16. Helminth Infections Flashcards
Helminths (agent, epi)
Agent: Parasitic worms. Most common infectious agents of humans in developing countries (with high morbidity)
Epi: Have a long history.
Estimated that approx. 1/3 of almost three billion people living in the developing regions of sub-Saharan Africa, Asia, and the Americas are infected with one or more helminths.
2 types of helminths
Nematodes (round worms) 蛔虫科
soil-transmitted nematodes disease (eggs deposited and mature in soi): ascariasis, hookworm
Trematodes (flukes) 吸虫科
schistosomiasis
Ascariasis （蛔虫病）
agent, epi
Agent: large roundworm.
- Life cycle: Adult worms live in small intestine. Females produce 200,000 eggs/day and passed in feces. Fertile eggs become infective in 18 days to several weeks (depending on environment). If infected eggs are swallowed, larva hatch & travel to lungs via circulatory system. Larva mature in lungs (10-14 days), ascend the respiratory track and are swallowed. Develop into adult worms in small intestine; live 1 – 2 years.
Epi:
- Most common human worm infection with over 800 million individuals infected worldwide
- Infection occurs worldwide in warm and humid climates, where sanitation and hygiene are poor, including in temperate zones during warmer months
- Individuals in these areas are at risk if soil contaminated with human feces enters their mouths directly or via unwashed/uncooked fruits or vegetables
Ascariasis
symptoms, Diagnosis, Treatment
Symptoms:
- Adult worms may cause no acute symptoms.
- High worm burdens may cause abdominal pain and intestinal obstruction.
- During lung phase, respiratory symptoms may occur: cough, difficulty breathing. Impaired growth in children
Diagnosis: Microscopic identification of eggs in stool
Treatment:
- Anti-worm (anthelminthics) treatment is available which will clear infection in 1 to 3 days
- Surgical removal
Hookworm
agent, epi (at risk ppl)
Agent: Nematode (round worm)
- life cycle: Eggs passed in feces of infected person. In suitable environment (dark/shady, warm, & moist) eggs hatch within 24-48 hours. release first-stage larva. Larva molt twice resulting in third-stage larva (filariform stage). Filariform larva are infectious; can survive and remain infectious for several weeks. Enter host by penetrating skin between toes or through feet or ankles. enter blood circulation. Pass through the lungs. gain entry to the human gut. mate in intestines. Adhere to and lacerate intestinal tissue with oral plates or teeth and consume blood. Egg production begins 4-8 weeks following initial infection. Worms can live approx. three years
Epi:
- Estimated 576-740 million people are affected globally
- Most prevalent in warm, humid areas. widespread in tropics but also common in warm & wet areas of the temperate zones
- Once widespread in southeastern U.S., prior to improvements in living conditions
- At risk populations:
- People living in warm & moist climates with poor sanitation and hygiene that walk barefoot or allow skin to contact contaminated soil
- Children playing in contaminated soil
- pre-school & school-aged children, women of child-bearing age, at risk due to occupation
Hookworm
transmission, prevalence
Transmission: Soil transmitted. Eggs passed in feces and mature/hatch.release larva in soil. Infection mainly acquired by walking barefoot on contaminated soil.
