What is a parasite? Examples of Parasites in Humans and Plants

Table of Contents

What is a Parasite?

Parasites are organisms that live in or on another organism (its host), depriving it of nutrients and causing harm. The word parasite was derived from the Latin form of the Greek word parasitos which means one who eats at the table of another. In a parasitic relationship, the parasite lives on or in the body of the host, causing harm and possibly death. However, although these parasites harm their hosts, it is not intentional. This is because it depends on the body of the host and its functions, such as digestion or blood circulation to survive. As the parasite and its host evolve together, the parasite adapts to the host as its environment. Many parasites are pathogenic to their host.

However, parasites can be grouped based on their:

  • Size
  1. Macroparasites
  2. Microparasites
  • Location on the host body
  1. Ectoparasites
  2. Endoparasites
  3. Mesoparasites
  • Level of damage
  1. Necrotrophic
  2. Biotrophic
  • Life cycle
  1. Monogenic
  2. Digenetic

Types of parasites

  • Macroparasites
  • Microparasites
  • Ectoparasites
  • Endoparasites
  • Mesoparasites
  • Necrotrophic parasites
  • Biotrophic parasites
  • Monogenic parasites
  • Digenetic parasites


Macroparasites are those parasites that are large enough to be seen with the naked eye. Nematodes, flukes, ticks, fleas, tapeworms, biting flies, and parasitic arthropods are all examples of macroparasites.


Microparasites are those parasites that are too small to be seen with the naked eye. Hence they must be viewed under a microscope. Generally, they are unicellular. An example is the protozoa.


Ectoparasites are those parasites that live on or attack outside of the host’s body. They usually live on the body surface and do not usually cause disease in the host. Most times they cause the host to itch resulting in wounds.  Such examples of ectoparasites are:

  • Ticks– They attach to a host to consume blood for nutrients. Some of them attach to their host rapidly. Some move around in search of thinner skin like the ears of mammals to attach to.
  • Fleas– They feed on warm-blooded vertebrates like humans, cats, squirrels, rats, birds, dogs, rabbits, ferrets, and mice. Fleas bite their host and cause an itching sensation which makes the host result in biting, pecking, or scratching, just to get rid of these parasites. The fleas’ bites however cause a swollen and irritating nodule to form at each site of the bite on the skin.
  • Leeches– Many of them live in freshwater habitats, while some can be seen in terrestrial or marine environments. Leeches attach themselves to a host with a sucker and feed on blood.
  • Bedbug– affect skin and vision. Infection can be spread through sharing beddings and clothes.
  • Crab lice– They affect the pubic area and eyelashes and spread through sharing clothing or beddings, sexual activity, or skin-to-skin contact.
  • Demodex: They affect the eyelashes and eyebrows and can spread through prolonged skin contact.
  • Screwworm– They affect skin and wounds.
  • Head lice– They live on the scalp of the host and affect the hair follicles. Head lice can spread through head-to-head contact and causes itching. The itching is a result of the host reacting to the saliva of the lice.


Endoparasites are those parasites that live inside the host. They can be either intercellular or intracellular. The intercellular parasites live in spaces in the body of the host. Whereas the intracellular parasites live in the cells in the body of the host. Often times the intracellular parasites depend on another organism to transmit them to the host. Such organisms are usually referred to as a carrier or vector of the parasite. Examples of endoparasites are nematodes, bacteria, viruses, and hookworms.


Mesoparasites are those parasites that enter the external openings of the host like the outer ear, buccal cavity, or cloaca. For instance, some copepods enter an opening in the body of the host and remain there partly.

Necrotrophic parasites

Necrotrophic parasites are those parasites that basically feed on part of the tissues of the host until the host dies due to nutrient loss or the loss of tissues. These parasites are also called parasitoids. Necrotrophic parasites get their nutrients from dead host cells and this is why they kill the host in advance for penetration.

For instance, is the necrotrophic parasitic Lucilia cuprina (blowfly of sheep). This parasitic fly usually lays eggs on their living host and the larvae (maggots) of the fly eat into the host flesh and may kill it. Then, the maggots continue to feed on the carcass of the host after the host’s death.

Also, there are necrotrophic parasites on plants that attack the vulnerable seedling stage and cause symptoms known as damping-off of seedlings. An example is the Botrytis fabi, a known fungal necrotrophic parasite of plants. These parasites develop in the leaves of the bean (Vicia faba) plant, killing its cell in advance for penetration. However, spots and blotches of dead tissues are then formed on the pods and leaves of the bean plant. As the host tissues are dead, this fungus continues to develop as a decomposer. The spores are then formed and dispersed from the dead tissue. Therefore most necrotrophic parasites can be regarded as pioneer saprotrophs.

Biotrophic parasites

Biotrophic parasites are those parasites that do not do severe damage enough to kill their host. Unlike the necrotrophic parasites, they actually need to keep the host alive. This is because they can’t survive in a dead host. Since biotrophic parasites get their nutrients from living host cells, the death of the host means the end of the parasite’s active life. The rust fungi (Basidiomycota) and powdery mildew fungi (Ascomycota) are common examples of biotrophic parasites.

