Neutering Surgery in Dogs and Cats — Fossam Technique

What is "Fossam"?

Overview of Male Neutering (Orchiectomy / Castration)

Anatomy Review

• Testes — housed in the scrotum, suspended by the spermatic cord
• Spermatic cord — contains the vas deferens (ductus deferens), testicular artery & veins (pampiniform plexus), lymphatics, and nerves
• Tunica vaginalis — peritoneal serous layer encasing each testis; determines open vs. closed technique
• Cremaster muscle — thin muscle layer on the cord
• Scrotal fossa (Fossam) — the anatomical depression/pouch formed by the scrotum that cradles each testis

Surgical Approaches

1. Prescrotal Approach (Most Common in Dogs)

1. Place the dog in dorsal recumbency
2. Clip and aseptically prepare the prescrotal and scrotal region
3. Push one testis cranially through the scrotum to tent the prescrotal skin
4. Make a 2–4 cm midline incision over the tented testis through skin and subcutaneous tissue
5. Incise the subcutaneous fascia to expose the testis within its tunica
6. Closed technique: Exteriorize the testis with its intact tunica vaginalis; ligate the spermatic cord as a unit with the tunica intact
7. Open technique (via the Fossam approach): Incise the tunica vaginalis to expose the bare testis and cord components; ligate the vas deferens and vascular pedicle separately
8. Repeat for the second testis through the same or a second incision
9. Close subcutaneous tissue (absorbable suture) and skin (non-absorbable or intradermal)

2. Scrotal Fossa (Fossam) Approach — Cats & Some Dogs

CAT — Scrotal/Fossam Technique (Most Common)

Why open healing in cats? The scrotal fossa incision is small and the highly vascular scrotal tissue heals rapidly. Leaving it open reduces fluid accumulation.

Open vs. Closed Technique — Fossam Context

Anesthesia Protocol

Dogs

• Pre-medication: Acepromazine + opioid (butorphanol or buprenorphine), or dexmedetomidine + opioid
• Induction: Propofol IV or alfaxalone IV
• Maintenance: Isoflurane or sevoflurane via endotracheal tube
• Analgesia: NSAIDs (meloxicam), local infiltration (lidocaine/bupivacaine into spermatic cord), opioids

Cats

• Pre-medication: Dexmedetomidine + ketamine + butorphanol (IM)
• Induction: Often mask induction with isoflurane, or continued sedation is sufficient for short procedures
• Maintenance: Isoflurane via mask or intubation depending on duration
• Analgesia: Buprenorphine (sublingual or IM), meloxicam, local block of spermatic cord

Ligation Methods at the Fossam

Three-Clamp Technique (Cats & Dogs)

1. Place 3 hemostatic clamps across the spermatic cord sequentially
2. Pass absorbable suture (e.g., 2-0 or 3-0 Vicryl) in the groove created by clamp #1 and tie a circumferential ligature
3. Optionally place a transfixion ligature at clamp #2
4. Transect the cord between clamp #2 and #3
5. Release clamp #3 and observe the stump for hemorrhage

Figure-8 (Autoligation) in Cats

• Separate the vas deferens from the vascular cord
• Use the vas deferens itself to wrap around the vascular pedicle and tie
• No suture material required — reduces foreign body reaction risk
• Only appropriate for cats (small, pliable structures)

Postoperative Care

• Elizabethan collar for 7–10 days (dogs especially)
• Monitor for scrotal hematoma, swelling, or wound dehiscence
• Restricted activity for 10–14 days
• NSAIDs for 3–5 days post-op (dogs); buprenorphine for 1–3 days (cats)
• Suture removal at 10–14 days (if non-absorbable)
• Cats: inspect open wound daily; small amount of swelling/discharge is normal for 48–72 hours

Complications

Key Points Summary

• Fossam = scrotal fossa — the anatomical point of access in the direct scrotal incision technique
• In cats, the scrotal fossa (Fossam) approach is the standard — bilateral scrotal incisions, open technique, wounds often left open
• In dogs, a prescrotal midline approach is more common in adults; scrotal fossa may be used in puppies
• Open technique (incising the tunica at the fossam) allows separate ligation of vas deferens and vascular pedicle
• Proper analgesia (local nerve block of the spermatic cord) is critical for intraoperative and postoperative comfort

Why Rabies Vaccine is Given After 3 Months in Dogs and Cats

1. Maternal Antibody Interference (Most Important Reason)

• Colostrum (first milk) in the first 24–48 hours after birth — this is the primary source
• Transplacental transfer (minor contribution in dogs and cats)

How MDAs Block Vaccination

If rabies vaccine is given while MDA levels are still high, the maternal antibodies bind and neutralize the vaccine antigen — the animal appears vaccinated but has no protective immunity. This is called MDA interference.

2. Immune System Maturity

• B lymphocytes (antibody-producing cells) are present but not fully differentiated
• T lymphocytes lack full functional capacity for antigen presentation and memory formation
• Antigen-presenting cells (dendritic cells, macrophages) are not fully active

• Process rabies antigen via MHC class II pathways
• Activate CD4+ helper T cells
• Stimulate B cells to undergo class switching and produce IgG antibodies
• Generate immunological memory (long-lived plasma cells and memory B cells)

3. The "Window of Susceptibility"

Birth ──── 6 weeks ──── 8–10 weeks ──── 12 weeks ──── Vaccine immunity established
  |              |              |               |
High MDAs     MDAs waning   "Window of    MDAs low enough
(blocks vaccine) (inconsistent)  susceptibility"  for full seroconversion

4. Rabies Vaccine Biology

• It requires a competent adaptive immune response to generate neutralizing antibodies — it does not replicate in the host and thus cannot overcome MDA by spreading
• Compared to modified-live vaccines (used for some other diseases), killed vaccines are even more susceptible to MDA interference because they present antigen only once at the injection site
• Studies show that inactivated rabies vaccines given before 12 weeks have significantly lower seroconversion rates (some as low as 30–50%), while vaccination at ≥12 weeks achieves >95% seroconversion

5. Regulatory and Legal Reasons

• Regulatory agencies (USDA, EMA, national veterinary authorities) license rabies vaccines specifically for use at or after 12 weeks of age
• A vaccine given before the licensed minimum age is considered off-label and may not be legally recognized for travel, import/export certificates, or local anti-rabies ordinances
• If an animal vaccinated before 12 weeks bites a human, the vaccination may be deemed legally invalid, requiring quarantine as if unvaccinated

6. Duration of Immunity and Booster Schedule

The 1-year booster after the initial dose exists precisely because a small percentage of animals vaccinated at 12 weeks may still have had just enough residual MDA to partially blunt the first response. The booster at 1 year guarantees solid, verified immunity.

7. Why Not Earlier (e.g., 8 weeks)?

• They use modified-live virus that can partially overcome MDA
• A series of doses (every 3–4 weeks) is designed to "catch" the animal as MDAs progressively decline

• High probability of MDA clearance
• Early enough protection before peak exposure risk
• Legal and regulatory validity

Summary