ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaienko 2005

Chapter I. VIRAL INFECTIONS AND ORGANISMAL RESPONSES TO THEM (General Concepts)

1.4. Animal Viruses

Viral infections have been identified in all animals—from Protozoans to Multicellular Organisms, including humans. Some animals are capable not only of becoming infected but also of acting as viral vectors for other organisms, even those belonging to different taxa.

One of the distinctive features of animal Viruses, much like plant viruses and in contrast to bacterial viruses, is a relatively prolonged reproduction cycle. For instance, a single reproduction cycle even in rapidly replicating animal viruses takes 5–6 hours, and in some cases several days. Furthermore, many animal viruses cause persistent infections in which the host Cell does not die, although the virus continuously replicates within it and its progeny.

The most common animal viruses. The simplest of all viruses are arguably Parvoviruses. They contain a single-stranded DNA with a Molecular Weight of 1.5 • 10⁶ Da. Their diameter is approximately 20 nm. The capsid lacks Lipids and consists of three Polypeptides, the largest of which has a molecular weight of 9 • 10⁴ Da.

Papovaviruses infect A wide variety of animals. For example, one of these viruses causes hemorrhagic enteritis and myocarditis in dogs, resulting in a 25–30% mortality rate.

The Replication of papovaviruses is controlled by the respective host cell systems. There are two main classes of papovaviruses: autonomous and defective. Autonomous viruses utilize the Enzymes of the host cell—specifically those whose DNA is already undergoing replication—for METABOLISM/31.html">Transcription, replication, and other Functions, meaning they infect Cells currently in the S-phase of the Cell Cycle.

Defective papovaviruses reproduce independently of The Cell cycle phase in cells co-infected with an adenovirus, which supplies A number of functions and acts as a "helper" during parvovirus synthesis. The Genome of these viruses consists of single-stranded, mutually complementary DNA molecules.

Papovaviruses derive their name from three viruses: rabbit papilloma virus (pa), polyoma virus (po), and vacuolating (va) simian virus. They exhibit high host Specificity. Infection with these viruses can induce cell transformation and tumor formation in certain mammals, though not in humans. An example

of such viruses is the rabbit polyoma virus, which induces epithelial-derived tumors in various tissues. Three papovaviruses are widely prevalent in humans: JC, BK, and the wart virus. The JC virus causes a progressive degenerative disease of the Central Nervous system; the BK virus has not yet been linked to a specific pathology and is detected in the urine of immunosuppressed individuals; the human wart virus, much like animal papillomaviruses, causes benign epidermal proliferation.

Adenoviruses contain significantly more DNA than papovaviruses, yet they share similar mechanisms controlling molecular synthesis, with their mRNA being transcribed from both strands of viral DNA. At the same time, adenoviral DNA is linear, and its recombination frequency is higher than that of papovaviruses.

Adenoviruses have been detected in various animals. In humans, 31 serological types of adenoviruses have been identified. They cause acute respiratory infections, gastrointestinal disorders, and Conjunctivitis. Some strains induce malignant tumors in rodents, though not in humans.

Herpesviruses are among the most widespread. The primary factor determining the epidemiological and epizootiological characteristics of herpesviruses is their ability to persist in a latent form within cells for extended periods. Disease symptoms are triggered only upon viral reactivation. In mammals, reactivation can be induced by various factors (stress, hormonal imbalances, inflammatory processes, radiation, etc.) and is presumably associated with the suppression of cellular Immunity.

Herpesviruses are remarkably diverse in their reproduction patterns, although they are morphologically and genetically similar in terms of DNA content. Among them are strains that cause lysis of infected cells, as well as those that do not induce lysis yet lead to rhinotracheitis, rhinopneumonia, malignancies—such as lymphomas and carcinomas—and other pathologies.

All stages of poxvirus replication take place in the Cytoplasm, unlike other Introduction/6.html">DNA-containing viruses whose DNA is synthesized in the host Cell Nucleus. Poxviruses bear a closer resemblance to RNA viruses than to DNA viruses whose replication occurs in the cell nucleus. Much like certain RNA viruses, their reproduction begins with the transcription of viral DNA by its own RNA polymerase. It contains all the enzymes necessary to convert precursor RNA into functionally active mRNAs. Numerous poxviruses are known, one of the most notable being the smallpox virus.

