The 9 Types of Parasites and Their Characteristics

Parasitism is essential to understand evolution and our society from a biological point of view. We show you what the 9 types of parasites are.

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 05 March, 2023

The relationship between animals and the agents that cause disease is a bit like a strategy war game. The hosts (generally evolutionarily complex) develop a strong immune system and behavioral changes to avoid parasites, while the latter improve their way of exploiting the weak points of their counterparts. Here we’ll tell you the 9 types of parasites that exist and their traits.

What is parasitism?

The Oxford Dictionary defines a parasite as ‘an organism that feeds on the substances produced by a living being of a different species, living inside or on its surface, which usually causes it some damage or disease’. Therefore, parasitism will be the type of relationship that is established between the parasitic agent and its host.

Parasitism is a consumer-resource relationship, just as predation is. However, in this case, a stronger being subjugates its prey and ends its life. In parasitic relationships, the parasite is much smaller than the living being it takes advantage of (the host) and, in addition, tries to keep it alive for as long as possible.

This constant drainage of organic resources by the parasite reduces the host’s strength. This means that they will have less mobility, resistance, capacity for confrontation, and more susceptibility to suffering associated diseases. Parasitized specimens of almost any species are less likely to survive than other conspecifics.

The 6 types of parasites (from a terminological point of view)

Now that you know roughly what parasitism is, we can explore what types of parasites exist from a theoretical point of view. The examples will come later, but some foundations must be laid. Still, we will take a general anthropological approach.

1. According to its dependence on the host

Not all parasites require a host to carry out their life cycle. Therefore, dependence on it represents a classificatory criterion by itself.

1.1 Obligate parasite

As indicated by professional portals, obligate parasites are those that are completely dependent on the host to grow, develop, reproduce, nourish, and survive. Without an evolutionarily more complex species to exploit, these biological agents would end up dying sooner or later.

Viruses are the clearest example within the group of obligate parasites in humans, although many times they aren’t conceived as such because they aren’t even “alive” (they don’t have a single cell). Since these pathogens lack the necessary tools to replicate their own genome and synthesize proteins, they must hijack the mechanisms of other complex cells and use them to their advantage.

1.2 Facultative parasite

A facultative parasite is one that can parasitize another living being at some point in its development, but doesn’t need it. This type of relationship is very common in the plant world, since many species of fungi parasitize the stem of living trees if they have the opportunity, although they’re also capable of consuming dead wood present in the soil.

The clearest example of a facultative parasite in humans is Naegleria fowleri, also known as the “brain-eating amoeba”. This pathogen of the Excavata group lives in fresh waters and feeds on bacteria, but it can also establish itself in the human brain. This causes an infection and death of the nervous tissue known as primary amoebic meningoencephalitis.

2. According to its location

The physiological characteristics of the pathogen and the type of infection it causes are classified by its place of manifestation. Let’s see what types of parasites exist according to their location in the host’s body (human or non-human).

2.1 Ectoparasite

An ectoparasite is a living being that lives on the surface of another organism and subsists on it. Many ectoparasites are obligate, although others are classified as facultative because it’s relatively easy for them to “jump” off the host and lead a life of their own in the environment.

The most common ectoparasites in humans are head lice (Pediculus humanus), although sometimes we are also exposed to mites, ticks, fleas, mosquitoes and many other invertebrates. These pathogens are usually insects or arachnids and cause bites, inflammation, redness and itching, among many other things.

Not all ectoparasites are permanently attached to their host. Some, like bed bugs (Cimex lectularius), suck human blood at night and return to the recesses of the room during the day to rest. Without the blood they would die, but they are detached from the body at some points.

2.2 Endoparasite

An endoparasite is a living being that lives inside its host. When we say “inside”, we refer to many structures: tissues, organs, interstitial spaces, and even cells. For example, the viruses already mentioned are intracellular endoparasites.

One of the most famous endoparasites that affect humans is Ascaris lumbricoides, also known as intestinal worm because of its worm-like shape. This nematode establishes itself in the small intestine and feeds on the nutritional substances that are in the intestinal fluids. A high infestation can lead to anemia , paleness, and malnutrition in the host.

3. According to the complexity of its life cycle

Some of these living things have very simple cycles, while others must go through 1, 2, 3, or even more different hosts to develop their full life cycle. We present the types of parasites that exist based on this premise.

3.1 Monoxene parasite

A monoxene parasite fulfills its life cycle within or on the same host. The egg, the larva and the adult are capable of developing in the same living being. This doesn’t mean that the pathogen has only one type of host (perhaps it can infect more than one species at the conceptual level), but that its entire life takes place in the same specimen.

Let us take Ascaris lumbricoides as an example again. As indicated by the Centers for Disease Control and Prevention, the eggs of this pathogen are accidentally ingested and hatch in the intestinal mucosa of humans. After that, the larvae travel through the blood system, mature in the lungs, and are swallowed again as they travel to the throat.