The powdery mildew fungi, cause a powdery coating of white spores on the surface of the host’s leaf. These fungi grow on the host, virtually only penetrating the leaf epidermis. Although they extract considerable amounts of plant nutrients through their haustoria to use for sporulation. This lead to a rapid epidemic spread of powdery mildew diseases.

Monogenic parasites

Monogenic parasites are those parasites that complete their life cycle in just one individual host. Such example of a monogenic parasite is the Entamoeba histolytica.

Digenetic parasites

Digenetic parasites are those parasites that need more than one host in order to complete their life cycle. In addition to a primary host, they also need a secondary host to complete their entire life cycle. A typical example is the Plasmodium vivax, the protozoa that carry malaria. For the Plasmodium vivax to complete its life cycle, it must be a parasite of both humans and mosquitos. The asexual part of this parasite life cycle is completed in humans and the sexual part is completed in the female Anopheles mosquitoes.

Parasites Examples

  1. Parasitic plants
  2. Fungi
  3. Protozoa
  4. Bacteria
  5. Viruses
  6. Cnidaria
  7. Trematodes (flatworm)
  8. Cestodes (flatworm)
  9. Monogeneans (flatworm)
  10. Horsehair worm
  11. Nematodes
  12. Acanthocephala
  13. Leeches
  14. Ticks
  15. Mites
  16. Copepods
  17. Lice
  18. Fleas
  19. True flies
  20. Twisted wing insects
  21. Parasitoid wasps
  • Parasitic plants

Parasitic plants are in almost every biome globally. They make up about 1% of angiosperms. Approximately 4,500 species of plants in about 20 families of flowering plants are known to be parasitic. For a plant to be parasitic, it has to have modified roots called haustoria.

Parasitic plants use these haustoria to penetrate their host plant. The haustoria connect the parasitic plants to either the xylem or phloem of the host plants and at times both. This eventually helps them to extract nutrients and water from the host plants. Also, some of these parasitic plants can locate their host plants by detecting chemicals in the air or soil that have been given off by the shoots or roots of the host plants.

Parasitic plants can be classified depending on where it latches onto the host and the number of nutrients needed.

Does it latch onto the stem or the root and how much nutrient does it need?

  • Holoparasites

Holoparasites get all their food and nutrient from their host plants. They have no chlorophyll. Hence they cannot manufacture their own food by photosynthesis, making them obligate parasites. A typical example of a holoparasite is the Dodder. Cuscuta (dodder) is a stem holoparasite that infects an acacia tree.

  • Hemiparasites or partial parasites

Hemiparasites get some of their nutrient or food from other living plants. They get part and not all of their nutrients from their host plants. A typical example of hemiparasites is the mistletoe.

Mistletoe is a parasitic plant
Mistletoe is a hemiparasitic plant
Dodder is a holoparasitic plant
Dodder is a holoparasitic plant

Parasitic plants examples

  • Viscum coloratum
  • Orobanche alba
  • Viscum album
  • Orobanche flava
  • Stinking corpse lily
  • Orobanche ramosa
  • Yellow rattle
  • Orobanche minor
  • Cuscuta reflexa
  • Orobanche crenata
  • Cuscuta australis
  • Orobanche hederae
  • Cuscuta campestris
  • Orobanche cernua
  • Cuscuta pentagona
  • Greater Dodder
  • American mistletoe
  • Odontites vernus
  • Scaldweed
  • Cuscuta epithymum
  • Beechdrops
  • Cuscuta salina
  • Red-rattle
  • One flowered broomrape
  • Wyoming Indian paintbrush
  • Rafflesia patma
  • Chaparral dodder
  • Clustered broomrape
  • Rafflesia hessseltii

Many species within Orobanchaceae (broomrapes) are known examples of parasitic plants. They infect a wide range of important crops like peas, tomatoes, carrots, chickpeas, and varieties of cabbage, causing huge yield loss. Also, the Species of Striga (witchweeds) have been known to cause huge crop yield loss too especially in sub-Saharan Africa. They infect grains and grasses. Crops like corn, rice, and sorghum are widely infected by these parasitic species of Striga.

  • Fungi

A fungus is a single-celled or multicellular organism that cannot manufacture its own food (heterotrophic). They can be found on land, in soil, or on plant material. Fungi usually play a major role in nutrient cycling in the ecosystem as they help to circulate carbon and other elements. They can reproduce sexually and asexually. However, they have a symbiotic relationship with plants and bacteria. Also, as parasites, they cause some diseases in plants and animals.

Some fungi are single-celled and some are multicellular. The single-celled fungi are called yeast. However, depending on the stage of their life cycle, some fungi switch between single-celled yeast and multicellular forms. Since fungi are heterotrophs, they cannot make their own food. Hence they have to obtain their nutrients from organic material.

