Organ-Specific Parasitic Diseases of Dogs and Cats

Organ-Specific Parasitic Diseases of Dogs and Cats takes an organ-specific approach to the impact of parasitic diseases in dogs and cats. This book provides a detailed description of the key role that parasites play in the pathogenesis of diseases, explores the manifestation of symptoms and diagnosis, and offers prevention and treatment strategies to counteract parasitic infections.

An introduction by the editor is followed by chapters from various authors covering parasitic diseases in specific organs and organ systems. The book also provides an overview on the diagnostic evaluation of parasitic diseases. Parasite control, travel management, therapeutic measures, and disease control strategies round out the discussion.

With contributions from experts in the field, this book serves as a useful resource to researchers, academics, and postgraduates who wish to expand their knowledge on parasitic diseases, diagnosis, and treatment, and as a reference for scientists looking to develop new antiparasitic drugs.

• Provides a systematic overview of the pathogenesis of parasites in dogs and cats
• Evaluates parasitic diseases and their pathogenesis in specific organs
• Identifies pathogens and their role in the severity of disease manifestation
• Explains therapeutic strategies to combat parasitic diseases in dogs and cats
• Offers preventive and treatment strategies to counteract the etiopathogenesis caused by various types of parasites

• Language: English
• Publisher: Academic Press
• Release date: Mar 28, 2023
• ISBN: 9780323958639

Book preview

Chapter 1: Introduction

Vikrant Sudana; Deepak Sumbriaa; Tanmoy Ranab    a Department of Veterinary Parasitology, COVS, GADVASU, Rampura Phul, Punjab, India

b Department of Veterinary Clinical Complex, West Bengal University of Animal & Fishery Sciences, Kolkata, West Bengal, India

Abstract

Veterinarians have an opportunity to educate the clients about parasites and its prevention/control and answer various questions from the clients concerning parasites and the betterment for their pets daily. Veterinarians also provide varied information regarding the local prevalence of various parasitic diseases and also elucidate awareness about the zoonotic potential of parasites hampering their companion animal. The veterinarians also provide knowledge about the parasitic zoonotic importance for the sake of human healthcare providers. A parasite is known as an organism that habitats in or on another being (the host) and also acquires its nourishment from that host. There are various types of parasites, namely ectoparasites, endoparasites, permanent parasites, temporary parasites, facultative parasites, obligatory parasites, occasional parasites, and aberrant or wandering parasites that can affect dogs and cats. The climate conditions favor the pattern of life cycles of innumerable parasites, forging them year-round health concern and threats. This chapter emphasizes the general concept of parasites and parasitic diseases of dogs and cats.

Keywords

Parasites; Parasitic diseases; Host; Environment; Organ; Invertebrates; Companion animals; Nomenclature; Infection; Infestation

Parasitology is an important scientific discipline referring to the study of the biology of parasites as well as the parasitic diseases [1]. The characteristic features of parasites and their hazardous effect on host will be elucidated by understanding their behavior and life cycles. This section evolves to develop various strategies to treat as well as control parasitic disease.

Veterinary parasitology is a general study of animal parasites that signify the complex relationships between parasites and animal hosts [2]. The section of parasites of companion animals is also an important aspect of veterinary parasitology. The study also magnifies the important aspects regarding genesis as well as development of parasitosis in the animal hosts [3,4]. The taxonomical and systematics of parasites including the important morphology, life cycles, and necessity for living of parasites are well described in a systematic manner. Various research methods are applied to diagnose, treat, and protect companion animal parasites [5]. The most important strategies were implied to elucidate the veterinary practice and improve animal breeding [6]. The most significant goal of veterinary parasitology is to prevent the diseases of parasites in animals and thereby improve their health [7]. Veterinary Parasitology also provides public health significance by emphasizing the hazardous effect of parasites on animals and implies knowledge about the transmission from animals to humans.

