrophy as found in this study has not been previously described in
free-ranging cetaceans, including bottlenose dolphins previously studied in the
northern GoM [32].
The normal corticomedullary ratio of dolphin adrenal glands has been determined
to be approximately 1:1 [24].
Thus, the discovered high prevalence of adrenal cortical atrophy in dolphins
stranding during the ongoing GoM UME may be part of a syndrome that has not
been previously reported in dolphins during mortality events.
The prevalence of
adrenal cortical atrophy identified in this study is consistent with the high
prevalence (approximately 50%) of live Barataria Bay dolphins with evidence of
hypoadrenocorticism assessed during 2011, including a relatively high
proportion of dolphins with low blood cortisol, aldosterone, and glucose [11].
Follow up evaluation of adrenal glands from stranded dolphins in subsequent
years will help to determine the persistence of adrenal insufficiency observed
relative to the timing of the UME and the concurrent DWH oil spill.
There are a number of
different causes of adrenal insufficiency in mammals, including autoimmune
disease, metastatic neoplasia, fungal infections, stress, trauma, miliary
tuberculosis, corticosteroid toxicity, and contaminant exposure [33].
Additionally, infection with phocine herpesvirus-1 has been demonstrated to
cause adrenal cortical necrosis in marine mammals [34].
In the current study, only 2 of 46 UME dolphins had inflammation in the adrenal
gland, and with the exception of one case with a disseminated bacterial
infection, neither infectious agents nor neoplasia were identified in UME
dolphin adrenal glands. Further, there was no histologic evidence of autoimmune
adrenalitis or neoplasia in any UME dolphin adrenal glands, indicating that
adrenal cortical atrophy in UME dolphins was not due to direct infection of the
adrenal gland, autoimmune disease, or neoplasia.
In humans, chronic demand
on the adrenal glands, including chronic illness, has been postulated to lead
to cortical thinning and potential adrenal exhaustion associated with lipid
depletion of the fasciculata cells [35,36].
Previous evaluations of adrenal glands from stranded GoM dolphins from Texas
with both acute and chronic disease have been conducted, but no cases of
adrenal cortical atrophy were identified [32].
Instead, adrenal glands of dolphins dying from chronic disease (likely chronically
stressed individuals) were significantly heavier, and corticomedullary ratios
were significantly higher than those dying from acute disease or acute trauma.
Findings from Clark et al. (2006) suggest that adrenal gland enlargement and
cortical hyperplasia are common responses to chronic stress and disease in
bottlenose dolphins, similar to that noted in other cetaceans and other
mammalian species [24,
32,
37–39].
Of the 12 UME dolphins with a thin adrenal gland cortex, one-third had primary
bacterial pneumonia, leaving the majority of adrenal cortex cases without
evidence of active or chronic infections. Further, none of the UME dolphins
with a thin adrenal gland cortex had depleted cardiac adipose tissue,
indicating that UME dolphins were not in an advanced, debilitated nutritional
state. These results do not support general infection or chronic poor body
condition as underlying causes of adrenal gland cortex depletion.
Although the effects of
polycyclic aromatic hydrocarbon (PAHs) on the hypothalamus-pituitary-adrenal
(HPA) axis are poorly understood, the adrenal gland is reported to be the most
common endocrine organ to exhibit lesions with exposure to toxigenic chemicals
[40,
41].
In general, mechanisms of direct adrenal toxicity include impaired
steroidogenesis, activation of toxins by cytochrome p450 enzymes generating
reactive oxygen metabolites, DNA damage, and exogenous steroid action [42].
The adrenal gland can be a significant site for metabolism of PAHs, thus increasing
the adrenal gland to exposure from these contaminants and their metabolites [43].
Several studies have
shown that PAHs or oil can affect the HPA axis and adrenal gland function.
Hypoadrenocorticism has been reported in mink fed either bunker C or
artificially weathered fuel oil [44,45].
In these mink, adrenal cortical hypertrophy with vacuolation of corticocytes
was detected histologically.
