Helminthic Infections



Helminthic Infections


Carla S. Wilson, MD, PhD





Key Facts


Terminology



  • Helminth derived from Greek word for “worms”


Etiology/Pathogenesis



  • Most common human infection in developing countries


  • Climate and topography determine distribution


  • Chronic infection leads to constant immune activation



    • Dominant Th2 type of cytokine profile


    • High immunoglobulin E levels


    • Proliferation and activation of eosinophils


Clinical Issues



  • Morbidity is associated with worm burden


  • Individuals often have > 1 infection


  • Mass drug administration of antihelmintic drugs


Microscopic Pathology



  • Anemia common



    • Blood loss, iron deficiency, vitamin B12/folate deficiency, anemia of chronic disease


    • Iron deficiency anemia is particularly prevalent with hookworm infections and schistosomiasis


  • Increased eosinophils (peripheral blood, bone marrow, tissue)



    • Associated with presence of worms in tissues


  • Circulating microfilariae seen in infections with



    • Wuchereria bancrofti


    • Brugia malayi


    • Loa loa (African eye worm)


  • Speciate pathogenic microfilariae based on placement of nuclei in tail






Eosinophilia is seen in helminthic infections caused by worms that invade tissue. Worm-induced cytokine release activates the innate immune system in which eosinophils play a key role.






5 species of microfilariae image circulate in the blood. They can be separated into sheathed (Loa loa, Wuchereria bancrofti, Brugia malayi) and unsheathed (Onchocerca volvulus, Mansonella) types.


TERMINOLOGY


Synonyms



  • Parasitic worms


Definitions



  • Helminth derived from Greek word for “worms”


  • 2 major phyla infect humans



    • Nematodes (roundworms)


    • Platyhelminths (flatworms)



      • Trematodes (flukes)


      • Cestodes (tapeworms)


ETIOLOGY/PATHOGENESIS


Infectious Agents



  • Worm lifecycle includes egg, 1 or more larval forms, adult



    • Does not replicate in human host (except Strongyloides stercoralis)


  • Human transmission occurs through



    • Ingestion of eggs


    • Penetration of skin by larvae


    • Inoculation of larvae by insect bites


  • Important circulating microfilariae include



    • Wuchereria bancrofti


    • Brugia malayi


    • Loa loa (African eye worm)


  • Heterogeneity in worm burden seen among individuals



    • Intensity of infection for many parasitic worms has genetic basis



      • S. mansoni infection intensity linked to chromosomal region 5q31-q33


      • Household clustering


Immunobiology



  • Type 2 immunity important in host defense against helminths



    • Rapid activation and employment of cells from



      • Innate immune system (eosinophils and basophils)


      • Adaptive immune system (CD4[+] T cells, Th2 pathway)


    • Synthesis of multiple cytokines (i.e., IL-4, IL-25)


CLINICAL ISSUES


Epidemiology



  • Incidence



    • Most common infection in developing countries


    • Climate and topography determine distribution


    • If transmitted by vector, need host and vector in same location



      • Loiasis: Chrysops spp. tabanid flies


      • Lymphatic filariasis: Anopheles, Culex, Aedes spp. mosquitoes


      • Onchocerciasis: Simulium spp. blackflies


  • Age



    • Reduction in intensity of some human helminth infections occurs with age



      • Ascaris lumbricoides, Trichuris trichiura, and schistosomes


      • Possibly related to host immunity


    • Some infections increase in intensity with age, peaking in adulthood



      • Hookworm


      • Filarial infections


Presentation

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Jun 13, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Helminthic Infections

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