06: Feed Toxins

Species that are most susceptible to nitrate poisoning

Ruminants

Why do plants produce nitrate

Most important nitrogenous nutrient

How do plants uptake nitrate

Constitutively for maintenance with low affinity transport system (LATS) and rapidly during growing phases with high affinity transport system (HATS)

Nitrate transport system that is energy dependent

HATS

How do plants utilize nitrate

Converts it to ammonium via nitrate reductase for protein synthesis

When do plants have a toxic level of nitrate concentration

When they turn off the activity of nitrate reductase. The conversion stops, but uptake doesn’t

When do plants turn off nitrate reductase activity

Stress

Types of plants that tend to accumulate more nitrate

Plants with the genetic potential to grow rapidly

Risk factors for nitrate accumulation

• Peak growing season

• Presence of nitrate in the soil

• Drought

• Lack of light

• Unseasonal temperatures

How do herbicides contribute to nitrate toxicity risk

They shut down growth and metabolism, and if a plant was sprayed when it had high concentrations it can’t convert the nitrate to ammonia → protein

Common sources of nitrate in water

• Fertilizers

• Septic tanks

Why are nitrates so dangerous to ruminants

Rumen microbes efficiently convert nitrate to nitrite but less efficiently convert the nitrite to ammonium. The nitrite can build up and get absorbed

Species that are much less susceptible to nitrate toxicity

Monogastrics

Nitrite toxicity mechanism of action

Oxidizes HgB to MgB

Animals that are more susceptible to nitrite toxicity

Animals with a high metabolic rate (swine, late gestation fetuses) tolerate MgB worse

CS associated with acute nitrite toxicity

• Hypoxia

• Tachypnea

• Exercise intolerance

How is nitrite toxicity different from onion toxicity

Onions cause oxidative damage tot he RBCs, nitrite oxidizes the iron, which does not result in hemolysis or renal failure

CS associated with chronic nitrite toxicity

Abortions and decreased production parameters

Specific treatment for nitrite toxicity

Methylene blue

Cons of using methylene blue for nitrite toxicity

It is possibly a carcinogen and is ELDU → be careful in food animals

Postmortem samples that can be submitted for nitrate testing

Serum and aqueous humor in the eye

Purpose of NPN in the ruminant diet

Converted by microbes into MCP

What has to be matched with NPN intake

Quickly available source of CHO

Most common source of NPN

Urea

Mechanism of NPN poisoning

Overwhelming build up of ammonia (NH₃) that reacts with water to form NH₄OH

How does NH₄OH result in poisoning

Can rapidly cause an extremely alkaline rumen environment → damage and necrosis → absorption of NH₄OH into the blood → alkalhosis

Species that are susceptible to NPN poisoning

Ruminants and occasionally hindgut fermenters

How long should you take to convert your ruminants to a high NPN diet

2-3 days

CS associated with NPN toxicity

RAPID CNS signs with mild bloat and alkalosis

Best sample to test if you suspect NPN poisoning

Rumen fluid

Which animals should you treat first if you suspect/confirm NPN poisoning

Those that are still unaffected and where treatment is more likely to work

Treatment for NPN poisoning

• Vinegar (lower pH)

• Cold water (lowers pH)

• Formaldehyde (kill microbes)

Prognosis for NPN

Not great, because even if you treat the rumen is damaged and the microbial population is destroyed

Post-mortem lesions associated with bloat

“Bloat line”: gas pressure pushes blood causing cranial congestion and caudal anemia

Risk factors for NPN poisoning

• High levels of NPN

• Abrupt diet shift

• Poor adaptation or CHO matching

• Fertilizer contamination

How much of the diet should be NPN

No more than 3% of the diet

How much of dietary nitrogen should be NPN

No more than 1/3

Why is ammonia toxicity a problem

You can increase the quality of low quality feed if you expose it to ammonia gas

Mechanism of ammonia toxicity

Converts sugars and proteins to toxic pyrazines and imidazoles

CS associated with ammonia toxicity

Episodic bouts of crazy behavior

Why are nursing animals more susceptible to ammonia toxicity

The toxic pyrazine and imidazoles are efficiently excreted in milk, which is protective for mom but not for baby

Treatment for ammonia poisoning

• Remove feed

• ± Sedatives

• Supportive treatment for any lasting trauma

Syndrome caused by sulfur toxicity

Polioencephalomalacia (PEM)

Damage caused by PEM

Multiple areas of softness in the brain

Supplemental feed that provides an increased risk of sulfur tox

Brewer’s grains

Natural conditions that increase the sulfur content in feed

Geological history of the area and sulfur containing salts

Common sources or sulfur risk

• Brewer’s grains

• Water contamination

• Fertilizer

• Ration mixing error

• Fish die-offs

How does sulfur exist in the body

Converted to sulfide → H₂S

Mechanism of sulfur toxicity

Interferes with E metabolism, interferes with blood flow, and causes oxidative damage

CS associated with sulfur toxicity

• Various CNS signs depending on when you catch the poisoning

• Apparent blindness

DDx for apparent blindness

Lead and rabies

Clin path abnormalities seen with sulfur toxicity

• Metabolic acidosis

• Hypokalemia

• Hypochloremia

Post mortem lesions seen with sulfur toxicity

• Gray/black sulfide deposits

• Brain swelling ± brainstem prolapse

• Brain necrosis, often in the cerebrum at the junction of the gray and white matter, potentially fluorescent!

Feed sulfur content that is considered a high risk

>0.4%