Interactions between Host and Parasite

In this article we will discuss about the interactions between host and parasite.

Host-specificity: its dynamic quantities:

(i) Usually related species and genera of parasites are found to attack closely related host-species.

(ii) In some cases, the parasite may be mono-specific attacking a single host species, this situation often indicates an old association and co-evolution.

(iii) Such mono-specificity is recorded in Sporozoa, especially in Plasmodium and Eimeria.

(iv) Monogenean flukes also show narrow specificity; 75% of these flukes parasitise a single host-species, while 90% are restricted to a single genus or family.

(v) Kennedy (1975) reports that each or­der of birds has its own tapeworm parasite. The phylogeny of these para­sites shows a parallelism to the phylogeny of the avian hosts.

(vi) Three alternative paths are followed during the parasitic evolution:

(a) The parasite-host-mix does not speciate any further;

(b) Both parasite and host undergo speciation; or

(c) The parasite speciates but not the host.

(vii) In unusual situation, however, the host may speciate while the parasite re­mains unchanged. In those cases, the parasite may abandon the old host and establish into a new one.

(viii) For example, the sheep liver-fluke Fasciola hepatica adopts a new second­ary host Lymnea tomentosa (pond snail) in Australia where the typical (Euro­pean host L. truncatula is unrepre­sented.

Factors determining specificity:

1. Ecological factors:

Numerous and various but difficult to be generalised.

2. Barriers to infection:

Larvae of Triaenophorus infecting copepod hosts have to overcome the barriers of both alimentary canal and haemocoel.

3. Unfavourable physiological environ­ments:

Dirofilaria immitis can pass into the haemocoel of Anopheles mosqui­toes easily because the blood meal clots slowly. In Aedes, blood clots very quickly in the digestive tube and thus prevents such migration.

4. Host responses:

(a) It is more intense and specific against an unusual invader than against the natural parasite.

(b) Phagocytic activities are usually, found in invertebrate hosts. Spe­cific antibodies are produced in vertebrate species.

Immunity and susceptibility of hosts:

Immunity refers to the mechanism by which the host successfully combats with the invader (parasite) and often discards it.

Two types of immunity are found:

(i) Natural immunity:

When the parasite invades but fail to establish, the host is said to possess the natural or innate immunity against that parasite.

(ii) Acquired immunity:

When an initial infection by a parasite evokes some degree of resistance against further infection. The term “resistance” refers to the condition of the host which is not completely successful in eradicat­ing the parasite. But it presents a par­tial barrier to the establishment of the parasite.

Remark:

Insusceptibility of the host spe­cies against a particular para­site accounts for its morphol­ogy, ecology, behaviour and physiology that are suitable for the infection.

Molecular mimicry—genesis of host­-like antigens:

“Ideal parasites” can thrive in the hostile climate of the host’s body without stimulat­ing its immune system. This problem is solved by the parasite by synthesizing host-like antigens. The host, therefore, fails to recog­nise the parasite as an intruder and foreign body owing to the presence of identical or rather mimicking antigen molecules over the parasite’s body. This is called molecular mimicry.

Smyth (1976) discussed three hypotheses accounting for the origin of molecular mim­icry.

These are:

(a) Mimicry by natural selection:

This hypothesis was forwarded by Sprent (1969).

(i) Antigens are of parasitic origin.

(ii) Antigens are synthesized in the evolutionary course by mutation and natural selection.

(b) Mimicry by host induction (For­warded by Capron, 1975):

According to this hypothesis, the host in some manner is capable of inducing the parasite to produce host-like anti­gens. Acceptance of this hypothesis depends on the belief that the parasite possesses a series of genes which code for host antigens. Furthermore, these genes would be activated only when induced by factors of host origin.

(c) By incorporation of host’s antigens (Forwarded by Desowitz and Watson, 1954):

This model has been proposed from Trypanosomes infecting rats.

(i) Parasite’s antigens are capable of accepting host’s antigens.

(ii) Host’s antigens adhere to para­site antigens.

(iii) Finally, the parasite’s antigens are masked.

Effect of Parasites on hosts:

Parasites bring about profound changes on hosts but it depends upon parasitic spe­cies, host species, sites and intensity of infec­tion.

Some of the changes taking place by the attack of parasites on their hosts are men­tioned below:

1. Attack of Plasmodium vivax is not so serious but that of P. falciparum is al­most fatal.

2. The infection of Trypanosoma brucei produces no apparent symptoms in wild antelope but its attack is almost fatal in domestic cattle.

3. Entamoeba histolytica lives as a commensal in the intestinal lumen of man but it is pathogenic while enter­ing in the tissue invading phase.

4. The parasites also cause mechanical injuries to their hosts. Coccidians cause haemorrhage in the intestines of avian and manmalian hosts. Echinococcus (dog tapeworm) causes great damage in liver, brain and other organs. Heavy infection of Ascaris lumbricoides oc­cludes the intestinal lumen.

5. Toxic influences. Haemozoin is pro­duced by the trophozoites of Plasmo­dium exerting toxicity.

6. Alterations in resistance. Migration of Fasciola (liver fluke) through liver makes the organ susceptible to the attack of a bacterium (Clostridium oedematiens) producing “black dis­eases” in sheep.

7. Infertility—caused by Sacculina in host crabs by impeding maturation of go­nads.

8. Parasites, being composed of antigenic molecules, induce their hosts to form antibodies.

9. Tissue reactions in hosts.

(i) Hyperplasia:

Parasites induce cell division and increase in cell divi­sion rates. Fasciola stimulates the division of hepatic cells.

(ii) Metaplasia:

Metaplasia is the transformation of tissue type from one to another without the inter­vention of embryonic tissue. The oriental lung fluke, Paragonimus westermani lives in the lungs of man in a cyst-like pocket. The cyst contains epithelial cells and fibroblasts (fibre-like cells). It is known that the cells of epithelium and fibroblasts are changed and is called metaplasia.

(iii) Neoplasia:

Malignant tissue growth. For example, a cestode, Cysticercus fasciolaris causes tumours in the connective tissue of rat liver.