Parasitology: Introduction, Branches, and Significance

Parasite: A parasite is an organism that lives on or inside another organism called its host and derives its nourishment from the host, without giving any benefit to the host.

Host: It is a reservoir, which is an intermediate host where the parasites develop and from which infections can be acquired by other hosts. (e.g., the dog is the reservoir host for cysts of echinococcosis). The host may be of the following types:

• Definitive host: The host in which the adult parasites reproduce sexually (for example anopheles species), is referred to as definitive host. The hosts could be human or nonhuman living beings.
• Intermediate host: The host in which the parasite undergoes asexual multiplication is known as an intermediate host. (for example, in the life cycle of the malaria parasite, humans are intermediate hosts). A few parasites require an intermediate host to complete their life cycle. Some parasites need two intermediate hosts where the various larval stages of the parasite complete their development. These are referred to as the first and second intermediate hosts respectively (for example, Amphibian snails are the first intermediate host while Fasciola hepatica is the second intermediate host)
• Reservoir host: A host to which the parasite is available for transmission to another host and it does not get infected by the disease commonly.
• Paratenic host: The intermediate host in which the adult form of parasite lives but cannot grow and develop further, and which is not necessary for the parasite’s life cycle is called paratenic host Freshwater prawn for Angiostrongylus cantonensis, a big suitable fish for plerocercoid larva of Diphyllobothrium latum and freshwater fishes for Gnathostoma spinigerum). It serves as a transport or carrier host.

Vector: A biological vector is an organism capable of transmitting the disease agent or the causative organism from infected hosts to other hosts.

Parasitic life cycle

The scale of development of the extracorporeal and intracorporeal phases of the life cycle of the parasite starts from the point of entering the host and ends with exiting the host. It can either be direct (simple), when only one host is affected, or the mode is indirect (complex), which means involving one or more secondary hosts.

Direct/simple life cycle

If, however, the parasite can develop in a single host, its life cycle is considered to be direct/simple. 

Table 1: Examples of direct life cycle

Indirect/complex life cycle

When a parasite requires two hosts (one definitive host and another intermediate host) to complete its development, it is referred to as an indirect/complex life cycle. Eg: Leishmania spp., Taenia solium, Taenia saginata, etc.

Table 2: Examples of indirect life cycle

Parasite ecology

Special features creatures have numerous characteristics unique to parasites that have relevance to their behavior and distribution. They therefore include various life histories and behaviors that have developed as means of identifying available hosts, exist and move about within the hosts, breed, and be persistent. The parasites on one extreme are monoxenous meaning they can only infect one host while on the other extreme are polyxenous and can use many host species. Morphological and physiological changes can also be brought about by parasites on their preferred host as well as the host’s communities with the implication that parasites can impact virtually every aspect of the host and host community as well as sites and conditions of host predation. Hence, every aspect of parasite biology and ecology (reproduction, distribution, host specificity, morphology, parasite life cycle, population dynamics, and geographical distribution) influences the abundance, habitat distribution, or biogeography of a particular species of parasite.

Host-Parasite Interactions

The relationship between a host and a parasite can be further divided into the following categories:

Symbiosis: It is the close association competition between the host and the parasite. Both are mutually dependent on one another one cannot exist without depending on the other. None of them get hurt by each other.

Commensalism: It is a relationship in which the parasite has everything from the host’s side of a profit, without the host suffering any losses in the process. In other words, a commensal has the capability to lead a life on its own.

Parasitism: It is a relationship in which the host works to the advantage of the parasite while always being detrimental to the host. The host does not get anything in return.

Mutualism: It is simply defined as the coexistence between the host and a symbiont in that none suffers although both gain. Yet, this may last longer or may be short-term in nature of relationship may be. Mutualism is of species of symbiosis. One example is kind of simple where some flagellated protozoa that inhabit the gut of termites. As the protozoa are purely and simply carbohydrate feeders, they obtain all their food from termites.

Laboratory diagnosis of parasitic diseases

An important modality in the diagnosis of parasitic infections, the laboratory diagnosis is an important aspect of the management of the disease. The following diagnostic techniques are used for diagnosis of parasitic infections:

• The identification morphological procedures are either in forms large (macroscopically) enough or small (microscopically) enough to be detected under the microscope.
• Culture methods
• Immunodiagnostic methods
• Molecular methods

Morphological identification techniques

Looking at the morphology of the parasites it is possible to categorize them, macroscopically or microscopically. Of course, the different forms that various parasites may have can be observed at different morphological levels.

Examination of feces

• Stool specimens should be obtained from the well, non-leaking, screw-capped jar with a wide mouth and should be handled gently so as not to contact infections from organisms present in the stool. 
• Ideally, the specimen should be taken before the commencement of anti-parasitic drugs and, at a time closer to clinical manifestation.
• Three stool samples given at least 48 hrs apart are sufficient to help establish a diagnosis of intestinal parasitic diseases.
• Liquid stool samples need to be processed and get ready for examination in 15-30 minutes, while semisolid ones in about one hour; formed feces should be examined within 24 hours after they were collected.
• During storage, the trophozoites may rupture, lose motility, and may hence, appear as artifacts.
• Many preservatives may be used for this purpose for instance 10% formalin or polyvinyl alcohol to preserve the cysts and egg morphology of the parasite.

