The article discusses diagnostic challenges in parasitic infections and the value of combining clinical insights with advanced tools, exemplified by a rare amoebic ulcer case.

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The past 20 years have seen great progress in diagnosing parasitic infections with the introduction of serological tests, PCR and Now Proteomics techniques. While microscopy remains the mainstay and gold standard in the diagnosis of many parasitic infections, these emerging techniques offer large benefits over the shortcomings of microscopy.

However, the right choice of investigation should always be guided by the clinical scenario for optimum results. Let us have a brief look at the advantages and disadvantages of various techniques and tools for diagnosis of parasitic infections-

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History & clinical examination

1. As the old saying goes, a good and thorough history remains invaluable to any diagnosis.
2. While the similar syndromic presentation of multiple parasitic infections acts a as deterrent in forming a conclusive diagnosis, it still provides invaluable information needed to guide the investigations required.
3. A good travel history and thorough knowledge of parasitic epidemiology have become essential as the world continues to become a smaller place. It is not uncommon for non-endemic area healthcare professionals to miss obvious diagnoses of diseases endemic to unsuspecting places due to improper travel history.
4. Immunocompromised status due to iatrogenic and infectious causes affects the differential diagnosis due to opportunistic infections, the natural course of infection and also the patient’s response to treatment.
5. A good clinical examination can also provide invaluable clues to the diagnosis as in Enterobius vermicularis (pinworm), larva migrans and scabies.

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Microscopy

Microscopy remains the gold standard and most widespread modality of diagnosing parasitic infections to date. It is cheap, requires minimal infrastructure and has good sensitivity and specificity when performed by experienced personnel.

Wet mounts of sputum, urine, vaginal swabs, duodenal aspirates, sigmoidoscopic material, abscesses, and tissue biopsies are usually examined using microscopy. Mounting specimens in saline or iodine are valuable for detecting trophozoites of Entamoeba histolytica/dispar, Giardia duodenalis, Balantidium coli, Trichomonas vaginalis, Naegleria fowleri, and Acanthamoeba, larvae (e.g. Strongyloides sp.), adults and eggs of helminths, and protozoan oocysts (e.g. cysts of Entamoeba histolytica, Giardia duodenalis, and Balantidium coli, while oocysts of Cryptosporidium spp., Cyclospora cayetanensis, or Isospora belli can be visualised only in heavy infections.

The wet mount examination of blood is used to detect microfilariae of various nematode species and trypanosomes, while saline mounts of superficial skin snips are useful for the detection of Onchocerca volvulus.

The use of staining and fluorescent microscopy further augment the diagnosing abilities of light microscopy making it indispensable to diagnosis of parasitic infections, especially in resource-limited settings.

However, microscopy has several disadvantages like the requirement of highly trained professionals for reliable results, interpersonal variations, sample collection errors due to parasite-induced variations and handling problems, invasive sampling for better results, limitations in the identification of species as well as contamination and infectivity concerns.

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Serology & immunology

The use of antigens, antibodies and other parasite-specific markers has ushered in a new era in the diagnosis of parasitic infections, especially with the development of point-of-care tests (POCTs). The widespread use of POCTs for malaria is now an integral part of its management and a testament to the utility of POCTs.

POCTs are cheap, require minimal infrastructure, can be done bedside give rapid results and are useful even in field and remote areas. In comparison to microscopy, serological tests and POCTs require less trained personnel and are relatively easy to interpret. They are also useful in conducting epidemiological studies to identify endemic areas.

Despite all their advantages, the problems of cross-reactivity and false results limit the use of serological tests. Many parasites are yet to have identifiable serological markers and research under this category is still progressing. Many markers are also unable to differentiate present from past infections, and active from inactive infections limiting their utility.

Malaria, Leishmaniasis, Trypanosomiasis (Sleeping Sickness), Amoebiasis, Giardiasis, Trichomonas vaginalis and Schistosomiasis are some examples where serological and immunological tests can be used for diagnosis currently.

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Molecular tests

The advent of PCR-based assays which can reliably diagnose not only single parasitic infections but also screen for multiple infections at the same time undeniably form the future of diagnosis of parasitic infections.

They have high sensitivity and specificity and can be done over a wide variety of samples. While research under this category is ongoing with resource and infrastructure limitations, these are expected to be resolved with time.

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Case study

A 40-year-old male patient presented with a 1-year history of bleeding per rectum and dyspnea on exertion with no other relevant history.

1. A proctoscopy revealed small haemorrhoids with no signs of active bleeding despite multiple assessments. Investigations were suggestive of Iron Deficiency Anemia.
2. Stool R/M reported no Ova or cyst.
3. A colonoscopy was planned to rule out colon carcinoma.
4. A colonoscopy showed multiple large, discreet ulcers with active oozing of blood in the cecum with the intervening and the rest of the ileal and colonic mucosa appeared normal.
5. Colonic carcinoma was suspected and biopsies were taken. However, histopathology revealed flask-shaped ulcers with protozoal trophozoites engulfing red blood cells suggesting Amoebic infestation which was then managed with anti-amoebic agents.
6. This was one of the rare presentations where an Amoebic ulcer presented as frank bleeding per rectum confounded by the presence of haemorrhoids which were initially attributed as the cause of bleeding by previous doctors leading to a delay in diagnosis of around 1 year.
7. The reliance on stool microscopy to rule out infestation in the case and the absence of clinical features further contributed to the delay which would have probably not risen if the serology of the patient had been done at an earlier stage.

 

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Please read the related article: Parasitic Infections: Current Challenges in Managing Infections

Disclaimer- The views and opinions expressed in this article are those of the author and do not necessarily reflect the official policy or position of M3 India.

About the author of this article: Dr.Vyom Agarwal is a Senior Resident, MBBS/MD(General Medicine), at Government Medical College, Korba.