What exactly is a parasite?

Parasites are organisms that live on or inside of a host’s body and they get their nutrition from or at the expense of its host organism. There are 3 primary classes of parasites that can cause diseases in a variety of organisms: ectoparasites, helminths, and protozoans.

Ectoparasites

Even though the term ectoparasites can loosely include blood-sucking arthropods like mosquitoes (which are dependent upon a blood meal from a host for their survival), this definition is typically used to refer to organisms like lice, fleas, ticks, and mites that attach to and/or burrow into the skin and stay there for a lengthy period of time – such as days, weeks, or months. These arthropods are important in spreading and causing diseases, but are even more critical as vectors, or transmitters, of numerous pathogens that in turn cause immense anguish and even death from the diseases they transmit.

Just some of the diseases caused by ectoparasites are babesiosis, theileriosis, anaplasmosis, typhus, trench fever, relapsing fever, encephalitis, Lyme disease, and haemorrhagic fever. These are but a few of the long list.

Helminths

These are larger, multicellular organisms that are typically able to be seen by the naked eye in their adult stages. Like protozoa, helminths can be either parasitic or free-living. In their adult form, helminths do not have the ability to multiply in animals. There are 3 primary groups of helminths (a word which hails from the Greek word for worms) that are animal parasites.

Some of the known helminths are flatworms, which include the trematodes, aka flukes, and cestodes, aka tapeworms; thorny-headed worms, where the adult forms live in the gastrointestinal tract; acanthocephala are thought to be intermediary organisms that are classified between cestodes and nematodes; roundworms, aka nematodes, where the adult stage of these worms can live in the blood, gastrointestinal tract, lymphatic system, or subcutaneous tissue layers. The immature, aka larval, states can cause various diseases via their infection of a variety of bodily tissues. Some scientists consider the helminths to also encapsulate segmented worms like leeches.

Protozoa

These are microscopic, single-celled organisms that can be parasitic or free-living. They are able to multiply in animals, which helps to contribute to their survival and also allows major infections to arise from just a solitary organism. The spread of protozoa that live in an animal’s intestinal tract to another animal usually occurs through a feces to oral path (for instance, contaminated food or water or animal to animal contact). Protozoa that dwell in the blood or tissues of animals are spread to other animals by an arthropod vector, for instance due to the bite of a sand fly or mosquito.

The protozoa which are considered infectious to animals are classified into 4 groups based on their method of travel. They are as follows:

• Ciliophora, aka the ciliates (such as Balantidium, Coleps, Colpoda, Didinium, Paramecium, Stentor, Tetrahymena, Vorticella, etc.)
• Mastigophora, aka the flagellates (such as dinoflagellates – like Ceratium and Peridinium), chrysomonads (like Dinobryon, Mallomonas, and Synura); euglenids (Euglena), volvocids (Volvox and Eudorina), choanoflagellates (Astrosiga), and more.
• Sarcodina, aka amebas (like Entamoeba histolytica: An obligate parasite which causes amebiasis, aka amoebic dysentery, which can affect mammals in tropical areas. It can eat animal cells alive, and has the capability of killing up to 100,000 animals each year.
• Sporozoa, aka Plasmodium (this can cause death from severe anemia or even blockage of capillaries in the brain, lungs, and/or other vital organs. The liver & spleen are also noticeably enlarged and are typically discolored); and Cryptosporidium (which has up to a 50% mortality in calves and goats, and causes up to 496,650 lamb deaths per year).

Parasitic Infections

These can cause a monstrous burden of disease in tropical, subtropical, and temperate climates. Out of all the parasitic diseases, malaria (which is caused by micro-organisms called Plasmodium) causes the most deaths globally by far. Malaria kills more than an estimated 600,000 people each year, most of them young children in sub-Saharan Africa. Animals are also affected by malaria.

The Neglected Tropical Diseases (NTDs), that have endured from a lack of attention by the public health community, include zoonotic parasitic diseases like echinococcosis, foodborne trematodiases, rabies, and taeniasis/cysticercosis, to name a few. Zoonotic diseases are those that can “hop” to and from an animal to a human.