Page 249 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

216 SECTION | II Organ Toxicity

VetBooks.ir The mucociliary clearance pathway can adapt to changing through litter use or upper respiratory tract infections
were blamed as likely causes of feline upper airway
environmental conditions, and forms a line of defense
inflammation. However, only certain cats become clini-
against a variety of airborne insults.
Upper conducting airways (bronchi, progressively cally ill, suggesting varying degrees of hypersensitivity to
smaller generations of bronchioles, and terminal bronch- stimulating antigens in cat populations (Dye, 1992).
ioles) are lined with pseudostratified columnar epithelial Cattle, especially the channel island breeds, and occasion-
cells with a few goblet cells interspersed. Their support ally sheep have a seasonal allergy that resembles this con-
tissue contains fibrous connective tissue and cartilaginous dition. Cattle are believed to be allergic to pollen
plates (Dungworth, 1993). Animals adapted for strenuous antigens. Histologically, the epithelium is hyperplastic,
physical activity have larger diameter airways to accom- eroded, and/or infiltrated with eosinophils.
modate additional airflow. As bronchioles become pro- A canine model of nasal congestion and allergic rhini-
gressively smaller, resistance to airflow increases. In tis has been described (Tiniakov et al., 2003). A very high
these small conducting airways, diseases reflecting altered prevalence of inflammatory gastrointestinal tract problems
airflow resistance are said to be diseases of small airways in brachycephalic dogs that presented with upper respira-
or small airway resistance. As the airways narrow, the tory problems was observed clinically, endoscopically
epithelium changes from pseudostratified ciliated colum- and histologically. Some histologically observed inflam-
nar cells with goblet cells to simple columnar epithelium, matory lesions were not macroscopically visible at endos-
reflecting the form, function, and stresses of these smaller copy. Respiratory and digestive signs correlated closely in
airways. The smallest bronchioles may collapse near the French bulldog males and heavy brachycephalic dogs
end of expiration, unless there is sufficient collateral cir- (Poncet et al., 2005). It is thought that systemic inflamma-
culation to keep them open. tory disease can lead to secondary pulmonary infiltration
Upper airways (bronchi and bronchioles) have bron- and inflammation (Bedenice et al., 2003).
chial associated lymphoid tissue; this tissue is similar to Moderate smoke inhalation injury to the upper airway
gut-associated lymphoid tissue (Dungworth, 1993). is usually independent of temperature damage; after as lit-
Bronchial associated lymphoid tissue is responsible for tle as 1 h, endothelin-1 in cells that line the airway
local immunity in the lung. increase (Cox et al., 2001). Such changes may contribute
to airway inflammation, mucus secretion, pulmonary
hypertension, increased airway resistance, and decreased
Toxic Diseases of the Nasal Passages
lung compliance in an ovine model of smoke inhalation
Epistaxis is a typical clinical effect associated with anti- injury (Cox et al., 2001).
coagulant pesticide toxicosis in animals (Berny, 2007).
High doses of ethylene glycol (.10 mg/kg) have pro-
THE GAS EXCHANGE REGION
duced bovine epistaxis and hemoglobinuria (Crowell
et al., 1979). The likelihood of equine exercise-induced Normal Function
pulmonary hemorrhage is increased by airborne reactive
Pulmonary parenchyma is divided into units of structure
oxygen substances (ROS), which can destroy the capillary
and function called acini (Dungworth, 1993). An acinus is
barrier and reduce vasoactive nitric oxides (Manohar
the gas exchange unit of the lung and of pulmonary
et al., 1993; Mills and Higgins, 1997). Rattle snake
parenchyma. The acinus includes all branches of respira-
envenomation is another toxic cause of epistaxis in horses
tory bronchioles, alveolar ducts and alveoli associated
(Dickinson et al., 1996). These horses have fever, tachy-
with ventilation; in addition, it contains the vasculature
cardia, cardiac arrhythmia, thrombosis, and hemorrhage.
associated with perfusion. Lobules are many acini
Since horses are often bitten on the nose and are obligate
grouped together and surrounded by connective tissue; in
nose breathers, the nostril may swell and must be kept
cattle, sheep and horses they form lobules visible at
patent.
autopsy. In other species lobules are less well outlined.
Cells most important to the gas exchange are epithelium
Toxic Diseases of the Conducting Airways (type II and type I epithelial cells). Fibroblasts provide
connective tissue and structural support for the alveolus;
Allergic rhinitis that clinically resembles hay fever in
vascular endothelial cells line the pulmonary capillaries.
humans can occur in horses, cats and dogs (Dungworth,
1993). Nasal granuloma is a chronic form of allergic rhi-
nitis, characterized by degranulation of mast cells and Ventilation
infiltration by eosinophils. Horses with chronic obstruc- Oxygen-rich inhaled air flows through the conducting air-
tive pulmonary disease have high transpulmonary pres- ways to reach the acini. Oxygen and carbon dioxide are
sures (Ammann et al., 1998). Constant exposure to dust exchanged between capillary blood and air across the