how are parasites transmitted

7/27/2019 How Are Parasites Transmitted

1/5

How are Parasites transmitted?

People can get parasites and worms is a lot of different ways:

Through transmitting agents such as mosquitoes, fleas and flies. In the air we breathe.

Pets and farm animals are often carriers of parasites.

From contaminated food.

Badly cooked or uncooked foods such as beef, pork, poultry and fish.

Some fruit and vegetables can also contain parasites.

Poor water supplies and poor sanitation can transmit parasites.

In areas where sanitation or sewer systems are lacking or badly maintained,

parasites and worms can quickly multiply.

Parasites can commonly be found in mud pools, stagnant ponds, streams, rivers

and other small or large pools of water that has remained there for a while. Bad hygiene in toilets and other public places can also be the cause.

Most protozoa have enormous reproductive potential because they have short generationtimes, undergo rapid sequential development and produce large numbers of progeny byasexual or sexual processes. These characteristics are responsible for many protozoaninfections rapidly causing acute disease syndromes. Parasites may multiply by asexual division(fission/splitting or internal/endogenous budding) or sexual reproduction (formation of gametesand fertilization to form zygote, or unique process of conjugation where ciliates exchangemicronuclei).

Protozoan developmental stages occurring within hosts generally consist of feedingtrophozoites, and they may be found intracellularly (within host cells) or extracellularly (inhollow organs, body fluids or interstitial spaces between cells). While trophozoites are ideallysuited to their parasitic mode of existence, they are not very resistant to external environmentalconditions and do not survive long outside of their hosts. To move from host-to-host, protozoanparasites use one of four main modes of transmission: direct, faecal-oral, vector-borne andpredator-prey transmission.

direct faecal-oral vector-borne predator-prey

> direct transmission of trophozoites through intimate body contact, such as sexual


2/5

transmission (e.g. Trichomonas spp. flagellates causing trichomoniasis in humans andbovine infertility in cattle).

> faecal-oral transmission of environmentally-resistant cyst stages passed in faeces of onehost and ingested with food/water by another (e.g. Entamoeba histolytica, Giardia

duodenalis and Balantidium coliall form faecal cysts which are ingested by new hostsleading to amoebic dysentery, giardiasis and balantidiasis, respectively).

> vector-borne transmission of trophozoites taken up by blood-sucking arthropods (insectsor arachnids) and passed to new hosts when they next feed (e.g. Trypanosomabruceiflagellates transmitted by tsetse flies to humans where they cause sleepingsickness, Plasmodium spp. haemosporidia transmitted by mosquitoes to humans wherethey cause malaria).

> predator-prey transmission of zoites encysted within the tissues of a prey animal (e.g.herbivore) being eaten by a predator (carnivore) which subsequently sheds spores into

the environment to be ingested by new prey animals (e.g. tissue cysts of thesporozoan Toxoplasma gondiibeing ingested by cats, and tissue cysts of themicrosporan Thelohania spp. being ingested by crustaceans).

Helminths form three main life-cycle stages: eggs, larvae and adults. Adult worms infectdefinitive hosts (those in which sexual development occurs) whereas larval stages may be free-living or parasitize invertebrate vectors, intermediate or paratenic hosts. Nematodes produceeggs that embryonate in utero or outside the host. The emergent larvae undergo 4metamorphoses (moults) before they mature as adult male or female worms. Cestode eggsreleased from gravid segments embryonate to produce 6-hooked embryos (hexacanth

oncospheres) which are ingested by intermediate hosts. The oncospheres penetrate hosttissues and become metacestodes (encysted larvae). When eaten by definitive hosts, theyexcyst and form adult tapeworms. Trematodes have more complex life-cycles where larvalstages undergo asexual amplification in snail intermediate hosts. Eggs hatch to release free-swimming miracidia which actively infect snails and multiply in sac-like sporocysts to producenumerous rediae. These stages mature to cercariae which are released from the snails andeither actively infect new definitive hosts or form encysted metacercariae on aquatic vegetationwhich is eaten by definitive hosts.

nematode cycleegg - larvae (L1-L4) -

adultcestode cycle

egg - metacestode - adult trematode cycleegg-miracidium-sporocyst-redia-cercaria-(metacercaria)-adult


