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Introduction to Parasitic Infections

Part 5: Infectious Diseases | Part 5 – Infectious Diseases: Parasitic · Part 5 – Infectious Diseases: Parasitic · Chapter 228

Key Clinical Points

  1. Parasites are eukaryotic organisms (helminths and protozoa) distinct from bacteria and viruses.
  2. Helminths are multicellular (worms); Protozoans are single-celled and can multiply within the human host.
  3. Travel history is the critical component of diagnosis for parasitic infections in non-endemic areas.
  4. Malaria (Plasmodium falciparum) is the most common parasitic infection with higher mortality than any other parasite.
  5. Immunocompromised hosts are at high risk for disseminated protozoan infections (Leishmania, Toxoplasma, Cryptosporidium) and Strongyloides.
  6. Fever onset timing: >10–14 days post-travel → Malaria likely; <10 days → Dengue/Chikungunya/Zika more probable.
  7. Duffy antigen negativity → resistance to Plasmodium vivax and ovale.
  8. Strongyloides infection can persist for decades → risk of dissemination during immune compromise.
  9. Table 228-1 provides a comprehensive mapping of parasites by organ system, symptoms, and geography.
  10. Specific clinical markers include: Ring-enhancing CNS lesions (Toxoplasma), Amastigotes (Leishmania), and Calcified cysts (>1cm) (Echinococcus).

1. DEFINITION & OVERVIEW

• Etymology: Parasites derived from Greek 'para' (beside) and 'sitos' (food); organisms living at another’s expense. • Biological Distinction: Unlike bacteria/viruses, parasites are eukaryotic; their structure resembles human cells, complicating drug development.

• Classification of Internal Parasites: • Helminths: Multicellular worms. • Two phyla: Platyhelminthes (flatworms) and Nemathelminthes (roundworms). • Life Cycle: Often require intermediate hosts; Exceptions: Strongyloides and Capillaria complete life cycles in humans. • Protozoa: Microscopic single-celled organisms. • Pathogenesis: Utilize mechanisms like antigenic variation (Trypanosoma) or intracellular survival (Plasmodium, Toxoplasma). • Risk Factor: Immunocompromised hosts lack partial immunity → severe disease.

1.1 Helminths vs. Protozoa

• Helminths: - Multicellular organisms visible to the naked eye. - Two phyla: Platyhelminthes (flatworms) and Nemathelminthes (roundworms). - Humans as definitive hosts for intestinal helminths; intermediate hosts for tissue-invading species. - Exceptions: Strongyloides and Capillaria complete life cycles in humans.

• Protozoa: - Microscopic single-celled organisms capable of intracellular replication. - Major pathogenic mechanisms include immune evasion via antigenic variation (Trypanosoma) or intracellular survival (Plasmodium, Toxoplasma). - Immunocompromised hosts experience severe disease due to lack of partial immunity.

2. EPIDEMIOLOGY

• Prevalence: Intestinal roundworm infections are the most common globally; prevalent in resource-poor regions with poor sanitation. • Travel Risk: Low risk for general travelers unless exposed to high-risk environments (war zones, refugee camps). • Malaria: Highest mortality rate among all parasitic infections.

• Risk Factors: • Prolonged exposure to endemic regions. - Immunocompromise (HIV, organ transplant) → risk of disseminated infection (Leishmania, Toxoplasma). - Adventure travel, rural exposure, or refugee settings. - Immigrant status from developing countries.

• Prevention: First-generation P. falciparum vaccine targets children in high-prevalence regions.

3. ETIOLOGY & PATHOPHYSIOLOGY

• Helminth Classification: • Tapeworms (Cestodes): Taenia saginata (beef), T. solium (pork), Diphyllobothrium latum (fish); Echinococcus granulosus (hydatid disease); Echinococcus multilocularis (sub-Arctic). • Flukes (Trematodes): Clonorchis sinensis, Opisthorchis spp., Fasciola hepatica (liver flukes); Paragonimus spp. (lung flukes); Schistosoma spp. (blood flukes: S. mansoni, S. haematobium, S. japonicum). • Roundworms (Nematodes): Ascaris lumbricoides, hookworms, Strongyloides stercoralis; Filariasis (Wuchereria bancrofti, Onchocerca volvulus, Loa loa); Trichinella spiralis, Gnathostoma spp., Angiostrongylus spp.