About parasites

Parasites are the eukaryotic unicellular or multicellular organisms, which are heterospecific and are dependent on the other organism for food and shelter [8]. Unicellular are protozoa and multicellular are helminths and arthropods. Though bacterial and viral are prokaryotes, and they cause acute infection, whereas parasite cause subchronic infection due to which lot of economic loss of farmers occurs. It is said that parasites has been evolved before human evolution. Francesco Redi and Antonie van Leeuwenhoek began their study in 16th–17th century. In year 1681, Antonie van Leeuwenhoek investigated his own stool and found protozoan parasite, i.e., Giardia lamblia. Francesco Redi worked on ticks, deer nasal fly’s larvae, and liver fluke of sheep and is thus also known as father of modern parasitology. Some parasites reside inside the body of the host (endoparasite) while others are present on the surface of host body (ectoparasite) (Fig. 1). As parasites live on the expense of the host for their nutritional requirement, so usually they don’t cause death of their host but in rare condition, severe mortality is also observed.

Fig. 1

Fig. 1 Tick infecting a dog: An example of heterogenic association (schematic).

Parasites are divided into many groups such as helminths, protozoan, and arthropods. Helminths constitute platyhelminthes and nemathelminthes. Platyhelminthes include flukes (leaf-like) and cestode (tape-like worm) while nemathelminthes include round worms (rounded ends). They are further divided into protozoa, helminths, and ectoparasites (arthropods). Protozoa are single cell organisms and are very primitive while helminths and arthropods are more advanced and are multicellular. Helminths include Platyhelminthes (flat worm, i.e., flukes and tapeworm) and nemathelminthes (round worm). Parasitic diseases are considered as immemorial and are attributable to continue to evade and plaque mankind. Helminthic infections occurred in ancient Egyptian mummies (1210–1000 BC). In ancient Egyptian history, Schistosoma haematobium, Taenia saginata, Dracunculus medinensis (guinea worm), and Ascaris lumbricoides were identified by the famous ebers papyrus in the era of 1550 BC [9]. Parasites in dogs and cats existed since long time and made a strong relationship by depriving the host of their nutrients. In the world history, parasitic diseases in dogs and cats are the most significant issue for causing mortality and morbidity. Ancylostomosis and toxocariosis were most common parasitic diseases in dogs. Puppies generally died due to Ascarids and hookworms infection, whereas heart worm infection in mature dogs gives the veterinarians a great challenge in the world. Toxoplasma gondii in cat is having zoonotic significance throughout the World. Mosquitoes play a key role in causing diseases in pet animals [10].

Scientific nomenclature of parasites

After the discovery of parasite, its naming is a crucial step. In early days, the name of parasite has been kept on the basis of place, person who discovered it. Local name of parasite is also given, which may differ from place to place; moreover, many organisms have the same name; so in order to overcome these flaws, a need of uniform nomenclature of parasite was felt. In year 1758–59, Carolus Linnaeus (a Swedish botanist) introduced Bionomial nomenclature in his book Systema Nature. Later on in the year 1895, International Code for Zoological Nomenclature (ICZN) was established to form rules and regulation for scientific nomenclature of parasite. Till now, it includes 27 members from 21 countries.

A binomial system of nomenclature in which the scientific name has two parts, i.e., first part of genus and second part of species was used in Greek or Latin language as these changes are very rare in these languages [11]. This system uses life history, morphological character and designates connection between various parasites. Similar species of parasites are kept under similar genus, linked genera are kept under similar family, linked families are kept under similar order, linked orders are kept under similar class, linked classes are kept under similar phylum, and at last, similar phylum is kept under animal kingdom [12]. In binomial system of nomenclature sometimes names are given on the base of the scientist (Leishmania donovoni donovoni), name of the place (Eimeria bombayensis), shape (Eimeria ellipsoidalis), site in the body (Schistosoma nasale), host (Toxocara canis) [13]. There are some basic rules in scientific nomenclature system:

(1)Name of the parasite should be underlined or italicized, e.g., Fasciola hepatica or Fasciola hepatica

(2)If the text is already in italic, then the scientific name should be nonitalic, e.g. A common cause of bloody diarrhea is Balantidium coli/Balantidium coli, a ciliated protozoan.

(3)First letter of genus must begin with an upper-case letter, and first letter of species name always begins with a lower-case letter, e.g., Fasciola hepatica.

(4)If scientific name is derived from the name of the scientist, then it may begin with upper or lower-case letter.

(5)In case the parasitic name has subspecies, then also its name must begin with a lower-case letter, e.g., Hyalomma anatolicum anatolicum.

(6)If two or more scientists give name to the same parasite than the person who so ever published it first will be given all due credit, e.g., Leishmania donovani.

(7)If it is necessary to provide the scientist name, then it should follow the scientific name followed by comma and year of publication, e.g., Taenia Linnaeus, 1758.

