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Neutering in the Cat (Feline Gonadectomy)

• Males → Castration (orchiectomy)
• Females → Ovariohysterectomy (OHE) or ovariectomy (OVE)

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1. Terminology & Definitions

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2. Indications

• Population control (primary indication)
• Prevention of reproductive diseases: pyometra, mammary neoplasia, testicular neoplasia, prostatic disorders
• Elimination of unwanted sexual behavior (roaming, spraying, vocalizing, aggression)
• Treatment of hormonally responsive conditions (e.g., estrogen-dependent dermatoses)
• Cryptorchidism (mandatory surgical correction)

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3. Recommended Age

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4. Pre-operative Considerations

Fasting

• Adults: 8–12 hours food; 2 hours water
• Pediatric (<12 weeks): 1–2 hours fast maximum (hypoglycemia risk)

Physical Examination & Labs

• Full PE mandatory; blood work recommended (especially in older cats or those with systemic signs)
• Rule out cryptorchidism in males (inguinal or abdominal palpation)

Anesthesia Protocol (per Fossum & Tranquilli's Veterinary Anesthesia and Analgesia)

• Medetomidine or dexmedetomidine (5–20 µg/kg IM) + butorphanol (0.2–0.4 mg/kg IM) or buprenorphine (0.01–0.02 mg/kg IM)
• Alternatively: acepromazine (0.05 mg/kg) + opioid

• Propofol IV (4–6 mg/kg) or alfaxalone (2–3 mg/kg IV)
• Ketamine/diazepam or ketamine/midazolam IM for fractious cats

• Isoflurane or sevoflurane via mask or endotracheal intubation (ET intubation preferred for OHE)

• NSAIDs (meloxicam 0.2 mg/kg SC) perioperatively
• Local blocks (intratesticular lidocaine for castration, incisional block for OHE)
• Buprenorphine for post-op pain

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5. Male Cat Castration (Orchiectomy)

Anatomy (Fossum, Chapter 26)

• Testes descend shortly before or at birth
• Located caudal to penis in a superficial scrotal position
• Covered by tunica vaginalis
• Spermatic cord contains: ductus deferens, testicular artery & vein, lymphatics, nerves

Positioning & Preparation

• Dorsal or perineal recumbency (dorsal most common)
• Surgical scrub of scrotum; clip hair minimally or pluck

Surgical Technique — Open Castration (most common in cats)

1. Incision: Two separate paramedian scrotal incisions (or single midline), one over each testis
2. Exteriorize testis: Apply pressure at base of scrotum to push testis into incision
3. Incise tunica: Incise parietal tunica vaginalis to expose testis and spermatic cord
4. Ligate: Three-clamp or figure-of-eight ligation technique:
   • Vascular cord (artery + veins) ligated separately from ductus deferens, OR
   • Combined ligation of the entire spermatic cord
   • Use absorbable suture (2-0 or 3-0 chromic gut, polyglycolic acid)
5. Transect cord distal to ligatures
6. Return cord stump to scrotum; verify hemostasis
7. Skin closure: Scrotal incisions typically left open to heal by second intention (reduces seroma/hematoma formation) — this is the standard feline approach per Fossum

• Vas deferens and vascular bundle are tied together in a knot without suture material in some practices; however, Fossum recommends proper ligation to avoid hemorrhage, especially in mature toms

Closed Castration

• Tunica vaginalis is NOT incised; ligature placed around spermatic cord + intact tunica
• Less risk of contamination; preferred in dogs; less commonly described for cats

Cryptorchid Castration

• Inguinal cryptorchid: Incision over inguinal ring; testis exteriorized and removed
• Abdominal (retained): Ventral midline celiotomy or laparoscopic approach; locate testis near caudal pole of kidney along the path of descent; ligate and remove
• Both testes must always be removed — retained testis has ~13x higher risk of neoplasia (Sertoli cell tumor, seminoma)

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6. Female Cat Ovariohysterectomy (OHE / Spay)

Anatomy (Fossum, Chapter 26)

• Ovaries located caudal to kidneys, suspended by mesovarium and ovarian ligament
• Uterus: Y-shaped, two uterine horns + body + cervix
• Broad ligament supports uterine horns; contains uterine vessels
• Ureter courses medial to the ovarian pedicle — critical landmark to avoid