Macroscopic examination

• Present in acute amoebic dysentery, intestinal schistosomiasis and invasive balantidiasis.
• The dark red colour of the stool means upper GIT bleeding while the bright red stool indicates bleeding from lower GIT.
• Non-bloody, pale offense, frothy stool with fat formed in the small intestines in case of giardiasis.
• This may contain round worm, thread worm or segments of tapeworm may be seen, in adults as well.

Microscopic examination: Direct wet mount method

Saline is dropped and Lugol’s iodine is put into different corners of one slide. A small amount of feces is manually stirred using a stick so that it becomes a homogeneous smooth liquid suspension. A cover slip is placed on the mount and observed at the low power of a lens (10X) or high power of a lens (40X).

Molecular methods of identification

There are two types of molecular methods for the identification of a parasite. They are:

• DNA probe
• Polymerase chain reaction (PCR)

DNA probe

The DNA probe is also a nucleotide sequence attached to each of its radioactive isotopes. It will involve a portion of the parasitic DNA detectable within the clinical samples available. This is highly specific and reproducible. At present, there are protocols for the detection of specific DNA sequences using DNA probes for parasites such as P. falciparum, W. bancrofti, and so on.

Polymerase chain reaction (PCR)

PCR-based diagnostic assays involve several additional steps after the choice of oligonucleotide primers and reporter probes. Cyclospora cayetanensis, Entamoeba histolytica, and E. dispar are detected by both conventional PCR and real-time PCR. Standard PCR is possible when it comes to Microsporidia.

Conventional PCR: PCR with diagnostic primers is nevertheless carried out to test the DNA preparations derived from fecal samples. The obtained DNA fragments are amplified and submitted to electrophoresis on an agarose gel for result analysis.

Real-Time PCR: In real-time PCR, the amplification of the DNA is however continuously assessed by measuring the fluorescence signal inherent in the reaction vessel. At the end of every cycle, the fluorescence signal is compared to the reference and is proportional to the PCR product formed. The real-time PCR assays that are employed in the detection of parasites at CDC employ SYBR Green or Taqman probes as fluorogens. The advantages of probe-based assays are their high specificity and the ability to detect multiple targets in the same vessel when the probes are labeled with dyes of distinct fluorescence spectra.

Phylogeny, systematics, and evolutionary relationships among parasites

Phylogeny and parasite evolution

The word phylogeny means the evolution history of a given group or organism lineage and is reconstructed based on morphological molecular and other characteristics. The set of taxa admitting a genealogical relationship is represented as a phylogenetic tree.  Consequently, for parasites, the question about the evolution of parasitism in the case of descendants of different lineages arises. For example, parasitism has evolved in various taxonomic divisions including helminths, arthropods, as well as protozoa with clear evidence of convergent evolvements, in which unrelated taxonomic subdivisions of the host achieve comparable methods of parasitic exploitation. Common methods, for setting up parasite phylogenies involve sequencing of ribosomal RNA genes or mitochondrial DNA. These phylogenies make it possible to make assessments about the early evolution of parasitism, the approximate time of key divergence events, and the nature and tempo of the co-evolutionary process with hosts.

Host-parasite coevolution

The term ‘co-evolution’ was originally used to describe the kind of inter-evolutionary connection that subsists between plants and the insects that feed on them. Phenomena considered coevolutionary are noted in such examples as pollinator insects and flowers, mimics and their models, and parasites and their hosts. The definition of coevolution could be summarized as the coevolution of two or more species whose relations are relatively easy to define and always involve mutual benefit which may be partial but can also be used to describe any kind of intricate relation between the species, for example, predator and prey or host and parasite. However, such coevolution takes place only specifically, simultaneously, and alternately between two organisms though there may be many exceptions. Loyalists are hard to come by when it comes to parasites and other organisms that live among their hosts. If a parasite can reinvent itself on a new suitable host species, then, of course, there are better chances of evolutionary success. Jumping of a parasite to an otherwise unrelated host is known as ‘capture’, ‘host switching’, or ‘shift’.

Systematics in parasitology

Systematics is the branch of biology whose works entail the arrangement of the organizational structure of living organisms. The study of systematics presents many difficulties to parasites because such organisms are most often small in size, difficult to differentiate morphologically, and often have intricate life cycles. It has been customary to systematize parasites based on their physical characteristics such as body color, kind of organization, or form of gonads. Until recently, this approach has been popular due to the inability of molecular systematics to resolve the relationships between closely related species or to cut through cryptic species complexes. Systematics is important not only in taxonomy but also in medicine and agriculture because the correct determination of parasites is significant in controlling infections.

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