The pneumonias of small mammals (Proceedings)

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The pneumonias of small mammals (Proceedings)

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Aug 01, 2009

Pneumonia is a common presenting complaint in exotic companion mammals. Although there are some species that present with pneumonia more often than others, there are no reported breed or gender predilections to it. Pneumonia usually occurs in young sick, debilitated, or immunodeficient animals when natural defense mechanisms have been eroded. It can be precipitated by stress such as shipment, overcrowding, or social conflict among cage mates. Improper environment and husbandry has a significant impact on an individual's resistance to pneumonia. Extremes in temperature, humidity, exposure to waste, and poor nutrition all tend to increase one's susceptibility to it. Likewise, the stress of concurrent infection, advanced age, dental disease, or other concurrent illness can lead to breakdown of the immune system and onset of pneumonia.

Clinical signs

Patients with pneumonia may exhibit haunched posture, unkempt appearance, lethargy, unfocussed eyes, disinterest in the surroundings, reduced appetite, weight loss, and/or diarrhea. Other clinical signs might include coughing, wheezing, or rapid respirations. Increased respiratory effort is usually manifested as pronounced abdominal movement when breathing. Discharge from the eyes or nose, and/or diarrhea may be present. A complete physical examination may reveal ocular or nasal discharge (found either on the face or the medial aspect of the front feet) and/or wheezing, and crackles, increased bronchovesicular sounds, or rhales may be ausculted. There may be tachypnea or overt dyspnea, and possibly a fever.

Diagnosis

Diagnosis of pneumonia in small mammals begins with thorough history and careful observation. The clinician should ask about diet, nutritional supplementation, the type of cage, bedding material, how often cage is cleaned, the presence of other animals, and any new animals that may have been introduced. Also ask about the routine and whether or not any changes to the routine (e.g. pet sitter, new diet, death of bonded cage mate) have occurred recently.

Diagnostic workup for pneumonia generally begins with thoracic radiographs. In a normal small mammal the caudal lung lobes are large and well aerated. Evaluation of the cranial lung lobes may difficult because in some species (e.g. guinea pigs) they are small. The classic radiographic appearance of bacterial pneumonia is an alveolar pattern, with air bronchograms in severe cases. Lesions can be diffuse, localized to a general area, or lobar in nature. If solitary masses are identified, differential diagnosis should include abscessation or consolidation of lung due to bacterial infection.

When pneumonia is suspected, culture and sensitivity testing should be done before antimicrobial therapy is started. Since small exotic mammals often present in severe respiratory distress, nasal swabs are often preferred because they can usually be obtained with minimal handling of the patient. However, nasal passages are prone to bacterial contamination from the environment, thus nasal cultures may not accurately represent tracheal and pulmonary flora. A tracheal wash is generally considered to be more accurate than a nasal swab for diagnosing pneumonia because it can provide material for cytologic exam and bacterial culture. Identification of degenerative neutrophils containing bacterial debris in a tracheal wash specimen is highly supportive of the diagnosis of bacterial pneumonia. While not a widespread practice, transthoracic needle aspiration is another way to obtain lung samples for cytology and culture.

Culture and sensitivity should test for both aerobic and anaerobic organisms, and empirical treatment with broad-spectrum antibiotics should start before results are available. In general, light growths of mixed bacterial populations are less important than the heavy growth of a single species in conjunction with a pathogenic response. Caution must be used when interpreting results; some labs will not report normal flora. However, in an immunocompromised individual, infections may result from normal flora that are opportunistic pathogens. In addition, the normal flora of many species still have not been established or are obscure. If culture can't be done because antibiotics have already been started, then PCR should be considered. Bacterial PCR can also be helpful in identifying anaerobes (because these frequently do not survive the transport to the lab) and for getting results from an area where bacteria are likely to be dead (e.g. abscess, caseous pus). Where Chlamydophila is suspected (i.e. guinea pigs), a conjunctival scraping from affected individuals by will contain intracytoplasmic, coccoid, basophilic organisms (i.e. Chlamydia elementary and reticulate bodies), or a PCR test is available for this infection, as well.

Necropsy findings from small mammals with pneumonia will vary by duration and severity of the disease, and by the organism(s) involved. Mild and acute cases will result in lung congestion, atelectasis. More severe and chronic cases develop supperative lesions, fibrin adhesions and fibrosis. Severely affected individuals can develop pulmonary abscesses, granulomas, and consolidation. There may also be other organ involvement: lymphadentitis, myocarditis, peritonitis, meningitis, septicemia, tracheitis, bronchitis, otitis media, etc.

There are a number of non-infections conditions that can cause respiratory distress in exotic companion mammals including heat stress, diaphragmatic hernia, pregnancy toxemia, and gastric torsion. Dyspnea and weakness may also be found with heart failure or pulmonary neoplasia.