Diseases (zoology)

A disease is essentially any disturbance in the structure or function of the body and may be accompanied by characteristic and well-defined areas of damage, or lesions, of specific tissues. The study of diseases is called pathology, and the causes of a disease are referred to as its etiology. An agent that can cause a disease is a pathogen. A particular disease has characteristic objective physical signs that may be seen by the examining physician or other outside observer, such as redness or swelling. Subjective feelings associated with disease, which can be described by an affected human but not by an animal or outside observer, are symptoms that might include pain or weakness. A set of signs and symptoms associated with a particular disease is called a syndrome. Diseases are often diagnosed by recognizing the signs and symptoms that characterize a particular disease and are not seen together in other diseases.

Inflammation and Immunity

Diseases related to tissue damage may be caused by pathogens such as bacteria and viruses, or by trauma, toxins, heat or cold, or exposure to chemical pollutants. When the body first responds to damage, certain cells release chemicals that produce a localized, nonspecific reaction called inflammation. Histamine causes blood vessels to dilate and become leaky, so inflamed areas show redness, swelling, pain, and heat, and may have a loss of function. Inflamed tissues also send out chemical messages that call in wandering cells (neutrophils and macrophages) from the blood and connective tissues to engulf pathogens and cellular debris by phagocytosis. This response is usually restricted to a localized area where tissue damage has occurred, and the area may be walled off from the rest of the body to restrict movement of pathogens or toxins.

The immune system involves other cells from the blood, lymph, and connective tissues called lymphocytes, which respond to specific antigens, usually small fragments of proteins that are not part of the “self” antigens the lymphocytes recognize. A bacterium may have thousands of different antigens associated with it. Fragments produced by a macrophage during processing of the bacterium after phagocytosis. Each lymphocyte can respond against only one kind of antigen, but millions of different kinds of lymphocytes in each individual produce a widespread immune response specific to each foreign antigen associated with the invading pathogens, their toxins, or other materials.

The first time the immune system encounters a foreign antigen, its primary response is slow, and a disease may result from a pathogen’s metabolic effects. Eventually, the immune response generates activated lymphocytes and antibodies that kill the bacteria or the virally infected cells to end the disease process. Memory lymphocytes are also produced that will respond against the same antigen if needed later. When the animal recovers, it will usually be immune to a second infection by the same disease-producing agent. The ability to resist a second infection is called immunological memory, and it may last for the life of the individual, as long as the memory lymphocytes live. Modern disease prevention techniques use immunizations to prevent the first experience of disease caused by a pathogen. In immunization, a derivative of the pathogen is injected into the individual to produce the slow primary response, so that memory is generated, and the individual will be immune when the same agent is encountered naturally in the environment. In some cases, a booster shot must be given regularly to maintain memory and immunity to that agent, as in repeated immunizations of pets against rabies.

In some cases, however, diseases are caused by an overreaction by the immune system and the inflammation that it helps to generate. Allergies, for example, are not directly caused by pollen or dust particles, but by the body’s responses to these allergy-producing antigens, or allergens. An allergic reaction is an immediate hypersensitivity response that may just cause an irritating, itchy swelling of the mucous membranes or skin, or may be extreme and even life-threatening. In highly allergic individuals responding to allergens, the respiratory passages close, blood vessels leak fluid into the tissues, and death can result in a hyperallergenic process called anaphylactic shock. Much more often, though, both the inflammatory and immune responses are protective, causing the destruction of invading pathogens or other foreign materials that get into the body past the barriers of the skin and mucous membranes. The extent to which the immune system protects against disease can be seen when it is not functioning, as in humans who have acquired immunodeficiency syndrome (AIDS) and die of infections or cancer that would be prevented by a fully functioning immune system. Viruses like the human immunodeficiency virus (HIV) cause related immunodeficiency diseases and leukemias or anemias in animals as well, including simian immunodeficiency virus (SIV) in monkeys and feline leukemia virus (FeLV) in cats or murine leukemia viruses (MuLVs) in mice. Studying these related viruses has been very important in scientists’ understanding of HIV.

Bacterial Diseases

Most inflammatory diseases are caused by pathogenic or infectious microorganisms. Infections occur when pathogens enter the body, replicate, and cause metabolic changes or toxic damage. Pathogens are often easily spread directly from an infected host to uninfected individuals, or may spread indirectly through air, food, water, soil, or other materials. Infectious and other disease-causing agents include bacteria, fungi, viruses, prions, protozoans, and parasitic animals.

