Infectious Disease and Microbiology - Ancylostoma

Basics

Ancylostoma is a genus of parasitic nematodes, or roundworms, that includes important human and animal hookworms. Human disease ranges from a localized pruritic skin eruption caused by migrating animal hookworm larvae to true intestinal hookworm infection associated with chronic blood loss and iron-deficiency anemia.

Important species include:

• Ancylostoma braziliense — primarily a hookworm of dogs and cats

• Ancylostoma ceylanicum — a zoonotic hookworm capable of establishing intestinal infection in humans

• Ancylostoma duodenale — one of the major human hookworms

Microbiologic Characteristics

Ancylostoma species are multicellular helminths belonging to the nematodes.

They have an elongated, cylindrical, unsegmented body and undergo several developmental stages:

Egg → larva → adult worm

The clinical manifestations depend strongly on the species and whether the larvae remain confined to the skin or mature into adult intestinal hookworms.

Epidemiology

Ancylostoma braziliense

A. braziliense occurs predominantly in tropical and subtropical regions.

Dogs and cats are its major hosts. Humans are accidental hosts, usually becoming infected when exposed skin comes into contact with soil or sand contaminated by animal feces.

This organism is an important cause of cutaneous larva migrans.

Ancylostoma ceylanicum

A. ceylanicum occurs particularly in Asia and the Pacific region and is an important zoonotic hookworm of dogs and cats.

Unlike A. braziliense, it can mature into an adult intestinal hookworm in humans.

Ancylostoma duodenale

A. duodenale has historically been prevalent in areas including:

• Mediterranean regions

• North Africa

• South and East Asia

• China

• Southeast Asia

• Parts of South America

• Pacific regions

It is an important cause of human intestinal hookworm disease.

Transmission

Infective hookworm larvae develop in warm, moist soil contaminated by feces.

A. braziliense

Humans typically acquire infection when bare skin contacts contaminated soil or sand.

Common exposure settings include:

• Beaches

• Sandboxes

• Moist soil

• Areas contaminated with dog or cat feces

Because humans are not the normal definitive host, the larvae generally cannot complete their life cycle. Instead, they migrate superficially through the epidermis.

A. duodenale

Infective larvae can penetrate human skin and eventually migrate to the intestine, where they mature into adult worms.

Oral acquisition can also occur with A. duodenale.

Cutaneous Larva Migrans

Clinical Presentation

A. braziliense is a classic cause of cutaneous larva migrans, also called creeping eruption.

After penetrating the skin, the larvae migrate within the superficial epidermis.

The characteristic lesion is:

Intensely pruritic + erythematous + serpiginous + progressively migrating skin track

The feet, legs, buttocks, and other areas exposed to contaminated sand or soil are commonly affected.

Appearance

The eruption usually consists of:

• Raised erythematous tracks

• Curved or serpiginous lesions

• Progressive extension of the track

• Marked pruritus

• Local inflammatory reaction

Scratching may result in excoriations and secondary bacterial infection.

High-Yield Clinical Clue

Recent tropical beach exposure

• ●

Barefoot contact with sand

• ●

Extremely itchy serpiginous migrating lesion

→ Think cutaneous larva migrans due to animal hookworm, classically A. braziliense.

Intestinal Hookworm Disease

Ancylostoma duodenale

A. duodenale produces true intestinal hookworm infection.

Adult worms attach to the mucosa of the small intestine and consume blood. Continued intestinal blood loss can eventually produce clinically significant iron deficiency.

Clinical Manifestations

Light infections may be asymptomatic.

More substantial infections can cause:

• Abdominal discomfort

• Dyspepsia

• Diarrhea

• Fatigue

• Weakness

• Iron-deficiency anemia

• Protein loss in severe infection

Heavy chronic infection is particularly important in patients with poor nutritional reserves.

Anemia

The most important systemic consequence of intestinal hookworm infection is chronic blood loss.

This may produce:

Iron-deficiency microcytic anemia

Patients may consequently develop:

• Pallor

• Fatigue

• Exercise intolerance

• Weakness

• Dyspnea with severe anemia

The severity depends largely on the intensity and duration of infection and the patient’s underlying iron stores.

