What Are Parasites?-Classification, Mode of Transmission, And Examples

Organisms that depend on another organism for food and shelter are called parasites. This interaction between the parasite and the host can cause harm to the host. Protozoa, helminths, and ectoparasites are the three main categories of parasites that result in illness in humans.

Protozoa are single-celled organisms that can cause diseases such as malaria and amoebiasis. Helminths are multi-cellular parasites, including worms, which result in illnesses like ascariasis and trichinosis. Ectoparasites are parasites that live on the surface of their host and cause diseases such as Lyme disease and typhus.

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Key Points

• Parasitism is a symbiotic relationship between two species, where one organism (parasite) relies on the other organism (host) for survival.
• Parasites can cause harm to their host, and the relationship can result in illness in humans.
• Protozoa, helminths, and ectoparasites are the three main categories of parasites that can result in illness in humans.
• Protozoa are single-celled organisms that can cause diseases such as malaria and amoebiasis.
• Helminths are multi-cellular parasites, including worms, which can result in illnesses like ascariasis and trichinosis.
• Ectoparasites are parasites that live on the surface of their host and can cause diseases such as Lyme disease and typhus.
• Parasites can be classified into 5 classes based on their effect on the host and transmission mode, including parasitic castrators, directly transmitted parasites, trophically transmitted parasites, vector-transmitted parasites, and parasitoids.
• Parasitic castrators are parasites that destroy the reproductive abilities of their host.
• Parasites can be transmitted through physical contact, fecal-oral route, arthropod vectors, and other means.

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Diversity of Parasites

Parasites are organisms that live on or inside other organisms and have been very successful in their way of life. There are many different types of parasites. It is important to study their diversity because it helps us to understand how they have evolved. Factors such as the traits of the host and the parasite itself can affect how many parasite species there are.

However, we still have a lot to learn about how parasites can thrive and diversify. To better understand the diversity of parasites, we need to combine different approaches and study the historical and ecological factors that have shaped their evolution. [1]

Parasites are tiny organisms that live in or on other organisms and can affect their health, behavior, and population. There are different types of parasites that are found in marine mammals, like seals and sea otters.

Protozoan parasites, which are a type of microscopic parasite, are not very common in marine mammals. The severity of the damage caused by parasites depends on the type of parasite, how many there are, and the health of the host. Some parasites can cause severe disorders in the hosts, while others only produce minor reactions. Parasites can also affect valuable marine animals in aquaria, causing diseases that need to be treated with expensive measures.

Anisakid nematodes are the most economically significant parasites because their larvae can be found in commercial fish and squid. [2]

Parasitism

Parasitism is a symbiotic relationship between two species where one organism, known as the parasite, relies on the other organism for survival. The host experiences harm as a result of this relationship.

Parasitism is a form of consumer-resource interaction just like predation. However, some parasites, excluding parasitoids, are usually much smaller than their hosts and do not cause their death.  They live on or within their host for a long period. Parasites that infect animals are highly specialized and have a faster reproductive rate compared to their hosts. FOR example interaction between tapeworms, flukes, the Plasmodium species that cause malaria, and fleas with their vertebrate hosts.

Classification of Parasites

Parasites are classified into 5 classes based on their affect on host and transmission mode.

Parasitic Castrators

Parasitic castrators are parasites that partly or completely destroy the reproductive abilities of their host, diverting the energy that would have gone into reproduction into host and parasite growth. One example is the parasitic crustacean Sacculina that specifically causes damage to the gonads of crab hosts.

Directly Transmitted Parasites

 Directly transmitted parasites are parasites that do not require a vector to reach their hosts. They are endoparasites or ectoparasites that have a single host species, and most individuals of the host species are free or almost free of these parasites. Examples of directly transmitted parasites are lice, mites, copepods, nematodes, and bacteria.

Trophically Transmitted Parasites

Trophically transmitted parasites are parasites that are transmitted by being eaten by a host. They have complex life cycles involving hosts of two or more species, and often modify the behavior of their intermediate hosts to increase their chances of being eaten by a predator. Examples of trophically transmitted parasites are trematodes, cestodes, and roundworms.