Retroviruses contain RNA as well as an RNA-dependent DNA polymerase (Reverse Transcriptase). A wide variety of retroviruses have been identified. Some of them cause fever, ulcerative lesions of the oral and nasal mucosa, diarrhea, muscular degeneration, and other ailments. These viruses are also capable of inducing malignant tumors in certain animals. Examples include the Rous Sarcoma virus and viruses responsible for leukemia in cattle, chickens, cats, mice, and other animals. Like other viruses, Different types of retroviruses vary in size, morphological characteristics, and the range of susceptible host cells.

Togaviruses cause a range of diseases. Some affect the mucous membranes of the gastrointestinal tract, leading to diarrhea, while others, belonging to the alpha-type, induce encephalitis.

Coronaviruses affect many mammals and are particularly hazardous to pigs and cattle. Infection of newborn piglets can result in a 100% mortality rate during acute disease courses. These viruses serve as the etiological agents of acute respiratory infections, diarrhea, hepatitis, nephritis, and encephalitis.

Paramyxoviruses cause respiratory diseases, parainfluenza, and distemper in mammals. Distemper caused by these viruses is particularly dangerous for cattle, accompanied by a 90–100% mortality rate.

Orthomyxoviruses cause an illness known as Influenza in mammals, including humans. This is an acute, rapidly spreading disease characterized by damage to the respiratory, nervous, and cardiovascular systems, among others.

Among the diseases caused by Rhabdoviruses, the most perilous are rabies and ephemeral fever. Rabies is an acute mammalian disease, affecting humans as well, characterized by severe central nervous system impairment. Bovine ephemeral fever triggers a sharp rise in body Temperature, inflammation of the cranial mucous membranes, and muscular damage.

Picornaviruses cause numerous diseases, including hepatitis and encephalomyelitis. One notable disease caused by these viruses affecting artiodactyls, particularly cattle, is FOOT-and-Mouth disease. This condition is marked by The formation of erosions on the mucous membranes of the digestive tract and Skin. Swine vesicular disease resembles foot-and-mouth disease in cattle, but it specifically targets pigs.

The aforementioned list of common animal viruses by no means exhausts all known and investigated viruses. It is worth noting that no single disease manifestation is caused exclusively by a single type of virus. Furthermore, no group of viruses targets exclusively one specific tissue or organ. For instance, Upper Respiratory Tract infections can be caused by picornaviruses, adenoviruses, myxoviruses, reoviruses, and others. Liver diseases are induced by togaviruses (yellow fever virus), hepatitis viruses, and others. The Nervous System is affected by togaviruses (encephalitis agents), rhabdoviruses (rabies virus), picornaviruses (poliovirus), and others. Viral diseases presenting with skin rashes are caused by poxviruses (smallpox), paramyxoviruses (measles), herpesviruses (chickenpox), and so on.

As previously mentioned, oncogenic DNA viruses include papovaviruses, adenoviruses, and herpesviruses, while retroviruses represent the oncogenic RNA viruses. They drive The Development of a wide array of tumors. In most cases, these viruses induce cellular transformation. The primary manifestations of their activity include altered growth regulation, extended lifespan, and modified Structure/108.html">Surface Properties. Transformed cells fail to enter a quiescent state; instead, they undergo continuous division, proliferate chaotically, and form multilayered, amorphous masses.

Some Oncogenic Viruses, in particular retroviruses that cause leukemias, replicate in host cells without transforming them.

Animal Iridoviruses. Iridoviruses (family Iridoviridae) get their name from the Greek word *iridos*, meaning rainbow. The sediment of particles of this virus exhibits an iridescent hue, which is also characteristic of certain insects whose bodies display a similar rainbow sheen. These viruses feature an icosahedral structure and are DNA-containing cytoplasmic viruses. Iridoviruses have been detected in a wide variety of animals—protozoans, insects, amphibians, fish, and mammals—and have also been identified in Higher Plants and Algae.

By size, iridoviruses are divided into two groups: small, with a diameter not exceeding 130 nm, and large, with a diameter exceeding 130 nm. The largest is likely the fish lymphocystis disease iridovirus, the virions of which reach up to 3000 nm in diameter.