After reaching the small intestine as adults, these nematodes reproduce and the females lay about 200,000 eggs a day, which will be expelled into the environment with the feces. As you can see, the entire cycle takes place within the same individual, but the human being isn’t its exclusive host. A similar process occurs between Ascaris suum and pigs.

3.2 Heteroxene parasite

The heteroxene parasite requires more than 1 host to carry out its life cycle. This term means we should distinguish 3 types of hosts: the definitive one is the one that harbors the sexual phase of the pathogen (and implies the closure of its cycle), while the intermediary carries the asexual or larval phase. There’s also the paratenic host, which only serves as a vehicle.

Without a doubt, the parasites that best exemplify this biological strategy are those of the genus Anisakis. The larvae of these nematodes must pass through a crustacean to develop and also use a series of paratenic hosts in order to reach the definitive one. These organisms can pass between crustaceans and fish through the food chain until they infect aquatic mammals, such as dolphins or seals.

The 3 types of parasites (from a practical point of view)

On a theoretical level, parasitism is perfectly understood, but in “the real world” the situation changes drastically. Humans are accidental hosts to many pathogens and, although they cause us diseases, ending up inside our body is a pretty bad move for them! If they aren’t specialized in exploiting H. sapiens, they can’t close their cycle.

Again, the perfect example to show this biological dilemma is found in the Anisakis complex. If a human being ingests a raw infected paratenic host (such as a fish), the larvae attempt to penetrate the intestinal mucosa, but are unsuccessful. Not having the tools to infect us, they become entrenched and die.

What causes the symptoms of anisakiasis in this case is the immune reaction and the physiological failures derived from the infection, not the natural biological cycle of the parasite. In any case, anisakis are considered parasites for use in human medicine, since they’re capable of generating adverse reactions in us.

With all these ideas in mind, here are the types of parasites that can cause imbalances in humans (whether we’re their definitive hosts or not).

1. Protozoa

Protozoa are microscopic unicellular microorganisms that can be free-living (in humid environments) or parasites. Most are heterotrophs, that is, they engulf bacteria (phagotrophs), prey on other microorganisms (predators) or feed on debris from the environment (detritivores). Sometimes they are also mixotrophs.

Parasitic protozoa of humans can be classified into the following 4 groups:

1. Sarcodinos: This is an artificial group that includes about 200 species. They carry digestive and contractile vacuoles, are heterotrophic, and have the ability to encyst.
2. Mastigophores: This heterogeneous group of protozoa is characterized by having one or more flagella throughout its life cycle. They can also become encyst to withstand unfavorable conditions in the environment.
3. Ciliophores: This is one of the most famous groups of protozoa and has about 3,500 different species. These microorganisms get their name because they are lined with cilia.
4. Sporozoans: These are obligate parasites belonging to different groups. Some are strictly protozoa, but others aren’t.

Many types of human protozoan parasites are confined to the intestinal environment. An example is Cryptosporidium spp., which causes cryptosporidiosis with symptoms such as abdominal pain and diarrhea.

2. Helminths

The term helminth is used to designate any parasitic organism that has the shape of a worm. This is not a taxonomic group as such, so it includes various living beings from different families and with disparate ancestors. The most important are the following:

• Nematodes: These pseudocoelomated worms have more than 25,000 species described to date. Some of the most famous parasites in humans (such as Ascaris, Anisakis or Trichuris ) are found in this group. They are generally known as “worms” due to their body composition.
• Trematodes (flatworms): Fasciola hepatica, Paragonimus, and Schistosoma are some of the most common.
• Cestodes (cestoda): This group of flatworms is also very famous in human culture, as it includes the classic tapeworm (Taenia solium) and other related species.

Many of the listed nematodes are located in the intestine, but others colonize organs such as the liver (Fasciola) and can even reach the ocular apparatus and other soft tissues.

3. Ectoparasites

We have already explored these types of parasites, so we aren’t going to spend much more time on them. Some of the invertebrates that exploit the surface of the human body are the head louse (Pediculus humanus), the bed bug (Cimex lectularius), various species of ticks (Ixodoidea) and various mosquitoes (Diptera), among others.

Some of these invertebrates feed on blood, while others feed on flakes and dead cells. Mosquitoes and ticks stand out for their vectorial capacity, that is, they carry other simpler pathogens in their digestive system and inject them into human blood when biting them. Malaria is the clearest example of this process.

The types of parasites and their diversity

There are many, many types of parasites from both a theoretical and a practical point of view. Here we have presented a total of 9, but there are many more depending on the approach you want to take.

Although in human culture they’re eminently negative, the reality is that parasites carry out a vital function in nature. They indirectly kill the weakest and prevent them from reproducing, allowing the survivor to always be the strongest. Without them, evolution as we know it wouldn’t be possible.