Prevalence:
- Lower among children under 5, but gradually increases with age, then diminishes later in life
- Highest prevalence among males, teenagers and young adults; potentially related to occupational hazards e.g. tending crops in rice paddies
Hookworm
symptoms, diagnosis, treatment
Symptoms:
- Itching and localized rash at the site where the larva penetrated the skin
- Morbidity is related to the number of worms infecting the individual
- Light infections (fewer worms) usually are asymptomatic
- Heavier infections (more worms) can present with diarrhea, abdominal pain, weakness & loss of appetite
- Anemia and protein deficiency can result from worms feeding on host tissue/blood
- May involve malabsorption of nutrients
- In children, physical and cognitive growth can be affected
Diagnosis: Microscopic identification of eggs in stool
Treatment: Anti-worm (anthelminthics) treatment is available which will clear the infection
Prevention & Control for nematodes
Ascariasis
- Avoid ingesting soil that may be contaminated with human feces, including where night soil or wastewater is used to fertilize crops
- Appropriate handwashing
- Wash & peel or cook raw vegetables and fruits prior to consumption
Hookworm
- Wearing adequate footwear
Both
- Appropriate use and maintenance of latrines
- Minimize use of night soil for fertilizer
- Provide education regarding transmission of worm infections
- Periodic deworming
- WHO recommends periodic deworming treatment to at-risk people living in endemic areas
- Once annually where prevalence of helminth infections is over 20%
- Twice annually where prevalence of helminth infections is over 50%
- Often implemented in school setting
shistosomiais
agent, epi (high risk pop)
Agent: Schistosome Parasites (trematode flatworms/fluke)
- Life cycle: Infected individuals shed eggs in urine or stool. If deposited in fresh-water, eggs hatch and release miracidia. Miracidia seek out host snails & penetrate tissue of snail. Development continues within the snail. Cercariae emerge after 30 days & seek out human host. Penetrate human host & enter circulatory system. Parasites migrate through host tissue & develop into adult worms. Reside in veins/capillaries of the bladder, intestines or liver. Worms mate and produce eggs, some of which are passed into urine or stool.
Epi:
- Greater than 200 million people affected worldwide
- Schistosomiasis is prevalent in tropical and subtropical areas, including regions in: Africa, South America, Middle East, Southeast Asia, and Southern China
- Estimated that 90% of those requiring treatment for schistosomiasis live in Africa
- In Africa, local community prevalence rates can exceed 50%
- Poor communities without access to safe drinking water and adequate sanitation are especially at risk
- Schistosomes are not present in the U.S.
shistosomiais
transmission, symptoms
Transmission:
- occurs when humans expose themselves to fresh-water inhabited by snails carrying the parasite
- Swimming, bathing, wading, fishing and domestic chores (e.g. washing clothes) can put individual at risk
- School-aged children at high risk due to time spent swimming or bathing in fresh-water
Symptoms:
- Penetration of cercariae can cause a rash – develops hours to 1 week after exposure
- Infection may be acute or chronic
- Acute infection can cause fever, headache, muscle ache, diarrhea and sometimes swelling of the liver or spleen
- Chronic infection is the result of host immune response to schistosome eggs
- Intestinal – diarrhea, constipation, blood in stool, bowel wall ulceration, intestinal polyps
- Urinary – painful urination, blood in urine, bladder calcifications, bladder cancer
- Hepatic – liver enlargement and inflammation, liver fibrosis
- Children can develop anemia, malnutrition, and learning disabilities
- WHO estimates 20,000 deaths annually
shistosomiais
Diagnosis & Treatment
Diagnosis
- Microscopic identification of eggs in urine or stool
- Antibody detection via serologic testing
Treatment
- Safe and effective medication is available for infections caused by all Schistosoma species (Praziquantel)
- Taken for 1 – 2 days
- Re-infection may occur after treatment but risk of developing severe disease is diminished
shistosomiais
Prevention & Control
The WHO strategy for schistosomiasis control focuses on reducing disease through periodic, targeted treatment with praziquantel through the largescale treatment (preventive chemotherapy) of affected populations
- Groups targeted for treatment:
- School-aged children in endemic areas
- Individuals with occupations involving contact with infested water (fisherman, farmers, irrigation workers)
- Women whose domestic tasks bring them in contact with infested water
- Entire communities living in highly endemic areas
- Periodic treatment of at-risk populations will cure mild symptoms and prevent severe, late-stage chronic disease
- In high-transmission areas, treatment may have to be repeated every year for a number of years to be successful
- Challenge – limited availability of praziquantel
- 2014 – Only 20.7% of individuals requiring treatment obtained it
Additional preventive measures include: access to safe water, improved sanitation, hygiene education, and snail control
