The following points highlight the four main parasitic adaptations of helminths. The adaptations are: 1. Morphological Adaptations 2. Physiological Adaptations 3. Life Cycle Adaptations 4. Immunological Adaptations.
1. Morphological Adaptations:
The Helminths, though are of lower grade of organisms, show structural modifications or adaptations along two lines:
(a) Degeneration or loss of organs or systems;
(b) Attainment of new organs.
(a) Degeneration or loss of organs:
The endoparasitic helminths undergo loss or simplifications of unused organs or parts.
Many parasites are large compared with their free-living relatives. This could be related to increased egg production.
Most parasites are dorso-ventrally flattened and this is related to the need to cling on to the host. Fleas are laterally flattened and rely on escape through the hairs. Nematodes are the obvious exception to the trend of flattening in parasites and parasitic nematodes, as a whole, show little morphological specialization.
3. Locomotor organs:
As the adult parasites live for entire life in the body of the host, the locomotor-organs are usually not necessary for them.
Consequently the locomotor-organs are completely reduced except in the free-swimming larval forms:
(i) General loss of movement,
(ii) Ectoparasites are capable of some free movement but endoparasites have little to do so when detached from the host,
(iii) Loss of cilia from the body surface is an indication in this direction. In parasites and particularly in endoparasites there is loss of locomotory organs.
4. Sense organs:
In many parasites, particularly endoparasites, there is often a reduction in the CNS and sense organs.
In endoparasites, again there is a trend to reduce the gut and absorb nutrients through the whole body surface.
6. In those intestinal parasites, which do not absorb nutrients through the body surface, there is usually a thick cuticle. So helminths tend either to loose their gut or absorb nutrients through their teguments, or else retain their gut and have a thick resistant cuticle.
7. Simplification to eventual loss of food-tube and where food-tube is present; the pharynx becomes extremely muscular, e.g., Ascaris sp. (Fig. 17.2).
8. Glands in buccal region develop anti-coagulatory secretion; e.g. Hook-worm.
(b) New attainment:
Parasitic existence leads to the attainment of new structures, helpful in food absorption, protection, attachment and vast reproduction.
1. For entry inside the host:
i. They secrete a liquid from unicellular gland to dissolve host tissue and thus making a microscopic passage for the parasite, e.g. Miracidium larva of Fasciola sp.
ii. Secretion also helps the hooklet to dissolve the tissue; e.g. Hexacanth larva of Tapeworm.
2. Organs for attachment:
i. Hooks are arranged as a crown around the rostellum in double rows, e.g. Taenia solium (Fig. 17.3).
ii. Acetabula or suckers found in adult parasitic flatworms like liver-fluke (Fasciola sp.) has two suckers on the ventral side of the body—one anteriorly and one posteriorly placed.
iii. In Ophiotaenia—four suckers and sometimes an apical one are also present.
iv. In some Cestodes and Nematodes hooks or hook-like structures develop at the cephalic end, e.g. Coracidium (Fig. 17.5).
v. In Taenia solium the rostellum contains a basal circlet of hooks.
vi. In Dipylidium canium several rows of hooks are present around the retractile rostellum (Fig. 17.6).
3. For protection—Cuticle:
i. Cyst membrane forms around the body, e.g., Metacercaria larva of liver-fluke.
ii. The cuticle of Helminth is highly modified and adapted to resist against digestive juices and for adhesion. The cuticle becomes thin, partly or wholely for food absorption. The parasites live in rich nutritious environments; such as liver- flukes (in bile), blood flukes (in blood), Sporocyst larva and Cysticercus (in vertebrate muscles) and other larval forms (developing in lymph spaces and blood stream).
iii. In gut parasites, however as in Tapeworms, Gnathostomes, Amphistomes and Nematodes—the cuticle becomes thick, impregnated with impermeable chitin like substances and enzyme resistant, so that it is not digestable by the digestive juices of the host but is permeable to water.
iv. Presence of spinous integument—in many Trematodes.
The well-developed musculature in Tapeworms (Taenia sp.) helps them to distribute their elongated snake like bodies throughout the length of the intestine of the host. Similarly due to specialised musculature, power of locomotion enables the roundworms (Ascaris sp.) to counteract gut peristalsis and to maintain their position in the intestine. The advantage is that the worms can get pre- digested nutrients of the host.
5. Simplified systems:
i. Nervous system is greatly reduced. Ladder-like nervous system is the characteristic feature of Platyhelminthes (Fig. 17.8).
ii. Excretory system particularly in Nematodes exhibits very little adaptation to parasitic mode of life.
The characteristic feature of excretory vessel in Cestoda is ladder like. The longitudinal lateral canals are provided with a number of flame cells (Fig. 17.4 and 17.9).
iii. Reproductive system:
In many parasites there is a tremendous elaboration of the reproductive organs, associated with increased gamete production. Cestodes, for example, basically consist of a small head and neck region and the rest is serially repeated gonads. Parasites can be described as being solely adapted for reproduction.
2. Physiological Adaptations:
Physiologically Helminths show striking adaptation to lead the parasitic life in the body of the host and to enjoy their life in simplest ways.
1. Intra-cellular digestion:
The flukes feed on tissue elements and inflammatory exudates and have probably intracellular digestion.
The osmotic pressure in the interior of the parasitic worms remains less than or same to the host, so that there is no difficulty in exchange of water. Cestodes have well-developed water osmoregulatory systems and their pH is also high.
