Helminths and Antihelminthics Flashcards
1
Q
Helminths
Overview
A
Nematodes, Trematodes, and Cestodes
- Complex multicellular animals
- Differentiated tissues
- Highly developed reproductive systems
- Specialized organs for attachment (suckers, hooks and teeth)
- Do not replicate in the host
-
Transmission through:
- Fecal oral route
- Swallowing larval stages in tissues of another host
- Active penetration of the skin
- Insect bite
-
Immunoregulatory aspects of helminth infections:
- Hygiene hypothesis
- Improvements in hygiene and management of infectious diseases ⇒ ↓ worm burden in developed world ⇒ ? ↑ in autoimmune disorders
- Hygiene hypothesis
2
Q
Nematodes (Roundworms)
Overview
A
- Widespread distribution: 500k species
- Mostly free-living, some parasitic
- Unsegmented, cylindrical and elongated
- Covered w/ protective cuticle
- Complete digestive tract
- Sexes are separate
- Differentiation from egg → larva → adult
-
Caenorhabditis elegans
- Free-living nematode
- Model for a simple metazoan
3
Q
Caenorhabditis elegans
Lifecycle
A
Free living nematode.
Model for a simple metazoan.
4
Q
Intestinal Roundworms
A
-
Transmission by ingestion:
-
Enterobius (pinworm)
- Also transmitted via inhalation
- Trichuris (whipworm)
- Ascaris (large roundworm)
-
Enterobius (pinworm)
-
Transmission by skin penetration:
- Necator and Ancyclostoma (hookworms)
- Strongyloides (small roundworms)
5
Q
Enterobius vermicularis (Pinworm)
Overview
A
- Most common roundworm in the temperate zone
- May be considered a commensal
- Most common worm infection in the US
- Infections in children account for the highest percentage of cases
- ♀ 8-13 mm, ♂ 2-5 mm
-
Life cycle confined to humans: eggs → larva → adult
- Autoinfection common
- Worms live in colon
- Gravid female → several inches out of the anus ⇒ lay eggs perianally
- Eggs deposited as worms crawl or released as worms dry out and explode
- Local irritation and itching, especially during the night when eggs are laid
- Most frequently infections are asymptomatic
6
Q
Enterobius vermicularis (Pinworm)
Clinical Characteristics
A
- Transmission: Fecal/oral, ingestion of eggs, inhalation
- Diagnosis: Scotch tape test, egg ID
-
Treatment and Control: Two doses (10 mg/kg; maximum of 1g each) of Pyrental Pamoate two weeks apart gives a very high cure rate
- Mebendazole is an alternative
- Bedding and underclothing must be sanitized between doses
- Personal cleanliness most effective in prevention
- Tx the whole family
- Re-infections common
7
Q
Soil-Transmitted
Intestinal Helminth Infections
A
Ascaris lumbricoides (large roundworm), Trichuris trichiura (whipworm), and hookworm affect ~ 1 billion people
In children aged < 5 years, these infections cause malnutrition and anemia
Anthelminthic treatment (deworming) improves nutritional status of school-aged children
8
Q
Trichuris trichiura (Whipworm)
A
- Transmission: ingestion of fully embryonated eggs
- ~10 d for full embryonation
- Larva adhere to small intestine temporarily
- Adults 3-5 cm long
- Adult anterior ends embedded in fecal mucosa
- Light infections ⇒ usu. asymptomatic
-
Heavy infections ⇒ ulceration and hemorrhage
- ± Prolapsed rectum, esp. in children w/ heavy infections
- May resemble IBD
- Diagnosis: observing football-shaped eggs in feces
- Treatment: Mebendazole x 3 days, 200 mg for adults,100 mg for children
- Accompanying infection treated accordingly
9
Q
Ascaris lumbricoides
(Large Roundworms)
A
-
Epidemiology:
- Worldwide distribution, common in tropical or subtropical environments
- Linked to poor sanitation
- ~ 1.4 billion infected people
- ♀ 20-35 cm, ♂ 15-25 cm
- Avg. worm burden hundreds per capita
- Transmission: contaminated soil, ingestion of embryonated eggs
- Each ♀ releases ~ 200k eggs/day
- Eggs need to be outside host for 2-3 wks to become infective
-
Clinical/Pathogenesis:
- Larvae → lung → maturation → coughed up → swallowed → small intestine
- Do not attach to intestinal wall
- Swims against peristaltic movement
- Can penetrate through intestinal wall
- Asymptomatic, Pneumonitis, 2°asthma attacks, GI perforations, bowel obstructions
- A few worms living in the intestine may not cause problems
- Migration intestine → pancreas, bile ducts or into esophagus problematic
- May live for a year
- In absence of reinfection, will be gone
- Usually not lethal, but ~60k die/yr
- Diagnosis: usu. via eggs in the feces
- Treatment: Mebendazole x 3 days, 200 mg for adults, 100 mg for children
10
Q
Hookworms
A
-
Necator americansus (New World hookworm)
- North and South America, Africa, Asia, Australia, common in Southern US
-
Ancylostoma duodenale (Old World hookworm)
- Africa, Asia, Australia
- ~1 billion infected individuals
- Transmission: penetration of the skin
-
Clinical/Pathogenesis:
- Lungs → swallowed → attach to intestinal lining ⇒ suck blood
- Loss of blood ⇒ iron deficiency ∝ to worm burden
- Up to 0.3 ml of blood per day by each adult; ≈ 1 million liters of blood given to the worms each day
- Pneumonia and eosinophilia as larva migrates
- Diagnosis: eggs in stool, ± occult blood in stool
- May live in host for 8-16 years
- A. caninum (dog hookworm) infections abortive, but causes cutaneous larva migrans
- Treatment: Mebendazole x 3 days, 200 mg for adults, 100 mg for children
11
Q
Strongyloides stercoralis
(Small Roundworms)
A
- arasitic and free-living forms
- Geographic distribution: Tropical, subtropical, poor sanitation, rural areas
- Transmission: Filariform larvae infiltrate through the skin
- Skin → lungs → small intestine
- Autoinfection d/t differentiation of rhabdiform larvae → infective filariforms
- Disseminated hyperinfection in immunosuppressed pts & usu. fatal
12
Q
Trichinella spiralis
(“Pork Worm”)
A
-
Worldwide distribution
- ~2% infected population
-
Morphology:
- Adult ♀ 3.5 mm x 60 μm
- Larvae in the tissue (100 μm x 5 μm) coiled in a lemon-shaped capsule (400 μm by 200 mm)
- Transmission: eating undercooked or raw pork containing encysted larvae
-
Symptoms: causes trichinosis
- Mild infections may be asymptomatic
- Larger bolus of infection ⇒ sx according to severity and stage of infection & organs involved
- Low mortality rate
-
In the intestines: Larvae → mature adults ⇒ additional larvae → muscle
- Initial infection w/ ± diarrhea
- Convert muscle cells → nurse cells ⇒ support larval growth
- Acute inflammatory reaction as larvae go into muscle
- ± Myocarditis, encephalitis (can be d/t vasculitis), eosinophilia
-
Pathogenesis: d/t large # of larvae in vital muscles & host reaction to larval metabolites
- Muscle fibers become enlarged, edematous, and deformed
- Paralyzed muscles infiltrated w/ PMNs, eosinophils and lymphocytes
- Worm ⇒ strong IgE response + eosinophils ⇒ parasite death
- Diagnosis: Biopsy
13
Q
Toxocara
A
- Nematode parasite of dogs (T. canis) and cats (T. catis)
-
Geographic distribution: Worldwide, highest in developing countries
- ~ 14% of US population infected w/ Toxocara
- Incidence is higher in inner cities
- Humans are incidental hosts
- Children get infected when playing in egg-contaminated dirt
- Most infections are asymptomatic, esp. in adults
-
Migration of larva in tissue can cause damage:
- Ocular Lava Migrans (damage to the eye)
- Visceral Larva Migrans (in heavy infections)
- Possible effects on cognitive development
14
Q
Wuchereria bancroftii and Brugia malayi
A
Filarial Parasites
- 1st parasite identified as being transmitted by mosquitoes
- Geographic Distribution: Sub Saharan Africa, Madagascar, Caribbean, sporadic in South America
- Adult worms live in lymphatics ⇒ inflammation ⇒ lymphatics obstruction
- Produce living embryos called microfilariae
- Periodically released related to feeding habits of vector
