Lecture 28 Flashcards
The liver is
A key organ for nutrient homeostasis
1o detoxication and toxication centre
Downstream of GI tract
Passive entry of toxicants into hepatocytes
The functions of the liver include nutrient homeostasis such glucose and lipid
metabolism as well as storage of glycogen minerals and vitamins, synthesis of
proteins and blood coagulation factors, excretion of waste products of
metabolism e.g., ammonia and hemoglobin breakdown products and bile
formation and secretion
The liver is also the primary detoxication and toxication center and therefore it
plays a major role in all toxicoses.
Because the liver is downstream of the GI tract, all toxicants/toxins absorbed
following oral exposure are channeled to the liver before any significant
biotransformation has occurred
Secondly, the entry of most toxicant into hepatocytes is passive and does not
require specialized transport systems. This means that most toxicants within the
blood circulation can gain access to hepatocytes
Lastly there is the process of enterohepatic recirculation in which toxicants
absorbed in the GI tract are transported via the hepatic portal vein to liver and
then excreted and taken back to the GI tract. The toxicant can then cycle
between the 2 systems repeatedly thus slowing clearance and facilitating
hepatocyte re-exposure
Overall, although the liver usually protects the individual against injury from
xenobiotics, it is the main site of metabolism where some chemicals concentrate
and bioactivated, leading to hepatic injury.
Enterohepatic recirculation
slows toxicant clearance
and facilitates hepatocyte
re-exposure
Mechanisms of hepatotoxicity
Mechanisms of toxicant-induced liver injury
are either intrinsic or idiosyncratic
Intrinsic injury
◦ A predictable, reproducible, and dose-dependent
response to a xenobiotic
◦ Accounts for majority of toxic liver injuries
Idiosyncratic injury
◦ An unpredictable response to a xenobiotic
◦ Rare and not dose-dependent. Can be associated
with extrahepatic lesions
Fatty degeneration (steatosis)
↑Fat in hepatocytes
Hepatic steatosis/lipidosis or fatty liver in which there is increase the
accumulation of fat vacuoles within hepatocytes. In severe cases the vacuoles
fill the cytoplasm of hepatocytes. It is due to an imbalance in uptake/supply and
secretion/utilization of fatty acids in hepatocytes. Grossly, the affected liver is
swollen with rounded edges, friable, and light brown to yellow. Due to the fat
accumulation, sections of the affected liver will float in formalin
Hepatocyte death
↑ALT, ↑AST
Hepatocyte death: Necrosis is the predominant form of hepatocyte death in
most toxic insults. It is characterized by rupture of cellular membranes and
leakage of cell contents, including cytosolic enzymes such as alanine
transaminase and aspartate aminotransferase. Necrotic liver injury can be focal,
zonal, bridging, or panlobular/massive. Focal necrosis is randomly distributed
and involves hepatocytes individually or in small clusters. Zonal necrosis
usually occurs in the centrilobular area due to a higher concentration of phase I
enzymes in this region. Bridging necrosis manifests as confluent areas of
necrosis extending between zones of the lobule or between lobules.
Panlobular/massive necrosis denotes hepatocyte loss throughout the lobule and
loss of lobular architecture.
Hepatic megalocytosis
↑ hepatocyte size
Megalocytosis is characterized by markedly enlarged hepatocytes due to
impaired cell division. It is caused by toxins that have antimitotic effect (e.g.,
pyrrolizidine alkaloids) on the hepatocytes but do not inhibit DNA synthesis.
Because hepatocytes normally proliferate to replace the damaged cells, DNA and
proteins are synthesis, but the new hepatocyte cannot divide resulting in megalocytosis
Cholestasis
↑bilirubin, ↑bile salts, yellow-green liver
Cholestasis is blockage of bile flow due to damage of the structure and function of bile
canaliculi or from physical obstruction of bile ducts. It characterized by increased
bilirubin and bile salts in blood and icteric or yellow-green liver
Bile duct damage
↑ALP, ↑GGT, ↑bilirubin, ↑bile salts
Bile duct damage results in leakage of enzymes associated with the bile duct and bile.
Therefore, it is characterized by elevated serum alkaline phosphatase, gamma-
glutamyltransferase, bilirubin and bile salts.