(8)If a species is to be transferred from one genus to new genus, then author name should be in parenthesis followed by new author name along with year, e.g., Taenia taeniaeformis (Batsch, 1786) Wolffhuegel, 1911.

(9)Always use sp. for any species, spp. for numerous species, and these short forms should not be italicized; e.g., Eimeria sp. or Eimeria spp.

(10)Once generic or species name gets published, even author cannot reject it.

(11)If the generic name is already in use for some organism, then only it can be rejected.

(12)Various suffixes can be added, which are given in the following table.

Origin of parasites

Parasites are very primitive living creatures and had evolved a long way. In Egypt, Schistosoma eggs are found in mummies that were preserved from 1200 BC [10]. In 1500 BC, Papyrus Ebers has noted intestinal worm and Schistosoma infection from man. Bible also gives reference of Guinea worm. Analysis of Egyptian mummy from Ptolemaic period (200–100 year BC) showed the presence of cysticercosis [14]. Analysis of samples from Neolithic lakeside settlement in Spain (5320–4980 BC) revealed the presence of parasites such as Taenia/Echinoccocus, Diphyllobothrium, Trichuris, Capillaria, Ascaris, Paramphistomum, Macracanthorhynchus, Dicrocoelium dendriticum, and Enterobius vermicularis[15].

Phenomenon parasitism

Parasite is derivative from the Greek word parasitos meaning one that eats at the table of another and designates the feeding habit of the organism. The word parasite is also meant for besides food (Para—beside, sitos—food). The parasite generally lives at the expense of their host without hampering or endangering the host. Interestingly, a predator may kill its prey to take food upon the flesh, whereas a parasite plays a key role for taking advantage from the host for their survival without killing it. Parasitism is a complex phenomenon, in which three subsystems may get involved and are interlinked such as the parasite, the host, and the environment. Parasites are metabolically dependent on the host for their survival [16]. Many definitions are in the literature, which try to define parasitism [17]. In a simple way, we can say that parasitism is an obligatory relationship between organisms (heterospecific), in which one organism lives at the cost of other organism and is also metabolically dependent on the other. Usually in parasitism, one organism is small and is known as parasite while other organism is large and is known as host [12]. In parasitism, parasite usually does not kill the host but harms the host by either direct feed or competing with the absorption of nutrient in the intestine.

There are many types of parasitism such as:

1.Obligate parasitism—It is a type of parasitism in which the parasite cannot live without the availability of host; in this association, parasites usually do not result in mortality of host, e.g., head lice.

2.Facultative parasitism—It is a type of parasitism in which the parasite can survive without the availability of host. Parasite uses these phenomena as a form of existence policy due to the scarcity of assets in the environment, e.g., Strongyloides stercoralis.

3.Ectoparasitism—It is a type of parasitism in which parasite lives on the surface of host body. Sometimes parasites in this phenomenon are obligate (Lice) and sometimes parasites only require host at the time of for feeding (Mosquito). Some ectoparasites may carry and transmit other pathogen or parasite (Mosquito transmit plasmodium during blood feeding).

4.Endoparasitism—It is a type of parasitism in which parasite lives inside the organ/tissue/body fluid/body cavity of host. They may be intracellular (Plasmodium sp.) or intercellular (Trypanosoma evansi).

5.Meso-parasitism—It is a type of parasitism in which parasite lives partly within the host’s body. They enter body of the host via openings such as ear, etc., and live within the body for a specific period of time. Otobius megnini causes aural myiasis in dogs.

6.Epi-parasitism—It is a type of parasitism in which one parasite parasitizes the parasite. Epi-parasites are also known as hyperparasites, e.g., Hunterellus hookeri on ticks.

7.Brood parasitism—It is a type of parasitism in which parasite uses host body to raise the young one. It is mainly seen in insects.

8.Social parasitism—It is a type of parasitism in which parasite gets benefit from social animals such as ants and bees, e.g., an ant species called Tetramorium inquilinum shows social parasitism and spends its life cycle on the back of the other ant species, thus making them slaves [18].

