Pneumocystis pneumonia in a Thoroughbred colt

Kristiina Ruotsalo

Animal Health Laboratory, University of Guelph, Guelph, ON

AHL Newsletter 2021;25(3):22.

A 60-day-old Thoroughbred colt presented with a history of pneumonia, pyrexia, and swollen legs.  A complete blood count (CBC) revealed evidence of a mild left shift with mild neutrophil toxicity characterized by cytoplasmic basophilia and occasional Doehle bodies.  Inflammation was further supported by a markedly increased serum amyloid A concentration of 4721 mg/L (reference interval 0-20 mg/L).

Concentrated slide preparations of bronchoalveolar lavage fluid (BAL) from this colt were evaluated.  The slides contained scant mucus in which the majority of sample material was entrapped.  This material included numerous clusters of clear to slightly basophilic, round, variably intact cystic structures which were occasionally found to contain internal evidence of four to eight, round, deeply basophilic, peripherally located intra-cystic bodies.  These structures were consistent with Pneumocystis jirovecci (P.cariini) cysts.  Concurrent inflammation consisting of a predominance of macrophages was also identified (Fig. 1).

Figure 1. Three macrophages and associated, variably intact P.jirovecci cysts containing intra-cystic bodies (arrows). Wright’s stain.  Image credit Dr. Janet Beeler-Marfisi.

Figure 1. Three macrophages and associated, variably intact P.jirovecci cysts containing intra-cystic bodies (arrows). Wright’s stain.  Image credit Dr. Janet Beeler-Marfisi.

Pneumocystis is an opportunistic extracellular fungus that can be seen as a thin-walled trophozoite-like form with a single nucleus, as well as a thick-walled, cyst-like organism with multiple inner bodies.  The procyst is considered either a product or a subtype of the trophozoite stage.

Pneumocystis pneumonia has been documented in several species, including: humans, horses, foals, goats, pigs and dogs.  Historically, disease has been thought to be a problem primarily in immunocompromised hosts.  However, in addition to SCID (severe combined immunodeficiency) Arabian foals and foals concurrently infected with other bacterial and viral pulmonary pathogens, immunocompetent foals from 4 months to one year of age and adult horses have also been diagnosed with pneumonia due to Pneumocystis.  Recent evidence supports that Pneumocystis may be a component of the normal upper respiratory flora of immunocompetent animals, and humans are typically exposed to P.jirovecci early in childhood with primary infections ranging from asymptomatic to mild upper respiratory signs in immunocompetent individuals.

Pneumocystis infection in horses is associated with two disease conditions: fulminant acute infections, and secondary infections with pulmonary fibrosis (pneumocystosis).  Localization of Pneumocystis to the alveoli results in accumulation of proteinaceous fluid which impairs oxygenation.  In equine infections, BAL is essential for detection of fungal elements as this organism cannot be grown in culture.  PCR testing is also possible, but not widely available.

The identification of Pneumocystis infection should prompt investigation of the immune status of the horse.  Pneumocystis infections in immunocompetent humans and horses with normal numbers of CD4+ lymphocytes have been documented, but the requirement for CD4+ cells for protection against disease has also been demonstrated in murine models and in humans with acquired immunodeficiency syndrome.

Flow cytometry of this colt’s peripheral leukocytes revealed a decreased proportion of CD8+ lymphocytes relative to adult values, but CD4+ lymphocyte proportions were unremarkable.  Evaluation of serum IgG revealed a decreased value of 637 mg/dl (reference interval for adult horses 984-1685 mg/dl, and the expected value for foals having received adequate colostrum is 800 mg/dl or greater).

The colt was treated with a systemic antibiotics and an anti-inflammatory drug, and was provided supportive care.  It is clinically well three months following initial diagnosis and treatment.  It is unknown if the immunodeficiency suggested by the decreased IgG concentration was transient or persistent, as follow-up testing has not been possible to date.   AHL


Punsmann S, et al. Acute interstitial pneumonia in foals: A severe, multifactorial syndrome with lung tissue recover in surviving foals. Equine Vet J 2021;53:718-726.

Zakrzewska M, et al. Pneumocystis pneumonia: Still a serious disease in children. Dev Period Med 2019;3:159-162.

Sellon D, Long M, Kohn C. Miscellaneous fungal diseases. In: Equine Infectious Diseases, 2nd ed. Sellon D and Long M, eds. Elsevier, 2014:446-448.