Fundamentals of veterinary parasitology, classification of zooparasites and types of hosts

Parasitology (Greek: hanger-on, parasite; study) is a complex science that studies parasites, the diseases they cause, and methods of controlling them.

Parasitism is a historically established association of genetically heterogeneous organisms, based on food connections and mutual exchange, where one (the parasite) uses the other (the host) as a habitat and a source of nutrition, with both partners in an antagonistic relationship of varying intensity. Out of 1.5 million total animal species, approximately 60–65 thousand (4–5%) are parasites.

Parasites are the cause of numerous diseases in humans, livestock animals, game animals, wild animals, and plants. Parasites that belong to the animal kingdom are called zooparasites, and the diseases they cause are called invasive or parasitic.

Zooparasitology consists of protozoology — the science of parasitic protozoa and the diseases they cause; helminthology — which studies parasitic worms and the diseases they cause; and arachnology and entomology, which study arachnids and insects — both as pathogens and as vectors of infectious and parasitic diseases.

Types of parasites. Animal organisms that lead a parasitic lifestyle are usually divided into temporary and stationary.

Temporary parasites are organisms that complete their entire developmental cycle, from egg to adult stage, outside the host organism (they use the latter only for nutrition). These include horseflies, mosquitoes, and certain mites.

Stationary parasites infest the host for a long period (or throughout its entire life) and use it not only for nutrition but also as a habitat. They live on the surface of the body or within the host organism, where they are most often localized in internal organs, and are subdivided into permanent and periodic.

Permanent parasites (lice, itch mites, chewing lice, biting lice, etc.) live on the host or in its body throughout their entire life, completing their entire biological developmental cycle.

Periodic parasites infest the host only at a certain stage of their development. For example, botflies settle in the host organism only at the larval stage, while adult insects and pupae do not lead a parasitic lifestyle. The broad tapeworm in its tape stage settles in humans and certain carnivorous animals, its larval stages — procercoids — in cyclops, and plerocercoids — in different species of fish, while the eggs of this helminth are found in water bodies. The overwhelming majority of helminths are periodic parasites.

Characteristics of parasite hosts. A host is called a human or an animal in whose body a parasite temporarily or permanently resides and feeds. Parasites that infest one host (for example, parasitic only in a rabbit) are called monoxenous. There are parasites that require two or more different hosts to complete their life cycle. Such parasites are called polyxenous. For example, Piroplasma or Theileria parasitize in the blood of animals and in the body of tick vectors; the tape stage lives in the human intestine, while its larva lives in pigs. In this case, the change of hosts is determined by the stages of the parasite's life cycle; the larval stage parasitizes in one host, and the sexually mature stage infests another.

The host in whose body the parasite reaches the sexually mature stage is called the definitive host. In this host, the parasite reproduces sexually. The host in whose body the parasite lives in the larval stage is called the intermediate host. In its body, the parasite undergoes metamorphosis and reproduces asexually. Some parasites have several intermediate hosts. The second intermediate host is called the paratenic (or supplementary) host.

Hosts in which parasites find the best conditions for their development are obligate for them. An obligate host is characterized by the fact that the parasite is ensured the best survival, rapid growth, and maximum fecundity within it. Hosts in whose body a parasite can reside, but to which it is not fully adapted, are called facultative. In these, parasites are found rarely and usually in small numbers. For example, the broad tapeworm is adapted to the human body, in which it parasitizes for a long time and reaches large sizes. However, this cestode also parasitizes the body of foxes, but its sizes are small, and its lifespan in these carnivores does not exceed two months.

Pseudoparasitism is the ability of free-living organisms to live for some time inside the body of another animal if they accidentally enter it.

A host in which no development of the parasite occurs, but only the accumulation of its invasive stage is noted, is called a reservoir host.

As with diseases of any etiology, a distinction is made between ante-mortem and post-mortem diagnosis of helminthiasis. It is mainly based on the detection of the pathogen of the disease in the examined object: whole helminths or their fragments, eggs, or parasite larvae. In this regard, the diagnosis of helminthiasis is more reliable than, for example, for infectious and non-communicable diseases.