- Most infections asymptomatic
- Tropical pulmonary eosinophilia frequently seen in endemic areas
- Elephantiasis (extreme manifestation)
15
Q
Onchocerca volvulus
A
Filarial nematode
- Causes river blindness
- Transmitted by Simulium species of black flies
- Subcutaneous nodules
- ± Microfilariae (living embryos) in cornea and anterior chamber
- African and American forms known
16
Q
Loa Loa
(African Eye Worm)
A
Filarial nematode
- Transmitted by Chrysops species of the mango flies
- Active migration throughout subcutaneous tissues
- Migration through tissues is painless
- Noticed when they traverse conjunctival tissue
17
Q
Dracunculus medinensis
(Guinea worm)
A
Filarial nematode
- Confined to a small part of W. Africa
- Will probably be extinct in 5-10 years
- ♀ up to 1 m long
- Live in the intestine
- After mating, gravid ♀ → skin ⇒ ulcer
- Use a small stick to slowly wind the worm out of the body through the skin
- When worm contacts water via ulcer ⇒ larvae released
- Larvae infect crustaceans (intermediate hosts)
- Infective larvae come out of the crustaceans into the water
- People infected by drinking this water
19
Q
Trematodes and Cestodes
(Flatworms)
A
Belong to Phylum Plathyhelminthes
Multicellular flatworms w/ bilaterally symmetrical body
20
Q
Schistosomes
(Blood Flukes)
A
Trematode
- 3 species infect humans: S. mansoni, S. japonicum, S. hematobium
- Certain non-human Schistosoma spp. can have abortive infections
- Geographic distribution: Southeast Asia
-
Transmission: cercariae from snails, swimming, penetration
- Humans are definitive hosts
- Snails are intermediate hosts
- Snails release cercaria(e) ⇒ motile and infect humans by penetrating skin
- Cercariae → adults in veins
-
Clinical/pathogenesis:
- S. mansoni and S. japonicum: mating pair live in veins of liver, eggs cause inflammatory damage
-
Rectal and colonic polyps (S. mansoni)
- Dx S. mansoni: stool, egg w/ lateral spine
- Dx S. japonicum dx: stool, ovum
-
S. haematobium: mating pair live in veins of bladder, eggs cause damage
- Hematuria
- Dx: urine, eggs w/ terminal spine
- Worms may persist for decades
- Produce eggs at prolific rate:
- Most secreted in feces or urine
- Some get deposited in tissues (freq. in liver) ⇒ pathology ⇒ fibrosis
- Host TNF induces egg production
- Schistosomes covered w/ host antigens, especially MHC
- Treatment: Praziquantel
21
Q
Cestodes
(Tapeworms)
A
- Resemble a measuring tape
- Adults ⇒ head (scolex) and segments (proglottids)
- scolex w/ hooks or other means ⇒ attach to host intestinal wall
- Tapeworms are hermaphrodites
- Each proglottid contains male and female reproductive organs
- Two categories: intestinal tapeworms and larval tapeworms
Life Cycles of Taenia saginata and T. solium:
24
Q
Cysticercosis
A
- Caused by Taenia solium
- Larval cysts in lung, liver, eye and brain ⇒ blindness and neurological d/o
- Cerebral cysticercosis ⇒ up to 20% of neurological case (Mexico)
- Ocular cysticercosisis ⇒ 2.5%
- Muscular involvement ⇒ up to 10% (India)
- ↑ Neurocysticercosis in the US
- Ass. w/ Hispanic ethnicity, immigrant status, and exposure to endemic neurocysticercosis areas (Southwest)
25
Q
Echincoccus granulosus
A
- Dogs: definitive hosts
- Humans: incidental hosts
- Small tapeworms
-
Fluid-filled cysts in liver, lungs
- Contain hydatid sand
- Dissemination can have severe consequences
- Caution during surgical procedures
- Geographic distribution: Africa, Europe, Asia, Middle East, Central and South America, rarely North America
26
Q
Diphyllobothrium lattum
(Fish Tapeworm)
A
- Acquired by eating undercooked fish containing larval forms
- Worldwide distribution, common in northern hemisphere
- Adult worm can reach >10 m in length
- Adults do minimal damage
- Occasional attachment to the proximal jejunum ⇒ clinical vitamin B12 deficiency
