BIOL 2421 Microbiology Lecture Notes: Parasitic Worms and Arthropods

BIOL 2421 Microbiology Lecture Notes: Parasitic Worms Dr. Weis

Helminth is a general term for a parasitic worm and comprise a group of multi-cellular organisms known as the flatworms and the roundworms. They are found world wide and are some of the most common parasites in poor, less developed countries in the tropical and subtropical regions, causing serious disease due to migration in the body. Worms are relatively large and have well developed organ systems and can be dioecious (different sexes) or have both sexes (hermaphrodites) in the same worm.

The life cycle of many helminthes is complex and may involve more than one host. The definitive host is the host in which the sexually mature adult helminth lives. In order for the parasite to continue its life cycle, the larval (developmental) form(s) of the parasite may need to be passed from one intermediate host to another.

They belong to two major groups of animals:

A. Platyhelminths [flatworms]à

a. Flat body covered with a plasma membrane

b. mainly hermaphrodites

c. There are two kinds of parasitic flatworms

i. Flukes (Trematodes)

Leaf-shaped

Ventral and oral suckers

Named for location

ii. Tapeworms (Cestodes)

Long body

Scolex head with suckers and / or hooks

Proglottid: reproductive segments

B. Nematoda [roundworms]à

a. cylindrical body covered with a cuticle

i. Roundworms [Ascaris]

ii. Hookworms [Ancylostoma]

iii. Strongyles [Stronglyloides]

iv. Whipworms [Trichuris]

INFECTION

Transmission

· ingestion of infective eggs or larvae

· larval penetration of the skin

· intermediate host vector

Host Susceptibility

^ Behavior regarding hygiene

^ Proper preparation of food

^ Immune system response: Nonspecific and specific mechanisms

^ Number of infective stages encountered

^ Increased susceptibility

- pregnancy / lactation

Parasite Factors

~ Avoid host defenses

outer membrane (cuticle)

movement through tissue

~ Size of worm

~ Life cycle

~ Longevity of worm

Pathogenesis

Many infections are asymptomatic. Pathological manifestations depend on size, activity, and metabolism of the worms. Immune and inflammatory responses can cause pathology.

I. Direct Damage from Worm Activity

a. Blockage of internal organs

i. Size

ii. Migration

iii. Immune response {granulomas}

b. Increased pressure from worm burden

c. Tissue Damage

i. Anticoagulants

ii. Necrosis

iii. Malabsorption/Maldigestion

iv. Protein loss

v. Migration [skin, liver, lung]

vi. Hyperplastic / metaplastic

II. Indirect Damage from Host Response

a. Hypersensitivity to antigen à granuloma

b. Inflammatory changes

c. Local allergic responses

d. Mucosal structural changes

III. Defenses Against Infection

A. Nonspecific

* Physical Barriers

* Secretions [HCl of stomach]

* Inflammation

* Complement activation

* Macrophages

B. Specific

- Antibodies: block enzymes secreted by worms

- Eosinophils

- Cytotoxic T cells

IV. Avoidance of Host Defenses

a. Acquire host molecules to reduce antigenicity

b. Acquire serum albumin on outer surfaces for a disguise

c. Secretions that suppress the immune system

i. Depress lymphocytes

ii. Inactivate Macrophages

iii. Digest Antibodies

iv. Anitcomplement factors

v. Decoy antigenic materials to create antigen overload

PLATYHELMINTHS

FLUKES (Trematodes)

Flukes have a flat, leaf-like body with a ventral sucker and oral sucker. Flukes are named according to where they live in the host.

Snail mediated intermediate host.

Schistostomiasis: Schistosoma mansoni, S. japonicum, S. haematobium

Obligate parasite of humans and snails.

Eggs are passed in the urine or feces and hatch in natural waters. Miracidia hatch from eggs, penetrate suitable snails and develop two generations of sporocytes à cercariae. Cercariae penetrate the skin when a new host comes in contact with the contaminated water. When the cercariae penetrate the skin, they shed their tail and become schistosomulae that then migrate through the tissues of the liver. Male and female flukes copulate and migrate to either the gall bladder or rectum where eggs are laid.

Signs include: fever, skin rash, abdominal pain, organ enlargement.

Schistosomiasis can result in chronic liver, spleen, and bladder damage.

DX: eggs in urine or feces

TX: Praziquantel, Albendazole, Mebendazole

Prevention: Adequate sewage treatment, parasite control using predatory snails.

Related Schistosoma Disease:

Swimmer’s Itch

Humans are not the definitive host, so as parasite penetrates skin it is unable to complete it’s life cycle migration. It then dies, leaving a small, inflamed papule.

