A parasite is not just an Oscar award-winning film, it is a creature in a very particular relationship with another organism. While we may never want to be accused of being a parasite, parasitic organisms don't seem to mind their classification, as they benefit from their lifestyle greatly. We can learn a lot about the relationships between different creatures in nature by studying the features and factors of parasites and parasitism.
Parasitism definition in biology
Parasitism is defined as a particular kind of symbiotic relationship, in which one creature benefits from the relationship, while the other creature is worse off (harmed) due to the relationship. The creature who benefits is termed the parasite, and the creature who is harmed is called its host.
Generally speaking, a symbiotic relationship is one in which two (or more) organisms of different species live together, in tandem. One of the organisms benefits from this relationship and, depending on the specific kind of symbiosis, the effect on the other organism is positive (mutualism), neutral or no effect (commensalism), or harmful (as in the case of parasitism).
Additional features of parasitism
Besides the definition of a parasitic relationship, in which one organism benefits while the other is worse off due to their relationship and proximity, there are other features of parasitism that tend to occur.
Firstly, parasites are not predators. This distinction is made by the direness of the relationship between the parasite and its host. Predators, whether immediately or penultimately, kill their prey. This is what defines their relationship. Parasites do not kill their hosts directly, they only lead to increased harm and damage to the host. Typically, parasites would not want their hosts to die, because much of the host's body functions are being used by the parasite to survive as well. From the host's body itself, to the host's digestion of food to release nutrients, to the host's pumping blood and circulation; many of these mechanisms are utilized by different parasites. Thus, parasitism and the predator-prey relationship are different.
Secondly, parasites are often smaller than their hosts. This is another difference that distinguishes parasitism from the predator-prey relationship, in which predators are most often larger and more massive than their prey. Parasites being smaller than their hosts gives them the ability to bother and detract from their hosts, but do not often kill them.
Thirdly, parasites may need a vector to transmit themselves and their disease to their hosts. This is most relevant in microbiology and medicine, and most common in parasites that cause disease. A vector is an agent of transmission and a good example of a vector is the deer tick that transmits Lyme disease to humans. The vector is the tick, the host is the human, and the parasite is the microbe that causes Lyme disease - a bacterium called Borrelia burgdorferi.
Parasitism in microbiology
We mentioned Lyme disease as an infection that can be passed to humans due to parasitism. Humans and other mammals are the host, the vector is the deer tick, and the parasite is the bacterium. But what are other examples of parasitism on display in microbiology?
Many of these microbes can cause disease and be parasites, and others can be the hosts for parasites themselves! We will examine some examples below.
Are viruses organisms? The debate rages on in science, but most agree that they are in a grey area between living and non-living things. They do replicate, but only inside a host, and they do have tremendous effects on the organisms they infect.
Parasitism in Malaria:
Malaria is an infection spread by mosquitoes. It can cause high fevers that come and go in a cyclical pattern, muscle aches, weakness, chills, fatigue, and headaches. Sometimes malaria infections go to the brain, causing cerebral malaria which has even worse outcomes. But did you know that malaria is a parasitic infection?
Parasitism in Larva Migrans:
Larva migrans is a disease that comes in two forms. First, there is a cutaneous infection, in which the hookworm Necator americanus burrows into the skin. This causes a serpiginous (wavy, snake-like) rash, and some infections stop here (Fig. 1(. Others progress to the respiratory and digestive tract where they stick to the walls of organs and suck blood, causing anemia.
Figure 1. Larva migrans (Necator Americanus) can cause a serpingous rash.Parasitism in Salmonella-Schistosomiasis:
Schistosomiasis is an infection caused by a fluke called Schistosoma. These flukes are a type of worm and they are found in fresh (not salty) water. People who drink or bathe in this fresh water are at risk for schistosomiasis, in which a fluke lives as a parasite in their liver, munching on the liver's tissues and nutrients. This can make your liver inflamed and enlarged, causing illness. However, while these liver flukes are parasites themselves, they can also have parasites of their own. Sometimes salmonella, a bacterium, is present within the body of the fluke. Salmonella infections usually cause gastrointestinal symptoms like vomiting, nausea, and diarrhea, but can also cause bone infections and high fever. This is a double-whammy for those with salmonella-Schistosoma parasitic infections.
The host - human beings
The parasite - Schistosoma, a fluke
The parasite of the parasite - Salmonella, a bacterium
Example of parasitism in biology on a macro level
Parasitism doesn't only happen at the microscopic level. There are many parasitic relationships in nature that involve two macroscopic beings, as we'll see in this section.
Barnacles and crabs
Barnacles are the parasites, crabs are the host. What are barnacles? These are crustaceans that live in seawater.