Vector-Transmitted Parasites

Vector-transmitted parasites are parasites that rely on a third party, an intermediate host, to carry them from one definitive host to another. The intermediate host does not reproduce sexually, and the parasites are microorganisms such as bacteria, viruses, and protozoa. Vectors of vector-transmitted parasites are mostly hematophagic arthropods such as fleas, ticks, and mosquitoes.

Parasitoids

Parasitoids are insects that eventually kill their hosts and their relationship is close to predation. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts. Idiobiont parasitoids sting their prey, immobilize it, and lay eggs on it, while koinobiont parasitoids lay eggs inside young hosts, which are allowed to grow until the parasitoids emerge as adults.

Types Of Parasites

There are three main types of parasites that can make people sick: protozoa, helminths, and ectoparasites.

Protozoa

Protozoa are single-celled organisms that can be free-living or parasitic. They are microscopic and can multiply in humans, causing serious infections. They are typically transmitted through a fecal-oral route or by an arthropod vector. Protozoa that infect humans can be classified into four groups based on their mode of movement: Sarcodina (ameba), Mastigophora (flagellates), Ciliophora (ciliates), and Sporozoa (non-motile).

Examples of protozoa include Entamoeba, Giardia, Leishmania, Balantidium, Plasmodium, and Cryptosporidium.

Helminths

Helminths are large multicellular organisms that can be free-living or parasitic. They are visible to the naked eye and cannot multiply in humans.

There are three main groups of helminths that are human parasites: flatworms (flukes and tapeworms), thorny-headed worms, and roundworms. These worms can reside in the gastrointestinal tract, blood, lymphatic system, subcutaneous tissues, or infect body tissues in their immature form.

Leeches, which are segmented worms, are also considered helminths but are not typically classified as parasites.

Ectoparasites

Ectoparasites are small organisms that live on the outside of an animal’s body. They live in the skin, fur, feathers, and scales. These parasites feed on the blood and other bodily fluids of their host and can cause significant harm, such as infection, irritation, and discomfort. Some common examples of ectoparasites include ticks, fleas, and lice.

Examples Of Parasites And Their Transmission Method

• The parasite Gyrodactylus turnbulli infects the host Poecilia reticulata (guppy) through physical contact during social behavior.

• Nematodes such as Strongyloides are transmitted to the host Macaca fuscata (Japanese macaque) through fecal-oral contact during social behavior (grooming).

• The nematode Heligmosomoides polygyrus infects the host Apodemus flavicollis (yellow-necked mouse) through fecal-oral transmission, with a bias towards infecting males.

• The tick Amblyomma infects the host Sphenodon punctatus (tuatara) through free-living infectious stages during social behavior.

• Plasmodium, the malaria parasite, infects birds and mammals (including humans) through a vector, the Anopheles mosquito, which is attracted by the odor of an infected human host.

Latest Research About Parasites

• Scientists have made progress in the artificial culture of parasitic ciliates affecting fish, such as Ichthyophthirius multifiliis and Cryptocaryon irritans, using novel technologies. This review discusses in vitro and in vivo culture, cryopreservation/preservation, challenges, and future applications of parasite culture for effective prevention and control strategies. [3]
• Parasite-host evolutionary models assume constant ecological dynamics, but in many systems, fluctuations occur due to intrinsic or extrinsic factors. Scientists discussed challenges and approaches to modeling evolution in systems with fluctuating ecological dynamics and provides work examples and interactive Python code. Feedback between ecological and evolutionary dynamics is important in driving infectious disease outcomes. [4]
• Scientists reconstructed a century-scale time series of metazoan parasite abundance in Puget Sound, United States, using natural history collections of eight fish species collected between 1880 and 2019. They found that parasite taxa using three or more obligately required host species declined in abundance at a rate of 10.9% per decade, and the decline was negatively correlated with sea surface temperature, diminishing at a rate of 38% for every 1 °C increase. These findings suggest that climate change is contributing to a massive loss of parasite biodiversity in Puget Sound. [5]
• The Acta Tropica Special Issue “Emerging parasites and vectors in a rapidly changing world: from ecology to management” focuses on the biology, ecology, and management of emerging parasites and vectors of human and veterinary importance. The issue highlights the importance of implementing management strategies for parasitic diseases in response to anthropogenic environmental changes. The final section outlines key research challenges in this rapidly changing scenario. [6]