FLUKE Summary

Fluke infections are usually acquired after ingesting an infective form.

· drinking infective water

· transmission of wet hands to mouth after exposure to infected water

· eating infected aquatic vegetation

· eating raw meat or raw intermediate hosts (snails, shellfish, crustaceans, fish)

Fascioliasis: Fasciola hepatica

Fluke found in herbivores that graze in wet pastures where intermediate snail hosts are found. F. hepatica eggs shed from the infected primary host and mature into the embryonated form in the environment. The embryo hatches and release motile miracidia that seeks out and penetrates the tissue of the intermediate snail host. Cercaria are produced in the snail that when released into the environment can encyst to produce metacercariae.

Man is often infected by eating wild watercress on which the metacercariae have collected. After being ingested, the metacercariae pass through the duodenal wall and penetrate the liver capsule. Adult flukes develop and live in the bile ducts causing damage to the biliary tract and adjacent liver tissue. Eggs are passed with the bile into the feces to continue the cycle.

Intestinal Fluke

Fasciola buski

Eggs in feces contaminate water and miracidia form and invade suitable snails.

Miracidia then form cercariae that escapes from the snail to form metacercariae.

Human exposure results from contaminated raw or poorly cooked vegetables.

Upon ingestion of metacercariae, adults form and migrate to the duodenum causing diarrhea and potential blockage.

Chinese Liver Fluke Disease

Clonorchis sinensis

Adult flukes live in the liver and produce eggs that contain miracidia that are released and penetrate snails to eventually produce cercariae that infect the muscle of fish that will encyst.

Humans are exposed when they ingest the cysts in raw or poorly cooked fish. Once exposed the metacercariae will mature into adults and migrate to the liver and infect the gall bladder and bile ducts that can cause blockage.

Lung Fluke

Paragonimus westermani

Adult flukes live in the lungs and reproduce to release eggs upon coughing and reswallowing. Eggs pass into the feces which contaminate water and develop into cercariae that develop in suitable snails. Metacercariae that escape from snails form in crabs.

Humans are exposed when they ingest the meatcercariae from poorly cooked crabmeat.

Flukes pass from the intestine to blood and then the lungs causing dsypnea and cough.

General Fluke Treatment: Praziquantel

Prevention:

Thoroughly cook meats

Thoroughly cook vegetables if taken from suspect waters

TAPEWORMS

Tapeworms have a long body consisting of a scolex, which is the head of the body, and proglottids, the individual segments. The scolex is equipped with suckers and possibly hooks for attaching to the intestine of their host. Definitive hosts include cats, dogs, cows, whales, and humans.

Tapeworm infection is an intestinal disease caused by one of several species of tapeworms, including

Taenia saginata (beef tapeworm)

Taenia solium (pork tapeworm)

Diphlyllobothrium latum (fish tapeworm)

Diplydium caninum (dog tapeworm)

Tapeworms are large, flat worms that live in the intestines and can grow to 15-30 ft and live up to 25 years.

Egg bearing sections of the worm, called proglottids, are passed in the stool. Eggs released into the environment can be eaten by intermediate hosts, such as pigs, cattle, or small crustaceans. The eggs hatch in the intermediate host à larval phase which invades the intestinal wall à bloodstreamà skeletal muscle / other tissues à cysts (cysticercus)

People acquire the parasite by eating the cysts in raw or undercooked meat or fish. The cysts hatch à develop into adult wormsà intestine à grow and reproduce proglottids.

People may act as the intermediate host of the pork tapeworm, Taenia solium. Pork tapeworm eggs reach the stomach when a person swallows them in food or water contaminated with human feces or from contact of an infected person with unclean hands.

Once the larvae are released, they penetrate the intestinal wall and travel to muscles, internal organs, brain, or skin where they form cysts [neurocysticerci].

People can acquire the dog tapeworm if they ingest an affected flea (the intermediate host). Usually this is seen with children.

Symptoms

Usually do not cause symptoms

Can have upper abdominal discomfort, diarrhea, and loss of appetite, nausea

Can interfere with the hosts ability to absorb vitamin B12, which will cause pernicious anemia

Sometimes can feel passage of proglottid from the anus.

Cysts in the brain / meninges can cause inflammation à headache, confusion, seizures

Finding worm segments or eggs in stool sample

CT or MRI scan of brain to locate cysts

TX : Praziquantel or Albendazole

Prevention: Meat inspection, thorough cooking of meat or prolonged freezing to kill cysts

HYDATIDIASIS

Echinococcus granulosus

The adult tapeworm inhibits the small intestine of canines from which the eggs of the species are passed. These eggs can be ingested by herbivorous animals (sheep) and hatch in the duodenum. Embyos enter the circulation where they are carried to various sites to develop into hydatid cysts, primarily the liver and lungs.