Sinusoidal damage
dilation or blockade
Sinusoidal damage can occur following toxicant exposure and manifest as dilation or
blockade of sinusoids with impaired blood flow
Fibrosis
↑fibrous/scar tissue
Fibrosis results from repeated or continuous liver damage e.g., following chronic
toxicant exposure. Hepatocytes are lost and replaced with fibrous (collagen) connective
tissue. Fibrosis usually occurs around the portal area, in the space of Disse, and around
the central veins
Cirrhosis
↑↑↑fibrous/scar tissue; firm liver; loss of function
Hepatic cirrhosis is end-stage liver disease following long term toxicant exposure and
is characterized by excessive collagen deposition (excessive fibrosis) which disrupts
hepatic architecture. The liver is firm and difficult to cut with a knife. Serum
transaminase concentrations are low due to the lack of functional hepatocytes. Bile acids
and ammonia are markedly elevated due to loss of hepatic function.
Neoplasia
Tumors of hepatocytes, bile ducts, sinusoid cells
Hepatic Neoplasia: Toxicant induced neoplasms can originate from hepatocytes, biliary
epithelium, and very rarely from sinusoidal endothelium. Neoplasms occur months or
years after toxicant exposure.
Toxicant-induced liver failure can be acute, subacute, or chronic.
Hypoproteinemia reduces the blood oncotic pressure resulting in fluid loss from
intravascular compartment. The fluid accumulates in tissues causing edema or in
body cavities such as the abdomen and thorax.
Acute liver failure: Abdominal pain, liver
enlargement, vomiting, hypovolemic shock,
hypoglycemia, icterus, 2o hepatoencephalopathy
Subacute liver failure: Intermittent GI upset,
reduced appetite, poor condition, icterus, possibly
liver pain and enlargement
Chronic liver failure: Recurrent GI upset,
chronic weight loss, hypoproteinemia, shrunken
liver, cirrhosis, icterus is variably present, 2o
photosensitization
→fluid loss from blood vessels and its accumulation in tissues & body cavities
Pyrrolizidine Alkaloids (PAs)
(Seneciosis, Hepatic Cirrhosis)
Sources: more than 6000 plants in the families
Boraginaceae, Compositae (Senecio) and
Leguminosae contain PAs. >350 PA alkaloids, half are toxic
These plants are found throughout the world
Exposure: Contaminated feed, young plants
indistinguishable from grasses and when
favorable forage is not available
PAs are the most common plant toxins
affecting livestock
Susceptibility to PAs
Influenced by species, age, sex, nutrition, and
biochemical and physiological factors
Pigs are the most sensitive followed by cattle and
horses
Sheep and goats are resistant
◦ Used to graze pastures that are unsafe for cattle and
horses
Differences in species susceptibility likely due to
◦ Species-specific differences in enzymatic activation of PAs
◦ Species-specific differences in rumen metabolism
Young animals are more sensitive than adults while males are more sensitive
than females. Animals in poor plane of nutrition are more sensitive than those in
good plane of nutrition. Lastly, experiencing physiological stresses such as
pregnancy and lactation or pathological stresses are more sensitive than those
not experiencing the stresses.
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ADME
PAs are bioactivated by mixed function oxidase (MFO) in the liver to toxic __________ alkaloids (______). MFO inducers and inhibitors affect toxicity. Detoxification also occurs in the liver e.g. by binding to ____ and by ______/______. Nontoxic metabolites are _______ while the ______ metabolites damage the liver. MFO inducers ______ while MFO inhibitors ______ the toxicity of pyrrolizidine alkaloids.