Positioning & Preparation

• Dorsal recumbency
• Clip: ventral abdomen from umbilicus to pubis (wider clip in obese cats)
• Aseptic prep

Surgical Technique — Standard Ventral Midline OHE

• Skin incision: 1–3 cm, midline, 1/3 of the distance from umbilicus to pubis
• Linea alba incised; peritoneum entered bluntly

• Use a spay hook (Snook hook) to locate the uterine horn — pass hook along lateral body wall, sweep medially to catch the horn
• Alternatively, use index finger to sweep horn into view

1. Exteriorize right uterine horn → follow to ovary
2. Create window in mesovarium (avascular area) using blunt dissection
3. Triple clamp technique for ovarian pedicle:
   • Clamp 1: most proximal (stays, guides ligature)
   • Clamp 2: middle (ligature placed in groove made by this clamp after it is slid proximally)
   • Clamp 3: most distal (for traction)
   • Place circumferential ligature (2-0 or 3-0 absorbable) in groove left by clamp 2
   • A transfixation ligature through the pedicle is preferred for larger pedicles
4. Transect between clamp 2 and 3 → inspect pedicle for bleeding → release into abdomen
5. Repeat on left ovary
6. Ligate uterine body: double ligature at uterine body (cranial to cervix)
   • Miller's knot or transfixation ligature
   • Transect uterus cranial to ligatures
7. Inspect all pedicles for hemorrhage before closing
8. Body wall closure: linea alba with simple interrupted or simple continuous 2-0 absorbable
9. Subcutaneous layer: simple continuous absorbable
10. Skin: intradermal (subcuticular) continuous absorbable or skin staples/sutures

Flank Approach OHE

• Used in some countries, particularly for feral cat programs
• Left paralumbar incision through abdominal muscles
• Advantages: less invasive, faster healing, easier post-op monitoring in ferals
• Disadvantage: limited exposure (right ovary less accessible)

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7. Ovariectomy (OVE) as Alternative

• Laparoscopic OVE is increasingly used in cats when uterus is normal
• Evidence supports OVE = OHE outcomes when performed on a healthy uterus
• Advantages: smaller incision, less pain, faster recovery
• Fossum (5th ed.) acknowledges this approach; Tobias & Johnston's Veterinary Surgery provides expanded coverage of laparoscopic OVE

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8. Suture Materials & Instruments

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9. Complications

Intraoperative

Postoperative

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10. Post-operative Care

• Analgesia: Buprenorphine (0.01–0.02 mg/kg q8–12h × 3 days) or meloxicam (0.05 mg/kg PO SID × 3–5 days) — Fossum emphasizes multimodal analgesia
• Activity restriction: 10–14 days
• E-collar (Elizabethan collar): Prevent self-trauma to incision
• Suture removal: 10–14 days (if non-absorbable skin sutures used)
• Dietary counseling: Neutered cats have reduced caloric requirements (~30% reduction); recommend controlled feeding or neutered-specific diet

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11. Special Considerations

Estrous / Pregnant Cat

• OHE can be performed during diestrus or early pregnancy (elective termination)
• Uterine vascularity is greatly increased during estrus/pregnancy → greater hemorrhage risk
• Pedicles are larger; use transfixation ligatures; consider postponing 4–8 weeks post-estrus if elective

Feral / Trap-Neuter-Return (TNR) Programs

• Ear-tip (tipping the left ear) is universal identifier for neutered ferals
• Short anesthetic protocols (ketamine/medetomidine IM) used for field conditions
• Flank spay preferred by some programs for visibility of healed scar

Pediatric (Early-Age) Neutering

• Safe from 8 weeks per current evidence (Howe, JAVMA 2006)
• Faster surgery and recovery
• Anesthetic modifications: maintain normoglycemia (dextrose supplementation), active warming

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12. Summary Comparison Table

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Key References

1. Fossum TW et al. Small Animal Surgery, 4th & 5th editions. Mosby/Elsevier. (Primary reference — Chapter on Reproductive & Genital Surgery)
2. Tobias KM, Johnston SA (eds). Veterinary Surgery: Small Animal. Elsevier Saunders, 2012. (Chapters 95–96)
3. Hedlund CS. Surgery of the Reproductive and Genital Systems. In: Fossum TW, Small Animal Surgery.
4. Howe LM. Surgical methods for neutering cats and their effects. JAVMA, 2006.
5. AVMA/AAHA. Early spay-neuter guidelines. Position statement.
6. Tranquilli WJ, Thurmon JC, Grimm KA (eds). Lumb & Jones' Veterinary Anesthesia and Analgesia, 4th ed.
7. Root Kustritz MV. Clinical Canine and Feline Reproduction. Wiley-Blackwell.

