Hookworm

 

Hookworm

Introduction

• Hookworm infection is a parasitic infection caused by the soil-transmitted helminths Necator americanus and Ancylostoma duodenale. Hookworms are blood-feeding nematode parasites that can cause mild diarrhea in both people and animals. Severe infection morbidity includes intestinal blood loss, anemia, and protein malnutrition.
  
• Zoonotic infection with hookworm species that do not use humans as a definitive host, such as Ancylostoma braziliense and Ancylostoma caninum (whose normal hosts are dogs and cats), can cause cutaneous larval migrans.

Geographical Distribution

• Hookworms are widespread in the tropics and subtropics. They are mostly found in areas with moist, warm climates where larvae can survive in the environment.
• N. americanus is the major cause of hookworm infection in the Americas, sub-Saharan Africa, and Asia.
• A. duodenale is found in more scattered, focal environments, specifically Europe and the Mediterranean.

Habitat

Adult hookworms live in the small intestine. They reside particularly in the jejunum The common site of infection for adult worms is the upper small intestine. Adult worms live in the lumen of the small intestine.

Morphology

• Adult worms:
• They look like an odd piece of thread and are about 1cm long. When living, they are white or light pinkish.
• The anterior part is curved, giving them the name hookworm. This bend forms a definitive hook shape.
• The head is slightly bent (hook). They have hooked mouthparts with which they attach to the wall of the gut, puncturing blood vessels and feeding on blood. The mouth carries characteristic teeth.
• Sex differences: The female is slightly larger than the male. Females are typically 12.5 mm, while males are 8mm. N. americanus males are usually 5-9 mm long, and females are about 1 cm long, making N. americanus generally smaller than A. duodenale.
• The male's posterior end is expanded in an umbrella-like fashion to form a copulatory bursa. This bursa is supported by fleshy rays. The pattern of distribution of these rays helps distinguish species. The cloaca is in the bursa, with two long, bristle-like spicules projecting from it.
• 
  Species differences in mouthparts: The mouth capsule of Ancylostoma duodenale shows the presence of four "teeth," two on each side. Necator americanus possesses a pair of cutting plates in the buccal capsule.
• Species differences in shape: A. duodenale has a single curve, looking like a 'C', while N. americanus has double curves, looking like an 'S'. The hook shape is much more defined in Necator than in Ancylostoma

                                    

• Eggs:
• Hookworm eggs are oval.
• They have a thin, colorless, hyaline shell. They are not bile-stained.
• They measure about 60–75 µm in length and 35–40 µm in width.
• Freshly passed eggs contain segmented blastomeres, typically 2-8 cells. When released in the intestine, they contain an unsegmented ovum, which develops during passage.

Life Cycle

• Eggs are passed in the stool.
• Under favorable conditions (moisture, warmth, shade), larvae hatch from eggs in 1 to 2 days.
• The released rhabditiform larvae (L1, the feeding non-infective stage) grow in the feces and/or soil, feeding on microbes.
• After 5 to 10 days and two molts, they become filariform (third-stage or L3) larvae that are infective. The L3 stage is non-feeding.
• On contact with the human host, these infective larvae penetrate the skin. This is the primary cutaneous route of transmission.
• (Note- Larvae can also infect orally by ingestion of L3 larvae present in the soil, although this occurs less frequently. Ingested larvae in the mouth develop directly into adult worms. A. duodenale can infect both through skin penetration and orally, while N. americanus larvae only infect through skin penetration.)
• Once they penetrate the skin, the larvae are carried through blood vessels to the heart and then to the lungs, and moulting occurs. They then penetrate the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed.
• The larvae reach the small intestine, where they reside and mature into adults. The process from skin penetration to adult development takes about 5-9 weeks.
• Adult worms live in the lumen of the small intestine. In the small intestine, male and female worms mate.
• Female adult worms release eggs (approximately 9,000-10,000 eggs/day for N. americanus and 25,000-30,000 eggs/day for A. duodenale).
• Eggs appear in the stool and are passed in the feces.
• Most adult worms are eliminated in 1 to 2 years, but longevity may reach several years.

Pathogenesis

The pathogenesis and clinical symptoms depend on the site of the worms and their burden. Light infections may not be noticed.