Symbiosis, commensalism, and predatorism

Symbiosis constitutes mutualism, phoresis, commensalism, parasitism, or predatorism. Symbiosis is attributed to the association between two different organisms living in a close physical association usually in the advantageous manner of both as opposed to free-living organisms or simply or living together [19] (Fig. 2). Symbiont is an organism associated with the intimate relationship with other organism, whereas phoresis is such a complex relationship between two symbionts that can travel together without having biochemical as well as physiological dependence on each other [20]. Smaller phoront can adhere with the larger one mechanically (e.g., mites travel with the adherence of beetles or bacteria on the legs of a fly). Mutualism is such a relationship where both symbionts are mutually dependent on each other for any food as well as shelter. This complex relationship is normally obligatory with each other dependency for the survival. Interestingly, [18].

Fig. 2

Fig. 2 A schematic diagram of symbiosis (schematic).

Commensalism is an important type of relationship, whereas one symbiont takes an advantage from the other symbiont (host) without hampering host at any means. Commensalism may be either facultative (host not required for survival) or obligatory (host required for survival) and both taking food at the same table.

On the other hand, predatorism is such a relationship between prey and predators, whereas predators kill the prey for their food. Predators are generally invertebrates, animals that prey on another invertebrates, or animals for their food [21]. Interestingly, predators as well as parasites can survive at the expense of the prey or host at any means. Besides, parasites rarely kill their host and are smaller in size with a good complex elation to the size of the host, whereas predators play a key role in killing the prey (Fig. 3). Parasites are numerous and have a good reproductive potentiality in comparison to their hosts whereas predators are less numerous and have a lower reproductive potentiality rather than their prey [22]. In addition, parasitoids are those symbionts whose immature stages are developed on another parasite and emerge by killing the parasite.

Fig. 3

Fig. 3 An example of homogenic association (schematic).

Characteristic features of the parasites

Parasites have to make many characteristics changes in order to match its suitability of host [23] (Fig. 4). Some of morphological, physiological, and reproductive characteristic features of parasites are shown below:

Fig. 4

Fig. 4 An example of heterogenic association (schematic).

Reproductive:

1.To produce many young generations in a short interval of time, adopts asexual mode of reproduction, e.g., protozoa.

2.In order to ensure higher fertilization of egg even in the absence of opposite sex, adopts hermaphrodite mode of reproduction, e.g., Cestodes and Trematodes.

3.Have high fecundity rate.

4.Produces resistant form to escape harsh environmental condition (metacercaria) or host immune system [24].

Morphological:

1.Parasites have special modification such as no GIT in tapeworm, no wings on lice, etc.

2.Have specialized mouth part and organ of attachment, e.g., sucker, hooks, etc.

3.Size is smaller than host.

4.They are generally site-specific.

5.Have tick outer covering, which is resistant to many enzymes, e.g., Fasciola.

6.Nervous system is reduced and in some parasite body is flat, and it helps in attachment [9].

Physiological:

1.Have developed method to escape host immune response such as antigenic variation due to variable surface protein in Trypanosoma.

2.Can change behavior of host, e.g., Toxoplasma infection in mice.

3.Amplified respiratory efficiency for existing in places with low oxygen concentration [25].

Taxonomy/classification of the parasites

The word taxonomy is derived from Greek language, i.e., taxis (meaning order, arrangement) and nomos (law or science). The discipline of naming, describing, and classifying organisms is known as Taxonomy. It is applicable to all flora and fauna present on the earth [26]. Taxonomists use number of ways such as morphology, behavior, genetic and biochemical finding, and organize species into its classification. Generally, parasitic species showing similar character are kept under same genus, similar genera are kept under same family, similar families are kept under same order, similar orders are kept under same class, similar classes are kept under same phylum, and at last, similar phylum is kept under animal kingdom. General taxonomy of parasite is given in the table below with example.

Morphological structure of ectoparasites and endoparasites

Ecto- and endoparasites are those parasites that live on the surface and inside the body of their host, respectively. Ectoparasites are generally arthropods while endoparasites can be Platyhelminthes (trematode and cestodes) (Fig. 5), nemathelminthes (roundworm), or protozoa [27]. We will discuss general morphological structure of these parasites in the following subsections:

Fig. 5

Fig. 5 Mites traveling on beetles: An example of phoresis (schematic).

Morphological structure of trematode:

1.Body is unsegmented, leaf-like, dorso-ventrally flat, and is covered with spiny or smooth tegument.

2.General body cavity is absent.

3.Alimentary canal is present but incomplete and consists of mouth, pharynx, esophagus, incomplete intestine in the form of two caeca.