Diagnosis

Intestinal Hookworm

Diagnosis is primarily established by detecting hookworm eggs on microscopic examination of stool.

Concentration techniques can increase diagnostic sensitivity, particularly when the parasite burden is low.

Because the eggs of different hookworm species may look very similar, routine stool microscopy may identify hookworm infection without reliably establishing the exact species.

Diagnosis of Cutaneous Larva Migrans

Cutaneous larva migrans is generally a clinical diagnosis based on:

Characteristic serpiginous lesion

• ●

Compatible exposure history

Routine stool examination is generally not useful for A. braziliense cutaneous larva migrans because the larvae do not normally mature into egg-producing adult worms in humans.

Treatment

Cutaneous Larva Migrans

Systemic antiparasitic therapy rapidly improves most cases.

Commonly used agents include:

Ivermectin

Ivermectin is often given as a short oral regimen and is highly effective for uncomplicated cutaneous larva migrans.

Albendazole

Albendazole is another effective treatment, particularly when lesions are multiple or extensive.

The source regimen lists:

Albendazole 400 mg orally every 12 hours for 3 days.

Exact dosing and duration may vary according to contemporary guidelines and the clinical situation.

Treatment of

A. duodenale

Intestinal hookworm infection is treated with an anthelmintic agent.

The source regimen lists:

Mebendazole 100 mg orally every 12 hours for 3 days.

Albendazole is also widely used for intestinal hookworm infection.

Additional Treatment

Pyrantel pamoate represents another potential treatment for intestinal hookworm.

More importantly, patients with significant anemia require correction of the consequences of chronic blood loss.

Management may therefore include:

• Oral iron replacement

• Nutritional rehabilitation

• Treatment of severe anemia when clinically necessary

Successful eradication of the parasite without correction of substantial iron deficiency may leave the patient symptomatic for some time.

Prevention

Prevention centers on interrupting contact with infective larvae and reducing environmental fecal contamination.

Important measures include:

• Wear footwear in areas where hookworm contamination may occur.

• Avoid sitting or lying directly on potentially contaminated sand or soil.

• Properly dispose of dog and cat feces.

• Regular veterinary care and deworming of pets can reduce environmental contamination.

• Improve sanitation and prevent human fecal contamination of soil in endemic regions.

A. braziliense

vs

A. duodenale

A. braziliense

Animal reservoir:

→ Dogs and cats

Human role:

→ Accidental host

Major disease:

→ Cutaneous larva migrans

Characteristic finding:

→ Intensely pruritic serpiginous skin tracks

Diagnosis:

→ Usually clinical

Treatment:

→ Ivermectin or albendazole

A. duodenale

Major host:

→ Humans

Major disease:

→ Intestinal hookworm infection

Characteristic complication:

→ Chronic intestinal blood loss

Classic laboratory consequence:

→ Iron-deficiency anemia

Diagnosis:

→ Hookworm eggs in stool

Treatment:

→ Albendazole or mebendazole

Exam Essentials

Organism:

→ Nematode helminth

Classic animal hookworm:

→ A. braziliense

Reservoir of A. braziliense:

→ Dogs and cats

Classic disease caused by A. braziliense:

→ Cutaneous larva migrans

Typical lesion:

→ Intensely pruritic, serpiginous, migrating skin track

Classic exposure:

→ Barefoot walking on contaminated tropical beach or soil

Major human intestinal species:

→ A. duodenale

Major complication of intestinal hookworm:

→ Iron-deficiency anemia from chronic intestinal blood loss

Diagnosis of intestinal infection:

→ Stool microscopy for hookworm eggs

Diagnosis of cutaneous larva migrans:

→ Usually clinical

Treatment of cutaneous larva migrans:

→ Ivermectin or albendazole

Treatment of intestinal hookworm:

→ Albendazole or mebendazole

Key distinction:

A. braziliense → skin disease

A. duodenale → intestinal disease + anemia