Dogs are exposed by eating the meat of sheep or deer containing the tapeworm cysts.

Humans are infected if they accidentally ingest eggs from infected dogs after petting them and so humans will act as the intermediate host. Other locations besides liver and lungs, will be the brain and bone.

Hydatids are slow growing and the infections may be asymptomatic for years.

Damage arises from the size of the cyst as it crowds normal tissue. If the cyst ruptures, it can create many daughter cysts or cause anaphylactic shock due to sudden release of proteinaceous material.

TX: Albendazole

Surgery to remove, but only if intact cyst can be excised

NEMATODES

FILARIASIS

Wuchereia bancrofti

Onchocerca volvulus

These are arthropod (insect)-borne infections caused by filarial worms.

Elephantitis is caused by the larvae of W. bancrofti which develop in the mosquito. When bitten, the larval forms pass through the lymphatics and mature to thread like adults in the lymph glands. After the male and females mate, eggs are released and larvae form as microfilaria in the general circulation where they are picked up by mosquitoes during feeding. Location of mature adults in the lymph glands causes obstruction of lymphatic drainage which then results in massive swelling of the extremities. This worm can also affect the pituitary gland, causing dwarfism.

Loaisis (Loa Loa or Eye worm) is caused by the adult worms of O.volvulus and is spread by biting deer flies. The adult worms migrate along C.T. planes reaching the conjunctiva of the eye where they produce microfilaria that go into the general circulation and picked up by other biting flies.

River Blindness is transmitted by a black fly and adults create tumor like nodules under the skin. Usually asymptomatic, but can cause disfigurement and blindness.

DX: Id microfilaria in blood

TX: Diethylcarbamazine to kill the microfilaria and adults

Prevention: Control intermediate hosts (flies, mosquitoes)

ASCARIASIS

Ascaris lumbriocoides

Most common roundworm infection in people, especially in areas of poor sanitation.
Endemic in the SE U.S.

Transmission
Swallowing of food contaminated with Ascaris eggs, usually contaminated through contact with soil or other objects. Ascaris eggs are hardy and can survive in the environment for years. Once swallowed, the eggs hatch and release larvae into the intestine. Each larva migrates through the intestine à lymphatic à blood à lungs.

Larvae can also get lost and migrate to other parts of the body.

Once in the lung, the larvae pass into the alveoli and moves up the respiratory tract where it is swallowed. The larvae then mature in the small intestine where it remains as an adult worm (6- 20 inches in length) and feeds on the intestine’s liquid contents.

Symptoms

Asymptomatic

Migration can cause: coughing, wheezing, fever

Large worm burdens can cause abdominal cramps and blockage

Eggs in stool

Vomiting or passing of worms in the stool

Medendazole, Albendazole, Pyranel pamoate

Prevention

Sanitation

Avoid undercooked foods

TOXOCARIASIS

Toxacara canis, Toxacara cati

aka Visceral Larval Migrans

Zoonotic

Transmission:

Eggs in soil or sand boxes contaminated with feces of dogs or cats. Children directly transfer eggs from hands to mouths after handling contaminated substances.

Eggs swallowed àlarvae hatch in instestines à penetrate wall to gain access to blood à liver à lung à any tissue. Larvae migrate and damage tissue

Symptoms

Resp: Fever, cough, wheezing, pneumonia

GI: Liver enlargement, spleen enlargement

Skin: rash

Eye: inflammation, decreased vision

DX:

Eosinophilia

Ab titers

Biopsy

TX:

Usually none, will resolve

Diethlycarbamazine, mebendazole

Corticosteroids for inflammation

Prevention

Deworm dogs, cats

Clean environment of animal feces

ANCYLOSTOMIAIS

Hookworm infection

Ancylostoma duodenale [US]

Necator americanus [Worldwide]

Intestinal roundworms, common in moist, warm places where sanitation is poor.

Transmission

Eggs are passed in the human stool and hatch in the soil after 2 days. Larvae emerge to live in the soil and when fully mature can penetrate the skin. Infection is direct contact with infected soil, especially in areas where human waste is used as a fertilizer.

Skin à lymphatic à bloodstream à lungs àairway to throat à swallowed.

Reach intestine where they develop into adults and attach themselves by their mouths to the lining of the upper small intestine (jejunum). Produce an anticoagulant in order to live off the blood of the host.

