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# Cyanide Poisoning in Animals - Veterinary Medicine

> **Prepared by:** Orris AI | **Date:** July 13, 2026  
> **Sources:** MSD Veterinary Manual, Tintinalli's Emergency Medicine 9e, ROSEN's Emergency Medicine, World Veterinary Journal (2024), PubMed PMID: 40286826

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## 1. Overview

Cyanide poisoning (cyanide toxicosis) is an acute, often fatal condition most commonly seen in **ruminant species** (cattle, sheep, goats), though all domestic and exotic animals are susceptible. It is characterized by rapid onset of cellular asphyxia, with death potentially occurring within 15 minutes to a few hours of exposure.

Four distinct syndromes are recognized:

| Syndrome | Description |
|---|---|
| **Acute cyanide poisoning** | Most common; sudden death after cyanogen exposure |
| **Chronic hypothyroidism** | From long-term low-dose cyanide exposure competing with iodine uptake |
| **Cyanide neuropathy** | Sorghum cystitis-ataxia syndrome (horses, cattle, sheep, goats) |
| **Musculoskeletal teratogenesis** | Arthrogryposis/ankylosis and abortion - associated with chronic *Sorghum* spp. exposure |

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## 2. Sources and Etiology

### A. Cyanogenic Plants (Most Common Cause)

Plant cyanogenesis occurs in over **2,500 species** across 110+ families of ferns, gymnosperms, and angiosperms. About **300 species** are clinically significant, containing roughly **75 known cyanogenic glycosides** (all O-beta-glycosidic derivatives of alpha-hydroxynitriles).

**Key cyanogenic plants affecting livestock:**

| Plant | Cyanogenic Compound | Species Affected | Notes |
|---|---|---|---|
| **Sorghum spp.** (*S. halepense*, *S. bicolor*) | Dhurrin | Cattle, horses, sheep | Most common; stressed/wilted plants are most dangerous |
| **Sudangrass, Johnsongrass** | Dhurrin | Cattle, horses | Young regrowth after frost or drought - highest risk |
| **White clover** (*Trifolium repens*) | Linamarin, lotaustralin | Cattle, sheep | |
| **Wild/black cherry** (*Prunus* spp.) | Amygdalin | All species, pigs, birds | Wilted leaves especially toxic |
| **Flaxseed/linseed** (*Linum usitatissimum*) | Linamarin | Ruminants | Raw seed > processed meal |
| **Cassava** (*Manihot esculenta*) | Linamarin | Cattle, pigs | Improperly processed roots |
| **Arrow grass** (*Triglochin* spp.) | Taxiphyllin | Cattle, sheep | Wetland species |
| **Elderberry** (*Sambucus* spp.) | Sambunigrin | Various | Unripe berries |
| **Lima beans** | Phaseolunatin | Pigs, poultry | |

**Risk factors that increase cyanide release from plants:**
- Wilting after frost, drought, or trampling
- Rapid regrowth after cutting (young, lush shoots)
- Acidic rumen pH (promotes enzymatic hydrolysis of glycosides)
- Low phosphorus or potassium soil content
- High nitrogen fertilization (promotes rapid growth with high glycoside concentration)

### B. Industrial and Non-Plant Sources

- Metal processing wastes, electroplating effluents
- Fumigants (hydrogen cyanide gas used in pest control)
- Building fires (combustion of wool, silk, polyurethane, certain plastics releases HCN)
- Rodenticides (cyanide salts)
- Mining operations - sodium/potassium cyanide in gold and silver extraction
- **Insects:** Eastern tent caterpillars (*Malacosoma americanum*), burnet moths (*Zygaena* spp.), certain centipedes and millipedes - hazard for exotic pets

---

## 3. Pathophysiology and Mechanism of Toxicity

### Biochemical Mechanism

1. **Cyanide ion (CN-)** enters cells and binds with extremely high affinity to the **ferric iron (Fe³⁺)** in **cytochrome c oxidase** (Complex IV of the mitochondrial electron transport chain)
2. This blocks electron transfer to molecular oxygen, halting **oxidative phosphorylation**
3. Cells cannot produce ATP via aerobic respiration - **histotoxic (cellular) hypoxia** results
4. Tissues switch to anaerobic metabolism → lactic acid accumulates → **severe metabolic (lactic) acidosis**
5. Despite normal or elevated blood oxygen levels and hemoglobin saturation, tissues starve for oxygen
6. The brain and heart (highest oxygen demands) fail first

