Introduction to Parasitology

• Parasitology is the study of parasites, their hosts, and the relationship between them
• Parasites are organisms that live on or in a host organism and get their food from or at the expense of its host

Types of Parasites

• Ectoparasites live on the surface of the host like lice, fleas, ticks, and mites
• Endoparasites live inside the host like helminths and protozoa
• Obligate parasites cannot complete their life cycle without a host
• Facultative parasites can live freely or as a parasite

Hosts

• A definitive host harbors the adult or sexually reproductive phase of the parasite
• An intermediate host harbors the larval or asexual stage of the parasite
• A paratenic host serves as a temporary refuge for the parasite, but is not necessary for the parasite's development
• A reservoir host is an animal that harbors a parasite and serves as a source of infection for humans or other animals

Protozoa

• Protozoa are unicellular eukaryotic organisms
• They can be free-living or parasitic
• Reproduction can be asexual via binary fission or schizogony, or sexual via conjugation or syngamy

Protozoan Groups

• Flagellates possess flagella for motility; examples include Giardia lamblia, Trichomonas vaginalis, Trypanosoma spp., and Leishmania spp.
• Amoebae move using pseudopodia; examples include Entamoeba histolytica, Naegleria fowleri, and Acanthamoeba spp.
• Ciliates possess cilia for motility; an example is Balantidium coli
• Apicomplexans are characterized by an apical complex used for penetrating host cells and all are parasitic, like Plasmodium spp., Cryptosporidium parvum, and Toxoplasma gondii

Helminths

• Helminths are multicellular parasitic worms
• They are generally large and visible to the naked eye in their adult stage

Helminth Groups

• Nematodes (roundworms) have a cylindrical body shape and a complete digestive system; examples include Ascaris lumbricoides, Necator americanus, Ancylostoma duodenale, Trichinella spiralis, Enterobius vermicularis, Strongyloides stercoralis, and filarial worms
• Cestodes (tapeworms) have a flat, segmented body and absorb nutrients through their tegument; examples include Taenia solium, Taenia saginata, Diphyllobothrium latum, and Echinococcus granulosus
• Trematodes (flukes) have a leaf-shaped body with oral and ventral suckers; examples include Schistosoma spp., Fasciola hepatica, and Paragonimus westermani

Arthropods

• Arthropods are invertebrate animals with an exoskeleton, segmented body, and jointed appendages
• Many arthropods are ectoparasites or vectors of parasitic diseases

Medically Important Arthropods

• Insects include lice, fleas, mosquitoes, flies, and bed bugs
• Arachnids include ticks and mites

Modes of Transmission

• Ingestion of contaminated food or water transmits parasites like Entamoeba histolytica, Giardia lamblia, Ascaris lumbricoides, and Taenia solium
• Vector-borne transmission involves parasites like Plasmodium spp. transmitted by mosquitoes, Trypanosoma spp. transmitted by tsetse flies or reduviid bugs, and Leishmania spp. transmitted by sandflies
• Direct contact transmits parasites like Enterobius vermicularis and Sarcoptes scabiei
• Skin penetration is a mode of transmission for Necator americanus, Ancylostoma duodenale, and Schistosoma spp.
• Congenital transmission occurs with parasites like Toxoplasma gondii

Pathogenesis

• Tissue damage results from mechanical obstruction, feeding on tissues, or production of toxic substances by parasites
• Inflammation is triggered by parasitic infections, leading to further tissue damage and clinical symptoms
• Immune suppression can be caused by some parasites, making the host more susceptible to secondary infections
• Nutrient deprivation results as parasites deprive the host of essential nutrients, leading to malnutrition and anemia

Diagnosis of Parasitic Infections

• Microscopy involves direct examination of stool, blood, urine, or tissue samples to identify parasites or their eggs/larvae
• Molecular methods like PCR-based assays detect parasite DNA or RNA
• Serology detects antibodies against parasitic antigens in serum
• Antigen detection identifies parasite antigens in samples
• Culture involves growing parasites in vitro for identification
• Imaging techniques like X-rays, ultrasound, CT scans, or MRI detect parasitic lesions in tissues

Treatment of Parasitic Infections

• Anti-protozoal drugs include metronidazole, tinidazole, chloroquine, mefloquine, primaquine, pentamidine, suramin, melarsoprol, nifurtimox, and benznidazole
• Anti-helminthic drugs include mebendazole, albendazole, pyrantel pamoate, ivermectin, praziquantel, and niclosamide
• Treatment strategies vary depending on the parasite and the severity of the infection

Prevention and Control

• Improved sanitation and hygiene practices include proper disposal of human waste, access to clean water, and regular handwashing
• Vector control includes mosquito control measures like insecticide spraying and bed nets, as well as tsetse fly and reduviid bug control
• Food safety includes proper cooking of meat and fish to kill parasites, and washing fruits and vegetables
• Chemoprophylaxis involves anti-malarial drugs for travelers to endemic areas
• Education entails public health campaigns to educate people about parasitic diseases and how to prevent them