Symptoms

Skin: Itchy, red, rash

Resp: coughing, wheezing, fever

GI: pain in upper abdomen, loss of appetite, malnutrition

Blood: Anemiaà blood loss and iron deficiency

Eggs or larvae in stool sample

Albendazole, Mebendazole, Pyrantel pamoate

Prevention:

Environmental exposure

Wear shoes, don’t sit directly on ground

PINWORM

Enterobiasis vermicularis

Intestinal roundworm

Most common parasite of U.S. children

Transmission

Ingestion of pinworm eggs à source: anus, clothing, bedding, toys, sand boxes,

Fingers of infected child to mouth of uninfected child, contaminated food,

Re-infect themselves [fecal – oral] due to itching.

Eggs can be in the environment for 3 weeks.

Eggs hatch in intestinal tract à worms migrate to lower intestines / rectum to mature within 2-6 weeks. Female worm moves to perianal region to deposit eggs in a sticky material.

Symptoms

GI: rectal itching, eggs at perianal region

DX:

White small worms @ anal region during night

Eggs – use scotch tape to trap for Id

TX:

Mebendazole, Albendazole, Pyrantel pamoate

Anti-itch creams to prevent re-infection

Prevention:

Wash clothing, bedding, toys

Hygiene

TRICHINOSIS

Trichinella spiralis

Zoonotic

Transmission

Trichinella larvae live in the skeletal muscle tissues of animals, typically pigs and carnivores.

People develop trichinosis if they eat uncooked or poorly cooked meat from an animal that carries the parasite in the cyst form à cyst is digested and larvae released and migrate throughout the bodyà mature to adult and mate in the intestine.

Female worms burrow into the intestinal wall and after 1 week produce larvae for 4-6 weeks.

Larvae are carried through the body via the lymphatic and bloodstream à penetrates muscle [eye, tongue, intercostals, arm, leg] and form cysts.

Symptoms

Asymptomatic

GI: abdominal cramps, diarrhea

Generalized: muscle pain / soreness in affected muscles, weakness, fever

Skin: rash

Eye: eyelid swelling, injected sclera, pain, sensitivity to light

Resp: dyspnea

Death due to myocardial damage or kidney failure

DX:

Symptoms, Muscle biopsy, Ab titer, Eosinophilia

TX:

Mebendazole, Albendazole

Analgesics

CCS

Prevention:

Thoroughly cooking meats, especially pork products

Freezing meats for several (minimum 3) weeks

TRICHURIASIS

Whipworm infection

Trichuris trichiura

Intestinal round worm

Location: moist, warm subtropics and tropics where poor sanitation

Eggs incubate in soil

Transmission:

Ingestion of contaminated food containing eggs that have incubated in the soil.

Larvae hatch in the small intestine à migrate to large intestine (cecum) à embed head into the intestinal lining à mature to adults that feed on blood à eggs passed in feces.

Symptoms

GI: abdominal pain, diarrhea, weight loss, bleeding, anemia, rectal prolapse

Blood: anemia

Eggs in stool

Colonscopy to view adults

Albendazole, mebendazole

Prevention

Sanitation

Personal Hygiene

Avoid unwashed vegetables

STRONGYLOIDIASIS

Strongyloides stercoralis

Female worms live in the mucosal glands of the small intestine. Eggs hatch in these glands and the larvae are passed in the stool into the soil. Exposure is due to burrowing through the skin (soles of the feet). The larvae then migrate to infect the lungs à coughed up à swallowed à intestines.

Clinical signs:

GI: pain, vomiting, diarrhea

Resp: cough, inflammation

Skin: pruritis

GUINEA WORM DISEASE

Dracunuculus medinensis

Round worm that lives in the skin causing ulcers.

Larvae are released into the water and picked up by a crustacean (cyclopoid) and spread to humans when they ingest the infected crustacean and re-exposed to the larvae that migrates to the skin. The male dies after mating and the female prefers axillary and inguinal regions where a blister containing the female with juvenile worms forms. Cold water is the trigger for the females to release the juvenile from the blister to continue the cycle.

TX: remove worm from blister

Metronidazole, Thiabendazole, Mebendazole

ARTHROPOD INFESTATIONS

SCABIES

Sarcoptes scabiei

Tiny mite that burrows under the skin to lay its eggs causing a variety of inflammatory skin lesions and intense itching. The mite is transmitted by direct contact of any type.

TX: Insecticides [permethrin] or antihelminthics [ivermectin]

PEDICULOSIS

Pediculus humanus capitis [head louse]

Pediculus humanus corporis [body louse]

Direct contact spread

Louse life cycle involves a blood meal from the host several times a day.

Female louse lives about a month and lays eggs (nits) at the base of the hair shaft

Clinical sign: Pruritis

TX: Insecticides [permethrin, pyrethrin]