3. Anaerobic respiration:
As intestinal parasites lead their life in an environment completely devoid of free oxygen (O2), evolutionary adaptations have resulted in a very low metabolic rate requiring a minimum amount of oxygen. Thus, respiration is anaerobic, which consists of extracting oxygen from food-stuffs—they absorb and assimilate through their cuticle. But the manner of O2 liberation from food is not yet clearly known.
In the absence of free oxygen, energy is obtained by the process of fermentation of glycogen, in which by glycolysis it is broken down into CO2 and fatty acids. The glycogen and lipid contents in their tissues are, therefore, high whereas the protein content is low.
Most Helminth parasites, specially intestinal ones, secrete anti- enzymes to protect themselves from the gastric juices and digestive enzymes of the host. So a worm is unable to produce antienzymes quickly digested by the host.
ii. For perpetuation of the race—vast reproduction:
There is a vast increase in the reproductive capabilities through greater egg production. In flatworms—the interior of the body is mostly occupied by the genital organs. The flatworms are hermaphrodites and roundworms are dioecious and this ensures fertilization.
Self-fertilization is more common than cross fertilization in flatworms. In tapeworms body consists of a large no. of proglottids, each containing single or double sets of genetalia. Thus we see that the life-history usually includes several larval stages for multiplication and for easy and sure transfer from one host to another. Larval stages have power of locomotion helping in dispersal mechanism.
3. Life Cycle Adaptations:
i. Simple in Turbellaria and Monogenic. Trematodes.
ii. In digestive Trematodes a larval stage occurs.
iii. In Cestodes one to three hosts may occur.
iv. Nematodes may often show 1 to 2 hosts.
A series of asexual reproduction, such as fission, exogenous or endogenous budding etc. can be traced in many Flatworms to increase the rate of multiplication. The use of many hosts in an individual life cycle is indicated as the expansion of parasite.
1. There is usually an increase in reproductive potential compared with free-living relatives. Parasites usually produce more eggs and sperms than their free-living relatives do and there may be a great elaboration of the reproductive organs. Other adaptations, which increase egg production, are hermaphroditism and parthenogenesis, where every individual produces eggs and loss of seasonal reproductive cycles, so eggs and sperms are produced all the year round.
Rapid maturation and extended life span also increase total reproductive capacity. The reproductive potential of the parasite can again be increased by asexual reproduction at different stages of the life cycle. One of the best examples are the digeneans where a single Sporocyst can give rise to daughter Sporocysts each of which can give rise to several generations of Redia, before the Cercariae are produced.
It has been estimated that the reproductive potential of a single liver fluke (Fasciola hepatica) is four hundred million offspring in its lifetime.
2. Infection of secondary and tertiary hosts:
This has three advantages:
i. Increased reproductive potential, since asexual reproduction can take place in the alternative host.
ii. It increases the range of the parasite in space and time. That is infection of more than one host which can increase the geographical range of a parasite, particularly if one host is say terrestrial and the other aquatic. By infecting more than one host species the parasite can survive periods when one host is temporarily scarce.
iii. An intermediate host can channel the parasite towards its definitive host since the intermediate host is frequently part of the final host’s food chain or else closely related ecologically.
3. There is a marked trend amongst the major parasitic groups to reduce the extent of the free-living phase of the life cycle (this avoids the variable external environment).
4. Many parasites have no provision for infecting new hosts beyond the provision of large numbers of eggs or larvae. However, the infective stages of many parasites show adaptations that help to increase their chances of infecting a host.
i. Behavioural response to locate favourable environments.
ii. Responding to chemical stimuli from their host.
iii. Changing the behaviours of the infected intermediate host to increase the chances of them being eaten by the final host.
5. Regulation of infection by the host:
Many parasites require a specific pattern of stimuli from their host before they are able to infect them. This is particularly clear in those parasites that infect their hosts passively via the gut in the form of cysts or eggs. Such stages may require pre-digestion with host enzymes and the presence of specific bile salts as well as the correct pH, temperature, redox potential, pO2 and pCO2 before they can hatch.
6. Regulation of the adult parasite by the host:
That is reproduction of the parasite is controlled by hormonal or physiological changes in the host (e.g. the periodicity of microfilaria, and Polystoma in the frog).
4. Immunological Adaptations:
Vertebrates react to the presence of foreign material in their tissues by the production of a humeral and cell mediated response and this depends on the ability of the host to recognize the difference between self and non-self. In mammals it takes approximately 9 days for the immune response to become fully effective, so any parasite that persists for significantly longer than 9 days must have some mechanism for avoiding or mitigating the host’s immune response.
1. Absorption of host antigen
2. Antigenic variation
3. Occupation of immunologically privileged sites
4. Disruption of the host’s immune response
5. Molecular mimicry
6. Loss or masking of surface antigens.
Host parasite relation—the environment plays a key role.
The following sorts of relation are observed between parasite and a host:
It makes the environment suitable for parasitic development and in its absence the parsites die.
2. Resistance of host:
This is of two folds— Firstly making the parasite itself immature to the host enzyme and secondly to have full capacity to reproduce.
3. End of virulents:
Parasites when not well adapted may lead to virulents and both the parasite and the host try to develop reciprocally an immunity.
4. Mutual help:
The reciprocal immunity on the other hand develops a sort of relationship which does not destroy any of the animals and thus makes “Parasitism” perfect.
Viewing the groups of parasitic Helminths as a whole with respect to successive stages of adaptation, which they have undergone and are undergoing, one is able to appreciate the vastness and profoundness of the principles of adaptations and at the same-time how marvelously and splendidly the parasitism has become successful in Helminths.