4.4 Toxic Plants

Question 1

plant toxicity is a function of:

Answer 1

Specific chemical compound(s) present
Concentration in the plant (influenced by growing conditions, health of plant etc.)
Dose and duration of exposure (amount ingested may vary depending on the presence of alternate food sources)

Question 2

Are all domestic species equally susceptible to these plant toxins?

Answer 2

• Species differences in metabolism can have a major effect on susceptibility:
• Monogastrics vs. ruminants: ruminal flora may either inactivate or activate compounds
• Small ruminants less affected than cattle
• Horses more discriminating
• Some toxins affect most vertebrate species; others are uniquely harmful only to particular species

Question 3

Why are some plants toxic?

Answer 3

• Evolutionary adaptation to discourage herbivory
• Many toxic compounds also are bitter, reducing immediate palatability of the plant, in addition to more specific toxic effects

Question 4

plant compounds that have been used to treat diseases

Answer 4

• Cardenolides (digitoxin, digoxin from foxglove); documented use since 1785 to treat cardiac disease
• Vinca alkaloids (vincristine from Madagascar periwinkle); antineoplastic, prevent microtubule assembly and disrupt cell division
• Taxols (from the yew family); antineoplastic, prevent microtubule disassembly

Question 5

How can we diagnose plant toxicities?

Answer 5

• History and clinical signs are important – many plant toxicities have
  no specific gross lesions on necropsy examination!
• Often there is no specific biochemical assay for the toxin.

Rely on the evidence:
* Availability (in the home, garden, barn or pasture)
* Observed ingestion, or signs of ingestion (e.g. present in vomitus)
* Compatible clinical signs

Question 6

these 3 plant toxin categories can cause sudden death:

Answer 6

Plants containing:
*Cyanogenic glycosides
*Toxic alkaloids
*Nitrates (which we’ve already talked about)

Question 7

Cyanogenic glycosides are found in what plants?

Answer 7

• Sorghum (Johnson grass, Sudan grass)
• Members of the rose family (chokecherry, pincherry, wild black cherry, serviceberry, crabapple, almond, plum, pear etc.)
• Part of plant: all, esp. damaged leaves (wilted, trampled, frozen, wind- falls)

Question 8

cyanogenic glycosides affect what species? what are clinical signs and lesions?

Answer 8

• Toxin: cyanogenic glycosides (prunasin, amygdalin and others)
• Species susceptible: all, esp. ruminants
• Clinical Signs: cyanide poisoning ➝ rapid onset, distress, weakness, labored respiration, collapse, convulsions, death.
• Lesions: non-specific; cherry red venous blood; smell of bitter almonds in rumen gas
  > Look for plant material in stomach(s)
  > Measure cyanide levels in GI contents

Question 9

Cyanogenic glycosides: mechanism of action

Answer 9

• Cyanogenic glycosides
  ➝ react w/ iron in cytochrome oxidase
  ➝ inactivation of cytochrome oxidase
  ➝ no release of oxygen from oxyhemoglobin
  ➝ inactivates cellular respiration
• Enzymes necessary for this conversion are present in plant cells and are also produced by rumen micro-organisms.

Question 10

treatment for cyanogenic glycoside toxicity, and mechanism it work by:

Answer 10

• Treatment: sodium nitrite and sodium thiosulfate ➝ breaks cyanide-cytochrome bond, nitrite forms methemoglobin
• Methemoglobin has greater affinity for cyanide than cytochrome oxidase ➝ binds cyanide and forms thiocyanate ➝ excreted in urine

Question 11

which plants toxic alkaloids

Answer 11

• Water hemlock (poison parsnip)
• Monkshood, larkspur

Question 12

what is the toxin in water hemlock? what species are susceptable? what are the slinical signs and lesions?

Answer 12

• Toxin: cicutoxin, a potent neurotoxic alkaloid
• Species susceptible: All, including humans! Spotted cowbane, ‘suicide root’
• Part of plant: Entire plant, esp. root
• C/S: Hypersalivation, tremors, convulsions, mydriasis, diarrhea, cardiac arrythmias, paralysis, respiratory failure, DEATH (within 45 min)
• Lesions: nonspecific skeletal muscle and myocardial degeneration

Question 13

plants containing cardiotoxins:

Answer 13

• Taxine alkaloids (*yew, *Taxus spp.)
• Cardiac glycosides (*foxglove, lily-of-the-valley, milkweed, oleander, Star of Bethlehem)
• Grayanotoxins (rhododendrons and laurels)
• Persin (avocado)

Question 14

are Taxus spp. (the yew family) very toxic? what is their toxin? What do they do? what species are susceptible?

Answer 14

HIGH risk plants
* Toxin: Taxine alkaloids – depresses cardiac conduction
* Species susceptible: livestock, dogs, humans

Question 15

what clinical signs does toxicity due to taxus spp. (yew) cause?

Answer 15

• C/S: trembling, dyspnea, bradycardia, diarrhea, cardiac failure, sudden death
• Lesions: usually none; find plant in stomach/rumen content

Question 16

what toxin do we find in cardiac glycosides? what species are susceptible? what are the clinical signs?

Answer 16

Toxin: cardenolides - interfere with cardiac conduction by inhibiting membrane Na+/K+ ATPase
-Species susceptible: All
-found in: foxglove plant

Clinical signs:
* GI – vomiting, diarrhea, salivation
*CV – bradycardia, AV heart block, arrhythmias > cardiac arrest and death at higher doses

Question 16

what toxin do we find in cardiac glycosides? what species are susceptible? what are the clinical signs?

