Menu

    1. Home
    2. About
    3. Learning Hub
    4. Clinical Reference
    5. Resources
    6. Contact

Screw Worm Infestation (Cochliomyia hominivorax)


Introduction

Screw worm infestation (myiasis) is a severe parasitic condition caused by the larvae of Cochliomyia hominivorax, commonly known as the New World screwworm. Unlike many other fly larvae that feed on dead tissue, screwworm larvae invade and feed on living tissue, causing rapidly progressive and often fatal wounds in animals.

Etiology
Causative Agent
  • Cochliomyia hominivorax (New World screwworm fly)
Parasite Characteristics
  • Adult flies resemble metallic blue-green blowflies
  • Larvae (maggots) are obligate parasites of living tissue
  • Larvae have a screw-like appearance (spiral rows of spines)
Life Cycle
  • Adult female lays eggs on edges of wounds or natural openings (e.g., navel, vulva)
  • Eggs hatch within 12–24 hours
  • Larvae penetrate and feed on living tissue
  • Larval stage lasts 5–7 days
  • Mature larvae drop to ground and pupate in soil
  • Adult flies emerge to continue cycle
Epidemiology
Species Affected
  • Cattle (most affected)
  • Sheep, goats, pigs, horses
  • Dogs and wildlife
  • Humans (zoonotic cases)
Geographical Distribution
  • Tropical and subtropical regions (Americas)
  • Eradicated in some regions through control programs
Transmission
  • Egg deposition on wounds, surgical sites, or mucous membranes
  • No direct animal-to-animal transmission
Risk Factors
  • Open wounds (castration, dehorning, branding)
  • Parturition (umbilical wounds, vaginal discharge)
  • Poor hygiene and wound management
  • Warm and humid climates
Pathogenesis
Larval Invasion
  • Larvae penetrate wounds and burrow deeply into tissues
  • Feed on living tissue, unlike most other maggots
Tissue Destruction
  • Extensive tissue necrosis and inflammation
  • Enlargement of wounds over time
Secondary Effects
  • Secondary bacterial infections
  • Toxemia in severe cases
  • Death if untreated
Clinical Signs
Local Signs
  • Rapidly enlarging wounds
  • Foul-smelling discharge
  • Visible maggots within lesions
  • Bleeding and tissue destruction
Behavioral Signs
  • Restlessness and discomfort
  • Reduced feeding
  • Isolation from herd
Advanced Cases
  • Severe debilitation
  • Systemic illness (toxemia)
  • Death
Summary for Practitioners

Screwworm infestation should be suspected in animals with rapidly worsening wounds containing actively feeding larvae and foul odor.

Diagnosis
Clinical Diagnosis
  • Observation of characteristic wound and larvae
  • History of recent injury or surgery
Laboratory Diagnosis
  • Identification of larvae (morphology)
  • Differentiation from other myiasis-causing flies
Differential Diagnosis
  • Other forms of myiasis (secondary blowfly infestations)
  • Abscesses
  • Traumatic wounds
Summary for Practitioners

Accurate identification of screwworm larvae is essential due to the severity and regulatory importance of the disease.

Treatment
Larval Removal
  • Manual removal of all larvae from wound
  • Thorough cleaning and debridement
Topical Treatment
  • Application of insecticides (organophosphates, ivermectin-based products)
  • Wound antiseptics
Systemic Therapy
  • Antibiotics for secondary infections
  • Supportive therapy (fluids, nutrition)
Summary for Practitioners

Immediate treatment is critical to prevent rapid progression and death.

Control and Prevention
Wound Management
  • Prompt treatment of all wounds
  • Use of insect repellents on surgical sites
Husbandry Practices
  • Avoid procedures during peak fly seasons
  • Maintain good hygiene
Biological Control Programs
  • Sterile insect technique (release of sterile male flies)
  • Successful eradication programs in some regions
Summary for Practitioners

Prevention relies heavily on wound management and participation in regional control programs.

Zoonotic Importance

Screwworm can infest humans, especially in untreated wounds, making it a significant zoonotic and public health concern in endemic regions.

Economic Importance

Screwworm infestation causes major economic losses due to mortality, reduced productivity, treatment costs, and trade restrictions in affected regions.

Summary

Screwworm infestation is a severe form of myiasis caused by Cochliomyia hominivorax, characterized by larvae feeding on living tissue. It requires urgent treatment and strong preventive measures due to its rapid progression and economic impact.