At the same time, it should be noted that the detection of certain helminths in livestock animals in the absence of clinical signs does not always serve as a basis for a diagnosis of the corresponding helminthiasis as a disease in the generally accepted sense of the word, which includes a specific syndrome with resulting pathogenetic consequences.

A diagnosis of a specific helminthiasis must be made comprehensively, taking into account epizootological data, symptoms of the disease, pathological anatomical changes, and special diagnostic laboratory research methods.

Intravital diagnosis and fecal sampling

Most helminthiases in livestock animals occur without pronounced clinical symptoms. A characteristic picture manifests only in some diseases: CNS disorders in coenurosis, skin hemorrhages in setariosis, as well as conjunctivitis and keratitis in thelaziasis. Symptoms are more often non-specific — digestive system dysfunction, a drop in milk yield, and overall productivity. For an accurate diagnosis, epizootological data (age, seasonality, geography, feeding and housing conditions) are taken into account and laboratory studies are conducted.

• Examination coverage — 10 percent of the livestock
• Fecal sample mass — 1020 g
• Maximum delivery time — 16–18 h
• Flotation loop diameter — 0.8 cm

The rules for collection, preservation, and shipment of material require precision. To obtain a reliable result, the following rules are observed:

• Samples weighing 1020 g are collected from the rectum with a gloved hand or taken from the top part of a freshly deposited pile that has not come into contact with the floor or soil.
• Solid samples are packed in parchment paper, while liquid samples are placed in glass jars or cellophane, indicating the sample number.
• For an objective picture, 10 percent of the livestock is examined.

Laboratory diagnostics are conducted in three main areas:

1. Macrohelminthoscopy — searching for whole helminths or their parts in feces (those discharged after deworming, as well as detached proglottids of tapeworms).
2. Helminthoovoscopy — detection of helminth eggs.
3. Helminthlarvoscopy — identification of parasite larvae.

Methods of helminthocoprological studies

In practical conditions, qualitative helminthocoprological methods are used more frequently. They are simpler than quantitative ones and allow for the rapid establishment of the presence of parasites in the body.

The simplest method is the native smear. A small piece of feces weighing 1–2 g (the size of a pea) is ground on a glass slide in 1–2 drops of a 50% glycerin solution. After removing solid particles, the specimen is examined under a microscope. The method does not provide high accuracy, therefore 2–3 smears must be examined from each animal.

To detect heavy eggs using the sedimentation method (successive sedimentation), a regulated order of actions is observed:

1. Place 5–10 g of feces in a container and dilute with water in a 1:20 ratio.
2. Stir and filter the suspension through gauze or a metal sieve.
3. Allow the solution to settle for 5 minutes, then carefully drain the top layer of liquid.
4. Repeat the filling with water and 5-minute settling 4–5 times until the supernatant liquid is completely clear.
5. Drain the supernatant layer and examine the remaining sediment under a microscope at low magnification on glass slides measuring 6–7 × 9–13 cm or in a Petri dish.

Combined methods. Flotation-sedimentation method. A fecal sample (3-5 g) is placed in a small beaker and thoroughly stirred with a saturated table salt solution (specific gravity — 1.18), allowed to settle for 15-20 min., and coarse particles floating on the surface are removed with a scoop or spoon. The supernatant liquid is suctioned off with a syringe or drained. Water is added to the sediment up to the brim and stirred. The suspension is filtered through a metal sieve or gauze into a beaker, and the filtrate is allowed to settle for 5 min. Then the surface layer is suctioned off, leaving 15-20 ml of sediment at the bottom. The sediment is stirred into a conical beaker (volume 30-40 ml, internal bottom diameter 1.5-2 cm), the suspension is allowed to settle for 5 min., the liquid is suctioned off, and the procedure is repeated. The sediment is transferred to a glass slide and examined. The method is used when testing for fascioliasis.

Preventive measures taken against invasive diseases differ significantly from those taken against infectious diseases. In the case of invasive diseases, vaccination and serotherapy have not yet found widespread practical application, and disinfestation is not used for all diseases. Quarantine and restrictions are established for a number of parasitic diseases. The basis of preventive measures against the causative agents of invasive diseases are biological and chemoprophylactic methods. Biological methods include: biothermal decontamination of manure, ploughing and reclamation of pastures, drainage of water bodies, isolation or rotation of pasture areas, and the creation of long-term cultivated pastures. In all cases, parasites and their intermediate hosts, as well as vectors, are destroyed.

