Intermediate host

Intermediate Host

When looking at the relationships amongst different biological members of our biosphere, we can notice a myriad of connections, dependences, interlinks, and alliances. Some are mutual while others are parasitic. Additionally, we can also notice some other relationships like commensalism, amensalism, and competition. The common subject among many of these is a “host”, for which the medical definition is:

“a harbouring organism that provides all the hospitality services to a guest organism”

To be specific, we can witness the presence of hosts in three relationships, namely mutualistic, commensalistic, and parasitic. The host plays a vital role in these relationships as it provides the 3Ns:- nutrition (food plus minerals), niche (shelter plus habitat), and nest egg (fund reserve for future)- for the optimal functioning of guest organism/s.

Now, if we try to categorize hosts into five broad groups, the concept clarity would greatly improve. So, before we define intermediate hosts, let’s look at the illustration below to understand how hosts are categorized.

So, in order to understand that among all these categories, what is an intermediate host, we should know what’s the basic difference between a definitive (primary) and intermediate (secondary) host. We should lay emphasis on the fact that these two hosts’ terminologies hold the most relevance when we are talking primarily about “parasite-host relationships”!!

Table 1. Definitive host vs. intermediate host
Definitive Host	Intermediate Host
It is that host that harbors the sexually active phases of the parasite.	It is that host that harbors the non-sexual (reproductively/sexually immature) phases of the parasite.
Also called the “primary” host.	Also called the “secondary” host.
Site of sexual reproduction for the parasite. (So, if asked, a definitive host harbors which stage of a parasite… it’s the sexually-mature adult stage!)	Site of asexual reproduction for the parasite.
Parasite spends a longer duration of its life cycle in this host.	Parasite spends a shorter/transient duration of its life cycle in this host.
Parasites’ gamete fusion and zygote formation happen in this host.	Parasites’ sexual differentiation happens in this host.
Examples: a) Plasmodium is a malarial parasite causing malaria. Definitive Host= Female Anopheles mosquito b) Dirofilaria immitis is a heartworm parasite causing dirofilariasis. Definitive Host= Dog c) Wuchereria bancrofti, Brugia timori and Brugia malayi are nematode parasites causing lymphatic filariasis. Definitive Host= Humans

There are selective pressures on both hosts and parasites that are “reciprocal” in nature. In order to escape the parasitism and resources’ extraction plus exhaustion by parasites, the host develops some adaptations. Simultaneously, the parasite in order to keep its nourishment and shelter needs secure, also develops certain adaptations. These adaptations are “reciprocal/antagonistic” ones and this sort of never-ending evolution is called coevolution. None of the species gives up, rather they keep adapting, evolving, and proliferating simultaneously down the evolutionary timelines.

Intermediate Host Examples

Some examples of intermediate hosts are shown in Table 2.

Table 2: Parasite and their intermediate host(s)
Parasite	Intermediate Host
Plasmodium	Humans
Dirofilaria immitis	Mosquito (Aedes spp., Culex spp., Anopheles spp., Mansonia spp.)
Wuchereria bancrofti, Brugia timori & Brugia malayi	Mosquito (Culex spp., Aedes spp.& Anopheles spp.)
Liver Flukes (Clonorchis, Opisthorchis)	Fish of Cyprinidae family (carps and minnows)
Tapeworm (Taenia solium)	Humans, Cow, Pigs
Schistosoma mansoni	Freshwater snails (Biomphalaria spp.)
Toxoplasma gondii	Many mammals and birds
Trypanosoma cruzi, Trypanosoma gambiense	2 intermediate hosts: a) Invertebrate host (Triatomine insects like tsetse fly- Glossina palpalis) b) Vertebrate host (Humans)
SARS-CoV Virus	Masked palm civet
MERS-CoV Virus	Dromedary camel