General Parasitology

An overarching framework for the clinical approach to parasitic infections, encompassing protozoal, helminthic and ectoparasitic disease. It gives the general principles of classification, risk assessment and diagnostic strategy that underpin rational clinical evaluation of a patient with suspected or confirmed parasitic infection.

It is particularly relevant in the assessment of the returning traveller or migrant from an endemic region presenting with unexplained symptoms, including eosinophilia.

Classification

• Protozoal infections, caused by single celled eukaryotic organisms, include malaria, toxoplasmosis, amoebiasis, giardiasis, leishmaniasis, trypanosomiasis and balantidiasis.
• Helminthic infections, caused by multicellular worms, are further subdivided into:
  • nematodes (roundworms, including strongyloidiasis, gnathostomiasis and various soil transmitted helminths)
  • cestodes (tapeworms, including echinococcosis)
  • trematodes (flukes, including schistosomiasis)
• The life cycle, transmission route and tissue tropism of each vary substantially. They directly inform both the clinical presentation and the appropriate diagnostic and treatment approach.
• Ectoparasitic infestations, involving organisms living on rather than within the host, include scabies and lice infestation, generally producing localised cutaneous manifestations rather than the systemic or organ specific disease patterns characteristic of the protozoal and helminthic infections.

• Eosinophilia is a hallmark, though non specific, laboratory clue strongly associated with helminthic infection specifically.
• It reflects the characteristic host immune response to multicellular parasitic organisms, particularly during tissue migration phases of the parasite life cycle.
• It is characteristically absent or minimal in most protozoal infections. This gives a useful initial discriminating feature when evaluating unexplained eosinophilia in a returning traveller or migrant.
• The specific pattern and timing of eosinophilia relative to symptom onset and geographical exposure can provide further diagnostic clues.
• Marked eosinophilia occurs during the acute, tissue migratory phase of many helminthic infections. An example is the pulmonary migratory phase of several soil transmitted helminths, producing transient pulmonary infiltrates with eosinophilia, termed Loeffler syndrome.
• Eosinophilia may be minimal or absent once the parasite has established chronic residence within its definitive tissue niche.
• Gastrointestinal symptoms (abdominal pain, diarrhoea, malabsorption) are common across many protozoal and helminthic infections affecting the gastrointestinal tract.
• Other parasitic infections produce organ specific manifestations reflecting the tissue tropism of the causative organism:
  • hepatic and pulmonary involvement in echinococcosis
  • urinary tract involvement in specific schistosomiasis species
  • central nervous system involvement in specific trypanosomiasis and, rarely, other parasitic infections
• A detailed and specific travel, dietary, water exposure, occupational and recreational history is essential in evaluating any patient with suspected parasitic infection.
• Individual parasitic pathogens have often highly specific geographical distribution, transmission route and exposure risk factors.
• This history frequently provides the single most valuable diagnostic clue guiding subsequent targeted investigation.

• Stool microscopy for ova, cysts and parasites remains a fundamental diagnostic investigation for suspected gastrointestinal parasitic infection.
• Sensitivity is imperfect with a single sample, given the intermittent egg or cyst shedding characteristic of many organisms.
• Multiple samples (typically three, collected on separate days) improve diagnostic yield where initial testing is negative but clinical suspicion remains significant.
• Serological testing is available and useful for a range of parasitic infections, particularly those where the causative organism resides predominantly in tissue rather than being readily accessible for direct microscopic identification (such as echinococcosis, schistosomiasis, and strongyloidiasis).
• Cross reactivity between related organisms can occasionally complicate interpretation.
• Blood film examination (thick and thin films) remains essential for diagnosis of malaria and can identify circulating microfilariae in suspected filarial infection.
• The timing of sample collection relative to the periodicity of microfilarial circulation (where relevant to the specific filarial species) is an important practical consideration for optimising diagnostic yield.
• Tissue biopsy, with histological examination and, where relevant, specific staining techniques, provides definitive diagnosis for parasitic infections with tissue dwelling stages not otherwise accessible to less invasive sampling.
• Molecular polymerase chain reaction based diagnostic techniques are increasingly available. They offer improved sensitivity and speed of diagnosis across an expanding range of parasitic infections compared with traditional microscopy alone.
• Given the frequently non specific presenting symptoms and the wide range of potentially relevant organisms, specialist tropical medicine or infectious disease input is valuable in guiding an efficient, appropriately targeted diagnostic approach.
• The approach should be based on the specific geographical exposure history and clinical presentation, rather than an exhaustive, unfocused panel of testing across the full breadth of potentially relevant parasitic organisms.

• Management of confirmed parasitic infection is directed according to the specific diagnosis established, with antiparasitic agent selection, dosing and duration highly specific to the particular causative organism and, in some cases, the specific tissue or organ system affected.
• Empirical antiparasitic treatment without a confirmed specific diagnosis is generally avoided, given the wide range of potentially relevant organisms and the specificity required for effective, targeted treatment.
• The exception is certain specific, well defined clinical scenarios. An example is empirical treatment of possible strongyloidiasis before initiating significant immunosuppression in a patient with relevant epidemiological exposure, given the risk of life threatening hyperinfection syndrome if unrecognised infection is present, as detailed under strongyloidiasis.
• Screening for and, where present, treatment of coinfection with other relevant organisms sharing similar epidemiological exposure risk factors is appropriate, given the frequent occurrence of polyparasitism in individuals from highly endemic settings.
• An example is screening for schistosomiasis, strongyloidiasis and other relevant parasitic infections in a patient from an endemic region presenting with any single confirmed parasitic diagnosis.
• Public health considerations form part of full management in appropriate clinical and public health contexts.
• They include notification requirements for specific parasitic infections in certain jurisdictions.
• Where relevant, they include screening and treatment of household or close contacts, particularly for infections with person to person transmission potential such as certain intestinal protozoal and helminthic infections.
• Follow up assessment, including repeat stool or blood testing where relevant, to confirm parasitological cure following completion of treatment, is appropriate for many parasitic infections given the potential for treatment failure or reinfection, particularly relevant for infections requiring specific test of cure protocols.