For helminthiases, a complex of therapeutic and preventive measures is carried out, aimed at destroying helminths at all stages of their development in the external environment and in the organism of livestock animals.

Among general measures, an important place is occupied by adequate feeding and proper housing of livestock animals, strict compliance with veterinary and sanitary rules, improvement of natural and creation of artificial cultivated pastures, organization of hygienic watering, biological treatment of manure, examination of animals for helminthiases before their removal from the farm, and quarantine and helminthocorpological examination of all newly arriving animals and, if necessary, their deworming. Pasture prophylaxis measures include: grazing animals on dry areas of the pasture, rotation of grazing areas, and separate grazing of young and adult livestock animals. Stall housing of animals prevents many geo- and biohelminthiases. It is necessary to carry out systematic cleaning and disinfestation of premises, farm areas (exercise yards), and manure, and to control intermediate hosts of helminths by destroying them using chemical and biological methods.

Chemoprophylaxis of invasive diseases. To prevent diseases caused by ectoparasites, various preparations called insectoacaricides are used. Once applied to the skin of livestock animals, they destroy mites and parasitic insects. This prevents their appearance for a certain period of time. For the prevention of helminthic diseases, it is recommended to add specific chemical preparations to the feed of livestock animals. They destroy larvae or inhibit the development of helminths in the host organism. To prevent certain protozoan diseases, a specific preparation is injected into the blood or under the skin of livestock animals, which destroys the pathogen or limits its activity.

Treatment is the destruction of helminths in the organism of livestock animals. The effectiveness of deworming mainly depends on good, high-quality anthelmintics.

Based on their purpose and results of application, the following types of deworming are distinguished: emergency, preventive, preimaginal, and diagnostic.

Emergency deworming is carried out at any time of the year during outbreaks of clinically expressed helminthiases for the purpose of treatment and prevention of livestock animal mortality.

Preventive deworming is carried out at specific times according to a pre-developed plan, taking into account the biology of helminths and the epizootiological features of helminthiases. Livestock animals are dewormed entirely. The purpose of deworming is to rid livestock animals of helminth carriage, thereby preventing the development of clinical signs and the spread of infestation.

Preimaginal deworming is performed during the period when helminths in the organism of livestock animals have not yet reached sexual maturity and have not yet shed eggs or larvae into the external environment. This prevents the spread of infestation and the occurrence of disease in livestock animals.

Diagnostic deworming confirms a suspected helminthiasis mainly in cases where the diagnosis cannot be made by the coprological method.

In large livestock complexes, at poultry farms, and in other specialized farms, veterinary and sanitary measures must be carried out in such a way as to protect livestock animals from the introduction and spread of not only infectious but also invasive diseases. In this regard, the veterinary service of a specialized farm develops a system of measures and implements them, starting from the selection of the site for farm construction to the full completion of construction, commissioning, and during the operation of livestock premises.

The stocking of young cattle, pigs, and other livestock animals is carried out only from breeding farms assigned to the complex that are free from infectious and invasive diseases. In these farms, the following are constantly carried out:

• preventive deworming;
• deacarixation;
• disinsection of livestock animals and premises.

This is done so that only healthy young animals are transferred to the complexes.

Sanitary control, devastation, and the use of anthelmintics

A strict sanitary regime is in force on the territory of specialized livestock complexes and farms. Keeping cats and dogs on the premises is prohibited, with the exception of guard dogs, which are mandatorily vaccinated against rabies and dewormed quarterly. The maintenance staff of the complex is required to undergo regular medical examinations.

To improve the health of farms against invasive diseases, two main approaches are used — active extermination of parasites and protection of livestock animals from infection:

• Devastation (from Latin "destruction", a term proposed in 1944) — a complex of offensive therapeutic and preventive measures for the systematic liberation of humans and livestock animals from the most pathogenic helminths. With total devastation, a parasite species is completely eliminated in a certain territory, while with partial devastation, the population size is sharply reduced.
• Preservation — a system of defensive, passive prophylaxis. It protects animals and humans from contact with pathogens without exerting a direct effect on the parasite itself.