Let us examine some of the iridoviruses that can be harbored by various animals.

Insect iridoviruses are typically designated by type along with a sequential number. For example, the iridovirus of the marsh crane fly *Tipula paludosa*, previously designated as TIV (Tipula iridescent virus), is referred to in this Classification as iridescent virus type 1. Accordingly, viruses of other mosquitoes—MTV (Mosquito iridescent virus)—include several types: 3, 4, 5, 11, 12, 14, and 15; the blackfly *Odagmia ornata*—type 7; biting midges *Culicoides* sp.—type 8; beetles of the family Scarabaeidae, specifically *Sericesthis pruinosa*—type 2, *Costelytra zealandica*—type 16, and others; hymenopterans of the family Apidae—type 24; moths of the family Hepialidae—type 9; owlet moths of the family Noctuidae—type 21; leafhoppers of the family Cicadellidae—type 17, and so on.

Mosquitoes constitute one of the most numerous groups of insects. Blood-sucking mosquitoes act as vectors for various animal and human diseases transmitted during blood feeding. Some non-blood-sucking mosquitoes, notably the larvae of the marsh crane fly (*Tipula paludosa*), feed on plant roots.

The primary site of replication for mosquito iridoviruses is the fat body, which disintegrates toward the end of viral replication. Along with hemolymph, a pale, iridescent fluid containing iridoviruses leaks from the mosquito larvae. In addition to the fat body, these viruses also infect hypodermal cells, particularly the tracheal epithelium, which contains modified hypodermal tissue. Virus replication presumably does not occur in the Midgut and Hindgut of insects, yet iridoviruses are present in the adjacent esophageal tissue. Large quantities of these viruses have also been detected in the Ovaries of adult mosquitoes, leading to infected offspring. Certain hemocytes apparently become infected upon engulfing cellular debris containing iridoviruses. Muscle and nerve tissues of insects are also infected to a minor degree. Overall, iridoviruses account for 15% of the total body mass of an infected insect, and sometimes even more.

An analysis of The chemical composition of insect iridoviruses indicates that they contain 80–85% protein, 12–14% DNA, and 3–5% lipids (primarily Phospholipids).

Iridoviruses of blackfly larvae, specifically *Odagmia ornata*, have a diameter of 160 nm and infect the fat body, the Connective Tissue of nerve ganglia, the hypodermis, oenocytes, tracheal cells, and abdominal Muscle Tissues. Vacuoles form in the cytoplasm of infected cells, and the Endoplasmic reticulum and Mitochondria are disrupted.

Larvae of phantom midges (Chaoboridae) and non-biting midges (Chironomidae), which are dipteran insects, are infected by iridoviruses with diameters of 127 nm and 165 nm, respectively. These viruses replicate in the cytoplasm of fat body cells, the hypodermis, and the Trachea. An iridovirus with a diameter of 125–135 nm has been identified in *Culicoides* sp. larvae.

Large accumulations of iridoviruses have been found in the fat body and subpharyngeal glands of bees (*Apis cerana*), although no signs of disease have been observed in these bees. Iridoviruses with a diameter of approximately 130 nm have been detected in the leafhopper *Pterostrictus madidus* and the giant Water bug *Lethocerus columbiae*.

Iridoviruses with a diameter of 130–140 nm have also been found in beetle larvae (*Sericesthis pruinosa*, *Costelytra zealandica*, *Opogonia* sp., etc.). Some of these replicate primarily in muscle cells rather than the fat body.

Caterpillars of the rice stem borer (*Chilo suppressalis*), the corn earworm (*Heliothis zea*), the cotton bollworm (*Heliothis armigera*), and other lepidopterans are infected by iridoviruses 130–140 nm in diameter.

An iridovirus with a diameter of 200 nm has been observed in chytridiaceous Fungi of the genus *Aphelidium* parasitizing the green alga *Scenedesmus armatus*. It is possible that this virus is native to the alga and enters the fungi during feeding.

Iridoviruses with a diameter of 140 nm have also been discovered in daphniids, specifically *Simocephalus expinosus*. They replicate primarily in the fat body, Nervous Tissue, and hypodermis. These viruses ($d = 80$ nm) have additionally been found in polychaetes, specifically *Nereis diversicolor*, where they exclusively infect spermatocytes, thereby causing male sterility.