In order to get nutrients, a fungus has to use its hyphae. Hyphae (singular: hypha) are branching filaments that a multicellular fungus has. The hyphae, however, elongate and branch off rapidly. This allows the mycelium of the fungus to quickly increase in size. Some fungi hyphae form root-like threads (rhizomorphs) that help tether the fungus to the substrate that it grows on, and still allows it to get more nutrients from other sources.

Fungi are seen as opportunists because they can get nutrients from a wide variety of sources and thrive well in a wide range of environmental conditions. Some fungi even get their nutrients from dead organic matter. Such fungi are decomposers and are called saprobes. They break down dead organisms and get rid of them. Other fungi however parasitize plants and cause plant diseases like the Dutch elm disease.

The parasitic fungi will usually obtain their nutrients from plants, animals, or other fungi. They are usually pathogenic except in the case of mycorrhizal fungi that have a mutualistic relationship with their host plant. The honey fungi, for instance, grow in the roots of trees and kills the trees. However, they eventually still live and feed on the deadwood.

Also, there is a widespread fungal infection among animals and humans. Microsporidia, for instance, affect large insects, vertebrates, and man. They are responsible for an intestinal infection called microsporidiosis. Other parasitic fungi examples are Endothia parasitica, Puccinia graminis, Ceratocystis ulmi, and Puccinia sparganioides that parasites plants. Fungi of the genus Aspergillus and  Candida albicans infect humans. Fungal infection (mycosis) kills approximately 1.6 million people every year.

  • Protozoa

Protozoa (protozoan) are microscopic single-celled eukaryotes that are either free-living or parasitic living in moist habitats. They can be seen in the soil, freshwater, and marine environments. Protozoans vary in shape and size. For instance, the Amoeba can change its shape, and the Paramecium has a fixed shape and complex structure. Protozoa are motile and move either with the aid of a cilium, flagella, or amoeboid movement. They feed on organic matter or organic tissues and debris.

However, Some protozoans are parasitic, living on other plants and animals, including humans. Examples of parasitic protozoans are Trypanosoma cruzi, Plasmodium, Entamoeba histolytica, and Balantidium coli. They are endoparasites and cause serious diseases in vertebrates. Protozoans are able to multiply in humans, which enables them to survive and also causes severe infection to develop from just a single organism.

Transmission of protozoa parasite in a human’s intestine to another human happens via the fecal-oral route. This involves transmission through egesting the parasite from contaminated food or water or by person-to-person contact.

Transmission of protozoa that live in the blood or tissue of humans to other humans occurs via an arthropod vector. For example, the transmission of the parasite through the bite of a mosquito or sandfly.

However, protozoans that infect humans can be classified into four main groups. The following classification is actually based on their way of movement:

  • Sarcodina (ameba)– Example is Entamoeba
  • Mastigophora (flagellates)– Examples are Giardia, Leishmania
  • Ciliophora (ciliates)– E.g Balantidium
  • Sporozoa– Protozoans whose adult stage is not motile. Examples Plasmodium, Cryptosporidium

The following diseases are caused by protozoans:

  • Primary amoebic meningoencephalitis (PAM or Naegleriasis)– This is caused by Naegleria fowleri
  • Malaria– caused by Plasmodium
  • Amoebiasis– caused by Entamoeba
  • Giardiasis– caused by Giardia lamblia
  • Trichomoniasis– caused by Trichomonas vaginalis 
  • Chagas disease– caused by Trypanosoma cruzi
  • Trypanosomiasis ( sleeping sickness)– caused by Trypanosoma
  • Leishmaniasis– caused by the trypanosome genus Leishmania
  • Toxoplasmosis– caused by Toxoplasma gondii 
  • Acanthamoeba keratitis– caused by Acanthamoeba
  • Heart rot disease in Coconut palm– caused by protozoa from the genus Phytomonas
  • Sudden wilt disease in oil palm– caused by Phytomonas stahelli
  • Phloem necrosis disease in coffee crops– caused by Phytomonas leptovasorum

Also, the protozoan Ophryocystis elektroscirrha parasitizes butterfly larvae. These parasites are passed from the female butterfly to the caterpillar. The infected individuals are usually weak and unable to expand their wings or eclose (i.e to emerge as an adult from the pupa or as a larva from the egg). However, those infected end up having shortened lifespans. Also, the infection gives a culling effect, whereby infected animals migrating are less likely to complete their migration.

  • Bacteria

Bacteria is an endoparasite

Bacteria are microscopic and single-celled organisms. They exist in millions inhabiting every environment, both inside and outside of other organisms. Some bacteria can be harmful, but some are useful. The useful bacterias support plants and animals and are used in medicinal and industrial processes. However many bacteria are parasitic, generally seen as pathogens causing disease.