4.Circulatory and respiratory systems are absent.

5.Nervous system is present.

6.With few exceptions, generally trematode has one oral and one ventral sucker.

7.Execratory system consists of excretory bladder, branched tubes, and in the end flame cells.

8.They are generally hermaphrodites and have two testes and one ovary, exception is Schistosoma sp.

9.Usually has five larval stages, i.e., miracidium, sporocyst, redia, cercaria, metacercaria.

Morphological structure of cestode [28]:

1.Body is segmented, long ribbon-like, dorso-ventrally flat, and is covered with tegument.

2.Body is divided into scolex, neck, and strobila.

3.General body cavity is absent and organs are present in parenchyma.

4.Alimentary canal is absent and absorption of nutrition occurs via tegument.

5.Circulatory and proper respiratory systems are absent.

6.Nervous system is present.

7.With few exceptions, generally cestode has four suckers at head region, which may or may not be armed with hooks.

8.Execratory system consists of nephridial system with flame cell and lateral canal.

9.They are hermaphrodites and each segment has numerous testes and one ovary.

10.Usually consist of one or sometimes two metacestode stages.

Morphological structure of nematode [29]:

1.Body is unsegmented, round, and cylindrical with tapering ends and is covered with cuticle.

2.General body cavity is present, i.e., Pseudocoelom.

3.Alimentary canal is present and consists of well-developed mouth, buccal cavity, esophagus, intestine, and rectum.

4.Circulatory and proper respiratory systems are absent.

5.Nervous system is present.

6.Execratory system is osmoregulatory type and lateral canal is present.

7.Sexes are separate and generally male is smaller than female.

8.Male has one testes and female has two ovaries.

9.Usually consist of four larval stages before becoming adult.

Morphological structure of arthropod [30] (Fig. 6):

1.Metamerically segmented animals and body is covered with chitin.

2.Body is divided into head, thorax, and abdomen.

3.General body cavity is present, i.e., hemocoel.

4.Alimentary canal is present and consists of stomodaeum, mesenteron, and proctodaeum.

5.Circulatory system is present and has heart and aorta.

6.Respiratory system is present and consists of branching tubes, trachea, and spiracles.

7.Well-developed nervous system is present.

8.Execratory system consists of malpighian tubes.

9.Sexes are separate and male has two testes while female has two ovaries.

10.Wings and legs are present in many species.

11.Usually consist of some larval stages before becoming adult.

Fig. 6

Fig. 6 Host and parasite (schematic).

Morphological structure of protozoa:

1.They are unicellular eukaryotic, organisms.

2.They divide by asexual and sexual modes. Budding, binary, or multiple fissions areforms of asexual reproduction, where gametogony occurs in sexual reproduction.

3.Cilia, flagella, and gliding act as locomotory organs.

4.Mostly found in blood as hemoprotozoans, in intestine as enteric protozoans, and in tissues as cystic protozoans.

5.Usually more pathogenic than other parasites and often turn fatal.

Different types of parasites and hosts

Types of parasites

Permanent parasite: The parasite spending its whole life in or on the host (e.g., Lice) (Fig. 7).

Fig. 7

Fig. 7 Endo-parasites and ecto-parasites (schematic).

Temporary parasite: The parasite living in or upon the host only during a part of its life (e.g., Bots, Fasciola) (Fig. 8).

Fig. 8

Fig. 8 Bed bugs: A temporary parasites (schematic).

Periodic Parasite (Intermittent or occasional parasite): The organism that parasitizes for a short period frequently (e.g., Mosquitoes).

Incidental parasite: The organism parasitizing an animal, which is not its definitive host (e.g., Ascaris in sheep).

Erratic or Aberrant Parasite: Is an organism that parasitizes an organ that is not its normal habitat (e.g., liver fluke in lungs).

Accidental parasite: Free-living organism, which lives as parasite for a short period due to accidental contact (e.g., larvae of flies).

Obligatory parasite: is a parasite, which is fully adopted for parasitic life and is not capable of independent existence (Trematodes).

Facultative parasite: Is an organism that can live either as a parasite or as free-living individual (e.g., Calliphorine larvae).

Hyperparasite: is a parasite living on another parasite (e.g., Huntrella hukeri on ticks).

Hermaphrodite/Monoecious parasite: is one in which both male and female reproductive systems are present.