### Absorption and Distribution

- **HCN gas:** Rapidly absorbed through the respiratory tract - extremely fast onset
- **Cyanogenic glycosides (ingested):** Must be hydrolyzed by plant beta-glucosidases or rumen microflora - ruminants are most susceptible because rumen bacteria efficiently cleave glycosides; monogastrics are somewhat less susceptible
- **Cyanide salts:** Absorbed from the GI tract; acid in stomach promotes HCN gas liberation
- **Lethal dose (cattle):** ~2 mg HCN/kg body weight; forage with >200 ppm HCN equivalent is considered toxic

### Species Susceptibility (most to least sensitive to plant cyanogenic sources)

1. Cattle and sheep (rumen hydrolysis very efficient)
2. Horses (cecal hydrolysis; less efficient than rumen)
3. Pigs (simple stomach; less efficient hydrolysis but can vomit)
4. Dogs and cats (least commonly affected from plant sources)
5. Poultry (can be affected from grain sources)

---

## 4. Clinical Signs

### Acute Poisoning - Timeline

**Within 15-20 minutes of ingestion** (up to a few hours with lower doses):

| Stage | Signs |
|---|---|
| **Early (Stimulatory)** | Excitement, apprehension, tachypnea, tachycardia, salivation, lacrimation, urination, defecation |
| **Middle (Respiratory distress)** | Dyspnea (labored breathing), muscle fasciculations/tremors, weakness, ataxia |
| **Late (Collapse)** | Prostration, convulsions/tetanic spasms, dilated pupils, **bright red/cherry red mucous membranes** (high venous oxyhemoglobin - oxygen cannot be utilized) |
| **Terminal** | Cardiovascular collapse, cessation of respiration, death (often within 30-60 minutes of severe exposure) |

**Classic signs to remember:**
- **"Bitter almond" smell** on breath (not all people can detect this)
- **Bright red/cherry red mucous membranes** - venous blood appears arterial due to high O₂ saturation despite cellular hypoxia
- **Rapid death** - animals may be found dead with no premonitory signs
- **Bright red venous blood** at necropsy

### Chronic Syndromes

- **Sorghum cystitis-ataxia syndrome (horses):** Posterior ataxia, urinary incontinence, cystitis, myeloencephalopathy - develops after weeks to months of sorghum grazing
- **Goitrogenic effects (hypothyroidism):** Low-level chronic cyanide exposure blocks iodine uptake → goiter, poor growth, reproductive failure - mainly in ruminants on cassava-heavy diets
- **Teratogenesis:** Arthrogryposis (joint fixation) and abortion in calves/lambs born to dams grazing sorghum

---

## 5. Diagnosis

### Ante-mortem Diagnosis

1. **Clinical history:** Access to cyanogenic plants or sources; sudden onset in multiple animals; recent frost, drought, or lush regrowth
2. **Clinical signs:** Dyspnea, cherry-red mucous membranes, bright red venous blood, rapid deterioration
3. **Whole blood cyanide levels:** >0.5-1.0 mg/L (0.5-1.0 µg/mL) considered toxic; normal <0.2 mg/L - collect heparinized blood in airtight container immediately
4. **Blood lactate:** Elevated (metabolic lactic acidosis)
5. **Venous oximetry:** Paradoxically high venous O₂ saturation

### Post-mortem (Necropsy) Findings

- **Cherry red blood** (does not clot well)
- **Petechial/ecchymotic hemorrhages** on heart, lungs, serosal surfaces
- **Bright red liver and muscles**
- **Rumen content testing:** Picrate paper test (turns from yellow to brick red in presence of HCN) - rapid, cheap field test
- **Congestion of viscera**, pulmonary edema

### Specimen Collection (Critical Points)

> **Must collect specimens within 4 hours of death** - cyanide is highly volatile and degrades rapidly.

- **Specimens:** Rumen/stomach contents, blood (heparinized, airtight), liver, muscle, suspected plant material
- Freeze specimens immediately; do **NOT** use formalin
- Test suspected plants separately

### Differential Diagnoses

| Condition | Differentiating Feature |
|---|---|
| Nitrate/nitrite poisoning | Chocolate-brown mucous membranes (methemoglobin); blood is brown |
| Organophosphate/carbamate | Cholinergic signs (salivation, miosis, bradycardia) |
| Carbon monoxide | Exposure history (barn fire); cherry-red membranes similar |
| Hydrogen sulfide | "Rotten egg" smell; similar acute onset |
| Urea/ammonia toxicity | Ruminants; tympany, frothy salivation |
| Clostridial sudden death | No prior clinical signs; specific lab confirmation |
| Yew (*Taxus*) poisoning | Plant access; cardiac arrhythmias |
| Heat stroke | Environmental history |

---

## 6. Treatment Protocols

### Immediate Management - General Principles

> **Time is critical** - cyanide acts within minutes. Treatment must be initiated immediately. In field settings where treatment cannot be administered fast enough, many animals will be found dead.