• Protozoan Classification: • Intestinal Protozoa: Entamoeba histolytica (amebic dysentery, liver abscesses); Cryptosporidium (waterborne, severe in AIDS); Giardia lamblia (foul-smelling stool/steatorrhea); Microsporidia (chronic diarrhea in AIDS). • Blood/Tissue Protozoa: Plasmodium falciparum (cerebral malaria, highest mortality); Trypanosoma cruzi (Chagas disease, cardiomyopathy); Leishmania spp. (visceral leishmaniasis, AIDS-defining); Toxoplasma gondii (reactivation in immunocompromise); Free-living amoebae (Acanthamoeba keratitis, Naegleria meningitis).

3.1 Helminth Classification

• Tapeworms (Cestodes): - Taenia saginata (beef), T. solium (pork), Diphyllobothrium latum (fish). - Echinococcus granulosus (hydatid disease from dog feces); - Echinococcus multilocularis (sub-Arctic regions, fox/dog transmission).

• Flukes (Trematodes): - Clonorchis sinensis, Opisthorchis spp., Fasciola hepatica (liver flukes); - Paragonimus spp. (lung flukes); - Schistosoma spp. (blood flukes: S. mansoni, S. haematobium, S. japonicum).

• Roundworms (Nematodes): - Ascaris lumbricoides, hookworms, Strongyloides stercoralis; - Filariasis (Wuchereria bancrofti, Onchocerca volvulus, Loa loa); - Trichinella spiralis, Gnathostoma spp., Angiostrongylus spp.

3.2 Protozoan Classification

• Intestinal Protozoa: - Entamoeba histolytica (amebic dysentery, liver abscesses); - Cryptosporidium (waterborne, severe diarrhea in AIDS); - Giardia lamblia (foul-smelling stools); - Microsporidia (chronic diarrhea in AIDS).

• Blood/Tissue Protozoa: - Plasmodium falciparum (cerebral malaria, highest mortality); - Trypanosoma cruzi (Chagas disease, cardiomyopathy); - Leishmania spp. (visceral leishmaniasis, AIDS-defining illness); - Toxoplasma gondii (reactivation in immunocompromise); - Free-living amoebae (Acanthamoeba keratitis, Naegleria meningitis).

4. CLINICAL FEATURES

• General Presentation: Varies by parasite and organ involvement; includes cutaneous lesions, CNS symptoms, ocular findings, and systemic complications.

• Patient Assessment: - Travel History: Critical for diagnosis in non-endemic areas; duration of stay correlates with risk. - Immune Status: Immunocompromise increases severity (e.g., disseminated Strongyloides, Toxoplasma). - Timing of Fever Decision Pathway: - If fever onset is >10–14 days post-travel → Malaria likely. - If fever onset is <10 days post-travel → Dengue/Chikungunya/Zika more probable. - Duration of Diarrhea: Persistent traveler's diarrhea (>2 weeks) → suggests parasitic etiology (Cryptosporidium, Giardia). - Physical Exam: Focus on organ-specific signs (e.g., hepatosplenomegaly, skin nodules).

5. DIFFERENTIAL DIAGNOSIS

• Amebiasis vs. Bacterial Diarrhea: Amebiasis features slower onset and potential liver abscesses; E. histolytica is indistinguishable from noninvasive E. dispar. • Malaria vs. Other Fevers: Malaria must be considered first in febrile post-travel patients; timing of fever (10–14 days) helps differentiate from Dengue/Chikungunya/Zika. • Traveler's Diarrhea: Acute bacterial/viral → resolves quickly; Chronic (>2 weeks) → parasitic (Cryptosporidium, Giardia). • Bacterial vs. Parasitic Jaundice: Hepatitis A/B unlikely in immunized patients; Malaria possible despite chemoprophylaxis due to drug resistance.