Parasitic bacteria are very diverse infecting their hosts via a variety of routes. Here are some examples of diseases caused by bacteria infection:

  • Anthrax: This is caused by Bacillus anthracis and is spread by contact with infected domestic animals. The bacteria spores can enter a host through abrasion or can be inhaled. However, these bacteria spores can survive for years outside the body.
  • Lyme disease and relapsing fever: is caused by Borrelia and is transmitted by vectors. These vectors are ticks of the genus Ixodes that are from the diseases’ reservoirs in animals like deer.
  • Gastroenteritis: is caused by Campylobacter jejuni and is spread by the fecal-oral route from animals. It can also be spread by eating poultry birds that are not properly cooked or by drinking contaminated water.
  • Bacterial meningitis, Influenza, and Bronchitis: This is caused by Haemophilus influenzae and is transmitted by droplet contact.
  •  Syphilis: This disease is caused by Treponema pallidum and is spread by sexual contact.

Also, most plant pathogenic bacteria belong to these genera:

  • Pseudomonas
  • Erwinia
  • Agrobacterium
  • Pectobacterium
  • Pantoea
  • Ralstonia
  • Burkholderia
  • Acidovorax
  • Phytoplasma
  • Clavibacter
  • Streptomyces
  • Xylella
  • Xanthomonas
  • Spiroplasma
  • Viruses

Viruses are parasites

Viruses are the smallest of all microorganisms. These parasites cannot reproduce by themselves. They are obligate intracellular parasites that are able to infect bacteria, archaea, plants, fungi, and animals. Viruses are different because they can only be alive and multiply inside the cells of other living organisms. However, the cell they multiply in is known as the host cell. They are able to latch onto host cells and enter them.

Viruses have either DNA or RNA as their genetic material and it may be single or double-stranded. The genetic material is covered in a protein coat called a capsid. Sometimes the coat is covered by an additional coat called the envelope.

There is a wide range of viral infections caused by viruses. Most viruses infect bacteria (bacteriophages). Diseases like chickenpox, smallpox, herpes, measles, flu (influenza), HIV/AIDS, Human papillomavirus (HPV), Infectious mononucleosis, mumps, viral pneumonia, hepatitis A, hepatitis B, hepatitis C, rubella, viral meningitis, shingles, and viral gastroenteritis (stomach flu) are all viral diseases.

Such viral diseases are contagious and can spread from one person to another. Once a virus enters the body, it begins to multiply. There are common ways that these viruses spread. These ways include:

  • Inhaling air-borne droplets contaminated with a virus
  • Egesting food contaminated with a virus
  • Drinking water contaminated with a virus
  • Having sexual contact with a person infected with a sexually transmitted virus
  • Indirect transmissions from one person to another via a virus-host, like a mosquito, mouse, or tick
  • Having contact with surfaces or body fluids contaminated with a virus
  • Cnidaria

Cnidarians are mostly marine animals and include corals, jellyfish, sea anemones, Portuguese men-of-war, sea pens, sea fans, and sea whips. The Class Myxosporea contains most of the parasitic cnidarians, which include parasites of fish and aquatic worms. However, the Class Malacospore of cnidaria too comprises several species that parasitize moss animals and fish. These obligately parasitic cnidarian animals can live in both freshwater and marine habitats. They are endoparasitic and exhibit complex life cycles. Myxozoans have a two-host lifecycle, which involves a fish and an annelid worm or moss animals.

The Parasitic relationship between myxosporeans and their hosts does not usually cause severe diseases. However, the most significant disease caused by them in cultured fishes are:

  • Proliferative Kidney Disease (PKD): This disease is caused by a Malacosporea member, Tetracapsuloides bryosalmonae. This disease affects salmonids.
  • Whirling disease: This disease is caused by a Myxosporea member, Myxobolus cerebralis. This disease affects salmonids.
  • Enteromyxosis: This disease is caused by Enteromyxum leei. This disease affects cultured marine sparids.
  • Proliferative Gill Disease (Hamburger disease): This disease is caused by Henneguya ictaluri. This disease is seen in catfish.
  • Sphaerospora renicola infections: This infection occurs in common carp fish.
  • Trematodes ( flatworm)

Trematoda is a class of Platyhelminthes that includes 2 groups of parasitic flatworms called flukes. They are endoparasites of vertebrates and mollusks. Most of these trematodes have a complex life cycle with at least 2 hosts. The flukes reproduce sexually in the primary host- a vertebrate. Then the asexual reproduction occurs in an intermediate host, which is usually a snail. These flukes can be found anywhere where untreated human waste is used as fertilizer.

Trematodes body surface has a tough syncytial tegument. This tegument helps protect flatworms that inhabit the gut of their hosts against digestive enzymes. However, these flukes can be classified into two groups based on the way in which they infect the vertebrate host:

  • Tissue flukes: These flukes infect the bile ducts, lungs, or other biological tissues of their hosts. Typical examples are the lung fluke( Paragonimus westermani) and liver flukes (Clonorchis sinensis and Fasciola hepatic).
  • Blood flukes: These flukes inhabit the blood of their hosts in some stages of their life cycle. Typical examples of blood flukes include species of the genus Schistosoma.