Unisexual/Diecious parasite: is one in which either male or female reproductive system is present [31].

Spurious parasites are such parasites normally observed transiently in the excretions of the host when a host consumes a parasite from another host (Eggs of Moniezia spp. in dog feces after ingestion of Moniezia-infected sheep, goat, and cattle intestines by the dog).

Pseudoparasites or artifacts are also important parasites exhibited in the clinical specimens such as yeast, pollen grains, and plant fibers, etc. (Fig. 9).

Fig. 9

Fig. 9 Yeast: A pseudoparasites (schematic).

On the other hand, monogenetic parasite is the parasite of no alternation in its generation (e.g., Trichomonads, amoeba, and ascarids, etc.). A digenetic parasite is considered as a parasite of alternation of generation (Plasmodium, coccidia, trematodes, piroplasmids of higher vertebrates, whereas asexual generation happens with the alteration of the sexual generation, as, e.g., in piroplasmids, sexual stages in invertebrate host, whereas the asexual stages are present in vertebrate host). Heteroxenous is a parasite that needs more than one host to complete its life cycle. Stenoxenous is considered as parasite of narrow host range, whereas Euryxenous parasites are considered as parasites of wide host range (T. gondii). Besides, Synanthropic parasites are found in human dwellings (e.g., houseflies) [32].

On the basis of activity in a day, parasites are further divided into

Nocturnal parasites: active at night, as per example Anopheline mosquitoes (Fig. 10A)

Fig. 10 Nocturnal and diurnal parasites (schematic).

Diurnal parasites: Active in the day, e.g., Tabanus sp. Musca domestica (Fig. 10B)

Crepuscular parasites: Active at twilight (dawn or dusk) (Culicoides sp. and biting midge)

Various types of parasitism

Infection is related to parasitism of host by internal parasites (Echinococcus granulosus in dog; Spirometra mansonoides in Feline).

Infestation is considered as parasitism of host by external parasites (fleas, ticks, lice, flies, etc.) (Fig. 11).

Fig. 11

Fig. 11 Culicoides (biting midges): An example of crepuscular parasites.

Autoinfection is generally considered as the condition where the juvenile infective form of a parasite infects the same host, without escaping from the host at any means.

For example, E. vermicularis in dogs.

Hyperinfection generally refers to such a condition where the juvenile form of a parasite without exiting the host can perforate in an area adjacent to the site of the predilection and also traditionally establishes in that same host, e.g., Strongyloides sp. [33].

Congenital (Transuterine or Transplacental) infection is the transmission of parasites from the mother (dam) to the fetus across the placental membranes, e.g., T. canis in dogs.

Transtadial (stage to stage) is such a transmission of parasite from one stage to another stage of the vector.

Transovarian transmission is transmission of parasite from the female parent to the progeny through the ova. For example, Babesia canis in ticks.

Transmammary (Transcolostral) is transmission of parasites from the mother to the young one through the milk of the infected dam (e.g., Toxocara cati in cats).

Zoonoses are diseases common to humans and animals (Singular—Zoonosis). According to WHO, zoonoses are considered as diseases transmissible between vertebrate animals and humans [2].

Anthropozoonoses is a disease of animals transmitted to humans, e.g., Cystic echinococcosis from animals to humans.

Zooanthroponoses is a disease of humans transmitted to animals.

Types of hosts

Host is one that harbors the parasite and is commonly larger of the two species in the parasitic relationship. There are different kinds, namely following are the different types of hosts.

Definitive/final host: The host that harbors the sexual stage of the parasite (e.g., Cattle for liver fluke).

Intermediate host: The host that harbors the asexual/immature stage of the parasite (e.g., Snails) (Fig. 12).

Fig. 12

Fig. 12 Parasites intermediate host: Snails.

Vector: A Vector is considered as a Latin word that means bearer. In the broader meaning, any agent or representative that carries/transports infectious organisms between animals. It is an invertebrate intermediate host (Mollusk, arthropods) [34]. It is also considered as an arthropod that carries the parasite between two vertebrate hosts (for example, Mosquitoes and Ticks for numerous blood protozoans) [35] (Fig. 13).

Fig. 13

Fig. 13 Active intermediate host: Tabanus fly.

Vector may be divided into mechanical vector and cyclical vector.