Bacteria are unicellular, prokaryotic organisms that have much simpler cells than those of multicellular organisms such as animals, which are eukaryotic (having a nucleus). Some bacteria are pathogenic in humans and other animals. Bacteria can reside normally on the skin or in the digestive tracts of animals without harmful effects, comprising the “normal flora” of those animals. A common bacterium, Escherichia coli, is found in the colons of humans and many other animals, but different strains may cause diseases in some hosts while being harmless in others. The E. coli strain O157:H7 causes no disease in cattle but has caused deadly infections in children who ate undercooked hamburgers contaminated with fecal material from the infected cattle. Salmonella are bacilli common in and on animals such as chickens (and their eggs) and turtles but can cause food poisoning in humans. In some cases, bacteria that are normal on the skin or in the internal organs of an animal may cause diseases when their number increases abnormally or when they are transferred to other body regions.

Certain bacteria can cause disease in both animals and humans, such as the anthrax bacterium Bacillus anthracis, which may be used in weapons for germ warfare. Infectious anthrax spores can persist in soil or on contaminated wool, leather, or other animal products for many years, then can enter through skin or lungs to cause a lethal disease for animals or humans. Domesticated animals most commonly associated with anthrax include sheep, goats, cows, camels, and horses. Wild animal populations may serve as reservoirs of bacteria that can cause epidemics in both humans and animals, such as plague bacteria in modern-day prairie dogs of the American Southwest. The same bacteria, Yersinia pestis, also became a major problem in medieval Europe when the Black Plague was carried to humans by dying rats and their fleeing fleas. Diseases that can be transmitted from animals to humans are called zoonoses, and they can be from nearly any pathogen, such as bacteria, viruses, parasites, fungi, or protozoans.

Viral Diseases

Viruses are not cells, and so are not actually microorganisms, but are still infectious agents. All viruses can infect only certain cells of particular host organisms, and they must be inside host cells to replicate. Viral infections can be latent (the cell is infected but uninjured); in some latent infections, the virus is silent and inactive, while in others, new virus particles may be released into the tissue fluid surrounding the cell. In cytopathogenic infections, host cells may increase in size and number, causing tissue enlargement (hyperplasia), or the cells may die. Some latent viral infections can become activated to produce disease, cell death, and viruses that infect other cells or organisms. Activation of latent viruses occurs when the host’s immune defenses are diminished by stress or illness. Antibiotics are ineffective against viruses, whether they are free in tissue fluids or inside host cells, because the actions of antibiotics are directed against bacteria. Drugs that counteract viral infections often kill the infected cells and may cause toxic reactions in the host. Most viral infections can be handled at some level by the immune system, but in some cases, the viruses may be present in the host organism for the rest of the host’s life span.

The para-myxo virus causes a common and deadly virus called Newcastle or Doyle's disease. It is most commonly found in chickens, ducks, and other poultry, and once one chicken is infected, the deadly virus spreads quickly. A similar virus, called Marek's disease virus (MDV) or Gallid alphaherpesvirus 2, is caused by a herpesvirus and leads to paralysis in poultry birds.

Prion and Fungal Diseases

A very unusual kind of infectious pathogen is an abnormal protein called a prion, not associated with either DNA or RNA. Neurodegenerative diseases such as scrapie in sheep and bovine spongiform encephalopathy (BSE, or mad cow disease), as well as kuru and Creutzfeldt-Jakob disease in humans, are prion diseases. The abnormal protein is a refolded form of a normal brain protein, and prions cause the normal proteins to fold into the prion form in brains of infected individuals. Prions are very stable and are not destroyed by heat, light, or acid, so prions eaten in animal proteins can cause rapid or very delayed disease in the individual who ingested the prions. Animal tissues have been banned in animal feed in Europe and elsewhere, and many Europeans have stopped eating beef because of outbreaks of mad cow disease in England.

Fungi are plantlike parasites without chlorophyll, subdivided into yeasts and molds. They may cause skin infections or localized or systemic internal infections. Yeasts are single cells that are much more complex than bacteria, while molds have branching filaments (hyphae) extending into host cells to obtain nutrients. Fungi that colonize the skin (dermatophytes) may cause diseases such as mange or ringworm, while yeasts infect mucous membranes or other moist surfaces. The immune response can often control or eliminate fungal infections, but such infections can be lethal in immunocompromised hosts.

