X
X symbol, Kienböck’s unit (of X-ray exposure).
xi, small letter; fourteenth letter in the Greek alphabet.
X-deletion X ovarian dysgenesis a chromosomal defect identified in mares; causes failure of germ cells to survive and the ovaries become inactive.
X disease see chlorinated naphthalenes.
X factor see hemin.
X-inactivation see the Lyon hypothesis, dosage compensation.
X-linked traits transmitted by genes on the X chromosome; sex-linked; the categories are X-linked dominant, X-linked recessive.
X-linked agammaglobinemia see Bruton’s agammaglobulinemia.
X-linked trait sex-linked, e.g. coat color in cats, hemophilia.
x radiation see radiation, X-ray.
X-ray [eks’ra] electromagnetic radiation of wavelengths ranging between 5.0 × 10−6 and 5.0 × 10−4 μm (including grenz rays). X-rays are produced by the collision of a beam of electrons with a metal target in an X-ray tube. Called also roentgen rays. The penetrability and hardness of the X-rays increases with the voltage applied to the X-ray tube, which controls the speed with which the electrons strike the target. For diagnostic radiography, tube voltages in the range 50–120 kilovolts peak (kVp) are normally used. For radiation therapy, voltages in the 1–2 megavolt range are used for most treatment. Accelerating electrons to speeds high enough to produce megavoltage X-rays requires a linear accelerator (lineac). The X-ray exposure is proportional to the tube current (milliamperage) and also to the exposure time. In diagnostic radiography, the tube voltage and current and exposure time are selected to produce a high-quality radiograph with the correct contrast and film density. In radiation therapy, these exposure factors are selected to deliver a precisely calculated radiation dose to the tumor. The total dose is usually fractionated so that tumor cells can be oxygenated as surrounding cells die; this increases the sensitivity of the cells to radiation. Body tissues and other substances are classified according to the degree to which they allow the passage of X-rays (radiolucency) or absorb X-rays (radiopacity). Gases are very radiolucent; fatty tissue is moderately radiolucent. Compounds containing high-atomic-weight elements, such as barium and iodine, are very radiopaque; bone and deposits of calcium salts are moderately radiopaque. Water; muscle, skin, blood and cartilage and other connective tissue; and cholesterol and uric acid stones have intermediate density. See also radiation and radiation therapy. A double contrast study uses both a radiopaque and a radiolucent contrast medium; for example, the walls of the stomach or intestine are coated with barium and the lumen is filled with air. The resulting radiographs clearly show the pattern of mucosal ridges.
soft x. beam made at low kVp output. The beam has low energy and weak penetration.
x. tube a glass vessel with a high vacuum and two electrodes. A very high voltage electrical current is passed across the tube and drives a stream of electrons produced by a tungsten filament set in the face of the cathode to collide with the anode and generate X-rays.
xanthemia hypercarotenemia.
xanthic [zan’thik] 1. yellow. 2. pertaining to xanthine.
xanthine [zan’thēn] a purine compound found in most bodily tissues and fluids; it is a precursor of uric acid. Xanthine compounds such as theophylline have diuretic properties.
x. alkaloids see caffeine, theobromine, theophylline.
x. calculi see xanthine urolith.
dimethyl x. theobromine.
x. oxidase key enzyme in the pathway for purine breakdown. Catalyzes the conversion of hypoxanthine to xanthine and then to uric acid. Generates hydrogen peroxide, which can be a generator of free radicals in biological systems through reactions with superoxide ions.
trimethyl x. caffeine.
xanthinuria xanthine in the urine.
Xanthium plant genus in the family Asteraceae; contain a potent toxin, carboxyatractyloside, which causes acute hepatic necrosis. Poisoning results from eating the seeds, or the cotyledons of the young seedlings that grow in profusion after rain falls. Includes X. ambrosioides, X. spinosum (Bathurst burr), X. strumarium (X. occidentale, X. pungens, X. californicum, X. cavanillesii, X. chinense, X. italicum, X. orientale, Noogoora burr, cockleburr).
xanth(o)- word element. [Gr.] yellow.