These studies, however, did not monitor changes in
response to higher level exposure and/or over longer periods of time. Chemicals
that induce adrenal cortical vacuolar degeneration can lead to loss of
adrenocortical cells due to necrosis and adrenal cortical atrophy.
It is possible
PAHs may act in a similar fashion [42].
Naphthalene, a common PAH associated with crude oil, reduced plasma
corticosterone in mallard ducks following ingestion of petroleum-contaminated
food, and a similar acute decrease in cortisol was detected in exposed eels [46,
47].
House sparrows exposed orally to 1% crude oil from the GoM exhibited decreases
in cortisol in response to stressors or to adrenocorticotropin hormone injection
[48].
Mammalian exposure to
PAHs can greatly increase hepatic metabolism of other compounds (e.g.
7,12-dimethylbenz(α)anthracene), which in turn can cause targeted and severe
injury to the adrenal gland, including necrosis and hemorrhage [49–51].
Removal of the inciting chemical, if the adrenal cortical injury is not too
advanced, may result in regained function and resolved lesions characterized by
fibrosis, atrophy, nodular regeneration or calcification [42,
47,
51].
Ultrastructural analysis can be beneficial in helping to identify direct toxic
damage. Unfortunately, optimally fixed, minimally autolyzed tissue from
affected dolphin adrenal glands was not available for ultrastructural analysis.
The lack of adrenal lesions beyond cortical atrophy suggests, however, that
potential chemical effects may be higher in the HPA axis [52].
During and following the
DWH oil spill, significantly elevated PAH levels were detected in the coastal
GoM waters, including Louisiana, Mississippi, and Alabama [53].
These locations coincide with the states most impacted by the ongoing UME since
the DWH oil spill [1].
Thus, northern GoM dolphins’ exposures to DWH spill-associated PAHs, especially
in Louisiana and Mississippi, may account for the observed effects on adrenal
function found in both live and dead dolphins [11].
Given the lack of evidence of alternative causes of adrenal cortical atrophy
and the high prevalence of this lesion among stranded dolphins following the
DWH oil spill, the leading hypothesis is that exposure to contaminants from the
DWH oil spill led to chronic injury of the adrenal gland cortex at least through
2012.
Chronic adrenal
insufficiency (CAI) is a life-threatening disease that can lead to adrenal
crisis and death in mammals [53].
Adrenal crises in people with CAI are triggered by infections, fever, major
pain, psychological distress, heat, and pregnancy [54].
Cold temperatures can also increase the risk of death among animals with CAI.
Angora goats with a genetically-driven high incidence of primary CAI lack
proper cortisol and glucose response and, as such, are susceptible to die-offs
from cold stress [54,
55].
GoM dolphins were exposed to colder than normal temperatures during early 2011,
and if those dolphins from the UME had pre-existing CAI, they may have been at
higher risk of cold stress-related deaths [6].
This hypothesis is further supported in that dolphins have a compensatory
adrenal response in cold temperatures, including increased cortisol levels,
presumably to help generate metabolic heat [56].
Adrenal crisis may have been the cause of death for many of the UME dolphins
with adrenal cortical atrophy following stress events to which a healthy
dolphin could have otherwise adapted. In addition to the cold weather during
2011, adrenal crisis could have also been precipitated by late-term pregnancies
and infections, including bacterial pneumonia [57].
Compared to reference
dolphins, UME dolphins were more likely to have a primary bacterial pneumonia.
Many of these pneumonias were much more severe than bacterial pneumonias in the
reference dolphins. These findings are consistent with the high prevalence of
moderate to severe lung disease detected in live Barataria Bay dolphins [11].
During the DWH oil spill and response period, numerous dolphins, including
dolphins in Barataria Bay, were observed swimming through visibly oiled waters
and feeding in areas of surface, subsurface, and sediment oiling [11].
As mentioned, the presence of increased coastal PAH levels associated with the
DWH oil spill, especially near Grand Isle, Louisiana in Barataria Bay, have
been confirmed, indicating an increased risk of inhaled PAHs in dolphins [4].