PAs are bioactivated by mixed function
oxidase (MFO) in the liver to toxic
dehydropyrrolizidine alkaloids (pyrroles)
◦ MFO inducers and inhibitors affect toxicity
Detoxication also occurs in the liver e.g. by
binding to GSH and by hydrolysis/oxidation
Nontoxic metabolites are excreted while
the toxic metabolites damage the liver
MFO inducers increase while MFO inhibitors reduce the toxicity of
pyrrolizidine alkaloids
Mechanism of Toxicity of Pyrroles
Pyrroles are potent _________ and powerful _______ agents. They ____-____ double-stranded DNA, ______, _____ acids –> ______ and ______ effects. ______ formation –> megalocytes die → replacement by _______ tissue –> liver ______
Pyrroles are potent electrophiles and
powerful alkylating agents
They cross-link double-stranded DNA,
proteins, amino acids –> antimitotic and
cytotoxic effects
◦ Megalocyte formation –> megalocytes die –>
replacement by fibrous tissue –> liver failure
Clinical Signs
Acute toxicosis: acute liver failure
◦ Anorexia, depression, icterus, diarrhea, rectal
prolapse, visceral edema/ascites
◦ Horses display “head pressing” or walking in straight lines regardless of obstacles in their path
Due to elevated blood ammonia from reduced liver function
Chronic toxicosis: photosensitivity, icterus,
and increased susceptibility to other liver
diseases, e.g., lipidosis and ketosis
◦ Affected animals are “hepatic cripples”
Easily develop liver failure
Clinical Pathology
In acute toxicosis there are:
◦ Marked elevations of aspartate
aminotransferase (AST), gamma-glutamyl
transferase (GGT), alkaline phosphatase (ALP)
and sorbitol dehydrogenase (SDH)
◦ Increased amounts of bilirubin and bile acids
In chronic toxicosis there are:
◦ Transient elevations of AST, GGT, ALP and SDH
◦ Mild elevations of serum bilirubin and bile acids
histological lesions caused by pyrrolizidine alkaloids in the liver.
The most characteristic effect of pyrroles is the induction of megalocytosis in
which there is nuclear and cytoplasmic gigantism.
This effect results continued synthesis of cellular components (DNA, proteins,
and other macromolecules) as hepatocytes attempt to replace those that have
undergone necrosis without cellular division due to the antimitotic effect of
pyrroles.
Continued nucleoprotein synthesis, coupled with mitotic inhibition, accounts for
the great increase in size of the nucleus and cytoplasm.
The volume of megalocytic cells can range up to 20 times that of normal
hepatocytes.
Note: Megalocytosis is not pathognomonic for pyrrolizidine alkaloid toxicosis
because other alkylating agents such as nitrosamine and aflatoxins can cause
megalocytosis.
Concurrent with the megalocytosis, there is bile duct hyperplasia and fibrosis.
Generally, the fibrosis is minimal in sheep, moderate in horses and marked in
cattle.
Dx
History of exposure to plants
Compatible clinical signs, biochemical changes, and
gross and histological lesions
Detection of PA in suspected plants and liver of
exposed animals
Tx
Usually not successful
Remove animals from plants source
Give diets high in carbohydrates and low in protein
Treat dehydration and photosensitization
Lantana spp. (Largeleaf lantana,
yellow/red sage, white brush etc)
Lantana spp. (Largeleaf lantana,
yellow/red sage, white brush etc)
Perennial shrub, 3-6 ft
Popular ornamentals in FL,
southern warmer US
Toxic principles:
Triterpene acids
◦ Lantadene, icterogenin,
dihydrolantadene
Photosensitizer
GI tract irritants
ADME
Lantadenes undergo slow absorption in
small intestine, stomach and rumen
Biotransformation occurs in the liver with
secretion into bile
Within the liver they damage bile
canalicular epithelium and hepatocytes
Mechanisms of Toxicity/Effects
Obstructive cholangitis and hepatotoxicity
◦ Reduction in canaliculi ATPase activity
◦ Collapse/blockage of bile canaliculi
◦ Loss of secretory function in hepatocytes
Hepatogenic photosensitization
GI tract irritation and cytotoxicity
Clinical Signs
Acute toxicosis
◦ Depression, anorexia, transient diarrhea,
decreased GI tract motility and constipation
◦ Prominent jaundice
◦ Photosensitization in 1-2 days of toxic exposure
◦ Sluggishness and weakness, death in 2-4 days
Chronic Toxicosis
Chronic poisoning is more common and
manifests mainly as photosensitization
◦ Lesions in muzzle, mouth and nostrils
◦ Swelling, hardening and peeling of nostril mucous
membrane
◦ Ulceration of cheeks, tongue and gums
◦ Invasion of photosensitized areas by blowfly
maggots and bacteria
Weight loss and death in 1-3 weeks
Dx
Identification of Lantana and evidence of their
consumption
Clinical signs and lesions
Tx: No specific treatment. Treat liver failure and
photosensitization
Prevention
Destruction of plants
◦ Clearing and grubbing
◦ Herbicide application
Kochia (fireweed, belvedere, fireball, red sage)
Found throughout North
America. Used as forage in
CO, OK, TX, NM
High in crude protein
Toxic principles:
thiaminase-like substances,
hepatotoxins,
nephrotoxins, sulfates,
nitrates, soluble oxalates,
saponins, others
Drought resistant, tolerates high soil [Na]
Toxicosis: most common in mid-summer and fall
Kochia has been termed “a poor man’s alfalfa” due to its high CP content (11.0–
22.0% DM basis) with a feed value that is proposed to be slightly inferior to
alfalfa
Mechanisms of Toxicity
Cause hepatotoxicity occasionally with
photosensitization
Sulfates –> laxative and CNS effects
H2S
Thiaminase –> CNS derangement
Nitrates –> methemoglobinemia
Soluble oxalates –>nephrotoxicity
Sulfates are reduced to hydrogen sulfide which causes degenerative changes in
the brain resulting in CNS signs.