Xanthocephalum Gutierrezia.
xanthochromatic [zan″tho-kro-mat’ik] yellow-colored. xanthochromia [zan″tho-kro’me-3] yellowish discoloration of the skin or spinal fluid. Xanthochromic spinal fluid usually indicates hemorrhage into the central nervous system and is due to the presence of xanthematin.
xanthochromic [zan″tho-kro’mik] yellow-colored.
xanthochromia [zan″tho-kro’me–
] yellowish discoloration of the skin or spinal fluid. Xanthochromic spinal fluid usually indicates hemorrhage into the central nervous system and is due to the presence of xanthematin.
xanthochromic [zan″tho-kro’mik] yellow-colored.
xanthogranuloma [zan″tho-gran″u-lo’m
] a tumor having histological characteristics of both granuloma and xanthoma.
xanthoma [zan-tho’m
] a papule, nodule or plaque in the skin due to lipid deposits; the co or of a xanthoma is usually yellow, but may be brown, reddish, or cream. Microscopically, the lesions show light cells with foamy protoplasm (foam cells, xanthoma cells). They occur most commonly in White Leghorn chickens and rarely in other species. The formation of xanthomas may indicate an underlying disease, usually related to abnormal metabolism of lipids, including cholesterol. In reptiles they are associated with high cholesterol diets.
xanthomatosis [zan″tho-md-to’sis] an accumulation of excess lipids in the body due to disturbance of lipid metabolism and marked by the formation of foam cells and Touton giant cells in skin lesions. Occurs in association with diabetes mellitus in the dog and is common in humans and chickens. See also xanthoma.
xanthomatous [zan-tho’m
–t
s] pertaining to xanthoma.
xanthomegrin nephrotoxin produced by Penicillium spp. fungi.
xanthophyll [zan’tho-f
l] yellow pigment in plants.
Xanthorrhoea Australian plant genus in the family Xanthor-rhoeaceae; contains an unidentified toxin which causes normally reversible posterior incoordination (wamps) with urinary incontinence in cattle; flower spikes most toxic. Includes X. australis, X. fulva (X. hastile, X. hastilis), X. johnsonii, X. quadrangulata, X. semiplana. Called also grasstree, blackboy, yakka.
xanthosine [zan’tho-sēn] a nucleoside composed of xanthine and ribose.
x.-5′-monophosphate purine nucleotide found in many animals; intermediate ion pathway to synthesis of guanosine 5′-monophosphate.
anthosis [zan-tho’sis] yellowish discoloration; degeneration with yellowish pigmentation.
Xanthosoma American plant genus in the family Araceae; probably contains calcium oxalate raphide crystals; causes stomatitis, salivation. Includes X. atrovirens, X. sagittifolium. Called also coco-yam, elephant’s ear, taro, tannia.
xanthurenic acid [zanth″u-ren’ik] a metabolite of L-tryptophan, present in normal urine and in increased amounts in vitamin B6 deficiency.
Xe chemical symbol, xenon.
Xenarthra an order of placental mammals that includes anteaters, sloths and armadillos.
xenobiothomochelidonine one of the toxic alkaloids found in the plant Chelidonium majus.
xenobiotic [zen″o-bi-ot’ik] any substance, harmful or not, that is foreign to the animal’s biological system.
x. transformation the principal mechanism for maintaining homeostasis during exposure to small foreign molecules such as drugs and toxins; the process deals with foreign chemicals which are too small for processing by the immune system; composed of enzyme systems evolved to render xenobiotics easily excreted, mainly in the liver; enzymic reactions classified as phase I (add to or expose functional chemical groups; includes cytochrome P-450 monooxygenases) and phase II (glucuronidation, conjugation and other reactions producing a large increase in water solubility to promote excretion). Cats lack the capacity for glucuronidation, making them more susceptible to certain poisonings, e.g. acetaminophen (paracetamol).
xenobiotics foreign organic compounds not produced in metabolism.
xenodiagnosis [zen″o-di″
g-no’sis] 1. diagnosis by means of finding, in the feces of clean laboratory-reared, parasite-free arthropod vectors allowed to feed on the host suspected of being infected by a protozoan, the infective forms of the organism causing the disease; used in diagnosis of trypanosomiasis. 2. diagnosis of trichinosis by means of feeding laboratory-bred rats or mice on meat suspected of being infected with Trichinella, and then examining the animals for the parasite.
xenogeneic in transplantation biology, denoting individuals or tissues from individuals of different species and hence of disparate cell type.
xenogenesis 1. heterogenesis (1). 2. production of offspring unlike either parent.
xenogenous [zen-oj’
–n
s] caused by a foreign body, or originating outside the organism.
xenograft [zen’o-graft] a graft of tissue or organ transplanted between animals of different species; a heterograft.
