Intermediate Host: The Delivery Boy of the Parasitic World

Why use an intermediate host?

Parasites have often been known to take a skewed path to their ideal host. What is the point of an intermediate host? What makes the longer path the better one? It seems more time and energy consuming, and overall inefficient. Well, sometimes it is. But more often than not, an intermediate host is beneficial, even necessary, to a parasite’s life cycle. Intermediate hosts can increase the probability of infection the definitive host[1], provide transport to the definitive host, or even supply nutrients for the parasite’s development until a suitable host comes along[2].  

Types of Transmission

Before we talk about all the different instances in which using an intermediate/ host is helpful to a parasite, it’s important to know that there are 2 kinds of transport through a host: passive and active. Passive transmission is essentially the host’s accidental ingestion/inhalation/etc of an infective source. For example, Dracunculiasis, or more commonly referred to as Guinea Worm Disease, is the result of passive transport. The nematode responsible for the disease, Dracunculus medinensis, is ingested by a copepod in its L1 stage. The parasite develops into the L3 phase while in the copepod, which can be swallowed by a human (the definitive host) when clean water is unavailable[3]. This parasite did not manipulate its host’s behavior in attempt to reach another host, but rather just let things happen as they did. Active transmission, on the other hand, is displayed when a parasite manipulates the paratenic host’s behavior or deliberately performs some kind of action to open the encounter filter and increase the probability of reaching its definitive host. The human Bot Fly, or Dermatobia hominis, is a good example of active transport. The fly lays its eggs on the abdomen of a mosquito (the intermediate host) that will be stimulated and released when the mosquito comes in contact, or takes a bloodmeal from, a human [4].

Compilation of Boy Fly removals. [8]

Infective vs. Infestive

Trichinella's Life Cycle [6]

Both passive and active transmission can be divided further into categories; active transmission, for example, can be attained by either an infected source or an infested source. An infested source essentially implies that the parasite is free-living, while infective sources utilize a living organism as an intermediate host [4]. Pigs and cows are both infective sources/intermediate hosts for certain Trichinella species. T. spiralisis the most common, and therefore most devastating, species in the Trichinella family. The T. spiralis life cycle is direct, meaning that it can be completed within a single host, and therefore does not need an intermediate host. However,  there often are multiple hosts that T. spiralis will go through during its life. T. spiralis larvae encase themselves in cysts inside the muscle of its current host (usually a pig), which affects the mobility of the infected muscle. If the host’s muscles are rigid and immobile, it will be more likely to get caught, and therefore eaten, which will pass the nematode onto another host. The parasite increases its chances of continuing its life cycle by shortening that of its current host.

Mind Control of Mice?

Several parasites exhibit mind control
 over their hosts.[7]

Another example of a parasite using an intermediate host to increase its chances of ingestion is Toxoplasma gondii. T. gondii can only reproduce in cats, but it is often found in rats and other rodents. Why on earth would a parasite be found in both rodents and cats? Because the protozoa controls its host’s (the rodent) mind to decrease fear of cats. Usually, when a rodent/rat smells cat urine, it turns and runs as fast as it can. But when infected with T. gondii, the rodent is intrigued by the smell, and it sticks around to find the source of the smell leading to its imminent consumption[5]. The effects of T. gondii on human brains is currently being studied, and so far it looks like humans are also susceptible to parasitic mind control. Apparently, humans infected with T. gondii are more likely to take on dangerous tasks and get into car accidents. Science North talks about T. gondii more in the video below.

                   

          More info about T. gondii. [9]

Sources:

1. Art of Being  A Parasite, by Claude Combes

2. http://books.google.com/books?id=eegdgaR81k0C&pg=PA18&lpg=PA18&dq=benefits+of+intermediate+host&source=bl&ots=yzlv3E-1zi&sig=nQpwIKisK1zhbC5S-CfJdGow1k0&hl=en&sa=X&ei=6d9RU-y8Eu7KsQTH_oDYCA&ved=0CCsQ6AEwAA#v=onepage&q=benefits%20of%20intermediate%20host&f=false

3. http://www.cdc.gov/parasites/guineaworm/biology.html

4. http://health.usf.edu/publichealth/gh/parasitology/sem/sourcesandmodes.htm

5. http://www.cdc.gov/parasites/toxoplasmosis/biology.html

6. http://commons.wikimedia.org/wiki/File:Trichinella_life_cycle.jpg

7. http://www.pidjin.net/category/fredo-pidjin/page/24/

8. https://www.youtube.com/watch?v=wDoB7uY7TFc

9. https://www.youtube.com/watch?v=Ri1-Y0rocSA