Aside from this classification, they can be classified too according to the habitat in which they are found. For instance, pond flukes (infect fishes in ponds).

There is a common infection of these parasites in humans in Africa, Asia, Latin America, and the Middle East. For instance, Schistosomiasis is an example of a parasitic disease caused by one of the species of trematodes belonging to the genus Schistosoma. Also, there are foodborne trematodes like Clonorchis, Opisthorchis, Fasciola, and Paragonimus species that cause disease. Additionally, other diseases are caused by fluke members that belong to the genus Choledocystus.

  • Cestodes ( flatworms)

Cestoda is a class of parasitic flatworms belonging to the phylum Platyhelminthes. The common cestode parasites are those in the subclass Eucestoda known as tapeworms. Also, the Species of another subclass Cestodaria, are mostly fish parasites.

Cestodes are parasitic and many of them have a complex life cycle. They have a stage in a definitive host. In this stage, their adults grow and reproduce, oftentimes for years. The adults usually live in the digestive tracts of their vertebrate host.

There is also one or two intermediate stages in which the larvae grow and develop in other hosts. Their larvae usually live in the bodies of other animals. The animal can be vertebrates or invertebrates.

An adult tapeworm has a scolex (head) with a short neck, and a segmented body formed of proglottids. These worms using their scolex attach themselves to the inside of their host’s intestine. Their scolex usually has hooks, suckers, or both. They lack a mouth and absorb nutrients directly from the gut of their host. The neck of the tapeworm continues to produce proglottids. Each of the proglottids contains a reproductive tract. However, a mature proglottid is full of eggs and falls off to leave the host. Leaving the host, either moving actively or passively in the feces.

There are infections of tapeworm parasites in humans. Humans are susceptible to tapeworm infection by several species if they eat undercooked meat and fish. Species of tapeworm like Taenia saginata have been seen in beef, Taenia solium in pork, and Diphyllobothrium in fish. Also, humans get subjected to tapeworm infection if they live or eat food prepared under poor hygiene conditions. Such tapeworms like Hymenolepis or Echinococcus species can infect humans in poor hygiene conditions.

  • Monogeneans ( flatworms)

Monogeneans are ectoparasitic flatworms mostly found on the skin, fins, or gills of fish. The lifecycle of monogeneans is a direct lifecycle and does not require an intermediate host.

In saltwater fishes, monogeneans can infect the gills and skin. This results in irritations to the host. Also, heavy infections of these parasites can result in erratic swimming behavior and affected gills may appear irritated and swollen.

The infected freshwater fishes become lethargic. These infected fishes swim towards the surface of the water. Also, fishes infected by these parasites may be seen rubbing the bottom or sides of their skin where the parasite is located. The infected skin harboring the parasites may show areas of scale loss and may also produce a pinkish fluid. Infected gills may appear swollen and pale. Also, the fish gulping for air at the water surface could indicate severe respiratory distress.

  • Horsehair worm

Horsehair worms are parasites of grasshoppers and crickets
Photo Credit: https://entomology.ca.uky.edu

Horsehair worms are also known as Gordian worms. These worms are similar to nematodes and belong to the phylum Nematomorpha. However, they are very thin and much longer than nematodes. Horsehair worms are found in water or wet areas. Hence they can be seen in streams, puddles, cisterns, or livestock watering troughs.

These parasites develop in the bodies of grasshoppers, cockroaches, crickets, and some beetles. As horsehair worm mature, it leaves the host to lay eggs. However, these worms are not parasites of pets, livestock, or humans.

  • Nematodes

Nematodes are also known as roundworms. They occur as parasites in plants and animals. Also, they can occur as free-living forms in soil, freshwater, and marine habitat.

Nematodes as parasites in humans and animals occur in almost all organs of the body. However, the most common sites they attack are in the alimentary, respiratory, and circulatory systems. Examples of these nematodes are eelworm, hookworm, whipworm, lungworm, pinworm, heartworms, and threadworms. They cause a variety of diseases such as filariasis, ascariasis, and trichinosis.

Heartworms are parasites
Heartworms extending from the heart of a dog

Nematodes also parasitize many crop plants. Such plant-parasitic nematodes are foliar nematodes, potato cyst nematodes, root-knot nematodes, lesion nematodes, dagger nematodes, and soybean cyst nematodes. They cause several diseases in plant crops like Pine wilt Disease, root-knot disease, cyst disease, and lesion disease.

  • Acanthocephala

Acanthocephalans are parasitic worms belonging to the Phylum Acanthocephala. They are spiny-headed worms or thorny-headed worms. Acanthocephalans have complex life cycles that involve at least two hosts. Their host may include invertebrates, amphibians, fish, birds, and mammals. These worms have an eversible proboscis that is armed with spines. They use a proboscis to pierce and hold their host’s gut wall.

  • Leeches

Leeches are segmented worms in the phylum Annelida. They are usually seen in fresh water and on land. Some of these leeches eat organic debris, some are predators of other animals, and the rest are parasitic.