Xenohaliotis californiensis a genus of intracellular bacteria in the family Rickettsiaceae that occurs in marine waters and infects the intestinal tract of abalone, causing the disease known as ’withering syndrome’. It is responsible for mortality in natural and cultured populations and is a disease notifiable to the OIE (see Table 22).
xenoimmunization development of antibodies in response to antigens derived from an individual of a different species.
xenoma a massive hypertrophic lesion caused in fish by the microsporidian protozoan parasites Nosema and Pleistophora spp. xeromyceteria dryness of the nasal mucosa. xenon (Xe) [ze’non] a chemical element, atomic number 54, atomic weight 131.30. See Table 4.
x.-133 (113Xe) a radioisotope of xenon having a half-life of 5.3 days and a principal gamma ray photon energy of 81 keV; used for pulmonary ventilation imaging.
xenoparasite [zen″o-par’
–sīt] an organism not usually parasitic on a particular species, but becomes so because of a weakened condition of the host.
xenophthalmia [zen″of-thal’me–
] inflammation caused by a foreign body in the eye.
Xenopsylla a genus of fleas, including more than 30 species, many of which transmit disease-producing microorganisms.
X. cheopis the rat flea, which transmits Pasteurella pestis, the causative organism of plague and Rickettsia typhi, the causative organism of murine typhus.
xenoreactivity the reaction of lymphocytes or antibodies with xenoantigens.
xenotransplantation the transplantation of an organ or tissue from an animal of one species to an animal of a different species. xer(o)- word element. [Gr.] dry, dryness.
xeroderma [zēr″o-der’m
] a mild form of ichthyosis; excessive dryness of the skin.
xerography [ze-rog’r
–fe] xeroradiography.
xeroma [zēr-o’m
] abnormal dryness of the conjunctiva; xerophthalmia.
xeromammography [zēr″o-m
–mog’r
–fe] xeroradiography of the mammary gland.
xerophagia the eating of dry food.
xerophthalmia [zēr″of-thal’me-
] abnormal dryness and secondary changes to the corneoconjunctival surface due to tear film deficiency; called also ophthalmoxerosis.
xeroradiography [zer″o-ra″de-og’r
–fe] the making of radiographs by a dry, totally photoelectric process, using metal plates coated with a semiconductor, such as selenium. The image produced by this process differs from conventional X-ray in that margins between tissues of varying densities are more enhanced. Hence, xeroradiography is especially beneficial in the diagnosis of mammary tumors. Structures such as tendons are also visually enhanced. It does, however, require higher doses of radiation. Called also xerography.
xerosialography [zēr″o-si″
–log’r
–fe] sialography in which the images are recorded by xerography.
xerosis [zēr-o’sis] abnormal dryness, as of the eye (xerophthalmia), skin (xeroderma) or mouth (xerostomia).
xerostomia [zēr″o-sto’me–
] dryness of the mouth from lack of the normal secretion.
xerotomography [zer″o-t
–mog’r
–fe] tomography in which the images are recorded by xeroradiography.
xi [zi, kse] fourtheenth letter in the Greek alphabet, 
or 
Xi-cleft points see accumulation points.
Ximenia americana plant in family Olacaceae; foliage of tree can cause cyanide poisoning; called also yellow plum.
xiphisternum [zif″i-stur’n
m] xiphoid process.
xiph(o)- word element. [Gr.] xiphoid process.
xiphocostal [zif″o-kos’t
l] pertaining to the xiphoid process and ribs.
xiphoid [zif’oid, zi’foid] 1. sword-shaped; ensiform. 2. xiphoid process.
x. process the pointed process of cartilage, supported by a core of bone, connected with the posterior end of the body of the sternum.
xiphoiditis [zif″oi-di’tis] inflammation of the xiphoid process.
xiphopagus [z
–fop’
–g
s] symmetrical conjoined twins united in the region of the xiphoid process.
Xiphophofus a genus of small fish that bear live young; they are popular as aquarium fish. Used extensively in research, particuar y genetics, genetic mapping and the study of melanomas. Includes platyfishes and swordtails.
XLD medium one used for isolation of salmonellae and Yersinia spp.
XO symbol for the karyotype in which there is only one sex chromosome, an X chromosome.
XO genotype in horses, pigs and cats causes infertile, usually anestrous, females; the uterus and external genitalia may be small and undeveloped.
XO ovarian dysgenesis the second female X chromosome is missing, hence the XO of the title, and the animal, most commonly a mare, has small inactive ovaries which lack germ cells, and small uteri and external genitalia.