### Step 1: Remove from Exposure

- Remove all animals from suspect pasture/feed immediately
- Move affected animals to fresh air if HCN gas suspected

---

### Step 2: Antidote Administration

#### Protocol A: Classical Nitrite-Thiosulfate Protocol (Most Widely Used in Large Animals)

**Mechanism:**
- Sodium nitrite oxidizes hemoglobin to **methemoglobin (Fe³⁺)** which has higher affinity for cyanide than cytochrome oxidase - cyanide is "lured away" to form **cyanomethemoglobin**, freeing cytochrome c oxidase
- Sodium thiosulfate provides sulfur substrate for the enzyme **rhodanese** (thiosulfate sulfurtransferase), which converts cyanide to far less toxic **thiocyanate**, excreted in urine

**Dosages:**

| Drug | Species | Dose | Route | Notes |
|---|---|---|---|---|
| **Sodium nitrite (3% solution)** | Cattle/sheep | 22 mg/kg | IV slow (5-10 min) | Do not exceed - MetHb >30% is dangerous |
| **Sodium thiosulfate (20-25% solution)** | Cattle/sheep | 660 mg/kg | IV or PO | Follow sodium nitrite immediately |
| **Sodium nitrite** | Horses | 22 mg/kg | IV slow | Use with caution - horses sensitive to nitrite |
| **Sodium thiosulfate** | Horses | 660 mg/kg | IV | |
| **Sodium nitrite** | Dogs/cats | 10 mg/kg | IV slow | Lower dose than large animals |
| **Sodium thiosulfate** | Dogs/cats | 1.65 g/kg | IV | |

**Important cautions:**
- If diagnosis of cyanide vs. nitrate poisoning is **uncertain**, use **methylene blue** first (reverses methemoglobin from nitrate; does NOT treat cyanide) - giving sodium nitrite to a nitrate-poisoned animal worsens methemoglobinemia
- Sodium nitrite must be given carefully - excessive methemoglobin formation will worsen hypoxia
- Half-dose of sodium thiosulfate may be repeated in 30-60 minutes if signs persist

---

#### Protocol B: Hydroxocobalamin (Vitamin B12a) - Gold Standard

**Mechanism:** Hydroxocobalamin binds directly to cyanide via its cobalt center, forming **cyanocobalamin** (vitamin B12), which is renally excreted. This is a "decoy receptor" approach - no methemoglobin is produced.

**Advantages over nitrite protocol:**
- Does NOT compromise oxygen-carrying capacity (no methemoglobin)
- Does NOT cause hypotension
- Safer when diagnosis is uncertain
- Safe to use in smoke inhalation cases (where CO poisoning co-exists)

**Species-specific dosages:**

| Species | Dose | Route | Duration | Notes |
|---|---|---|---|---|
| **Dogs** | 75-150 mg/kg | IV | Over 7.5 min (once) | Best evidence in companion animals; combine with O₂ |
| **Swine** | 65 mg/kg | IV | Over 2-3 min | |
| **General/cattle** | ≥70 mg/kg | IV | Over 15 min; repeat as needed | Limited large animal data |
| **Combination therapy** | Hydroxocobalamin 150 mg/kg + Sodium thiosulfate 413 mg/kg | IV | - | May be synergistic |

**Side effects:** Transient reddish discoloration of skin/mucous membranes/urine (harmless); transient hypertension

---

#### Protocol C: Thiosulfate Alone (Large Animal / Field Use)

- **Sodium thiosulfate** alone at >500 mg/kg PO has been reported to improve survival in cyanide-poisoned **swine**
- Oral sodium thiosulfate + glycine combination: improved survival in experimentally induced cyanide poisoning in pigs
- Practically useful when IV access is not available in field emergencies
- **Sodium tetrathionate** (2 mol/L, 18 mg/kg IM once) - experimental, limited availability

---

### Step 3: Supportive Care

| Treatment | Rationale |
|---|---|
| **100% oxygen supplementation** | Maximizes available oxygen for remaining functional cytochrome oxidase; especially important in dogs/cats |
| **IV crystalloids** | Correct hypovolemia/hypotension |
| **Sodium bicarbonate** | Treat severe metabolic lactic acidosis (pH <7.1) |
| **Vasopressors** | For refractory hypotension |
| **Anticonvulsants (diazepam)** | For seizures |
| **Cardiac monitoring** | Arrhythmias may occur |

