A very unusual kind of infectious pathogen is an abnormal protein called a prion, not associated with either DNAor RNA. Neurodegenerative diseases such as scrapie in sheep and bovine spon-giform encephalopathy (BSE, or mad cow disease), as well as kuru and Creutzfeld-Jacob 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 that 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, toxo-plasmosis, 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. Blood-sucking insects such as lice and mosquitoes, or other arthro pods such as ticks and mites, may 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 as well, such as malaria, yellow fever, hantavirus diseases, sleeping sickness, and several forms of encephalitis. All animals are subject to infestation by many parasites against which the immune system responds; 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.
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 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 defects 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.
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