Iridoviruses with diameters of 122–136 nm have been noted in the shrimp *Protrachypene precipua*, and with diameters of 170–180 nm in the crab *Macropipus depurator*.

Mollusks are also susceptible to iridoid infections. For instance, iridoviruses 145 nm in diameter have been found in the Cells of the hemolymphatic sinus of the proximal Kidney and in amoebocytes of the snail *Lymnaea truncatula*. In the infected mantle and gill cells of the Portuguese oyster (*Crassostrea angulata*), iridoviruses 230 nm in diameter have been found, causing a significant enlargement of the affected cells. Iridoviruses of the common octopus (*Octopus vulgaris*) have a diameter of 100–140 nm and induce tumors in the tentacle Muscles and adjacent tissues.

Among amphibians, iridoviruses have been observed in frogs (*Rana pipiens*, *Rana catesbeiana*, etc.), salamanders, and newts. For example, the frog iridovirus (135 nm in diameter) infects only tadpoles and young animals, whereas adults are susceptible solely to high doses of the virus. Infection primarily affects muscle tissues. Hemorrhagic changes also occur in various Organs, accompanied by the aforementioned degenerative alterations in Muscle tissue and Necrosis of the liver, Kidneys, and digestive tract. In its primary host—adult leopard frogs (*Rana pipiens*)—the frog iridovirus does not cause disease. In newts, specifically *Triturus dorsalis*, an iridovirus has been identified that establishes a persistent infection wherein the virus replicates within a seemingly healthy Organism and is shed into the environment.

Unlike newts, adult salamanders are susceptible to the iridovirus, which can be detected in the liver, kidneys, and Spleen, though it does not cause overt disease.

In over 100 species of fish, iridoviruses cause lymphocystis, a chronic disease affecting the skin and fins. The virus responsible for this condition is designated as FLDV (Fish lymphocystis disease virus). Upon infection with this virus, certain skin cells enlarge significantly. As a result, tumor-like nodules resembling wart clusters develop on the body surface, particularly on the fins. Similar formations occasionally arise within the body cavity and Internal Organs, though less frequently than On the surface. Over time (7–9 months post-infection), the giant cells (up to 2 mm in diameter) that make up these tumor nodules rupture, the affected skin scars over, and the sick fish survives while continuing to serve as a source of infection. The dimensions of the fish lymphocystis virus vary widely: their diameter is about 200 nm in the dab, and 300–380 nm in the sauger.

Gill necrosis, caused by an iridovirus 200–220 nm in diameter, has been detected in cyprinid fish. Sick fish become lethargic, lag behind in growth, and their gills become covered with a whitish coating. Later, hemorrhages and necrotic areas appear on the gills and may slough off. The kidneys of infected fish swell, the spleen enlarges, the liver becomes anemic, hemorrhages occur on the Pericardium and Meninges, ammonia accumulates in the blood, the leukocyte count increases, and the levels of erythrocytes and Hemoglobin decrease.

African swine fever is caused by an iridovirus with a diameter of 175–215 nm. The disease is characterized by high fever, damage to cells of the reticuloendothelial system, inflammatory processes, and dystrophic and necrotic changes in internal organs.

Under natural conditions, the host range of the African swine fever virus is limited to pigs. In Africa, it has been detected in warthogs (*Phacochaerus* sp.), bushpigs (*Potamochoerus* sp.), and giant forest hogs (*Hylochoerus* sp.). This virus also infects domestic pigs. In European wild boars and domestic pigs, the disease takes a severe form with a high mortality rate. The virus has additionally been found in hippopotamuses, hyenas, and porcupines.

Arthropods, particularly the ticks *Ornithodoros erraticus* and *Ornithodoros moubata*, are capable of retaining the African swine fever iridovirus in their bodies for 6–12 months. It is hypothesized that this virus may replicate within the tick organism.

Densonucleosis virus. Blood-sucking mosquitoes act as vectors for the causative agents of dangerous animal and human diseases, including malaria, tularemia, wuchereriasis, piroplasmosis, theileriosis, adenovirus infections, and many others.

