Parasitology: Parasites and Hosts

• Parasitology studies the relationship between parasites and their hosts

Parasites

• Organisms infecting other living organisms
• Depend on hosts for shelter, food and growth

Host

• An organism that harbors a parasite

Medical Parasitology

• Deals with parasites causing human infections and related diseases

Ectoparasite

• Resides on the host's body surface
• Does not penetrate the tissue

Endoparasite

• Parasite living inside a host's body
• Said to cause infection

Free-Living Parasite

• Nonparasitic active stages living independently
• Example: cystic stage of Naegleria fowleri

Endoparasites classification

• Obligate parasites like Toxoplasma gondii and Plasmodium
• Facultative parasites like Naegleria fowleri

Scientists Involved

• Antonie van Leeuwenhoek: First to use a single-lens microscope and observe Giardia in his stools in 1681 in Holland
• Louis Pasteur: Published first scientific protozoa study
• Prevented and controlled an epidemic silkworm disease in Southern Europe in 1870,
• Patrick Manson: Discovered mosquitoes role in filariasis in 1878
• First evidence of vector transmission
• Laveran: Discovered the malarial parasite in Algeria in 1880
• Ronald Ross: Discovered malaria transmission by mosquitoes in India in 1897

Definitive Host

• Host where the adult parasite lives and reproduces sexually
• Mosquitoes: Definitive host in malaria

Intermediate Host

• Host where parasite’s larval stage lives or multiplies asexually
• Parasites using humans as intermediate or secondary hosts include Plasmodium spp., Babesia spp., and Toxoplasma gondii

Paratenic Host

• Host where the parasite's larval stage remains viable without further development

Reservoir Host

• Host that maintains a parasitic infection in an endemic area.
• Acts as a significant infection source for susceptible hosts
• Dogs: Reservoir host for hydatid disease

Accidental Host

• Host not typically found to harbor the parasite
• Humans: Accidental host for cystic echinococcosis

Zoonosis

• Diseases and infections naturally transmitted between vertebrate animals and humans

Protozoal Zoonoses

• Toxoplasmosis, leishmaniasis, balanlidiasis, and cryptosporidiosis

Helminthic Zoonoses

• Hydatid disease and taeniasis

Anthropozoonoses

• Infections transmitted from lower vertebrate to man
• Example: cystic echinococcosis

Zooanthroponoses

• Infections transmitted from man to lower vertebrate animals
• Example: human tuberculosis to cattle

Exposure and Infection

• Pathogens: Harmful, causing mechanical injury to hosts
• Carrier: Harbors a pathogen without showing signs or symptoms
• Exposure: Process of inoculating an infective agent
• Infection: Establishment of the infective agent in a host
• Incubation Period: Time between infection and symptom appearance
• Pre-Patent Period: The time between infection acquisition and demonstration of infection
• Autoinfection: When an infected individual becomes their own source of infection
• Superinfection (Hyperinfection): Further infection intensifies the already parasitized individual with the same species

Infection Sources

• Contaminated soil, water and food
• Vectors that can be biological, where parasites develop/multiply, or mechanical, aiding transfer
• Soil with eggs penetrate uncovered skin
• Infected water transmits Cyclops containing worm larva
• Raw or undercooked meat consumption results in harboring parasite larvae

Transmission Modes

• Oral transmission
• Through contaminated food, water, fingers, and fomites
• Skin transmission
• Hookworm via larvae entering through bare feet on contaminated soil
• Schistosomiasis cercarial larvae penetrate exposed skin

Other Transmissions

• Direct transmission: person-to-person contact, like kissing for gingival amebae or sexual contact for trichomoniasis
• Vertical transmission: mother to fetus, as in malaria and toxoplasmosis
• Iatrogenic transmission: through blood transfusions or organ transplants

Nomenclature

• Animal parasites: According to the International Code of Zoological Nomenclature

Laboratory diagnosis

• Clinical features and physical examination, laboratory testing is required as parasitic infections are unable to be definitively diagnosed using only clinical assessment
• Essential tests: microscopy, culture, serological tests, skin tests, molecular methods, animal inoculation, xenodiagnosis, imaging, hematology

Specimen Diagnosis Tools

• Stool, blood, urine, sputum, cerebrospinal fluid, tissue aspirates, and genital samples
• Stool: identifies intestinal infections
• Cysts and trophozoites of parasites such as E. histolytica and Giardia, and eggs and larvae of worms are demonstrated in stool samples
• Blood: identifies parasites circulating in blood vessels. confirmed by demonstration of its morphological stages in the blood, parasiticides include Wuchereria bancrofti

Urine and Sputum Examination

• Lateral-spined eggs of S. haematobium, as well as trophozoites of T. vaginalis and microfilaria of W. bancrofti can be found in urine
• Eggs of P. westermani, larvae of S. slercoralis and A. lumbricoides can be identified in sputum

Bodily Fluids

• T. brucei, Naegleria, Acanthamoeba, Balamuthia, and Angiostrongylus identified in cerebrospinal fluid in CSF
• Larvae of Trichinella, eggs of Schistosoma identified in muscle biopsy
• Naegleria and Acanthamoeba detected by histopathological brain exam
• Leishman Donovan (LD) bodies in spleen, bone marrow aspirate indicates Kala-azar
• Giardia trophozoites identified via intestinal aspirates
• E. histolytica trophozoites identified in amebic liver abscess pus

Diagnostics

• T. vaginalis vaginalis and E. vermicularis are found by examining genital specimens
• Parasites culturing includes Leishmania, Entamoeba, Trypanosoma
• Serological tests detect protozoa, helminth infections

Tests

• Detecting antigen, examples include pLDH for malaria and ELISA for filarial antigens
• Antibody detection tests are usefull for amebiasis, echinococcosis, and leishmaniasis
• Skin tests by injecting parasitic antigens detect hypersensitivity
• Molecular methods, such as DNA probes and PCR techniques, are used to diagnose parasitic infections
• Animals are useful for detecting Toxoplasma, Trypanosoma and Babesia in blood
• Xenodiagnosis detects T. cruzi stages in bugs fed with patient blood

Imaging Techniques

• Procedures like X-rays, USG, CT scan and MRI used to diagnose cyst disease

Hematology

• Analyzing the ratio of White Blood Cells can also determine infection
• Helminth and hookworm can result in eosinophilia and anemia
• Immunodeficiency related to visceral leishmaniasis can result in occurrences of hypergammaglobulinemia
• Amoebic liver abscess presence can result in leukocytosis