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Mechanisms of Action of 7 Drugs in Dogs & Cats

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1. TRAMADOL

Drug Class

Mechanism of Action

A. Opioid Receptor Agonism

• Tramadol is a weak µ-opioid receptor (MOR) agonist (affinity ~6,000 times less than morphine)
• Its active metabolite O-desmethyltramadol (M1) — produced via hepatic CYP2D6 metabolism — has much higher µ-opioid affinity (200× greater than tramadol itself) and is responsible for most of the opioid analgesic effect
• Activation of µ-receptors in the dorsal horn of the spinal cord, periaqueductal gray, and limbic system reduces nociceptive transmission by:
  • Increasing K⁺ conductance → hyperpolarization of neurons
  • Decreasing Ca²⁺ conductance → reduced neurotransmitter release
  • Inhibiting adenylyl cyclase (↓ cAMP)

B. Monoamine Reuptake Inhibition

• Tramadol (parent compound) inhibits the reuptake of serotonin (5-HT) and norepinephrine (NE) at presynaptic nerve terminals
• This potentiates the descending inhibitory pain pathways from the brainstem (locus coeruleus → dorsal horn), which normally suppress nociceptive signal transmission
• The (+)-enantiomer preferentially inhibits 5-HT reuptake; the (−)-enantiomer preferentially inhibits NE reuptake

Species-Specific Considerations (Critical)

• Half-life: Dog ~1–2 hours (tramadol); Cat ~3–4 hours — cats have slower elimination
• Dose: Dog: 2–5 mg/kg PO q8–12h; Cat: 1–4 mg/kg PO q8–12h (bitter taste is a compliance issue in cats)

Summary Diagram

Tramadol
   ├── µ-opioid receptor activation (↑ K⁺, ↓ Ca²⁺, ↓ cAMP)
   │        └── Spinal cord & supraspinal pain inhibition
   └── Serotonin + NE reuptake inhibition
            └── ↑ Descending inhibitory pathway activity

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2. REMDESIVIR

Drug Class

Mechanism of Action

Step-by-Step Activation (Prodrug Mechanism)

1. Remdesivir is a phosphoramidate prodrug of an adenosine C-nucleoside analogue (GS-441524)
2. After cellular uptake, it undergoes intracellular hydrolysis by carboxylesterases (CES1) → releasing the nucleoside monophosphate
3. Cellular kinases sequentially phosphorylate the monophosphate → active triphosphate form (GS-443902)
4. GS-443902 acts as a competitive substrate and chain terminator for the viral RNA-dependent RNA polymerase (RdRp):
   • It competes with adenosine triphosphate (ATP) for incorporation into the nascent viral RNA strand
   • Once incorporated, RdRp continues synthesis for 3 additional nucleotides before stalling ("delayed chain termination")
   • This mechanism evades the viral proofreading exonuclease (nsp14), making it effective against coronaviruses and other RNA viruses

Viruses Targeted

• SARS-CoV-2 (COVID-19), Ebola, Marburg, RSV, MERS-CoV
• Feline Infectious Peritonitis (FIP) virus (feline coronavirus) — this is the primary veterinary application

Veterinary Context (Dogs & Cats)

• GS-441524 (the active nucleoside metabolite) is used extensively in cats for FIP treatment because it is more bioavailable orally and bypasses the prodrug activation step
• Remdesivir itself has been used in cats with FIP when GS-441524 is unavailable
• In dogs, remdesivir has been investigated for canine coronavirus and other RNA viral infections, but clinical use is limited
• Mechanism in feline coronavirus: inhibits the feline CoV RdRp (nsp12) → prevents viral genome replication → viral load reduction → resolution of FIP clinical signs