One of the promising approaches to controlling blood-sucking mosquitoes is infecting them with densovirus. This disease is also known as densonucleosis (or "dense-nuclei disease"). It was first discovered in the greater wax moth, Galleria mellonella. The causative agents are densoviruses belonging to the genus Densovirus of the family Parvoviridae. According to the updated classification, the Aedes aegypti densovirus, along with 14 other representatives, has been assigned to the genus Contravirus of the family Parvoviridae, where it serves as the type species. The Bombyx mori densovirus has been placed in the genus Dependovirus of the same family. Aside from the wax moth, these viruses have been isolated from various Representatives of the orders Lepidoptera, Coleoptera, Diptera, Orthoptera, Blattoptera, and Odonata.

Insect densoviruses are extremely small, with a diameter of about 22 nm. They contain linear DNA with a molecular weight ranging from 1.5 • 10⁶ to 2.2 • 10⁶ Da.

When larvae of the greater wax moth Galleria mellonella L. are infected with densovirus, the majority of them perish. Those that survive experience a delayed molt into the pupal stage. Most of these survivors also die before completing metamorphosis into adults. The high mortality rate among pupae results from active viral replication within their tissues.

In Aedes aegypti mosquitoes infected with densovirus during the larval stage, the disease manifests across all developmental phases. The larvae become sluggish, twitch when stimulated, and exhibit body deformation. In some individuals that manage to complete metamorphosis, the disease persists into the adult stage, though most mosquitoes die before they can even take flight from the water surface.

As noted earlier, densonucleosis—the disease caused by densoviruses—got its name due to the enlargement and hypertrophy of cell nuclei in the tissues where the virus replicates, primarily in the cells of the fat body. Nuclei increase in size by 6 to 10 times, lose their Chromatin and normal structure, and become packed with viral masses that occupy up to 3/4 of their volume. While most larval densoviruses are concentrated in the fat body, they have also been found in smaller quantities in the hypodermis, hemocytes, oenocytes, muscles, pharyngeal cells, Gonads, nervous system, hindgut, and molting glands. In pupae, densoviruses are also present in most persisting larval tissues, namely the fat body, larval musculature, peritracheal cells, and nerve ganglia. Furthermore, they are detected in developing adult (imago) tissues, such as adult musculature, the hypodermis, all sections of the gut, and the Salivary Glands. In surviving adults, the viruses additionally replicate in the cells of the Malpighian tubules, gonads, and accessory glands.

The fat body suffers the most intense damage across all developmental phases; in larvae, the peritracheal cells, foregut, and imaginal discs are affected to a somewhat lesser extent; in pupae, the adult musculature is primarily damaged; and in adults, the hindgut and gonads are also affected. A notable exception is the Bombyx mori virus, which replicates exclusively in the columnar epithelial cells of the larval midgut.

One of the transmission routes for densoviruses is alimentary, occurring when healthy insects ingest the virus with food contaminated by feces and other excretions from infected individuals. Another pathway involves infected insects completing metamorphosis and passing the virus vertically to their progeny via the eggs. In this case, the virus may adhere to the surfaces of eggs and newly hatched larvae, infecting the organisms at birth through contaminated Contact surfaces.

It has been established that the densonucleosis virus of blood-sucking mosquitoes, specifically Aedes aegypti, does not infect dipterans (Musca domestica, Phormia regina), lepidopterans (Lymantria dispar, Malacosoma neustria, etc.), honeybees (Apis mellifera), crustaceans (Daphnia sp., Cyclops sp.), or earthworms (Lumbricus sp.). Research has also shown that these viruses generally do not infect vertebrates (fish, amphibians, birds, mammals) or humans. Consequently, densoviruses specific to blood-sucking mosquitoes cause disease exclusively in these insects while leaving other invertebrates and vertebrates unharmed. This makes them highly promising for Practical Application as biocontrol agents against mosquitoes. To reduce populations of blood-sucking mosquitoes of the genera Aedes, Culex, and Culiseta, the Faculty of Biology at Taras Shevchenko National University of Kyiv developed the biopesticide "Viroden", which relies on the Aedes aegypti densonucleosis virus as its active ingredient. Field and laboratory trials have demonstrated that pre-imaginal mortality among blood-sucking mosquitoes reached 43–73%.