Medical Parasitology: Protozoa, Helminths, Arthropods

Medical Parasitology

• Medical Parasitology is a branch of medical science focused on parasites living on or in the human body, potentially causing infections
• Protozoa, helminths, and arthropods are the parasites included in medical parasitology
• Bacteria, fungi, and viruses are generally excluded from the field of medical parasitology, even though they are parasitic

Classification of Medical Parasitology

• Medical Protozoology deals with the study of medically important protozoa.
• Medical Helminthology deals with the study of medically important helminthes.
• Medical Entomology deals with the study of medically important arthropods that cause or transmit diseases to humans.

Parasites

• A parasite is a living organism that relies on another organism for its life cycle, growth, and reproduction without providing any benefit to the host
• The host harbors the parasite, providing necessary resources, but suffers from discomfort, infections, and diseases as a result

Types of Parasites

• An obligatory parasite is completely dependent on the host during its life cycle and cannot survive without it (e.g., roundworms, flukes, Plasmodium spp)
• A facultative parasite exhibits both parasitic and non-parasitic modes of living, adapting to a parasitic lifestyle if placed on a host (e.g., Strongyloides stercoralis)
• An accidental parasite attacks an unnatural host and survives (e.g., Hymenolepis diminuta and Toxocara canis in man)
• An erratic parasite wanders into an organ in which it is not usually found (e.g., Entamoeba histolytica in the liver or lung of humans)
• A temporary parasite visits the host only for feeding and then leaves (e.g., bed bug)
• A permanent parasite lives in or on its host without leaving it (e.g., lice)
• Opportunistic parasites can produce disease in an immune-deficient host (e.g., Toxoplasma gondii)
• A zoonotic parasite primarily infects animals but is transmittable to humans (e.g., Fasciola species).
• Mesoparasites live in an intermediate position, being half-ectoparasites and half-endoparasites
• An epiparasite feeds on another parasite, a relationship known as hyperparasitism (e.g., a protozoan living in a flea in a dog)

Hosts

• A host is an organism (man, animals, birds, fishes etc) that provides shelter, nourishment, or resources for a parasite's survival
• Hosts may experience discomfort, infections, and diseases, or may show no signs of infection

Host Types

• A parasite may need more than one host to complete its life cycle
• Hosts are classified according to their role in the life cycle of the parasite, including definitive, intermediate, paratenic, transport, reservoir, natural, and accidental hosts

Definitive Host

• The definitive host (DH) is where the parasite reaches its adult stage or where sexual reproduction occurs; also known as the primary host.
• Man is the DH for Schistosoma haematobium, while the female Anopheles mosquito is the DH for Plasmodium species (malaria parasites)

Intermediate Host

• The intermediate host (IH) harbors the larval, sexually immature, or asexual stages of a parasite, alternating with the definitive host
• Intermediate hosts can be arthropods, snails, fish, crustaceans, or animals
• Some parasites require two intermediate hosts to complete their life cycle

Examples of Parasites that lack intermediate hosts

• Entamoeba histolytica
• Giardia lambia
• Trichomonas vaginalis
• Ascaris lumbricoides
• Toxocara canis
• Enterobius vermicularis
• Strongyloides stercoralis
• Trichuris trichiura
• Ancylostoma duodenale
• Necator americanus
• Isospora belli
• Cryptosporodium parvum

Paratenic Host

• The paratenic host (transport host) is a vertebrate host in which a parasite remains viable and infective without development or multiplication.
• Paratenic hosts serve as temporary refuges and vehicles for reaching an obligatory host

Reservoir Host

• A reservoir host is an animal host serving as a source of infection for other animals, including humans, usually without being affected by the infection.
• Reservoir hosts harbor the same species and stages of the parasite as man and maintain the parasite's life cycle in nature.
• Sheep are a reservoir host for Fasciola hepatica.

Natural, Accidental & Dead End Hosts

• A natural host is naturally infected with certain species of parasite
• An unnatural or accidental host is not normally infected with the parasite
• A dead end host is unable to transmit parasites to other susceptible hosts

Host-Parasite Relationship

• The host-parasite relationship involves interaction between the host and parasite and competition between them
• Symbiosis is an intimate interaction between any organism (symbiont or symbiote) and another organism from a different species for a portion or all of its life cycle.

Types of Symbiotic Relationships

• Mutualism where both organisms benefit
• Commensalism where one organism benefits and the other is unaffected
• Parasitism where one organism benefits and the other is harmed

Overview of Intermediate Hosts

• Medical parasitology involves a significant number of intermediate hosts.