Given that the dolphin's blowhole is at the surface of the water, chemicals,
including volatile PAHs, could have been readily inhaled. In other animals,
inhaled PAHs can irritate airways, denude mucosal surfaces, and cause
peribronchial inflammation and systemic toxicity [58].
Damaged epithelium and cilia, in turn, can severely impair immune defenses.
In other animals, the
severity of chemical inhalation injury is dependent on breathing patterns, in
which deep breaths increase injury to tissues deeper within the lung [59].
This pattern is of particular importance given the dolphin's respiratory
anatomy and physiology.
While humans exchange approximately 10 to 20% of air
with each breath, dolphins exchange 75 to 90% of deep lung air [60–63].
They also lack nasal turbinates and cilia to filter the air prior to reaching
the lungs, and have deep inhalations followed by a breath hold that provides
potential for more prolonged contact and exchange between air-borne
particulates and the blood [60–63].
All of these factors would likely amplify the effects of inhaled chemical
irritants in dolphins compared to observations and studies involving other
mammals.
The severe bacterial
pneumonias found in UME dolphins could represent a chronic sequelae to
hydrocarbon inhalation or aspiration, or have been secondary to PAH induced
immune system compromise.
The most common sequela to hydrocarbon inhalation and
ingestion in humans and animals are aspiration pneumonia and pneumonia often
involving the bronchioles [64–68].
Inhaled hydrocarbon vapors or aspirated hydrocarbons may cause necrosis of
bronchial and bronchiolar epithelium, and pneumocyte and alveolar septal
necrosis which leads to inflammation and secondary infection [64–66].
During the 2007 firestorm in San Diego, dolphins and people living in San Diego
Bay area were exposed to high levels of PAHs [69,
70].
The month following the fires, these dolphins demonstrated decreased absolute
and percent neutrophils [70].
This change indicated that dolphins exposed to PAHs through inhalation may have
had a compromised immune response and an increased risk of acquiring bacterial
pneumonia.
In addition to
inhalation risks, hydrocarbon ingestion can lead to gastrointestinal mucosal
irritation, vomiting or regurgitation, and resultant aspiration pneumonia.
Cattle that ingest petroleum develop bacterial pneumonia due to
chemical-induced regurgitation and/or aspiration [67,68].
Correspondingly, based on histologic examination, one dolphin that stranded
during June 2010 in Mississippi during the DWH oil spill had suspected
aspiration pneumonia, secondary bacterial infection, ulcerative tracheitis, and
ulcerative gastritis with edema. Both the tracheal and gastric lesions,
although non-specific, could have resulted from mucosal irritation, such as may
occur with toxin ingestion.
Though there was no
difference in prevalence of liver lesions when comparing UME and reference
dolphins in this study, two UME dolphins had similar severe centrilobular liver
lesions characterized by hepatocyte loss, necrosis and vacuolation that could
potentially be associated with toxin exposure.
Both of these dolphins stranded
in Barataria Bay. Hepatocellular vacuolation, degeneration and necrosis have
been associated with exposure to crude oil and benzo[a]pyrene (BaP) [71
72].
Hepatotoxic liver injury may occur due to xenobiotic metabolism of substances
producing injurious metabolites and lesions most often occur in the centrilobular
zones where hepatocytes have the highest concentration of cytochrome p450
enzymes [71].
Other rule-outs for centrilobular degeneration and necrosis include hypoxia,
severe or precipitous anemia (e.g. hemolytic anemia), chronic heart disease, or
circulatory failure associated with septic shock [72,
73].
There was no other evidence of hypoxia, hemolytic anemia or heart disease in
either of the affected dolphins and lesions were more chronic than would be
expected secondary to acute hypoxia or shock. Some oiled sea otters that died
following the Exxon Valdez oil spill had centrilobular hepatic necrosis, though
whether the lesions were due to direct toxic insult or secondary to anemia is
unclear [74].
Biliary or periportal inflammation and fibrosis secondary to infection by the
trematode Campula spp. are common hepatic lesions noted in a number of cetacean
species, and periportal lesions noted in both UME and reference dolphins were
consistent with the chronic sequelae of biliary trematode infection [75].