6. INVESTIGATIONS & DIAGNOSIS

  1. Travel History Assessment:
  2. Duration of stay in endemic areas.
  3. Adherence to chemoprophylaxis.
  4. Timing of symptom onset relative to travel return.
  5. Laboratory & Imaging Findings:
  6. E. histolytica: Trophozoites/cysts (indistinguishable from noninvasive E. dispar).
  7. Leishmania: Amastigotes in tissue biopsies.
  8. Onchocerca: Microfilariae in skin snips.
  9. Naegleria: Motile trophozoites in CSF.
  10. Toxoplasma: Ring-enhancing lesions on CT.
  11. Taenia solium: Calcified cysts on imaging.
  12. Echinococcus: Liver > lung cysts; cysts >1cm.
  13. Cryptosporidium: Watery diarrhea in AIDS patients.
  14. Giardia: Foul-smelling stool with steatorrhea.
  15. Schistosoma haematobium: Hematuria with negative cultures; potential for bladder cancer.

6.1 Table 228-1: Parasitic Infections by Organ System

Table 228-1 provides a comprehensive mapping of parasites to clinical findings:

• Eosinophilic Meningitis: - Angiostrongylus (most common cause globally; spontaneous resolution). - Gnathostoma (migratory nodules). • Eyes: - Acanthamoeba (corneal opacification/ulcers). - Toxoplasma (retinal mass). - Onchocerca & L. loa (worms may cross eye during migration). • Heart: - P. falciparum (pulmonary edema as end-organ damage from severe malaria). - Trypanosoma cruzi (cardiomegaly, arrhythmias). • Gastrointestinal Tract: - Entamoeba histolytica (bloody diarrhea, liver abscesses; acute with fever/RUQ pain or chronic with enlarged liver/abscesses). - Schistosoma mansoni (portal obstruction, cirrhosis, varices). - Leishmania donovani complex (visceral leishmaniasis; AIDS-defining infection). - Echinococcus & Fasciola (common in sheep-raising areas). - Clonorchis (recurrent cholangitis and late cholangiocarcinoma). - Cryptosporidium (watery diarrhea, severe in immunocompromised). - Giardia (foul-smelling stool with steatorrhea). - Isospora belli (fever, abdominal pain, chronic diarrhea). - Microsporidia (chronic diarrhea in AIDS). - Capillaria (malabsorption, wasting; Southeast Asia/Egypt). - Pinworm (anal itching; eggs rarely detected by O&P). - Trichuris (rectal prolapse with heavy infection in children). - T. solium or Taenia saginata (passage of tapeworm segments >6 cm). - Diphyllobothium latum (pernicious anemia in genetically predisposed Scandinavians). • Genitourinary System: - Trichomonas vaginalis (common STD). - Schistosoma haematobium (hematuria with negative cultures; potential for bladder cancer).

7. MANAGEMENT & TREATMENT

  1. Malaria: Artemisinin-based combination therapies (ACTs) for P. falciparum.
  2. Leishmania: Liposomal amphotericin B.
  3. Toxoplasma: Pyrimethramine + sulfadiazine.
  4. Strongyloides: Ivermectin (primaquine for liver forms).
  5. Schistosomiasis: Praziquantel.

• Special Considerations: - Immunocompromised patients require prolonged therapy. - Drug resistance in malaria necessitates ACTs. - Vaccine availability limited to P. falciparum.

8. PROGNOSIS & COMPLICATIONS

• Malaria (P. falciparum): Highest mortality among parasites; can cause end-organ damage like pulmonary edema. • Immunocompromised Patients: Disseminated infections (Toxoplasma, Cryptosporidium) are often fatal without treatment. • Chronic Helminth Damage: Schistosomiasis leads to portal hypertension and cirrhosis. • Strongyloides: Can persist for decades → risk of dissemination during immune compromise.