3/5

Helminth eggs have tough resistant walls to protect the embryo while it develops. Mature eggshatch to release larvae either within a host or into the external environment. The four mainmodes of transmission by which the larvae infect new hosts are faecal-oral, transdermal,vector-borne and predator-prey transmission:

faecal-oral trasdermal vector-borne predator-prey

>

faecal-oral transmissionof eggs or larvae passed in the faeces of one host and ingestedwith food/water by another (e.g. ingestion ofTrichuris eggs leads directly to gut infectionsin humans, while the ingestion ofAscaris eggs and Strongyloides larvae leads to apulmonary migration phase before gut infection in humans).

> transdermal transmissionof infective larvae in the soil (geo-helminths) activelypenetrating the skin and migrating through the tissues to the gut where adults developand produce eggs that are voided in host faeces (e.g. larval hookworms penetrating theskin, undergoing pulmonary migration and infecting the gut where they feed on bloodcausing iron-deficient anaemia in humans).

> vector-borne transmission of larval stages taken up by blood-sucking arthropods orundergoing amplification in aquatic molluscs (e.g. Onchocerca microfilariae ingested byblackflies and injected into new human hosts, Schistosoma eggs release miracidia toinfect snails where they multiply and form cercariae which are released to infect newhosts).

> predator-prey transmission of encysted larvae within prey animals (vertebrate orinvertebrate) being eaten by predators where adult worms develop and produce eggs


4/5

(e.g. Dracunculus larvae in copepods ingested by humans leading to guinea worminfection, Taenia cysticerci in beef and pork being eaten byhumans, Echinococcus hydatid cysts in offal being eaten by dogs).

Adult arthropods are generally small in size, most are visible but some remain microscopic.Arthropod sexes are separate and fertilization is internal. A wide range of mating behaviours,insemination and egg production strategies are involved. In most species, the egg developsinto a larva: i.e. a life-cycle stage that is structurally distinct from the adult and must undergometamorphosis (structural reorganization) before becoming an adult. This metamorphosis maybe complete (involving major changes during a pupation stage) or incomplete (involving gradualchanges in nymph stages). For example, the grub-like larval stages of flies and fleas formcocoon-like pupae where they undergo complete metamorphosis and emerge as radically-different adult insects. In contrast, the larval instars (or nymphs) of lice, ticks and mites undergoincomplete metamorphosis through a series of moults gradually becoming more adult-like inappearance.

complete metamorphosis incomplete metamorphosis

hatch moult hatch hatch moult moultegg----------larva----------pupa----------adult egg----------larva----------nymph----------adult

Arthropods are involved in nearly every kind of parasitic relationship, either as parasitesthemselves or as hosts/vectors for other micro-organisms (including viruses, bacteria, protozoaand helminths). They are generally ectoparasitic on, or in, the skin of vertebrate hosts. Manyspecies are haematophagous (suck blood) while others are histophagous (tissue-feeders) andbite or burrow in dermal tissues causing trauma, inflammation and hypersensitivity reactions.Infestations are transmitted from host-to-host either by direct contact or by free-living larvae oradults actively seeking hosts.

direct host-seeking(larva or adult parasitic)

host-seeking(all feeding stages parasitic)


5/5

>Direct transmission of infective stages occurs when hosts come into close contact witheach other or share quarters, bedding or clothing. Larvae, nymphs or adults may crossfrom one host to another, while eggs or pupae may contaminate shared environments.Insects (fleas and lice) and arachnids (mites) rely on close contact between hosts.

>Many adult insects actively seek hosts in order to feed or lay eggs. Winged insects

(mosquitoes, flies) fly to new hosts to feed while fleas jump onto passing hosts. Someadult flies (botflies) do not feed on their hosts but deposit eggs from which larvae emergeand feed on host tissues and exudates.

> Tick larvae actively seek hosts by climbing vegetation and questing for passing hosts.Some species complete their life-cycle on the same host (one-host ticks) while othersdetach after feeding and drop to the ground to moult before seeking new hosts asnymphs or adults (two-host or three-host ticks).

how are parasites transmitted

Documents