Leeches are parasites that suck blood from their host

Land leeches feed only on the blood of mammals, whereas aquatic leeches can feed on the blood of fishes, birds, amphibians, and mammals, or even eat snails, worms, and insect larvae.

Nevertheless, leeches have a small sucker that contains a mouth, at the anterior end of their body and a large sucker located at the posterior end. It uses its proboscis as a hypodermic needle to suck blood from its host. The saliva of a leech contains substances that anesthetize the wound area. The saliva however dilates the blood vessels to increase blood flow and prevent the blood from clotting. Most times, the host victim is unaware of being bitten until blood is discovered running from the wound. The blood may continue to flow because of the anticoagulant saliva of the leech that is still present.

Leeches await their host victim in damp vegetation. However aquatic leeches like Limnatis nilotica, may enter the body in drinking water. Once ingested with drinking water, the L. nilotica may attach to the linings of the nose or throat first. Then is inhaled into the lungs.

Also, some leeches may enter the excretory openings of someone bathing in infested waters. Some leeches prefer the mucous membranes found in the nasal cavity of host mammals. Hence they attach to their victim as they bathe. They attach with the help of their single jaw made up of eight large teeth.

Therefore, these parasites can cause suffocation and death by blocking the breathing passages of the host. A host infected by many leeches may suffer from anemia due to the loss of blood. Also, a secondary infection may set in the external wounds.

  • Ticks

Ticks are parasites

Ticks are ectoparasites that consume blood to satisfy all of their nutritional requirements. They are obligate hematophagous parasites requiring blood to survive and move from one stage to another. However, ticks can fast for a long time, but will eventually die if they are unable to find a host. They can feed on amphibians, reptiles, mammals, and birds. Most of them prefer to have a different host at each stage of their life and the majority of ticks go through 4 life stages:

EggSix-legged larvaEight-legged nymphAdult

After hatching from the egg stage, ticks have to feed on blood at every stage to survive. However, ticks that require many hosts can take about 3 years to complete their life cycle. Eventually, most of them die when they don’t find a host for their next feeding.

They find their hosts by detecting their host animal’s body odor and breath. Also, they sense the host’s body moisture, heat, and vibrations.

Additionally, they pick a place to wait for their host by identifying well-used paths. However, resting on the tips of shrubs and grasses, they await a host. As they can’t fly or jump, many of them wait in a position called – questing. While questing, they hold onto grasses and leaves with their 3rd and 4th pair of legs. They outstretch their 1st pair of legs waiting to climb onto a host. As soon as a host brushes the spot where the tick is waiting, it quickly climbs onboard. Some of them will attach quickly to the host skin whereas others will wander around on the host body in search of places like the ear, or other areas with thinner skin.

  • Mites

Mites are tiny parasites of plants and animals. They can be seen in the water, in the soil as decomposers, and some on plants, sometimes creating galls. Also, others are predators or parasites. A typical example is the Varroa parasite of honey bees and the scabies mite of humans. Most species of mites are harmless to humans but a few are known to transmit disease or cause allergies.

These parasites attack plants and animals. For instance, is the nest mites known to live mainly in the nests of birds and other animals. They consume keratin, blood, and skin. There are also the Dust mites that feed primarily on dead skin and hair shed from humans rather than consuming them directly from the host organism.

mites are parasites
Photo credit: https://ucanr.edu

Some parasitic mites infest insects. Like the Varroa parasites of honey bees and Acarapis woodi that live in the honey bees’ tracheae. Plant-parasitic mites include spider mites, gall mites, and thread-footed mites. Mites that attack animals include members of the sarcoptic mange mites. They burrow under the skin of their host. Also is the Demodex mite that lives in or near the hair follicles of humans and mammals.

  • Copepods

Copepods are a group of small crustaceans with a number of species with parasitic phases. They are seen in nearly every freshwater and saltwater habitat. Many of them are parasites and feed on their host. About 3 out of the known 10 orders of copepods are parasitic. A typical example is the order Siphonostomatoida that parasitizes fishes.

  • Lice

Lice are parasites
Body louse and head louse
Photo Credit: https://entnemdept.ufl.edu

Lice are obligate parasites that live externally on warm-blooded organisms. Their common hosts include every species of bird and mammal, except for monotremes, bats, and pangolins. Generally, a louse cannot survive for long if removed from its host. Once its host dies, it uses phoresis to hitch a ride on a fly in an attempt to find a new host. However, the relationship this louse has with the fly for transportation is a symbiotic relationship called commensalism.

Lice are grouped into two:

  • Sucking lice: These lice get their nourishment from feeding on the sebaceous secretions of their host and their body fluids. They pierce the host’s skin, feeding on blood and other secretions.
  • Chewing lice: These lice are scavengers and feed on the host’s skin and the debris found on the skin. Also, they feed on fragments of feathers or hair. They live among the hairs or feathers of their host, feeding on skin and debris.

