Lecture 28

Question 1

The liver is
 A key organ for nutrient homeostasis
 1o detoxication and toxication centre
 Downstream of GI tract
 Passive entry of toxicants into hepatocytes

Question 2

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

Question 3

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

Question 4

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

Question 5

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

Question 6

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.

Question 7

Enterohepatic recirculation
slows toxicant clearance
and facilitates hepatocyte
re-exposure

Question 8

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

Question 9

 Fatty degeneration (steatosis)

Answer 9

↑Fat in hepatocytes

Question 10

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

Question 11

Hepatocyte death

Answer 11

↑ALT, ↑AST

Question 12

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.

Question 13

Hepatic megalocytosis

Answer 13

↑ hepatocyte size

Question 14

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

Question 15

Cholestasis

Answer 15

↑bilirubin, ↑bile salts, yellow-green liver

Question 16

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

Question 17

Bile duct damage

Answer 17

↑ALP, ↑GGT, ↑bilirubin, ↑bile salts

Question 18

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.

Question 19

Sinusoidal damage

Answer 19

dilation or blockade

Question 20

Sinusoidal damage can occur following toxicant exposure and manifest as dilation or
blockade of sinusoids with impaired blood flow

Question 21

Fibrosis

Answer 21

↑fibrous/scar tissue

Question 22

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

Question 23

Cirrhosis

Answer 23

↑↑↑fibrous/scar tissue; firm liver; loss of function

Question 24

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.

Question 25

Neoplasia

Answer 25

Tumors of hepatocytes, bile ducts, sinusoid cells

Question 26

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.

Question 27

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.

Question 28

Acute liver failure: Abdominal pain, liver
enlargement, vomiting, hypovolemic shock,
hypoglycemia, icterus, 2o hepatoencephalopathy

Question 29

Subacute liver failure: Intermittent GI upset,
reduced appetite, poor condition, icterus, possibly
liver pain and enlargement

Question 30

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

Question 31

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

Question 32

Question 33

Answer 33

Question 34

Answer 34

Question 35

Answer 35

Question 36

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

Question 37

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.
4

Question 38

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.

Answer 38

 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

Question 39

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 ______

Answer 39

 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

Question 40

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

Question 41

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

Question 42

Answer 42

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.

Question 43

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

Question 44

Tx
 Usually not successful
 Remove animals from plants source
 Give diets high in carbohydrates and low in protein
 Treat dehydration and photosensitization

Question 45

Answer 45

Lantana spp. (Largeleaf lantana,
yellow/red sage, white brush etc)

Question 46

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

Question 47

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

Question 48

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

Question 49

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

Question 50

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

Question 51

Dx
 Identification of Lantana and evidence of their
consumption
 Clinical signs and lesions

Question 52

Tx: No specific treatment. Treat liver failure and
photosensitization

Question 53

Prevention
 Destruction of plants
◦ Clearing and grubbing
◦ Herbicide application

Question 54

Answer 54

Question 55

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

Question 56

Mechanisms of Toxicity
 Cause hepatotoxicity occasionally with
photosensitization
 Sulfates –> laxative and CNS effects
H2S
 Thiaminase –> CNS derangement
 Nitrates –> methemoglobinemia
 Soluble oxalates –>nephrotoxicity

Question 57

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.

Question 58

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

Question 59

Dx
 Appropriate clinical signs and lesions
associated with ingestion of Kochia
 Rule out other causes of liver failure

Question 60

Tx
 Remove animals from source of Kochia
 Reduce stress and keep photosensitized
animals away from sunlight
 Treat polioencephalomalacia with thiamine

Question 61

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

Question 62

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.
9

Question 63

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

Question 64

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

Question 65

Toxicity
Rabbits: oral LD50 is 4-6 g/kg

Question 66

Toxicity
Ferrets; Anecdotal reports indicate that
xylitol ingestion causes hypoglycemia

Question 67

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

Question 68

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

Question 69

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

Question 70

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.
10

Question 71

 Dx
◦ History of exposure and expected clinical signs

Question 72

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

Question 73

 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)

Question 74

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

Question 75

Question 76

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.

Answer 76

Question 77

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.

Question 78

Answer 78

moldy grain

Question 79

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

Question 80

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

Question 81

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.
12

Question 82

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)

Question 83

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

Question 84

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.

Question 85

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

Question 86

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

Question 87

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

Question 88

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

Question 89

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

Question 90

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

Question 91

 DDx: pyrrolizidine alkaloids

Question 92

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

Question 93

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

Question 94

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

Question 95

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.

Question 96

Answer 96

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.
15

Question 97

Blue Green Algae
Toxicity: 0.05-11mg/kg dependent on animal
species, toxin analog, and route of exposure

Question 98

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

Question 99

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

Question 100

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

Question 101

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

Question 102

Hepatic Lesions
 Gross
◦ Enlarged, congested and hemorrhagic livers

Question 103

Hepatic Lesions
Microscopic
◦ Centrilobular to midzonal necrosis
◦ Breakdown of sinusoidal endothelium
◦ Intrahepatic hemorrhage

Question 104

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

Question 105

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

Question 106

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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