Soluble oxalates precipitate in the renal tubules as calcium oxalate resulting in
nephrotoxicity.
Clinical Signs
Species: cattle, sheep and horses
Loss of appetite, poor weight gains,
diarrhea, icterus, oral ulcerations, and
abdominal pain
Depression, weakness, excessive tearing,
and photosensitization
Elevated serum liver enzymes and bilirubin
levels
CNS signs: ataxia, circling, head pressing,
convulsions, and blindness
Dx
Appropriate clinical signs and lesions
associated with ingestion of Kochia
Rule out other causes of liver failure
Tx
Remove animals from source of Kochia
Reduce stress and keep photosensitized
animals away from sunlight
Treat polioencephalomalacia with thiamine
Food-associated Toxicants
Xylitol
Sources
◦ Xylitol, a 5-carbon sugar alcohol, is a
natural sweetener found in plants.
Commonly obtained from birch bark
◦ Found in >1900 products:
Sugar-free gum, dental spray, dental lozenges,
toothpaste, candies, and baked goods
Sweetener in products for diabetics
Dietary supplements and chewable vitamins
Low carb products, prescription drugs
Xylitol is present in over 1900 products. The use of xylitol has increased in
recent years because of the popularity of low-carbohydrate diets and low-
glycemic index foods.
Additionally, because xylitol prevents oral bacteria from producing acids that
damage the surfaces of teeth, it is widely used on toothpastes and other oral care
products.
Although its presence in gum, mints, and candies has been well known for years,
it is currently used in numerous additional products.
Therefore, veterinarians should be aware that xylitol may be found in several
common household products and even in medications.
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ADME
Xylitol is absorbed readily from the canine GI
tract with peak plasma levels in 30 min
Metabolism (80%) occurs in liver:
◦ Xylitol is first oxidized by NAD-xylitol
dehydrogenase to D-xylulose
◦ D-xylulose is then phosphorylated by the D-
xylulose-kinase to D-xylulose-5-phosphate, an
intermediate of pentose phosphate pathway (PPP)
◦ D-xylulose-5-phosphate is metabolized to fructose-
6-phosphate and glyceraldehyde phosphate which
are converted to glucose and then glycogen, and to
a lesser extent lactate
Toxicity
Dogs
◦ 75-100 mg/kg causes signs of hypoglycemia
50 mg/kg warrants decontamination and blood glucose
monitoring
◦ >500 mg/kg causes hepatic failure
◦ 2.96 g/kg of body weight resulted in lateral
recumbency, non-responsiveness, and gas in GI
tract
Toxicity
Rabbits: oral LD50 is 4-6 g/kg
Toxicity
Ferrets; Anecdotal reports indicate that
xylitol ingestion causes hypoglycemia
Mechanism of Toxicity
Hypoglycemia
The pentose phosphate pathway (PPP) is
believed to control insulin release
◦ PPP is the major source of NADPH which
reduces oxidized glutathione (GSSG) to GSH
which stimulates insulin secretion
◦ Metabolism of xylitol through PPP in dogs (but
not in humans) causes rapid release of insulin
resulting in rapid and profound hypoglycemia
Xylitol directly stimulates secretion of
insulin by pancreatic islet β cells in dogs
Mechanism of Toxicity
Hepatotoxicity
The mechanism of hepatotoxicity is not
known. It is proposed to be due to:
i). Depletion of ATP during metabolism of
xylitol
ii). Production of reactive oxygen species
Clinical signs
Related to hypoglycemia and/or hepatopathy
◦ Hypoglycemia may develop within the 1st h but can
be delayed to 12-48h depending on the xylitol source
◦ Signs of hypoglycemia include vomiting (first to
occur), lethargy, weakness, ataxia, disorientation,
depression, hypokalemia, seizures, collapse, and coma
◦ Hepatic dysfunction: elevated liver enzymes in 4-24 h.