Intermediate host - Domestic Animals

• Intermediate Host Domestic Animals hosts include:
  • Cow harbors Taenia saginata (Beef tapeworm) which cause taeniasis
  • Pig harbors Taenia solium (Pork tapeworm) which cause taeniasis
  • Dog harbors Echinococcus granulosus which causes Hydatid disease (Echinococcosis)
  • Rat harbors Toxoplasma gondii which cause toxoplasmosis

Intermediate host - Examples

• Wild game animals: Intermediate hosts for trypanosomes, which cause Sleeping sickness (Trypanosomiasis).
• Fish: 2nd Intermediate host for Diphyllobothrium latum (Fish tapeworm) which cause diphyllobothriasis.
• Snails: Intermediate hosts for Schistosomes and liver flukes.
• Arthropods: can also be an intermediate host

Snails

• Snails are not found in salty and acidic waters
• The water snails are an intermediate host of schistosomiasis worms

Examples of water snails that serve as intermediate host include;

• Biomphalaria species e.g B. glabrata harbors Schistosoma mansoni that causes intestinal schistosomiasis, have oval shells
• Bulinus species e.g B. africanus, B. truncatus harbor S. haematobium that causes urinary schistosomiasis
• Oncomelania species O. hupensis harbors Schistosoma japonicum a causative agent of intestinal Schistosomiasis
• Lymnaea species e.g L. acuminata and Succinea species e.g S. angustior are important snail host for Faciola hepatica (Sheep liver fluke)

Arthropods General Characteristics

• Arthropods are small, bilaterally symmetrical creatures with jointed legs, insects (e.g mosquitoes, flies), arachnids (e.g. ticks, and mites), crustaceans (e.g. Cyclops), lice, fleas, centipedes and millipedes
• Arthropods have bodies divided into segments with jointed appendages that may take the form of legs or antennae
• They have a hard chitinous exoskeleton (cuticle), which helps for protection.
• The exoskeleton is partitioned by chitinous plates (scleritis) in order to allow movement
• They have a body cavity called haemocele, which contains haemolymph (blood and lymph) that bathes internal organs
• Ecdysis or moulting is a phenomenon characteristic of all arthropods whereby the cuticle is shed at regular intervals in order to accommodate the growing tissues

Insect vectors

• Insect vectors serve to carry and transmit parasites directly or indirectly from an infected animal to a human from an infected human to another human.
• Transmission may occur through injecting of parasites into a blood meal or depositing it on a person's skin
• Mosquitoes, are an essential part of the reproductive life cycle and also also serve as a Definitive host
• Insect Vectors could be biological or mechanical in nature.

Transmission of Vector Borne Diseases

• When arthropods become biological vectors for transmission of diseases, certain stages in the parasite’s life cycle takes place in the insect,
• The parasites undergo change and multiplication in order to mature to an infective stage.
• It takes several days before they are capable of being transmitted to a new host (e.g. human malaria parasite).
• Infective stages of some endoparasites in the host's blood are transported to new hosts by bite of a biological vector

Examples of biological vectors

• Mosquito transmits Plasmodium species which cause malaria
• Tsetse fly transmits Trypanosoma Brucei Complex which cause African Sleeping Sickness
• Sand fly transmits Leishmania species which cause Kala-azar disease
• Reduviid bug transmits Trypanosoma cruzi which cause South American Sleeping Sickness
• Black fly transmits Onchocerca volvulus which causes River blindness

Mechanical Vectors

• Mechanical vectors are a vector which assists in the transfer of parasitic forms from one host to another but is not essential in the life cycle of a parasite
• They lodge the disease causative agent without altering its development or multiplication
• Vectors like housefly serve as passively carry amoebic cysts from feces to food which results in amoebiasis after ingestion

Classification of Vectors based on different characteristics

• Feeding Period: Nocturnal (night feeder), Diurnal (day feeder), Crepuscular (twilight feeder), Matutinal (before sunrise), Vespertine (after sunset)
• Host Specificity: Anthropophilic (feeds on humans), Zoophilic (feeds on animals), Ornithophilic (feeds on birds)
• Host Feeding Location: Exophagic (feeds outdoor), Endophagic (feeds indoor), Exophilic (rests outdoor), Endophilic (rests indoor)

RODENTS importance

• Rodents are important disease vectors located in almost all human communities, they feed on any kind of food
• Rodents cause destruction of foodstuffs by contamination with urine, faeces, or simple contact.
• Rodents can transmit parasitic infections like Toxoplasmosis, viral infections like Lassa fever and bacterial infections like Leptospirosis to humans.
• They also act as reservoirs for pathogens which cause Murine typhus, plague and relapsing fever
• Female rats give birth to 5-8 young which become adult and can get pregnant in two months

Medically Important Rodents

• Rattus norvegicus also known as Brown rat or Norway rat, weighs about 500g and about 45 cm principally found in sewers and feeds on garbage
• Rattus rattus also known as Roof rat or Black rat, weigh about 250g and about 40 commonly found under the rooftop of any type of building
• Mus musculus also known as House mouse, weighs about 20g and measures about 18 cm commonly found around supplies of grain, cereals, and flour

VECTOR CONTROL MEASURES

• Mechanical methods (e.g., bed nets, wire mesh)
• Ecological methods involve the removal, destruction, modification and isolation of materials that favour the survival of insect pests by making a site suitable for breeding.
• Chemical methods use of natural or synthetic chemicals that directly cause the death or repulsion of insects (e.g., Dichlorodiphenyltrichloroethane)
• Biological methods refers to the regulation of vector population of rats like cats or fish which feeds on larval stages of arthropods and parasites.
• Genetic control involves manipulation of the mechanisms of heredity (e.g., sterilized male mosquitoes)