Pneumocystis is an extracellular opportunistic ascomycete fungus which rarely causes disease in dogs.
This fungus causes pneumonia in immunocompromised hosts and is best known as a pathogen of human AIDS patients.
The only part of the life cycle of the organism that is known is that involving the mammalian lung, in which two main developmental stages can be identified: the trophozoite and the cyst. Cysts may be found with up to eight intracystic bodies (trophozoites) or be found ruptured and empty.
Species which are pathogenic to dogs include:
- Pneumocystis carinii
Pneumocystis carinii of the dog is thought to be genetically distinct from that found in humans and rats.
A breed predisposition has been reported in the Cavalier King Charles Spaniel, Miniature Dachshund and Yorkshire Terrier (associated with immunoglobulin deficiency) , but this disease can also occur in dogs with chronic demodicosis and canine distemper.
In the dog, most affected patients are young and often present with a febrile cough. The respiratory symptoms usually progress to the point where the dog suffers dyspnea, reduced exercise intolerance and weight loss.
In the Cavalier, IgG concentrations are significantly lower in affected dogs.
Diagnosis is difficult as hematological or biochemical parameters are usually nonspecific, although leucocytosis, a mature neutrophilia and monocytosis are common.
Lung auscultation often reveals respiratory crackles and wheezes and thoracic radiographs show diffuse interstitial and peribronchial densities throughout the lungs as well as air bronchograms. Pneumonia is not a common feature of this condition. In severe cases cor pulmonale may be present.
A definitive diagnosis requires direct visualization of Pneumocystis obtained from bronchotracheal lavage or biopsy specimens and PCR identification of Pneumocystis DNA. Several histochemical stains are useful, and diagnostic immunohistochemical kits are available.
Palliative relief may be obtained with prednisolone but specific therapy is aimed at
As a clinical disease, it often has a fatal outcome.
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