Hyperbilirubinemia and coagulopathy (prolonged PT
and APTT times, petechiae, ecchymoses and GI
hemorrhage)
◦ GI signs: diarrhea and intestinal gas production
◦ Other signs: thrombocytopenia, hyperphosphatemia
Hyperphosphatemia is a poor prognostic indicator
Adrenaline release and excess insulin during hypoglycemia stimulate uptake of
potassium from the bloodstream thus reducing plasma potassium concentration
(hypokalemia).
Hypokalemia has a profound effect on the heart and is associated with an
increased risk of cardiac arrhythmias.
Hyperphosphatemia is considered a poor prognostic indicator because it was
found in 4 of 5 dogs that died of liver failure after xylitol ingestion.
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Dx
◦ History of exposure and expected clinical signs
Tx
◦ Emesis if within 15-30 min of exposure and if
asymptomatic and no other contraindications to
emesis exist. Emesis is not recommended if >30
min after ingestion of 100% xylitol products
◦ Activated charcoal is not recommended due to
Rapid absorption of xylitol
It binds xylitol poorly
◦ Monitor blood glucose, chemistry, electrolytes,
liver enzymes and coagulation parameters
Symptomatic and supportive care
◦ IV fluids with dextrose followed by parenteral fluids with
2.5-5% dextrose
◦ Nutritional support
Addition of fiber to diet may help elimination of wrapper material
◦ K+ supplementation
◦ Hepatoprotectants (SAMe, silymarin or NAC)
◦ Tx coagulopathy with vitamin K1 or plasma transfusion
Dogs should be hospitalized for at least 12-24hr after
xylitol ingestion due to risk of delayed onset of
hypoglycemia (e.g., cases associated with chewing gum
ingestion)
Outbreak in USA 2005/6
December 20, 2005: Aflatoxin found in Diamond
Pet Food manufactured in Gaston, South Carolina
19 pet food varieties recalled
January 9, 2006: Fort Jackson, Columbia, SC; toxic
pet food kills dozens of dogs
Aflatoxins (AF)
Sources: Bisfuranocoumarin metabolites
produced by Aspergillus flavus, A. parasiticus and
A. nominus
◦ 13 aflatoxins identified. Aflatoxin B1, B2, G1, G2 are
most common
Found in crops with high energy content
◦ Almost any feedstuffs can support
aflatoxigenic fungi
Grains (corn, cottonseed, peanuts, rice, wheat,
oats, almonds, soybean, millet, etc), potatoes, etc.
The term “aflatoxin” comes from Aspergillus flavus, the fungal species from
which some of the compounds were first discovered.
Classification of aflatoxins is based on the fluorescence they emit under UV
light.
moldy grain
Conditions for Mold Growth
on Grains
Warm temp. (>24o – 35o C)
High moisture (>15%)
High humidity (>75%)
Sufficient oxygen (> 0.5%)
Damage to corn kernel
Not all moldy grain is toxic
Note: Fungal growth occurs in the field or during
storage when conditions are right
Susceptible species
All. Poultry, calves, sheep, swine, dogs
◦ Ducklings and trout are the most sensitive
◦ Adult ruminants are of low sensitivity due to
detoxification by rumen microbes
Historical note: Aflatoxins were
determined to be the cause of a
mysterious turkey “X” disease in Great
Britain in the 1950s and 1960s
A total of 100,000 turkeys died of so-called turkey “X” disease after being fed
with contaminated Brazilian groundnut meal on a poultry farm in London.
