Overview of Respiratory Diseases of Sheep and Goats
Diseases of the upper respiratory tract of sheep and goats include sinusitis caused by the larvae of Oestrus ovis, nasal foreign bodies, and nasal tumors. Clinical signs associated with sinusitis may include some or all of the following: unilateral or bilateral, serous to mucopurulent nasal discharge; decreased or lack of airflow through one or both nostrils; coughing; sneezing; and mild to severe respiratory distress. The types of nasal neoplasms reported include adenopapillomas (nasal polyps), adenomas, adenocarcinomas, lymphosarcomas (goats), and squamous cell carcinomas (sheep).
Enzootic nasal tumor is caused by an exogenous retrovirus referred to as enzootic nasal tumor virus (ENTV). It can be transmitted experimentally by tumor homogenates, which would explain the widespread occurrence of this condition within some flocks. This type of tumor generally affects mature animals (2–4 yr old), although it has been reported in animals as young as 4 mo old. The lesion may be unilateral or bilateral, resulting in either unilateral or bilateral serous, mucoid, or mucopurulent nasal discharge. Advanced unilateral tumors may cause deviation of the nasal septum, resulting in bilateral nasal discharge. Affected animals show progressive signs of dyspnea (inspiratory), including open-mouth breathing, decreased airflow as measured at the nares, dullness on percussion over the turbinates, sneezing, and head-shaking. Stridor may also be caused by compression of the larynx by enlarged retropharyngeal lymph nodes associated with abscessation of the head. Laryngeal chondritis also results in inspiratory dyspnea of varying severity. With advancing tumor growth, exophthalmos and facial deformity may occur. Metastatic spread is uncommon. Outcome depends on the tumor type, condition of the animal, and extent of the lesion, but in most commercial situations the animal is culled for animal welfare and commercial reasons. Surgical removal of a noninvasive tumor is rarely undertaken.
The most common problems associated with the pharynx are trauma and abscessation. Pharyngeal trauma usually results from overly aggressive use of equipment used to administer boluses. Injuries may result in the formation of discrete abscesses or extensive and diffuse cellulitis, both of which can interfere with swallowing and possibly lead to respiratory difficulty or distress. Bacteria commonly isolated after an incident of pharyngeal trauma include Trueperella, Pasteurella multocida, Mannheimia haemolytica, and Fusobacterium.
Laryngeal chondritis is an obstructive upper respiratory tract disease characterized by severe dyspnea most commonly encountered in meat-breed rams 18–24 mo old. Acute onset of severe respiratory distress with marked inspiratory effort and stertor is caused by edema of the arytenoid cartilages of the larynx, resulting in narrowing of the lumen. Affected sheep stand with the neck extended, head held lowered with flared nostrils, and mouth open; they are reluctant to move because of dyspnea. Delayed identification and/or inadequate duration of antibiotic therapy may result in abscess formation within the arytenoid cartilages.
The most common problem associated with the lower respiratory tract is pneumonia. Pneumonias can be caused by viruses, bacteria, or parasites. They can be acute, chronic, or progressive.
Viruses associated with acute pneumonia include parainfluenza type 3 (PI-3), adenovirus, and respiratory syncytial virus. These viral pneumonias most often affect lambs and kids.
PI-3 is an enveloped RNA virus (family Paramyxoviridae) that induces a mild interstitial pneumonia. Clinical signs may include coughing, serous nasal and/or ocular discharge, fever (104°–106°F [40°–41°C]), and an increased respiratory rate. The single PI-3 serotype for sheep that has been identified is distinct from the bovine PI-3 serotype. Infection with this virus can be confirmed by its isolation from nasal swabs from affected animals, or by comparison of acute and convalescent serum antibody levels. Treatment is usually not warranted in mildly affected animals. In severely affected animals in which secondary pathogens are suspected, antimicrobial therapy is recommended using drugs with efficacy against the most likely organisms, such as P multocida, M haemolytica, and Mycoplasma sp. There are no PI-3 vaccines specifically designed for use in small ruminants.
Chronic, progressive viral pneumonia is most common in adults and includes progressive interstitial retroviral pneumonia (in sheep, ovine progressive pneumonia or maedi [see Progressive Pneumonia in Sheep and Goats]; in goats, pneumonia induced by arthritis encephalitis virus [see Caprine Arthritis and Encephalitis]) and ovine pulmonary adenocarcinoma (see Ovine Pulmonary Adenocarcinoma), also known as jaagsiekte or the contagious lung tumor of sheep and, infrequently, of goats.
Chronic, progressive, proliferative changes in the lungs are usually associated with the lentiviruses (family Retroviridae), or so-called slow-virus infections. In both progressive pneumonia and pulmonary adenocarcinoma, the entire lung can change in a gradual process of abnormal cellular proliferation. In affected sheep, the loss of functional lung tissue results in progressive dyspnea, anorexia, and weight loss.
M haemolytica, P multocida, Mycoplasma spp, Chlamydia pneumoniae, and Salmonella spp are associated with either primary or secondary bronchopneumonia in sheep and goats. Both P multocida and M haemolytica can be cultured from the upper respiratory tract of normal sheep and goats. Not all factors predisposing to acute respiratory diseases are known, but acute viral infections in a susceptible population can alter the protective mechanisms in the respiratory tract so that certain bacteria may invade lung tissue, multiply, and cause serious disease. An initial infection with PI-3 virus may predispose an animal to infection with pathogenic M haemolytica. Also, Mycoplasma ovipneumoniae alone can cause a mild bronchopneumonia; however, it is often isolated along with M haemolytica from sheep and goats with severe pneumonia, suggesting that the Mycoplasma may predispose the lung to invasion by this organism. Additionally, introduction of new animals, high-density stocking, poor ventilation, and a sudden change to a high plane of nutrition can act as stress factors that predispose to development of pneumonia.
Caseous lymphadenitis (see Caseous Lymphadenitis of Sheep and Goats) caused by Corynebacterium pseudotuberculosis may result in abscessation of the lungs and mediastinal lymph nodes. This can result in a progressive debilitation in sheep and goats with or without obvious clinical signs of respiratory disease.
Parasitic or verminous pneumonias of sheep and goats are most commonly caused by infection with Dictyocaulus filaria, Muellerius capillaris, or Protostrongylus rufescens. (Also see Lungworm Infection.) In contrast to the acute viral and bacterial pneumonias, which result in a bronchopneumonia affecting the anterior ventral portion of the lungs, verminous pneumonia affects the margins of the diaphragmatic lung lobes. Dictyocaulus has a direct life cycle, whereas Protostrongylus and Muellerius have indirect life cycles and rely on a variety of snails and slugs to serve as intermediate hosts. Adult forms of Dictyocaulus and Protostrongylus live in bronchi but rarely cause clinical signs. Adult Muellerius live in alveoli and lung parenchymal tissue and are considered the least pathogenic of the three lungworms. Muellerius appears to cause more problems for goats than for sheep.
Diagnosis of lungworm infection requires Baermann examination of fecal material (also see Diagnostic Procedures for the Private Practice Laboratory:Internal Parasite (Endoparasite) Diagnosis in Small Animals). Treatment for lungworm infection is rarely indicated; however, it is likely that sheep with such infections will also carry other nematodes that will cause parasitic gastroenteritis and limit production.