Infectious Disease and Microbiology – Myiasis Agents

Overview

Myiasis is an infestation of humans or other vertebrates by the larval stages (maggots) of certain dipterous flies. The larvae may invade or temporarily inhabit the skin, subcutaneous tissues, wounds, or body cavities, where they feed on living or necrotic tissue or body fluids.

Disease is particularly associated with tropical and subtropical regions, although cases can occur elsewhere, especially after travel to endemic areas.

Classification

Type: Arthropod infestation

Organisms: Larvae of flies belonging to the order Diptera

Important genera capable of causing human myiasis include:

• Calliphora

• Chrysomya

• Cochliomyia

• Cordylobia

• Dermatobia

• Gasterophilus

• Lucilia

• Phormia

• Sarcophaga

• Wohlfahrtia

Numerous species within these genera can produce myiasis.

Microbiologic Characteristics

The causative organisms are:

Fly larvae (maggots)

and therefore are:

Arthropods

rather than bacteria, fungi, protozoa, or helminths.

Adult flies generally deposit eggs or larvae directly or indirectly onto the host, after which the larvae develop within or upon human tissues.

Important Terminology

Myiasis

= infestation of living humans or animals by dipterous fly larvae

The word describes the clinical infestation rather than one particular species of fly.

Epidemiology

Myiasis occurs most frequently in:

Tropical and subtropical climates

Risk depends heavily on the responsible fly species and its geographic distribution.

Risk Factors

Potential risk factors include:

• Residence in or travel to endemic regions

• Exposure to biting insects

• Sleeping outdoors

• Open wounds

• Poor wound care

• Contact with livestock or animals

• Inadequate protection from flies

Some forms occur in completely healthy travelers after ordinary outdoor exposure.

Transmission

Transmission varies considerably among fly species.

Some flies:

Deposit eggs directly onto skin or wounds

while others use an indirect mechanism.

For example, some species attach their eggs to another arthropod, such as a mosquito, which subsequently transports the eggs to human skin.

Dermatobia hominis

Dermatobia hominis, the human botfly, is an important cause of:

Furuncular myiasis

in tropical regions of the Americas.

Its eggs can be transported to humans by blood-feeding insects.

Dermatobia Transmission Pattern

Adult botfly

↓

Eggs attached to mosquito or another insect

↓

Mosquito contacts human skin

↓

Larva enters skin

↓

Furuncle-like lesion develops

↓

Larva matures beneath skin

Cordylobia Species

Cordylobia anthropophaga, sometimes called the tumbu fly, is another important cause of furuncular myiasis, particularly in:

Sub-Saharan Africa

Larvae may penetrate skin after contact with contaminated soil or clothing.

Clinical Forms of Myiasis

Myiasis can be classified according to the anatomical site involved.

Important forms include:

• Furuncular myiasis

• Wound myiasis

• Migratory myiasis

• Nasal myiasis

• Aural myiasis

• Ophthalmic myiasis

• Intestinal or other cavity-associated forms

Furuncular Myiasis

The classic presentation is a:

Furuncle-like skin lesion containing a developing larva

The lesion typically has a central opening that allows the larva to obtain oxygen.

Clinical Manifestations

Patients may experience:

• Local swelling

• Erythema

• Pain

• Pruritus

• Serous or bloody drainage

• Sensation of movement within the lesion

• Visible central opening

Occasionally, part of the larva may become visible through the opening.

High-Yield Clinical Pattern

Traveler returning from a tropical region

• ●

Persistent boil-like lesion

• ●

Central punctum

• ●

Sensation of movement

→ Think furuncular myiasis

Wound Myiasis

Some fly species deposit eggs or larvae into:

Open wounds

The larvae subsequently develop within the wound.

Clinical manifestations may include:

• Visible larvae

• Increased wound drainage

• Local inflammation

• Tissue destruction

• Secondary bacterial infection

Certain species can invade viable tissue and produce substantial tissue damage.

Local Inflammation

As described in the source, larvae may parasitize:

Skin and subcutaneous tissues

resulting in:

Local inflammatory reactions

The severity ranges from a small localized lesion to extensive tissue destruction depending on the species and type of myiasis.

Diagnosis

The primary diagnosis is made by:

Identification of larvae within the affected area

A careful physical examination is often sufficient to recognize the infestation.