Answer 16

Toxin: cardenolides - interfere with cardiac conduction by inhibiting membrane Na+/K+ ATPase
-Species susceptible: All
-found in: foxglove plant

Clinical signs:
* GI – vomiting, diarrhea, salivation
*CV – bradycardia, AV heart block, arrhythmias > cardiac arrest and death at higher doses

Question 17

what toxin do we find in cardiac glycosides? what species are susceptible? what are the clinical signs?

Answer 17

Toxin: cardenolides - interfere with cardiac conduction by inhibiting membrane Na+/K+ ATPase
-Species susceptible: All
-found in: foxglove plant

Clinical signs:
* GI – vomiting, diarrhea, salivation
*CV – bradycardia, AV heart block, arrhythmias > cardiac arrest and death at higher doses

Question 18

what are common digestive issues caused by toxins?

Answer 18

-excessive salivation
-vomiting
-diarrhea

Question 19

excessive salivation due to toxicity is often due to what types of problems? what plants cause these?

Answer 19

• Oral trauma (foxtail barley)
• Irritating compounds (dieffenbachia, jack-in-the-pulpit)

Question 20

what do lactones cause?

Answer 20

vomiting (eg. sneezeweeds, spewing sickness)

Question 21

what do glycoalkaloids result in, in terms of digestive symptoms?

Answer 21

diarrhea (eg. nightshades)

Question 22

what sort of digestive issue do lectins cause?

Answer 22

diarrhea (eg. castor bean, rosary pea, black locust)

Question 23

what symptoms are caused by toxicity due to Araceae family: Dieffenbachia spp.? what are some examples of these plants?

Answer 23

• Houseplants: Arrowhead vine, Calla lily, Dumbcane, Peace lily, Philodendron, Pothos, umbrella plant etc.
• Insoluble calcium oxalate crystals cause mechanical irritation
• Clinical signs: oral irritation, salivation, edema

Question 24

what is the toxin found in lectins eg. castor bean? what is its mechanism of action and what does it do? what species are susceptible? what are clinical signs and lesions?

Answer 24

• Toxin: ricin & ricinin (water-soluble lectin glycoproteins) inhibit ribosomal protein synthesis (lipid-soluble ricinoleic acid in castor oil is used as a lubricant and purgative)
• Species susceptible: All, including humans
• Part of plant: Whole plant; seeds have highest
  concentration of ricin but must be chewed to release toxin
• C/S: Abdominal pain, hemorrhagic diarrhea, hypovolemic shock
• Lesions: GI ulceration, necrosis of liver, kidney

Question 25

what is primary photosensitization?

Answer 25

• Photodynamic agent is ingested, absorbed, or injected
  > Reaches the skin in its native form
• Contact photosensitization: giant hogweed
• Ingested photosensitization: St. Johnswort, buckwheat etc.

Question 26

what is secondary photosensitization?

Answer 26

= compromised liver function reduces excretion of plant pigment metabolites from body

Question 27

what kind of issues does giant hogweed cause?

Answer 27

• Heracleum mantegazzianum
   Clear watery sap which sensitizes the skin to ultraviolet radiation

Question 28

what is the toxin in st. johns wort? what kind of issues does st. johns wort cause? what species are susceptible? what are the clinical signs and lesions?

Answer 28

(Hypericum perforatum)
* Perennial naturalized herb with clusters of yellow flowers; tiny translucent spots on leaf surfaces
* Droughty, poor, overgrazed meadows, pastures, waste areas.

• Toxin: hypericin (photodynamic compound, absorbed and deposited in tissues, photoactivated by sunlight)
• Species susceptible: cattle, sheep, horses, pigs
• Part of plant: all parts
• C/S: erythema ➝ ulceration of lightly
  pigmented areas (esp dorsally)
• Lesions: Primary photosensitization

Question 29

Plants containing neurotoxins:

Answer 29

• Locoweed
• Crown vetch
• White snakeroot
• *Yellow star thistle
• *Russian knapweed

Question 30

what toxin is contained in russian knapweed? what does it do? what species is susceptible? what are the clinical signs and lesions?

Answer 30

• Toxin: sesquiterpene lactone – “repin”, a neurotoxin
• Species susceptible: Horses, “chewing disease”
• Part of plant: entire
• C/S: abrupt onset, impaired eating/drinking; depression, hypertonicity of lips and tongue, fixed facial expression, constant chewing.
• Lesions: “nigropallidal encephalomalacia”

Question 31

what is nigropallidal encephalomalacia?

Answer 31

-lesions caused by certain neurotoxins, eg. repin from russian knapweed

• Malacia (necrosis) of the globus pallidus, substantia nigra; neuronal necrosis, bilaterally symmetrical.
• Repin causes glutathione depletion ➝ increased ROS ➝ oxidative damage ➝ neuronal cell death
• Areas of dopamine metabolism more sensitive to oxidative injury
• Inhibits dopamine release ➝ Parkinson’s like signs

Question 32

what type of musculoskeletal problems can plants cause, and what examples of these plants?

Answer 32

• Calcinosis due to vitamin D analogues (day-blooming jessamine)
• Muscle degeneration (cottonseed)
• Laminitis (black walnut)
• Selenosis (milk vetch)

Question 33

what is the toxin in black walnut? what does it do? what species are susceptible? what are clinical signs and lesions?

Answer 33

• Toxin: Unknown but suspected to be Juglones
• Species susceptible: Horses
• Part of plant: fresh shavings as bedding, leaves, bark, roots
• C/S: depression, laminitis, rapid onset within 12-24 hours of contact; increased respiratory rate and effort.
• Lesions: acute/chronic laminitis, limb edema