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3. METRONIDAZOLE

Drug Class

Mechanism of Action

Activation Pathway

1. Metronidazole enters cells by passive diffusion due to its lipophilicity
2. In anaerobic organisms (bacteria and protozoa), low-potential electron transport proteins (ferredoxin, flavodoxin) reduce the nitro group (–NO₂) of metronidazole to a toxic nitro radical anion (–NO₂⁻)
3. This reactive intermediate is:
   • Cytotoxic: directly damages DNA by causing strand breakage (single and double-strand breaks) and inhibiting DNA repair → cell death
   • Irreversible: the reduction product is not recycled back to the parent compound
4. Aerobic organisms cannot reduce metronidazole because their higher oxygen tension reoxidizes any intermediates before they cause damage → selective toxicity against anaerobes and microaerophiles

Spectrum of Activity

Veterinary Applications (Dogs & Cats)

• Dogs: Giardiasis, anaerobic infections, hepatic encephalopathy (reduces ammonia-producing gut flora), IBD (immunomodulatory effect)
• Cats: Giardiasis, anaerobic diarrhea, oral/dental infections, hepatic encephalopathy
• Additional anti-inflammatory mechanism: Metronidazole also inhibits cell-mediated immunity (suppresses neutrophil migration, lymphocyte proliferation) — clinically useful in IBD independent of antimicrobial action

Species-Specific Notes

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4. OMEPRAZOLE

Drug Class

Mechanism of Action

Step-by-Step Mechanism

1. Prodrug activation: Omeprazole is a substituted benzimidazole prodrug that is acid-labile and inactive at neutral pH
2. Administered as an enteric-coated formulation to prevent stomach acid degradation
3. After intestinal absorption, omeprazole diffuses to the secretory canaliculi of gastric parietal cells — the most acidic compartment in the body (pH ~1–2)
4. At low pH, omeprazole undergoes protonation and rearrangement to form an active sulfenamide (tetracyclic compound)
5. This active form irreversibly binds by covalent disulfide bonds to cysteine residues (Cys813, Cys892) on the H⁺/K⁺-ATPase (proton pump) — the final common pathway of gastric acid secretion
6. Inhibition of H⁺/K⁺-ATPase → prevents exchange of H⁺ into the stomach lumen and K⁺ into the parietal cell → profound, long-lasting suppression of gastric acid secretion (up to 24–72 hours)

Why It Is Superior to H₂ Blockers (e.g., ranitidine, famotidine)

• PPIs block the final step of acid secretion regardless of the stimulus (histamine, gastrin, acetylcholine)
• H₂ blockers only block histamine-stimulated secretion
• Effect is more complete and sustained

Veterinary Applications (Dogs & Cats)

Species-Specific Notes

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5. ERYTHROPOIETIN (Epoetin Alfa / Darbepoetin)

Drug Class

Mechanism of Action

Endogenous Erythropoietin Background

• Erythropoietin (EPO) is a glycoprotein hormone (MW ~34,000 Da) produced primarily by peritubular interstitial cells in the renal cortex in response to tissue hypoxia
• In anemia or hypoxia: HIF-1α (hypoxia-inducible factor-1α) accumulates → transcription of EPO gene → ↑ EPO secretion → stimulates red blood cell production

Receptor-Level Mechanism

1. Exogenous EPO (e.g., human recombinant EPO / rHuEPO) binds to the erythropoietin receptor (EPOR) on the surface of erythroid progenitor cells in bone marrow (burst-forming units erythroid [BFU-E] and colony-forming units erythroid [CFU-E])
2. EPOR is a homodimeric type I cytokine receptor — binding causes receptor dimerization
3. Dimerization activates JAK2 (Janus kinase 2) → phosphorylates STAT5 (Signal Transducer and Activator of Transcription 5)
4. Phosphorylated STAT5 translocates to the nucleus → activates transcription of anti-apoptotic genes (Bcl-xL, Bcl-2)
5. Net effects:
   • ↑ Proliferation of erythroid progenitors
   • ↑ Differentiation toward mature red blood cells
   • ↑ Survival of erythroid precursors (anti-apoptotic)
   • ↑ Reticulocyte release from bone marrow
   • ↑ Iron utilization (requires adequate iron, B12, folate as substrates)

Veterinary Applications

Species-Specific Notes (Critical)