Complete recovery of the livestock or maximum reduction of the infestation level is possible only through the complex application of the helminthological triad: treatment, prophylaxis, and devastation.

Classification of parasitic worms and mechanisms of pathogenesis

Helminthology studies parasitic worms (helminths) and the diseases they cause — helminthiases. In the system of the organic world, worms are grouped into several independent types:

• flatworms (all representatives of the classes of flukes and tapeworms are parasites);
• roundworms (contain a huge number of parasitic species);
• thorny-headed worms, or acanthocephalans (all representatives lead a parasitic lifestyle);
• segmented worms, or annelids (contain individual parasitic forms).

Helminthiases dangerous to humans and animals belong to anthropozoonoses. They are divided into two groups: in the first, the human is the obligate (obligatory) host of the parasite, in the second, animals act as obligate hosts, while in humans, the worms parasitize facultatively (including larval stages of taeniids, certain species of nematodes, etc.).

Pathogenesis in helminthiases includes a complex of damages inflicted by the parasite and the host organism's response. The mechanical impact begins from the moment the worms attach to mucous membranes or tissues using suckers, hooks, and cutting plates of the buccal capsule, which causes trauma and inflammation.

Tissue damage and larval migration lead to severe pathologies:

• Organ atrophy: the development of parasites in the liver, lungs, spleen, kidneys, brain, or muscles causes the working tissue to wither. In echinococcosis, organ atrophy can be almost complete.
• Obstruction and ruptures: the accumulation of nematodes or cestodes in the intestines (for example, in ascariasis) blocks its lumen, causing rupture of the walls, internal hemorrhages, and peritonitis.
• Traumatization during migration: larvae, moving along the walls of the gastrointestinal tract, peritoneum, blood, and lymphatic vessels, tear tissues and provoke focal inflammation.

Helminths exert a pronounced toxic effect on the animal's organism. In the process of their vital activity, they release poisonous metabolic products and toxic glandular secretions that affect organs and tissues. First of all, these poisons disrupt the work of the central nervous system, which is clinically manifested by convulsions, severe agitation, or deep depression. In addition, toxicosis disrupts metabolism and leads to disorders of the respiratory, cardiovascular, and endocrine systems.

Upon the introduction of worms and their metabolites, sensitization of the organism occurs, which is why helminthiases are classified as allergic diseases. The parasite's metabolic products possess the properties of antigens and form hypersensitivity. Upon repeated contact with the same pathogen, animals develop acute allergic reactions, accompanied by severe pathological changes.

Parasitic worms often provoke inoculation and the activation of secondary infections, especially during the period of larval migration. Helminths are capable of introducing pathogenic microorganisms from the external environment directly into the host's tissues. Passing through the intestinal wall, they open the way for intestinal microflora and simultaneously activate "dormant" opportunistic infections.

Varieties of immunity in helminthiases

The immune response of the organism in helminthiases can significantly reduce the harm from the infestation. Protective reactions are expressed in the reduction of parasite survival, delay in their development, restriction of egg-laying, and a reduction in the parasite's lifespan. In practical veterinary medicine, three main forms of immunological response are distinguished:

• Absolute immunity. Complete susceptibility to infestation, in which symptoms of the disease are absent. Invasive larvae are unable to overcome the intestinal barrier and are excreted from the organism into the external environment in transit.
• Barrier immunity. Animals are infected with parasites, but their larvae are held back by protective tissue barriers — the intestinal wall, skin, liver, lymph nodes, or lungs. In these organs, the larvae encyst and die. For example, when Ascaris eggs are ingested, the larvae penetrate the liver, where some of them are blocked by the tissue barrier and die.
• Limiting immunity. Helminths overcome protective barriers, but the immune system restrains their reproduction and vital activity. This is manifested in the slowing of parasite growth, a decrease in their survival, a reduction in lifespan, and uneven maturation of the population, thanks to which animals easily endure repeated infections (re-infestation and super-infestation).