• Pathogenicity of infective larvae:
• At the site of skin penetration, the infective filariform larva produces a local reaction called ground itch. This is a pruritic, erythematous, papular rash.
• Ground itch is also known as cutaneous larval migrans or creeping eruption. It is an allergic manifestation that can be more severe in Necator infection. Scratching can lead to secondary bacterial infections.
• For zoonotic hookworms (A. braziliense, A. caninum) that do not use humans as definitive hosts, the larvae migrate beneath the skin, which is the hallmark of cutaneous larva migrans. The movement beneath the skin leaves reddened, raised "tracts" or lines. As juveniles migrate, they cause serpiginous eruptions, a tingling sensation, and vesicles. Lesions are typically 3 mm wide but can expand to 15-20 cm. The larvae advance at a rate of a few mm per day and frequently change direction. The most common infection site is the feet. Zoonotic larvae usually die within weeks to months because humans are dead-end hosts.
• Migration of a large number of larvae through the lung produces minute hemorrhage and infiltration of leukocytes, potentially entrapping larvae in lung tissue.
• During the migratory phase in the lungs, minor cough and throat irritation may occur, though these symptoms are rare. Coughing, chest pain, wheezing, and sometimes fever can occur with large numbers of larvae in the lungs.
• Both eosinophilia (higher than normal level of eosinophils) and leukocytosis (high white blood cell count) occur at this stage.
• Pathogenicity of adult worm:
• The major pathological changes are caused by the attachment of adult worms to the intestinal walls by their buccal capsule. Attachment leads to mechanical disruption of the intestinal mucosa. The worm sucks in a portion of intestinal villi and utilizes gut epithelial cells and plasma for food.
• These parasites penetrate blood vessels with their mouth parts and suck blood for nutrition. These worms cause considerable loss of blood and tissue.
• Adult Ancylostoma can suck about 0.2 ml blood a day, while the smaller Necator suck about 0.03 ml per day.
• The worm's secretions contain anticoagulant activity, which adds to blood loss by causing continued bleeding from the attachment site. Excessive blood loss caused by heavy and prolonged infection leads to hypochromic microcytic anemia. Hypochromic means decreased red color, and microcytic means RBCs are smaller than usual.
• Major morbidity is caused by intestinal blood loss, iron deficiency anemia, and protein malnutrition, mainly resulting from adult worms ingesting blood, rupturing RBCs, and degrading hemoglobin.
• Pica may be caused by iron deficiency anemia.
• Early gastrointestinal symptoms include epigastric pain, indigestion, nausea, vomiting, and constipation. Diarrhea can occur early or later.
• Signs of advanced severe infection include those of anemia and protein deficiency, such as emaciation, cardiac failure, and abdominal distention with ascites.
• Light infections produce few or no symptoms but can include abdominal discomfort, diarrhea, and/or blood in the stool.

Laboratory Diagnosis

Microscopic Examination of stool- The most common method for diagnosing hookworm infection is microscopic identification of eggs in the stool. This is dependent on finding characteristic worm eggs. The recommended procedure for stool examination includes: collecting a specimen, fixing it in 10% formalin, concentrating it using the formalin–ethyl acetate sedimentation technique, and examining a wet mount of the sediment.

2.     Culture- Laboratory confirmation can also be made by microscopic identification of Ancylostoma or Necator species larvae cultured from the stool.

3.     Endoscopy- Identification of adult worms expelled after treatment or removed during endoscopy is also a method of laboratory confirmation. Adult worm identification may not be available at private laboratories.

4.     Imaging methods- can be used in the migratory phase of larvae in the lungs; a patchy infiltrate may be demonstrated on a chest X-ray.

5.     Blood tests are carried out to ascertain the nature of anemia (hypochromic microcytic anemia) and the presence of eosinophilia. Eosinophilia and leukocytosis occur during the larval migration stage.

6.     Occult blood test in the stool gives a positive reaction in case of hookworm infection. Charcot-Leyden crystals are often found in the stool in cases of hookworm infection. These are microscopic, needle-shaped structures made of galectin-10, a protein found in eosinophils, and are associated with eosinophil-rich inflammation and parasitic infections. They appear where eosinophil breakdown has occurred.

Treatment

• Mebendazole is a drug of choice, typically given orally at 100 mg twice daily for 3 days.
• Other antiparasitic drugs include pyrantel pamoate, thiabendazole, Albendazole, and levamisole. Albendazole and ivermectin can speed up recovery and alleviate symptoms of cutaneous larva migrans.
• Treatment of iron deficiency anemia with replacement iron therapy is necessary.

Prevention

• Sanitary disposal of human feces is essential. Avoid areas where human waste contamination of soil or water is likely. Do not defecate in places other than latrines/toilets. Do not use human excrement or raw sewage as fertilizer.
• Treatment of infected persons helps prevent contamination.
• Use of sanitary latrines and wearing footwear (shoes or other clothing) to prevent contact with soil are important measures. Do not walk barefoot in known infected areas.
• Good hygiene is important, including routine hand washing with soap and warm water, especially after touching animals or playing in soil. People may get hookworms by ingesting larvae from soil if they don't wash their hands.
• Thoroughly wash fruits and vegetables to remove soil/fertilizer residue.
• For animals, prompt removal of animal feces is the best prevention. Routine testing and deworming by a veterinarian are recommended for protecting animals. Deworming infected animals reduces environmental contamination and the chance of human exposure.