9. SPECIAL CONSIDERATIONS

• Geographic Variation: - Malaria: Sub-Saharan Africa (P. falciparum), Southeast Asia (P. vivax). - Schistosomiasis: Africa, China. - Leishmaniasis: India, Brazil. - Toxoplasma: Global; severe in immunocompromised.

• Diagnostic Clues: - Migratory skin nodules → Loa loa, Gnathostoma. - Ring-enhancing CNS lesions → Toxoplasma. - Calcified liver cysts → Echinococcus. - Serpentine rash/anemia → Hookworm. - Foul-smelling stool with steatorrhea → Giardia. - Hematuria with negative cultures → Schistosoma haematobium.

Reference Tables

TABLE 228-1 Parasitic Infections, by Organ System and Signs/Symptoms a

Harrison's 22e, p.1739

ORGAN SYSTEM, MAJOR
SIGN(S)/SYMPTOM(S)		 PARASITE(S) GEOGRAPHIC DISTRIBUTION COMMENTS
Naegleria Worldwide
Eosinophilic meningitis Angiostrongylus (rat lung
worm)		 Southeast Asia, Pacific,
Caribbean		 Most common cause globally of eosinophilic meningitis;
spontaneous resolution
Gnathostoma Southeast Asia and China Migratory nodules
Eyes
Acanthamoeba Worldwide
Corneal opacification Onchocerca Mexico, Central/South America,
Africa		 Immune response to microfilaria in cornea
Toxoplasma Worldwide
Retinal mass Toxocara Worldwide Ocular larva migrans
Visible roundworm in eye Onchocerca Mexico, Central/South America,
Africa		 Worms may cross eye during migration.
L. loa Western and central Africa Worms may cross eye during migration.
Gnathostoma Southeast Asia and China
Lungs
Pulmonary nodule/abscess Paragonimus Far East, Africa, Americas Ectopic migration to abdomen or central nervous system
Migrating helminths Worldwide
Heart
Pulmonary edema P. falciparum (complication) Tropics and subtropics End-organ damage from severe malaria
Trypanosoma cruzi Mexico, Central/South America
Gastrointestinal Tract
Hepatosplenomegaly Malaria (multiple episodes) Tropics and subtropics Splenomegaly with anemia and recurrent fever are hallmarks of
malaria.
S. mansoni Africa, Central/South America Portal obstruction with cirrhosis and late varices
Leishmania donovani complex Tropics and subtropics Visceral leishmaniasis; AIDS-defining infection
Entamoeba histolytica Tropics
Echinococcus Sheep-raising areas
Fasciola Sheep-raising areas
Cholangitis Clonorchis China, Southeast Asia Recurrent cholangitis and late cholangiocarcinoma
Microsporidia Worldwide AIDS
Cryptosporidium Worldwide AIDS-defining infection
E. histolytica Tropics
S. mansoni Africa, Central/South America
S. japonicum Far East
Watery diarrhea Cryptosporidium Worldwide Severe in immunocompromised patients
Giardia Worldwide Foul-smelling stool with steatorrhea
Isospora belli Worldwide Fever, abdominal pain, chronic diarrhea
Microsporidia Worldwide Chronic diarrhea with AIDS
Capillaria Southeast Asia, Egypt Malabsorption, wasting
Ascaris Worldwide
Small roundworms visible
around anus		 Pinworm Worldwide Anal itching; eggs rarely detected by ova and parasite (O&P) exam
Trichuris Worldwide Rectal prolapse with heavy infection in children
T. solium or Taenia saginata Worldwide
Diphyllobothrium latum Worldwide
Genitourinary System
Itchy discharge Trichomonas vaginalis Worldwide Common sexually transmitted disease of both sexes
Schistosoma haematobium Africa