Xoloitzcuintli a small-to medium-sized (10–50 lb) lively dog with large delicate ears held erect, a long neck and a long, thin tail. Most familiar as a hairless dog, there is also a coated variety and they both come in three sizes, toy, miniature and standard. The hairless variety has smooth skin without wrinkles, but with a few coarse hairs on the head, feet and tip of the tail. The coated variety has a short, flat coat in colors ranging from black to red or bronze. Known commonly as the Mexican hairless dog; called also Xolo.
3X rule in normal blood samples, hemoglobin × 3 = hematocrit (± 2%); a rapid check on the accuracy of laboratory results.
XX sex reversal inherited defect of sexuality occurring in goats and American Cocker spaniel puppies with an XX karyotype. They may be hermaphrodites if they also carry a Y-effect gene, e.g. the gene for polledness in goats.
XXX symbol for a chromosomal aberration found in mares and cows in which there is infertility because of ovarian hypoplasia.
XXX ovarian hypoplasia a chromosomal aberration found in mares and cows resulting in infertility caused by ovarian hypoplasia; estral cycles are long and irregular.
XX/XY chimerism see chimera.
XXXY males a cytogenetic chromosomal abnormality in bulls resembling human Klinefelter’s syndrome and causing testicular hypoplasia.
XXY symbol for a karyotype associated with hypoplasia of the testes and hermaphroditism. It is seen in male tortoiseshell cats and has also been observed in sexually normal rams. See also Y chromosome.
XY sex reversal mares chromosomal abnormality in which affected mares vary from phenotypically normal, sterile mares with normal tubular reproductive organs but inactive ovaries to mares with ovotestes and aplastic tubular organs.
xylazine [zi’id-zēn] an analgesic and sedative which has become very popular for the immobilization of adult ruminants and is also registered for use in dogs, cats, horses, deer and elk. Often used in combination with ketamine. Care is needed with its use in cattle because of the very low dose required in that species. Xylazine also causes a marked increase in plasma growth hormone concentrations.
x. stimulation test used in the diagnosis of abnormalities of pituitary function, e.g. pituitary dwarfism, and specifically deficiency of growth hormone. Plasma concentrations of growth hormone are measured before and after the intravenous administration of xylazine. In normal dogs, there is a marked increase but hypopituitary dogs fail to respond.
xylene [zi’lēn] an organic solvent used in processing histological specimens as a solvent for wax and a clearing agent. Narcotic and toxic if inhaled in high concentration.
xylidine [zi’l
–din] a compound used in blending gasoline; a potent hepatoxin.
xylitol [zi’l
–toi] a sweetener used widely by people as a sugar substitute. In dogs, cattle and goats it acts as a stimulant to release of insulin. Poisoning has occurred in dogs ingesting sugar-free chewing gum resulting in life-threatening hypoglycemia. At very high doses, it has been implicated in liver failure in dogs.
Xylocaine trademark for preparations of lidocaine, a topical and injectable anesthetic.
Xylohypha see Cladosporium.
Xylomelum a genus of Australian shrubs and trees in the family Proteaceae. They contain cyanogenetic glycosides and have potential for causing cyanide poisoning. Includes X. angustifolium (sandplain woody pear), X. pyriforme (woody pear).
Xylorrhiza North American plant genus in the family Asteraceae. All of its members are selenium indicators and usually contain sufficient selenium, in the form of selenocompounds, to cause poisoning manifested by alopecia, lameness, laminitis, hoof deformity. The recorded species are X. glabriuscula (X. parryi), X. tortifolia, X. venusta, X. villosa. All called also woody aster. Formerly classified in genus Aster.
xylose [zi’iōs] a pentose occurring in mucopolysaccharides of connective tissue and sometimes in the urine; also obtained from vegetable gum.
x. absorption test, D-x. absorption test D-xylose is absorbed primarily in the duodenum and cranial jejunum; after oral administration, determination of urine or plasma concentrations at timed intervals can be used as an indication of intestinal absorption. Used in dogs and horses, but largely superseded by other methods of assessing small intestinal absorptive capacity.
xylulose [zi’iu-iōs] a pentose sugar occurring as D-xylulose and as L-xylulose, one of the few L-sugars found in nature; it is sometimes excreted in the urine. See also pentosuria.
xysma [zis’m
] material resembling bits of membrane in stools of diarrhea.
xyster [zis’t
r] a filelike instrument used in surgery.
XY/XXY mosaic see mosaic.
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