Parasites may enter the body or remain on the body surface as they obtain nutrients from a living animal host. The condition of having parasites is called infestation rather than infection, but disease generally results from the parasites’ effects on the host. Diseases caused by protozoan parasites include amebic dysentery, giardiasis, toxoplasmosis, malaria, and pneumocystis pneumonia. Important parasitic worms are roundworms, tapeworms, and flukes, which infest nearly all animals, and often have multiple host species during their complex life cycles. Worms may reside in the digestive tract, heart, liver, lungs, eyes, lymphatic system, skeletal muscle, or other organs and systems, where they can cause malnutrition, tissue damage, and death of the host.

In vector-borne diseases, blood-sucking insects such as lice and mosquitoes, or other arthropods such as ticks and mites, act as vectors that transmit protozoan parasites, bacteria, or viruses to a host animal during blood removal. Arthropod-vectored diseases with animal reservoir hosts have caused many major diseases in humans, such as malaria, yellow fever, hantavirus diseases, Lyme disease, West Nile virus, sleeping sickness, and several forms of encephalitis. All animals are subject to infestation by many parasites against which the immune system responds. The reduction of parasite load in humans and domestic animals in industrialized societies is thought to be related to the increased incidence of allergy in both humans and pets.

Genetic and Congenital Diseases

Diseases that result from genetic abnormalities may be present at birth or may not become apparent until later in life when metabolic processes fail to function because of inherited errors. Both single gene mutations and chromosomal abnormalities may occur, or diseases can be caused by the interaction of genetic predisposition and environmental factors. Congenital diseases are present at birth and can be genetic or developmental in cause. Developmental problems may be associated with nutrient deficiency, intrauterine injury, inadequate placental support for the fetus, or environmental agents such as radiation, toxins, or pathogens. Genetic problems tend to occur particularly in inbred lines of animals, such as purebred dogs, where breeding selection for desirable characteristics also inadvertently produces recessive inherited diseases such as hip dysplasia and deafness. Some congenital abnormalities are considered desirably exotic in companion animals, such as curled ears or stubby tails in cats, droopy ears in rabbits, or short legs, flattened faces, or lack of hair in dogs.

Metabolic, Neoplastic, and Degenerative Diseases

Metabolic disorders include those that are strictly genetic, and those that have a combined etiology involving inheritance and environment. Disturbances of metabolism can include changes in endocrine functions or metabolic imbalances when an enzyme is missing due to a genetic mutation. The enzyme’s substrate would then build up and cause damage, while the enzyme’s product would not be formed, also causing problems.

Neoplastic diseases are characterized by abnormal cell division and tumors, enlarged growths that may be benign or malignant. Benign tumors do not spread throughout the body but are usually enclosed in a dense connective tissue capsule. While their cells remain relatively normal aside from their unrestrained growth, benign tumors may grow to enormous size and cause death by compressing organs or blocking passageways. Malignant tumors contain many more abnormal cells, which can leave the primary tumor site and form secondary tumors, especially in the liver, bone marrow, lungs, or brain, that usually cause death in cancer conditions. Malignant cells lose their original characteristics and become much less specialized and less efficient in using nutrients and energy, causing the body to waste away.

Degenerative diseases are associated with the aging process when body tissues lose the ability to repair themselves effectively. The immune system also becomes less functional in combating foreign antigens or even recognizing the difference between self and foreign antigens, thus attacking self-antigens by mistake. In some cases, degenerative diseases are not directly linked to aging, but to damage caused by pathogens, toxins, nutrient deficiency, or even nutrient excess. As normal tissues are damaged by the standard wear and tear of life, repairs become less effective and scar tissue replaces normal tissues such as muscle or liver. Nervous system damage is particularly problematic, since neurons are unable to replicate in mature animals, and lost cells are not replaced, producing sensory reception, muscle control, and memory loss. Problems caused by diseases and trauma lead to continued loss of function over time in aging animals, eventually reaching a point where repairs can no longer be made or infections resisted, and the animal dies.

Principal Terms

Bacteria: Single-celled microorganisms that are often the cause of infectious diseases in animals

Developmental Disorders: Diseases caused by embryonic or fetal mistakes in normal development

Diseases of Aging: Loss of functions required for health due to age-related degeneration of tissues

Genetic Diseases: Disorders caused by lack of enzymes or structural proteins caused by mutations

Immune System: System that produces antibodies and cells that attack foreign substances and pathogens that invade the body

Parasites: Protozoans, fungi, or animals that survive by obtaining nourishment from a living host, from inside the host or on its surface

Prions: Infectious proteins that cause neurological diseases such as “mad cow disease”

Viruses: Noncellular infectious agents that must enter a host cell to infect it

Bibliography

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