How does the relationship between barnacles and crabs work? Barnacle larvae grow up within the female crab, living where the crab's eggs normally should be. The female crab thus can't have crab babies and instead hatches more barnacle larva. This renders the female crab infertile. If barnacle larvae enter a male crab, they also sterilize them. Barnacles mess with male crabs' hormone balance, causing them to look and behave more like female crabs.
How the relationship harms crabs: Crabs with barnacle parasites can't reproduce. Both male and female crabs become sterile. This reduces fitness. Also, crabs with barnacles dwelling within them cannot molt or shed their shells. This stops them from growing properly and also stops them from being able to re-make any organs that are lost or bitten off (crabs can sometimes regrow their claws).
How the relationship benefits barnacles: Barnacles get to usurp the crab's reproductive mechanism of hatching and spraying eggs to use it for the propagation of their own larvae. Also, barnacles get a safer place to live, inside and atop a larger organism that may be more resilient against predators.
Fitness - in biology and population genetics, fitness is breeding success - the amount and quality of offspring that an individual has in its lifetime.
Fleas and dogs
As you probably already know, fleas are the parasite and dogs are the host.
How does the relationship between fleas and dogs work? Fleas live on and near dogs, sucking their blood and hence consuming their nutrients. Fleas jump on to dogs, live on them, and reproduce on them, laying their eggs and causing an ever-growing flea infestation on the dog (they can also do this on other mammals)!
How the relationship harms dogs: First of all, dogs lose energy and nutrients to blood-sucking fleas. If enough blood is lost, the dog can become anemic. Secondly, flea bites are not painless. Many dogs can be allergic to fleas and their bites will get red, inflamed, itchy, and bothersome, plus they'll loose hair in the areas of the flea bites. These bothersome skin problems can eventually spread all over the dog. Also, because of the damaged skin barrier, these dogs are more prone to other infections as well. Finally, some fleas carry tapeworms within them, and if a dog manages to swallow one of the fleas flying around its body, it can get a tapeworm infection. The tapeworm lives in the gastrointestinal system of dogs, stealing nutrients. Tapeworms can also be found in dogs' fecal matter, causing their butts to itch (Fig. 2).
How the relationship benefits fleas: Fleas are flightless insects. This makes it difficult for them to escape attempts to eat or kill them. Being housed on a dog, a much larger animal, provides more security for fleas. Fleas get onto dogs by jumping, not flying, and dogs provide warmth and nutrients for fleas.
Figure 2. Tapeworms and fleas are examples of dog parasites.
Types of parasitism
In Table 1 below, we summarize the meaning, common factors and provide some examples of different types of parasitism.
| Parasitism Type | Meaning | Common Factors | Example |
| Endoparasitism | The parasite is found within the body of the host. | Infectious microbes are common endoparasites. They utilize the host's resources and cause disease. | B. burgdorferi bacteria in Lyme disease. |
| Mesoparasitism | The parasite lives partially within and partially outside of the body of the host. | Also known as facultative parasitism: they don't require a host to complete their lifecycle. Arthropods may use this method. | Copepods only partially embed into the gills of their fish hosts. |
| Ectoparasitism | The parasite is found outside the body of the host. | Often found on the body surface of hosts, and often cause lesions and rashes on the host. | Lice in humans, fleas in dogs. |
Types of parasitic relationships
There are seemingly endless distinctions between types of parasitic relationships. We will outline the most common terms below.
Obligate parasitism - this is when the parasite needs the host to survive. It cannot complete its life cycle without having certain needs met by the host. Ex: human head lice that die when they're no longer on our heads!
Facultative parasitism - this is when the host helps the parasite, but symbiosis is not required to complete the parasite's lifecycle. Ex: Naegleria fowleri, a brain-eating amoeba that can cause death when it passes through the human skull, but normally lives freely in fresh water.
Secondary parasitism - also known as epiparasitism or hyperparasitism. This is when a parasite develops against a different parasite, that is actively harming its host. Ex: Salmonella-Schistosoma double infections.
Brood parasitism - this is when the parasite utilizes its host to raise its brood (young animals). Ex: The brown-headed cowbird often drops its eggs in the nest of the warbler bird, letting the warbler bird warm and raise its young.
Social parasitism - this is when the parasite utilizes its hosts for free labor. Ex: a colony of bees, in which some parasitic females lay their own eggs in cells of worker bees, which act as the hosts. They then force the worker bees to raise their young and perform labor for the hive.
Parasitism - Key takeaways
- Parasitism is a symbiotic relationship in which one organism benefits and the other is harmed.
- There are many kinds of parasitic relationships including obligate, facultative, epiparasitism, ectoparasitism, and more.
- Most infections in microbiology - whether by bacteria, viruses, fungi or protozoa are considered parasitic relationships.
- A classic example of a parasitic relationship that harms humans is human lice or Lyme disease.
- A classic example of a parasitic relationship that harms dogs is tick infections.
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