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Risk Factors
Nutritional status of the animal and feed quality are
important variables in the severity of toxicosis
Low protein diets increase hepatotoxicity & necrosis
Dietary amino acids: ↑lysine and ↑arginine increase
toxicity; ↓choline and ↓methionine increases the
carcinogenic effect; casein is protective
Vitamin A and carotene decrease toxicity
Antioxidants and Se reduce toxicity and promote
repair of macromolecules
Toxic dose
LD50 (mg/kg) values: 0.55 (cats), 0.62 (pigs), 1 (dogs),
2 (sheep)
ADME
Absorption: passive diffusion from small intestine.
AFB1 is most lipophilic absorption
◦ Absorption is more efficient in younger animals
Distributed to most tissues; highest
concentration in the liver. No accumulation
Metabolism: in liver, kidney, small intestine
◦ Aflatoxin B1 is activated to reactive AFB1-8,9-epoxide
intermediate by cytochrome P450 and hydroxylated to
aflatoxin M1 or bound by GSH
Excretion: bile, urine, feces, milk, eggs, semen
- Majority is excreted within 24 h after exposure
The main excretory product is aflatoxin M1
- Max aflatoxin M1 in milk: 0.05-0.5 ppb
Turkeys are a sensitive species because they activate AFB1 at a faster rate.
Younger birds activate AFB1 at a faster rate than older birds.
Conjugation of AFB1-8,9-epoxide with glutathione (GSH) is an important
detoxification pathway.
Mechanisms of Toxicity
Reactive metabolites (AFB1-8-9-
epoxides) bind with DNA, RNA,
proteins and organelles disruption of
anabolic and catabolic processes
◦ Loss of organelle function, carcinogenesis,
mutagenesis, teratogenesis, protein
synthesis and immunosuppression
Protein synthesis reduced production of essential
metabolic enzymes and structural proteins for growth
◦ Aflatoxins impair reproductive performance
Clinical Signs
Depend on dose and duration of exposure
Acute toxicity: anorexia, depression,
weakness, prostration, dyspnea, emesis,
diarrhea, epistaxis, fever followed by
subnormal temperature, convulsions
(dogs), hemorrhage and icterus
Death
Clinical Signs: Chronic Toxicity
More common
Anorexia, reduced production and feed
conversion efficiency, rough hair coat
Anemia, icterus, and depression
Bleeding disorders and ascites in dogs
High mortality in young birds
Clinical Pathology
Increased activity of hepatic enzymes in
serum: AST, ALT, ALP and GGT
◦ GGT is a biomarker of aflatoxicosis and is elevated
due to bile duct hyperplasia
Increased serum bilirubin
Serum proteins can decrease in aflatoxicosis
◦ Albumin and β-globulins decrease, and γ-globulins
increase
Coagulation defects: prolonged prothrombin and
activated partial thromboplastin times, and
thrombocytopenia
Hepatic Lesions
Livers may be swollen, friable, and
congested
In chronic toxicity livers are firm, fibrous
and pale
Bile duct proliferation, fibrosis, icterus,
megalocytosis, and hepatocyte necrosis
◦ Necrosis is periportal or centrilobular
depending on animal species
Dx
History, lab data, necropsy findings, and liver
microscopy
Black light test (screening test for aflatoxin
fluorescence in feeds)
◦ Significant number of false positives
◦ Must be followed by reliable analysis of aflatoxins
Many methods: ELISA, HPLC, TLC
Demonstration of fungus and aflatoxins with
compatible clinical signs and lesions
DDx: pyrrolizidine alkaloids
Tx
There is no antidote or specific Tx
Removal of contaminated feed
Optimize quality of the diet
◦ Protein, amino acids (choline and methionine),
vitamins (B12 and K1) and trace elements
Clinical outcomes of these supplementations are mixed
Liver protectants, e.g., SAMe, selenium and
vitamin E should be considered
Oxytetracycline reduces hepatic damage and
mortality
Activated charcoal soon after exposure is helpful
Control
Avoid contaminated feeds by monitoring
batches for aflatoxin levels
Monitor local crop conditions (e.g., drought)
as predictors of aflatoxin formation
Prevention of crop damage e.g., with
insecticides decreases fungal invasion
Ammoniation of feeds hydrolyses AFB1
Use of adsorbents, e.g., sodium calcium
aluminosilicate to bind aflatoxins
Blue Green Algae (Cyanobacteria)
Microcystis sp. produce microcystin. Many
other cyanobacteria produce microcystin
Nodularia sp. produce nodularin
Both toxins are highly hepatotoxic
Susceptible species: all
Conditions for toxicity: sunny and windy
weather, water high in nutrients
Toxicoses usually occur in late summer-early
winter but can occur any time
Stagnant freshwater, brackish water
In addition to the anatoxins and saxitoxins, cyanobacteria produce hepatotoxins.
