Lecture 15 - Helminths

Question 1

What is Schistosomiasis?

Answer 1

Human schistosomiasis is caused by 5 species (Schistosoma) three of which are widely distributed and two which are more localised.
It is spread through contact with water which is carrying (cercariae) the infective stage that burrow through the skin. (snail is the intermediate host)
Eggs from female worms are shed in faeces (and urine) and therefore it is a disease of poor sanitation and poverty.

Question 2

Describe the epidemiology of schistosomiasis.

Answer 2

• There is an estimated 240 million people infected
  
  • 85% of cases occur in Africa where prevalence can exceed 50%
  • It causes 300,000 deaths a year in Africa
    It can be treated with praziquantel but re-infection can occur if infecting water sources remain.

Question 3

Describe the clinical diagnosis of schistosomiasis.

Answer 3

• Cercarial dermatitis (after 2-3 days)
  
  • Kayatama syndrome after 2-8 weeks (lung infection, hepatosplenomegaly, fever, skin rash and weight loss)
    Chronic phase (occurs months/years later depending on species)
  • Urinary schistosomiasis (caused by S. haematobium)
    ○ Haematuria, painful or difficult urination, abdominal pain, bladder inflammation.
  • Other Schistosoma species cause intestinal schistosomiasis
    ○ Abdominal pain, diarrhoea/dysentery, hepatomegaly, splenomegaly.
    None of these symptoms are completely diagnostic

Question 4

Describe the parasitological dianosis of shistosomiasis.

Answer 4

The presence of eggs in either stool or urine samples is used in diagnosis - each species has a characteristic spine which means the species can be identified.
* Kato-Katz - thick stool smear used for intestinal schistosomiasis - good specificity, but poor sensitivity for low intensity infections (currently widely used)
Concentration techniques can improve sensitivity:
○ Formal ether, salt flotation for stools, centrifugation for stools and urine
○ FLOTAC is a commercially available floatation system to achieve higher concentration fo eggs and is more sensitive.
* Can also look for eggs by histology in biopsies from rectum or bladder tissues
Adult worms and/or the effects of fibrosis can also be detected by ultrasound

Question 5

How is antigen and antibiody detection used to diagnose schistosomiasis. Additionally how are molecular techniques used.

Answer 5

Antibody detection (indirect)
* Circumoval precipitin test (COPT) mixes lyophilised (dried) eggs with patient serum to form a precipitate - used for S. japonicum - it has good sensitivity and specificity but is complex to perform.
* Indirect fluorescent antibody test (IFAT) - works well but requires expensive equipment
* ELISA - most widely used antibody test, many variants available using different target antibodies
* Indirect haemagglutination test (IHT) uses red blood cells coated with schistosome antigens - mainly used for epidemiology due to low specificity for clinical use.

Antigen detection (from schistosomes released into circulation)
* Highly effective method of diagnosis
* Circulating adult worm and soluble egg antigens in blood, sputum or urine can be detected
* Antigens separated into two main categories (cathodic and anodic)
* Works well in ELISA format ; RDT lateral flow versions have also been developed by need improved sensitivity
* Some quantification of the antigen is possible - useful to monitor the effectiveness of therapy.

Molecular biology methods for Schistosoma
* DNA or RNA based conventional PCR (end point) and quantitative (real-time) PCR have been developed
* Show very good sensitivity and depending on gene target used good specificity
* Used in research and epidemiology studies but not yet for routine diagnosis.
Field-friendly Loop mediated isothermal amplification (LAMP) method under development.

Question 6

What is Ascariasis.

Answer 6

Roundworms or Ascaris lumbricoides are large (10-35cm) nematode worms that live in the lumen of the small intestine.
* They are an example of a soil-transmitted helminth and have no intermediate host
* The female worms produce a large number of eggs - up to 200,000 per day that are passed in faeces
* The eggs are very durable and can persist in the environment for months to years
* Spread by faeco-oral route (eating contaminated food)
Associated with poverty and poor sanitation

Question 7

Describe the epidemiology of ascariasis.

Answer 7

Epidemiology of ascariasis
* 800 million people are estimated to be infected
* It is the most common helminth infection found in all tropical regions of the world
* It causes approx. 20,000 deaths per year (if heavily infected and/or eliciting and allergic response)
* Mass drug administration programs using drugs like albendazole aim to reduce the burden of infection “Preventative chemotherapy”
Control is difficult because a few heavily infected individuals can maintain transmission.

Question 8

How is ascariasis diagnosed.

Answer 8

There are no antibody or antigen detection methods so clinical, parasitological and molecular methods are used.

Clinical diagnosis of ascariasis
* Larvae migrating through tissues can cause Loffler syndrome, an allergic hypersensitivity reaction similar to asthma.
* Heavy infection of adults in the gut can cause obstruction with abdominal pain, which can be detected using ultrasound
* Neither of these is definitively diagnostic
* Sometimes adults leave the small intestine ad move up the gut to cause effects like bocking the bile duct or even be vomited up or emerge from the nose.

Parasitological diagnosis of Ascaris
* Main type of method used, based on finding the distinctive eggs in a stool sample
* Eggs can be easily distinguished from all other common helminth infections by light microscopy
* Kato-Katz thick smear is the gold standard with excellent specificity but sometimes poor sensitivity.
* FLOTAC can also be used for Ascaris

Molecular biology methods for Ascaris
* Conventional PCR based methods for detection of eggs in stool samples
* Multiplex versions testing for several worms simultaneously available
* qPCR variants exist (quantitate)
* PCR is not yet used for routine diagnosis due to cost, used mainly for research studies.

Question 9

Compair direct and indirect methods of parasitological diagnosis

Answer 9

Comparison if direct and indirect methods
In theory direct tests are better as there is unequivocal demonstration of the presence of the parasite and they are better suited for quantitation. The main drawback is they show low sensitivity and a small number of parasites can have large effects on the body (serological responses (indirect) are often sensitive)

Diagnosis in a healthcare setting
The choice of test also depends of the healthcare setting/resources. Some tests requires training requirements, some are expensive. Need point of care tests.

Need for new methods
* The most commonly used methods are not always the best and are limited by cost and availability
* Investment in diagnostics is one of the most cost effective way of investing in disease research
Better to identify early and treat early.