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6. ONDANSETRON

Drug Class

Mechanism of Action

Background: The Role of 5-HT₃ in Vomiting

• Enterochromaffin (EC) cells in the GI mucosa release serotonin (5-HT) in response to chemotherapy, radiation, toxins, gastric distension, and vagal stimulation
• Released 5-HT stimulates 5-HT₃ receptors on:
  1. Vagal afferent neurons (in the GI wall) → signals travel to the nucleus tractus solitarius (NTS) → vomiting center activation
  2. Area postrema (chemoreceptor trigger zone, CTZ) neurons → central emetic trigger
• This pathway is the primary mechanism of acute chemotherapy-induced nausea and vomiting (CINV) and many other emetic stimuli

Receptor-Level Mechanism

1. Ondansetron is a highly selective competitive antagonist at 5-HT₃ receptors — an ionotropic ligand-gated ion channel (Na⁺/K⁺/Ca²⁺ channel)
2. Ondansetron binds to the orthosteric site on the receptor → prevents 5-HT from binding → channel remains closed → no depolarization of vagal afferents
3. At peripheral (GI) 5-HT₃ receptors: blocks initiation of the vagal afferent emetic signal
4. At central (CTZ/NTS) 5-HT₃ receptors: blocks direct central serotonergic activation of the vomiting center
5. Result: complete blockade of the serotonin-mediated emetic reflex arc

Veterinary Applications

Species-Specific Notes

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7. FILGRASTIM (G-CSF / Granulocyte Colony-Stimulating Factor)

Drug Class

Mechanism of Action

Endogenous G-CSF Background

• G-CSF is a naturally occurring glycoprotein cytokine (MW ~19,600 Da) produced by:
  • Monocytes/macrophages, endothelial cells, fibroblasts, stromal cells
  • Production is dramatically ↑ by infection, inflammation, endotoxin, TNF-α, IL-1β
• Primary role: regulation of neutrophil production, maturation, and function

Receptor-Level Mechanism

1. Filgrastim (recombinant G-CSF) binds to G-CSF receptors (G-CSFR / CD114) — a type I cytokine receptor — on the surface of:
   • Myeloid progenitor cells (granulocyte-committed progenitors: CFU-G)
   • Mature neutrophils
   • Endothelial cells, platelets (minor)
2. Receptor homodimerization upon binding → activates intracellular signaling:
   • JAK1/JAK2 → STAT3/STAT5 pathway → transcriptional activation of proliferation/differentiation genes
   • MAPK/ERK pathway → cell proliferation
   • PI3K/Akt pathway → cell survival (anti-apoptosis)
3. Net effects on bone marrow and blood:
   • ↑ Proliferation of neutrophil progenitors (CFU-G)
   • ↑ Differentiation and maturation of myeloid precursors → neutrophils
   • ↑ Release of mature neutrophils from bone marrow (mobilization)
   • ↑ Functional activation of neutrophils: enhanced phagocytosis, oxidative burst, ADCC
   • ↑ Neutrophil survival (inhibits apoptosis via Bcl-2 upregulation)
   • Mobilizes hematopoietic stem cells (HSCs) into peripheral blood (used for stem cell collection)
4. Rapid response: Neutrophil counts begin rising within 24 hours; peak effect at 2–5 days

Veterinary Applications

Species-Specific Notes (Critical)

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Comprehensive Comparison Table

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Key References

1. Plumb DC. Plumb's Veterinary Drug Handbook, 9th ed. Wiley-Blackwell, 2018.
2. Riviere JE, Papich MG (eds). Veterinary Pharmacology & Therapeutics, 10th ed. Wiley-Blackwell, 2018.
3. Boothe DM. Small Animal Clinical Pharmacology & Therapeutics, 2nd ed. Saunders/Elsevier, 2012.
4. Papich MG. Saunders Handbook of Veterinary Drugs, 4th ed. Elsevier, 2016.
5. Ettinger SJ, Feldman EC (eds). Textbook of Veterinary Internal Medicine, 8th ed. Elsevier, 2017.
6. Tizard IR. Veterinary Immunology, 10th ed. Elsevier, 2018.
7. Rang HP, Ritter JM, Flower RJ, Henderson G. Rang & Dale's Pharmacology, 9th ed. Elsevier, 2020.
8. Harrison's Principles of Internal Medicine, 21st ed. (Remdesivir — p. 5513).
9. Pedersen NC et al. Treatment of feline infectious peritonitis using the nucleoside analogue GS-441524. J Feline Med Surg, 2019.
10. Pypendop BH, Ilkiw JE. Pharmacokinetics of tramadol and its metabolite O-desmethyl-tramadol, in cats. JVIM, 2008.