This study did not
support that previously documented or suspected contributing factors for GoM
dolphin UMEs were primary contributors to the ongoing UME among non-perinatal
dolphins. All UME dolphins in this case study had biotoxin levels that were
below detectable levels except for one with low levels [12].
Relatively few morbillivirus cases were identified among UME cases. In previous
known dolphin morbillivirus-associated die-offs, more than 60% of cases tested
positive for the virus when using a similar PCR assay [14,16].
Exposure to morbillivirus has been documented in GoM dolphins, and the cases
identified in 2011 and 2012 may represent exposure to the virus in a small
number of susceptible individuals in the population [76].
Similarly, there were too few brucellosis cases in this study to explain the
ongoing UME, with only two cases that had Brucella identified in the lung,
demonstrating that Brucella was not the driver for increased primary bacterial
pneumonia. Despite global reports of Brucella infections in marine mammals,
there have been, to date, no documented brucellosis epizootics in cetaceans [17].
Due to the long duration
and large scope of the ongoing UME, there may be multiple factors affecting the
health of dolphins by region through time. Aside from the DWH oil spill, there
were two relatively smaller oil spills that occurred in and around Barataria
Bay during this study’s timeframe. Specifically, the T/V Pere Ana C spill in
Mud Lake, Louisiana on July 27, 2010 (approximately 7,000 gallons spilled) and
the Cedyco Manilla Village Spill in Bayou Dupont, Louisiana which occurred on
September 11, 2011 (approximately 10,500 gallons) [77,78].
In comparison, however, the DHW oil spill released approximately 126,000,000
gallons and was visible across 40 km and 366,000 m2 of Barataria Bay’s
shoreline in decreasing amounts over time for at least 2 years, demonstrating
the higher magnitude of the DWH oil spill’s likely impact compared to other
spills [7–10].
The Gulf of Mexico has
historically had documented dead zones with episodes of seasonal hypoxia
associated with nutrient loading from the Mississippi River watershed [79,80].
The geographic area and clusters of dolphin stranding identified from the
current UME, however, were not limited to single seasons or specific dead zone
hotspots [2].
Further, dead zones are often associated with fish die-offs and habitat loss;
the lack of emaciation as the primary contributor to the deaths of dolphins in
this study supports that the primary driver of this UME was not loss of prey [80].
The lack of baseline
diagnostic and histologic data on fresh stranded dolphins prior to 2010 in the
UME area, as well as during the pre-DWH oil spill period, paired with an
assumption that stranded dolphins should have similar lesion prevalence
regardless of location, are limitations in this study.
The surprisingly high
number of assessed lesions that were not significantly different between the
UME and reference dolphins, however, increased the confidence that the study
groups were indeed comparable. Continual assessment of trends and changing
disease states over time are needed, however, to better understand the
potential roles of multiple contributing factors to dolphin mortality during
the ongoing UME.
In summary, UME dolphins
had a high prevalence of thin adrenal gland cortices (especially in Barataria
Bay dolphins) and primary bacterial pneumonia. These findings are consistent
with endocrinologic and pulmonary-based observations of live bottlenose
dolphins from health assessments in Barataria Bay during 2011 [11].
Previously documented or suspected contributing factors for GoM UMEs (marine
biotoxins, morbillivirus, and brucellosis) were not supported by this study as
contributors to the ongoing UME. Due to the timing and nature of the detected
lesions, we hypothesize that contaminants from the DWH oil spill contributed to
the high numbers of dolphin deaths within this oil spill’s footprint during the
northern GoM UME following the DWH oil spill.
Direct causes of death likely
included: 1) affected adrenal gland cortices, causing chronic adrenal
insufficiency, 2) increased susceptibility to life-threatening adrenal crises,
especially when challenged with pregnancy, cold temperatures, and infections,
and 3) increased susceptibility to primary bacterial pneumonia, possibly due to
inhalation injury, aspiration of oil, or perturbations in immune function.
Source: http://journals.plos.org