---

### Treatment Decision Algorithm

```
CYANIDE SUSPECTED
       |
       v
Is diagnosis certain?
       |                          |
      YES                        NO (cyanide vs nitrate uncertain)
       |                          |
       v                          v
Sodium nitrite +           Methylene blue first
Sodium thiosulfate         Then reassess
  (+ O2)
       |
       v
Hydroxocobalamin available?
  --> Prefer hydroxocobalamin in dogs/cats/smoke inhalation
  --> Nitrite-thiosulfate in large animal field settings (cost/availability)
       |
       v
Supportive care (O2, fluids, bicarbonate, anticonvulsants)
```

---

## 7. Prevention and Control

1. **Pasture management:**
   - Avoid grazing sorghum, sudangrass, or known cyanogenic species when young (<45 cm height), stressed, or shortly after frost
   - Allow regrowth to reach >60 cm before grazing
   - Graze lightly to prevent selective ingestion of young shoots
   - Hay/silage fermentation reduces cyanogenic glycoside content (ensiling is effective)

2. **Feed testing:**
   - Suspect forages should be tested before feeding (prussic acid test, picrate paper)
   - Forages >500 ppm HCN equivalent are dangerous for ruminants

3. **Introduce slowly:**
   - Limit grazing time on suspect pastures initially; allow animals to fill on safe hay before access

4. **Industrial settings:**
   - Secure storage of cyanide salts; prevent water/feed contamination
   - Antidote kit availability on premises

5. **Breeding/genetic approaches:**
   - Low-cyanogenic cultivars of sorghum and cassava are commercially available

---

## 8. Species-Specific Summary

| Species | Main Source | Key Vulnerability | Treatment Priority |
|---|---|---|---|
| **Cattle** | Sorghum, sudangrass, clover | Most susceptible (rumen hydrolysis); high mortality | Immediate Na nitrite + Na thiosulfate; Na thiosulfate PO in field |
| **Sheep** | Clover, arrow grass, cassava | High susceptibility; similar to cattle | Same as cattle |
| **Horses** | Sorghum (chronic ataxia syndrome more common) | Cystitis-ataxia with chronic exposure | Nitrite-thiosulfate acute; management of chronic syndrome |
| **Pigs** | Cherry pits, cassava, industrial sources | Can vomit (somewhat protective); Na thiosulfate PO effective | Oral thiosulfate viable; hydroxocobalamin IV |
| **Dogs/Cats** | Industrial sources, building fires, insects | Rarely from plants; most often HCN gas exposure | **Hydroxocobalamin** preferred + 100% O₂ |
| **Poultry** | Flaxseed, lima beans, stone fruit pits | Lower susceptibility | Supportive; thiosulfate in water |
| **Exotic pets** | Insects (tent caterpillars, millipedes) | Unique hazard | Remove source; supportive |

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## 9. Key Points

- Cyanide kills by binding cytochrome c oxidase, blocking aerobic ATP production - it is **cellular/histotoxic hypoxia**, not hemoglobin-based hypoxia
- Ruminants are most susceptible because rumen microflora efficiently hydrolyze cyanogenic glycosides
- **Cherry-red mucous membranes and venous blood** are classic - caused by inability to unload oxygen at the tissue level
- Specimen collection must be within **4 hours post-mortem** as HCN is volatile
- **Hydroxocobalamin is the gold standard** antidote (preferred in companion animals, smoke inhalation); nitrite-thiosulfate kit remains practical and widely used for large animals
- Treatment may be impossible in severe acute field cases - **prevention is paramount**

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## References

1. MSD Veterinary Manual - [Cyanide Poisoning in Animals](https://www.msdvetmanual.com/toxicology/cyanide-poisoning/cyanide-poisoning-in-animals)
2. Tintinalli JE et al. *Tintinalli's Emergency Medicine: A Comprehensive Study Guide*, 9th ed. McGraw-Hill, 2020.
3. Walls RM et al. *Rosen's Emergency Medicine: Concepts and Clinical Practice*, 10th ed. Elsevier, 2023.
4. Vasconcelos de Sousa AL, Riet-Correa F, Botelho de Castro M. Sorghum poisoning in ruminants and horses: a review. *Toxicon*. 2025 Jul. PMID: 40286826
5. World Veterinary Journal. An Overview of Cyanide Poisoning in Humans and Animals. *WVJ* 14(4):645-651, 2024.
6. Ng PC et al. Cyanide medical countermeasure development: assessing the efficacy of intramuscular sodium tetrathionate. *J Med Toxicol*. 2025;21(1):9-14.
7. Giantin S et al. Overview of cyanide poisoning in cattle from *Sorghum halepense* and *S. bicolor* cultivars in Northwest Italy. *Animals*. 2024;14(5):743.

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*Document generated by Orris AI | July 13, 2026*