However, many lice are particular to a single species of host, and in some cases, they live on only a particular part of the body. Usually, they spend their whole life on a single host, cementing their eggs to feathers or hairs. Then, their eggs hatch into nymphs. These nymphs molt 3 times before becoming adults. This whole process takes about 4 weeks.

Some animal hosts are known to host up to 15 different species of lice. A mammal may host about 1-3 different species of lice. Birds may host about 1-6 different species of lice. However, humans host 2 species of louse. The subspecies of Pediculus humanus are the body louse and the head louse, and the pubic louse Pthirus pubis.

  • Fleas

fleas are parasites
Photo Credit: https://entomology.ca.uky.edu

Fleas are tiny flightless insects that survive as ectoparasites of birds and mammals. They survive by consuming blood from their hosts. Adult fleas have bodies that are narrow to enable them to move through the fur or feathers of their host. Although they lack wings, they possess strong claws that prevent them from being dislodged. Their mouthparts are adapted for sucking blood and piercing the skin. They possess hind legs well adapted for jumping.

A wide variety of warm-blooded vertebrates are hosts to fleas such as ferrets, cats, squirrels, dogs, rabbits, rats, birds, mice, and humans. Fleas normally specialize in one host species, although they can often feed on other species but do not reproduce on other species.

In many species, fleas cause an itching sensation which makes the host try to remove it by pecking,  biting, or scratching. Their bites cause a slightly raised, swollen, irritating nodule to form at the site of each bite on the skin. There is usually a single puncture point at the center of this nodule.

Their bites can lead to an eczematous itchy skin disease called flea allergy dermatitis. This disease is common in many host species such as cats and dogs. The flea bites usually appear in clusters or lines of two bites. Also, the bites can remain itchy and inflamed for several weeks. Additionally, fleas can lead to secondary hair loss on the host due to the frequent scratching and biting by the host. Plus, in extreme cases, they can also cause anemia.

  • True flies

Flies are insects of the order Diptera containing an estimated 1,000,000 species such as horse-flies, mosquitoes, robber flies, hoverflies, crane flies, black flies, and others. They have mouthparts adapted for piercing and sucking or for lapping and sucking.

Flies like tsetse flies, botflies, and screwworms attack livestock, causing wounds and spreading disease. Also, mosquitoes, warble-flies, horse-flies, and blow-flies cause parasitic diseases to domestic animals and also transmit pathogenic organisms.

  • Twisted wing insects

Twisted wing insects are endoparasites that parasitize other insects, such as wasps, silverfish, bees, leafhoppers, and cockroaches. Twisted wing females never emerge from the host after entering the host’s body. They end up dying inside their host.

However, the early-stage larvae do emerge because they must find an unoccupied living host. Also, the short-lived males must emerge to find a receptive female in their host.

  • Parasitoid wasps

Parasitoid wasps are parasitoids that lay their eggs in or on the bodies of other arthropods. After a while, they cause the death of their hosts. Different species of these parasitoid wasps specialize in hosts from different insect orders. Some select beetles, bugs, or flies, while the spider wasps (Pompilidae) attack spiders exclusively.

Also, they differ in which host life-stage they attack. Attacking the eggs, larvae, pupae, or adults of their host. They can be endoparasitic and koinobiont or ectoparasitic and idiobiont. However, some endoparasitic wasps have a mutualistic relationship with polydnaviruses. This virus helps the wasps by suppressing the host’s immune defenses.

Transmission methods for parasites

Parasites use several methods to infect animal hosts such as physical contact, the fecal-oral route, free-living infectious stages, vectors, life cycles, and ecological contexts.

However, there are six major parasitic strategies used by parasites for transmission:

  1. Parasitic castration
  2. Directly transmitted
  3. Trophically-transmitted
  4. Vector-transmitted
  5. Parasitoids
  6. Micropredation
  • Parasitic castration

Parasitic castrators completely or partly destroy the ability of their host to reproduce. Instead, they divert the energy that would have been used for reproduction into the host and parasite growth. Sometimes this causes gigantism in the host. However other systems of the host remain intact for it to survive and also sustain the parasite.

For instance, is the Parasitic crustaceans in the genus Sacculina that intentionally cause damage to the gonads of their crab hosts. Also, various species of helminth castrate their hosts such as snails and insects.

These parasites can do the castration of their host directly, either by feeding on the host’s gonads or by secreting a chemical that destroys the host’s reproductive cells. They also do it indirectly, either by diverting nutrients or by secreting a hormone. For instance, is the trematode (Zoogonus lasius) which castrates the marine snail( Tritia obsoleta) chemically, by developing in the snail’s gonad and killing its reproductive cells.

  • Directly transmitted

To get to their hosts, directly transmitted parasites do not require a vector. Typical examples of parasites directly transmitted are lice, mites, copepods, cyamid amphipods, monogeneans, nematodes, fungi, protozoans, viruses, and bacteria. Each of such parasites, whether they are endoparasitic or ectoparasitic has a single host species.

