Main Difference – Definitive Host vs Intermediate Host
Parasitism is a non-mutual relationship in which a species called parasite benefits at the expense of the host. Parasites can be classified based on their life cycle as obligate parasites, temporary parasites, and facultative parasites. Obligate parasites require a host to complete their life cycle. Most obligate parasites use two hosts called the definitive host and intermediate host. The main difference between definitive host and intermediate host is that the maturation and sexual reproduction of the parasite occur inside the definitive host whereas the maturation and persistence of the parasite occur inside the intermediate host.
Key Areas Covered
1. What is a Definitive Host
– Definition, Facts, Examples
2. What is an Intermediate Host
– Definition, Facts, Examples
3. What are the Similarities Between Definitive Host and Intermediate Host
– Outline of Common Features
4. What is the Difference Between Definitive Host and Intermediate Host
– Comparison of Key Differences
Key Terms: Definitive Host, Intermediate Host, Maturation, Obligate Parasite, Parasite, Parasitism, Primary Host, Sexual Reproduction
What is a Definitive Host
Definitive host refers to the organism that supports the sexual reproductive form of a parasite. It is also called the primary host. Some parasites like tapeworms only use the definitive host for the completion of their sexual reproduction. Generally, tapeworms lay eggs on grass, and these eggs move into the intestine of animals, and the larva grow to an adult inside the host. Then eggs come to the environment with feces. The life cycle of the tapeworm is shown in figure 1.
Figure 1: Tapeworm Life Cycle
Other parasites live their complete live cycle inside different hosts. They use the definitive host to complete their sexual reproduction, and the intermediate host to complete the asexual reproduction. The sexual reproduction of Plasmodium occurs inside the female Anopheles mosquito. The mosquito picks blood stage parasites called gametocytes during the blood meal. The two types of gametocytes are macrogametocytes (female) and microgametocytes (male). The multiplication cycle of the gametocytes occurs inside the sporogonic cycle. The penetration of the macrogametocytes by microgametocytes occur in the stomach of the mosquito, producing the zygote. The zygote is motile, and it invades the midgut wall to become the oocyst. The rupture of the oocyst produces sporozoites that migrate into the salivary glands of the mosquito.
What is an Intermediate Host
Intermediate host refers to the organism in which the parasite primarily grows to the point of sexual maturity. It is also called the secondary host. More often, the intermediate host acts as the vector of the parasite to reach its definite host. So, the parasite only spends a particular developmental stage inside the intermediate host. The life cycle of Plasmodium is shown in figure 2.
Figure 2: Plasmodium Life Cycle
Sporozoites are inoculated into the bloodstream of the human. Thus, the human is the intermediate host of the Plasmodium. The liver cells are infected by sporozoites where they mature into schizonts. The rupture of the schizonts releases merozoites that infect red blood cells of the human. The blood stage parasites are responsible for the clinical manifestation of the malaria disease.
Similarities Between Definitive Host and Intermediate Host
- Both definitive and intermediate host are two types of obligate parasites.
- Both definitive and intermediate host supply the requirements of the growth and reproduction of the parasite.
Difference Between Definitive Host and Intermediate Host
Definition
Definitive Host: Definitive hosts are organisms that support the sexual reproductive form of parasites.
Intermediate Host: Intermediate hosts are organisms that support the immature or non-reproductive forms of parasites.
Alternative Names
Definitive Host: Definitive host is also called the primary host.
Intermediate Host: Intermediate host is also called the secondary host.
Type of Reproduction
Definitive Host: Sexual reproduction of the parasite occurs inside the definitive host.
Intermediate Host: Asexual reproduction of the parasite occurs inside the intermediate host.
Stages of Sexual Reproduction
Definitive Host: The formation of the zygote occurs inside the definitive host.
Intermediate Host: Sexual differentiation of the parasite occurs inside the intermediate host.
Examples
Definitive Host: Definitive host of Plasmodium is female Anopheles.
Intermediate Host: Intermediate host of Plasmodium is human.
Conclusion
Definitive host and intermediate host are two types of hosts used by obligate parasites to complete their life cycle. Parasites undergo sexual reproduction inside the definitive host. Asexual reproduction or the multiplication of the parasite occurs inside the intermediate host. The main difference between definitive host and intermediate host is the stage of the life cycle of the parasite that takes place inside each type of host.
Reference:
1. “About Malaria – Biology.” Centers for Disease Control and Prevention, 1 Mar. 2016, Available here.
Image Courtesy:
1. “Plasmodium lifecycle PHIL 3405 lores‘ By CDC/Alexander J. da Silva, PhD/Melanie Moser – the Centers for Disease Control and Prevention’s Public Health Image Library (PHIL) (Public Domain) via Commons Wikimedia
2. “Hymenolepis nana Life Cycle” by SuSanA Secretariat (CC BY 2.0) via Flickr
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