Species Identification

When necessary, the removed larva can be examined morphologically to identify the causative species.

Features useful for identification may include:

• Body segmentation

• Spines

• Respiratory structures

• Posterior spiracles

• Other characteristic larval structures

Geographic exposure also provides an important diagnostic clue.

Imaging

Imaging is generally unnecessary for superficial disease.

However, ultrasound or other imaging can occasionally help demonstrate a larva when the diagnosis is uncertain or the organism is located relatively deeply.

Treatment

The major goal is:

Complete removal of the larva

without leaving significant larval material within the tissue.

Occlusion Technique

For appropriate forms of furuncular myiasis, the source describes occlusion of the larva’s respiratory opening using:

Petroleum jelly (Vaseline)

The principle is:

Cover central breathing opening

↓

Reduce larval oxygen supply

↓

Larva moves toward the surface

↓

Larva can emerge or become easier to extract

Removal with Forceps

Once accessible, the larva may be:

Carefully removed with forceps

The source cautions against forceful extraction.

This is particularly relevant when the larva is tightly anchored by backward-facing spines.

Why Forceful Removal Should Be Avoided

Forceful extraction may:

• Rupture the larva

• Leave larval fragments within the wound

• Increase inflammation

• Cause additional tissue injury

Therefore, removal should be controlled and complete.

Surgical Removal

When occlusion or gentle extraction is unsuccessful:

Surgical removal

may be necessary.

A small incision can enlarge the opening and permit intact extraction of the larva.

Wound Myiasis Treatment

For wound-associated infestation, management may require:

Mechanical removal of all larvae

• ●

Wound irrigation and debridement

• ●

Appropriate wound care

Secondary bacterial infection should be treated when clinically present.

Treatment Principle

Furuncular myiasis

↓

Identify breathing opening

↓

Occlusion may encourage larval emergence

↓

Gentle complete extraction

↓

If unsuccessful:

Surgical removal

Prevention

Prevention is particularly important when traveling in endemic tropical regions.

Measures include:

• Protective clothing

• Insect repellents

• Appropriate mosquito/fly control

• Covering open wounds

• Proper wound hygiene

• Avoiding exposure of wounds to flies

• Appropriate handling of clothing in areas where particular myiasis-producing flies occur

Myiasis vs. Cutaneous Larva Migrans

Myiasis

→ Fly larvae

→ Arthropods

→ Furuncle-like or wound-associated lesions

→ Larva may be directly visible

→ Central breathing pore may occur

Cutaneous larva migrans

→ Usually animal hookworm larvae

→ Nematode helminths

→ Characteristic serpiginous migrating tracks

→ No mature fly larva within a furuncular cavity

Myiasis vs. Tungiasis

Myiasis

→ Dipterous fly larvae invade tissue

Tungiasis

→ Adult female Tunga penetrans flea embeds within skin

Both are arthropod-related skin infestations but involve completely different organisms.

High-Yield Clinical Pattern

Tropical or subtropical exposure

• ●

Boil-like inflammatory skin lesion

• ●

Central opening

• ●

Visible or moving larva

→ Think MYIASIS

→ Dipterous fly larva

Exam Essentials

Disease: Myiasis

Cause: Larvae of dipterous flies

Organism type: Arthropod

Important genera: Dermatobia, Cordylobia, Cochliomyia, Chrysomya, Lucilia, Sarcophaga, Wohlfahrtia and others

Distribution: Especially tropical and subtropical regions

Major tissue involvement: Skin and subcutaneous tissue

Classic form: Furuncular myiasis

Classic lesion: Boil-like lesion with central respiratory opening

Other form: Wound myiasis

Diagnosis: Direct identification of larvae

Treatment principle: Complete larval removal

Source technique: Occlusion with petroleum jelly followed by gentle extraction

Important caution: Avoid forceful removal

If extraction fails: Surgical removal

Wound disease: Remove larvae, debride/clean wound, and manage secondary infection if present

Prevention: Fly avoidance, protective clothing, wound coverage, and appropriate hygiene

Key clinical pearl: Myiasis is an infestation by dipterous fly larvae. The classic clue is a traveler from a tropical region with a persistent furuncle-like lesion containing a central breathing pore and sometimes a sensation of movement. Treatment centers on complete, gentle removal of the larva; occlusion of the breathing opening can facilitate extraction, while forceful removal should be avoided.