Mechanical Vector: A mechanical vector is considered as an arthropod that carries the infectious agent from one vertebrate host to another vertebrate host without formation of any development in its body. Is one that transmits a causative organism in which the pathogen does not undergo any development or multiplication. (Tabanus flies for T. evansi.)

Biological/Cyclical Vector: Is one that transmits an infection in which the causative agent undergoes development or multiplication or both (e.g., Glossina for Trypanosomes).

Reservoir host: The host that harbors the sexually mature parasite of some other definitive host. But rarely suffers from the disease after recovery from infection (e.g., Brugia malayi can be transmitted from cats and monkeys to man via mosquitoes).

Carrier host: The host harboring residual parasite in a latent phase without showing any clinical symptoms (e.g., Theileria annulata in bovines).

Transport host/Paratenic host: the host conveying the infective agent (sexually immature stage of the parasite) from place to place without any development or multiplication in its body (e.g., Earthworm carrying Ascaridia galli) (Fig. 14).

Fig. 14

Fig. 14 Earthworm: An example of transport host.

Experimental host: The host that can be infected by a parasite under unnatural or experimental conditions.

Intercalary host: Is the predator that eats its prey containing the infective stage of a parasite. The infective stage is released in the GIT of predator and expelled in its feces.

Terminologies applied in parasitic diseases

Epidemiology is the study associated with the disease in relation to the population aspects of disease including incidence, prevalence, and transmission in a population.

Epidemic disease refers to diseases that happen at a higher level in an area at the particular time than expected for the disease in the same place at that particular time.

Endemic disease refers to a disease that happens with minor fluctuations at a considerably predictable frequency in a population [17].

Sporadic diseases refer to the diseases that occur widely as well as irregularly with dispersed incidence or a disease that occurs infrequently in an area.

Pandemic is an epidemic disease occurring in a wide/larger area in devastating form within a short period of time.

Incubation period is considered as the time lapse between the entry of the parasite and the first appearance or onset of clinical signs [36].

Prepatent Period is also considered as the period in which parasitic form is being demonstrated in the clinical material of the host.

Hibernation or winter Sleep is characterized by lower activity of parasites in winter season.

Aestivation or summer sleep refers to the period of lower activity in summer season.

Schizogony is a specific type of asexual multiplication or division in protozoa, whereas a parasite multiplies or divides into numerous individuals by multiple fission methods. The nucleus of the parasites undergoes repeated multiplications without any cytokinesis, e.g., apicomplexan parasites [23].

Hypobiosis is characterized by a temporary cessation in the development of immature stages of some nematode parasites in the host due to adverse environmental conditions.

Alternation of generation of Metagenesis defines the sexual multiplication or division through alteration of asexual multiplication, e.g., Trematodes. Sexual multiplication occurs in definitive host by alternating pedogenesis in intermediate hosts [34].

Diapause refers to a period of lower activity in arthropods.

Virgin birth or parthenogenesis refers to the formation of progeny without any fertilization, e.g., Strongyloides sp. of nematodes or Haemaphysalis ticks.

Juvenile multiplication or paedogenesis defines the multiplication or division of immature/juvenile stages of parasite in the intermediate hosts, e.g., Multiplication of immature stages of trematodes in snail.

Sporogony includes the differentiation of zygote to sporocyst, oocyst, and sporozoites, e.g., all apicomplexan parasites.

Syngamy is characterized by the union of the female and male gametes to form the zygote, e.g., apicomplexan parasites.

Host-parasite interrelationship

Host-parasite relationship is the most important term applied in the parasitology characterized by the immense relationship between two different organisms mainly the parasite and host, whereas parasites invariably take an advantage to be succeeded in life at ease. Parasites upon entering their hosts through multiple routes may die or survive in the hosts. If parasites survive in the host system, they multiply and develop and thereby cause infection or disease in the host [37]. There is a complex interrelationship in which both bear a mutual adjustment. Therefore, parasite control may be achieved without any pathological changes. Host tries to kill or expel the parasite through a complex immune-mediated mechanism by creating immunopathological lesions [38]. The nutrition status and energy resources are key factors for the determination of the host parasite relationship. Host may fail to determine immune-mediated mechanisms due to failure of counteraction against large and complex type of parasites. On the other hand, parasites enjoy their life by evading the immune responses of the host by various ways to ensure their survival as well as propagation [29]. Therefore, the relationship magnifies complex ecological setting where external environments as well as in vivo conditions alter