  • Trophically-transmitted

Trophically transmitted parasites are transmitted by a host egesting the parasites. Such parasites include trematodes (all except schistosomes), Toxoplasma, cestodes, acanthocephalans, pentastomids, and many roundworms.

They have complex life cycles that involve hosts of two or more species. In the juvenile stages of the parasites, they infect and usually encyst in the intermediate host. So that when a predator eats the intermediate-host animal, the parasite still survives the digestion process and matures into an adult. However, the predator of the intermediate host becomes the definitive host.

Some of these parasites live as intestinal parasites. Also, many trophically-transmitted parasites modify the behavior of their intermediate hosts to increase their chances of being eaten by a predator. For instance, is the Chinese liver fluke (Clonorchis sinensis) that is trophically transmitted.

  • Vector-transmitted

Vector-transmitted parasites depend on a third host (intermediate host) to carry them from one definitive host to another. This is because the parasite does not reproduce sexually in the intermediate host. Vector transmitted parasites are microorganisms such as protozoa, viruses, bacteria, and are often pathogens.

The vectors of these parasites are mostly hematophagous arthropods like fleas, ticks, mosquitoes, and lice. For instance, is the deer tick (Ixodes scapularis) that acts as a vector for diseases such as Lyme disease, anaplasmosis, and babesiosis. Also, protozoan endoparasites, like the malaria parasites (Plasmodium) and sleeping-sickness parasites (Trypanosoma) have infective stages in the host’s blood which are transported by biting insects to new hosts. Trypanosoma is vector-transmitted among human red blood cells.

  • Parasitoids

Parasitoids are insects that sooner or later kill their hosts. Hence, placing their relationship close to predation. Most parasitoids are phorid flier or parasitoid wasps. However, they vary as Idiobionts or koinobionts depending on how they treat their host.

Idiobiont parasitoids sting their prey on capture, either killing them instantly or paralyzing them immediately. The immobilized prey is then carried to a nest. Idiobiont parasitoids then lay eggs on top of the prey and seal the nest. The young parasitoid will then develop rapidly through its larval and pupal stages, feeding on the provisions left for it in the nest.

Koinobiont parasitoids lay their eggs inside young hosts, usually the larvae of the host. The larvae host is allowed to keep on growing. This is to ensure the host and parasitoid develop together for an extended period till when the parasitoids emerge as adults. As the parasitoid emerges as an adult it leaves the host dead, after eating it from inside.

However, some koinobionts control and regulate the development of their host. For example, these parasites may prevent the host from pupating or may make the host molt whenever the parasitoid is ready to molt. They can do this by producing hormones that mimic the molting hormones (ecdysteroids) of the host. Sometimes they regulate the endocrine system of the host.

  • Micropredation

A micro predator usually attacks more than one host. It reduces each host’s fitness by a small amount. A micro predator is only in contact intermittently with any one host. This behavior makes them suitable vectors because they can pass smaller parasites from one host to another. Moreso, most micro predators feed on blood. Such micro predators are leeches, gnathiid isopods, mosquitoes, tsetse flies, fleas, ticks, lampreys, and vampire bats.

Symptoms of parasites

  • Allergies
  • Anemia
  • Aches and pains
  • Abdominal pain
  • Constipation
  • Diarrhea
  • Dehydration
  • Fever
  • Flu-like symptoms
  • Generally not feeling well
  • Itching and redness
  • Irritation
  • Increased appetite
  • Nausea
  • Sleeping problems
  • Swollen lymph nodes
  • Skin bumps or rashes
  • Vomiting
  • Weight loss
  • Weakness

The above-listed symptoms are parasite symptoms that may occur. Since there are many types of parasites, their symptoms can vary widely. Oftentimes the symptoms of parasitic infection may resemble symptoms of other conditions. However, parasite symptoms will vary and depend on the disease at which the parasite causes.

Moreso, parasite symptoms are hard to predict because oftentimes there are no symptoms and sometimes symptoms appear long after infection. Whatever the case may be, the parasite can still be transmitted to someone else, who may develop symptoms.

How to get rid of parasites

  • Drink purified water.
  • Keep your environment clean and use antiseptics if possible when cleaning.
  • Use boiled or purified water for cooking or to wash your fruits and veggies.
  • Avoid sharing beddings and clothing.
  • Wash your hands regularly with soap and warm water before and after using the toilet.
  • Practice good and improved personal hygiene.
  • Avoid contact with soil that may be contaminated with feces.
  • Wash your hands properly before preparing or eating foods.
  • Avoid raw fish or meat and cook them thoroughly before eating.
  • Wash or peel all raw fruits.
  • Wash all vegetables before eating or cooking.
  • Reheat or wash any food that falls on the floor.
  • Clean and store fruits and veggies properly.
  • If possible, include natural foods like garlic, honey, pumpkin seeds, and papaya seeds in your diet. They are all touted as antiparasitic foods.