There include microcystin produced by Microcystis sp. in Stagnant freshwater
and nodularin produced by Nodularia sp. in brackish water.
Cyanobacteria blooms are predicted to become more severe and widespread due
to climate change if land use practices are not altered to reduce nutrient input to
surface waters.
Although microcystin concentrations may be highest when the growth of the
cyanobacteria is high, toxin concentrations do not necessarily correlate with cell
count.
Microcystis and Nodularia sp. have different morphologies.
Microcystis are small cells with gas filled vesicles that are usually organized into
colonies visible with the naked eye.
In contrast Nodularia may form solitary filaments or groups of filaments.
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Blue Green Algae
Toxicity: 0.05-11mg/kg dependent on animal
species, toxin analog, and route of exposure
ADME
Blue-green algae ingested with water are
broken down in GI tract to release toxins
The toxins are rapidly absorbed in the small
intestine, transported to the liver and enter the
hepatocytes through a bile acid transporter
◦ The transporter is critical for entry because
microcystin does not passively permeate hepatocytes
Other target organs are the kidney and gonads
Mechanisms of Toxicity
Disruption of hepatocyte cytoskeleton
◦ Inhibition of protein phosphatase 1 & 2A (PP1 &
PP2A) impaired phosphorylation of regulatory
intracellular proteins
◦ Impairment of structural integrity of the
cytoskeleton (microtubules, intermediate
filaments and microfilaments)
Induction of apoptosis via ROS formation
and mitochondrial dysfunction
Microcystin is a tumor-promotor
The hepatocyte is the specific target of microcystin, which enters the cell
through a bile-acid transporter.
Microcystin covalently binds to protein phosphatase, leading to the
hyperphosphorylation of cytoskeletal proteins, and deformation and loss of
function of the cytoskeleton
Clinical Signs
Appear within 1-4 h of exposure
◦ Lethargy, vomiting, diarrhea, GI atony, weakness, pale
mucous membranes, shock
◦ Death occurs within 24 h but may be delayed for
several days. In some cases, acute death may occur
with minimal clinical signs
◦ Hyperkalemia, hypoglycemia, nervousness,
recumbency, convulsion and development of 2o
photosensitization in animals that do not die acutely
◦ Serum concentrations of hepatic enzymes are
elevated
Hepatic Lesions
Gross
◦ Enlarged, congested and hemorrhagic livers
Hepatic Lesions
Microscopic
◦ Centrilobular to midzonal necrosis
◦ Breakdown of sinusoidal endothelium
◦ Intrahepatic hemorrhage
Dx
History and compatible clinical signs
Gross and microscopic lesions
Presence of algae in water
◦ Collect water/algae sample immediately after incident
GI contents may contain identifiable algae
Mouse bioassay with the water
Measurement of toxins in water by a colorimetric
assay (screening) and HPLC, TLC or GC-MS
DDx: Cycad palm, aflatoxin, xylitol, metals (e.g., Cu),
acetaminophen
Tx
There is no specific antidote or Tx
◦ Many Tx options have been evaluated but none has
been proven to be effective
Decontamination
◦ Dogs: Emesis, activated charcoal, cathartic and
bathing
◦ Large animals: activated charcoal, cathartic, bathing
Symptomatic and supportive therapy
◦ IV fluids, possibly blood transfusion, vitamin K1,
hepatoprotectants, corticosteroids and other
elements of shock therapy
Due to the rapid onset of acute hepatotoxicosis treatment is difficult, and
mortality rates are very high.
Despite the evaluation of numerous treatment options, no specific therapy has
been proven to be effective.
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