What is your diagnosis? Bronchoalveolar lavage fluid from a pig

Abstract

A 4-month-old castrated male mini pig was presented to a veterinary teaching hospital for evaluation of frequent, dry, nonproductive coughing for at least 3 weeks. The animal and littermate share an indoor enclosure with outdoor access, including daily walks and swims in a nearby lake. No other clinical signs were noted.

Initial physical examination was unremarkable, but when sedated, occasional crackles were auscultated in the caudal lung fields. A CBC revealed a mild to moderate microcytosis [41.8 fL; reference interval (RI): 49.89–75.21¹], suspected due to young age, hyponatremia, and/or iron deficiency; hematocrit (25.6%), red blood cell count (6.11 × 10⁶/μL), and MCHC (34.7 g/dL) were considered unremarkable for the sex and species.¹ Biochemistry abnormalities included mild hyponatremia (130 mmol/L, RI 136.73–150.25¹) and hypochloremia (94 mmol/L, RI 96.24–105.94¹), secondary to recent intravenous fluid administration. A head and chest computerized tomography (CT) scan revealed generalized bronchial thickening and right middle ventral lung lobe consolidation (suggestive of inflammation, e.g., infectious bronchopneumonia) and fluid/soft tissue debris in the trachea (possibly respiratory debris/phlegm or mucosal thickening/tracheitis). Bronchoalveolar lavage (BAL) was performed, and fluid was sent for cytologic evaluation (Figure  1A–D), aerobic bacterial culture, and mycoplasma PCR testing. Fecal direct mount and flotation were also performed.

Direct, sediment, and cytospin (Figure 1A–D) preparations of BAL fluid (BALF) were lowly cellular with minimal blood on a light purple background with mucinous material. Low numbers of large (approximately 40–50-μm long), extracellular, three-dimensional, ovoid, brightly basophilic structures, some of which had a scalloped edge, were noted on the cytospin preparation (Figure 1A,B). A leukocyte differential revealed approximately 86% large mononuclear cells/macrophages, 7% small lymphocytes, 6% eosinophils, and 1% nondegenerate neutrophils. Large mononuclear cells were occasionally vacuolated and rarely contained dark blue-black pigment (presumed hemosiderin); rare multinucleated cells were seen. Mixed bacterial populations (cocci in chains, plump rods/bacilli) were noted in the background in variably sized aggregates/mats and adhered to uniform squamous epithelial cells (Figure 1C,D).

Cytologic interpretation: Possible increased eosinophil proportion with extracellular, ovoid basophilic structures (parasitic ova vs contaminants); evidence of oropharyngeal contamination.

Given the age, species, and clinical signs of the patient, we speculated that the brightly basophilic structures could represent parasitic ova (i.e., lungworm).

The fecal flotation (Figure 2A,B) and direct mount revealed moderate numbers of larvated, thick-shelled ova measuring approximately 40–50-μm long, consistent with Metastrongylus sp. These ova displayed similar staining and morphologic features as the basophilic structures in the BALF when stained with Wright–Giemsa (Figure 3A,B), and upon re-examination, the BALF structures appeared to have a faint outline of a larva (Figure 3A).

Streptococcus suis and Mycoplasma spp. were detected on aerobic bacterial culture and PCR, respectively, of BALF. The animal was prescribed antimicrobials (oxfendazole, doxycycline) and was reported to have a substantial decrease in coughing episodes after 2 weeks and clinical resolution by 5 weeks. Physical examination was unremarkable at 8 weeks; repeat diagnostics were declined at that time and owners were instructed to discontinue antimicrobials.

This animal was diagnosed with both parasitic (Metastrongylus sp.) and bacterial (Streptococcus suis, Mycoplasma sp.) respiratory infections, either of which could have been responsible for the coughing clinical presentation, and co-infections are not uncommon in this species.² Streptococcus suis infection is usually seen in nursing or recently weaned pigs but can be isolated from clinically healthy pigs as it is a normal inhabitant of the upper respiratory tract.³ Mycoplasma spp. infection is a common, widely distributed disease and can affect pigs of any age, with secondary bacterial infections a common sequela.³ Both are considered causative agents of porcine respiratory disease complex (PRDC), with Mycoplasma spp. (specifically Mycoplasma hyopneumoniae) considered a primary pathogen, and Streptococcus suis considered a secondary/opportunistic agent.³

The most unique cytologic finding in this case was parasitic ova in the BALF, and the possible increased eosinophil proportion² was likely secondary to lungworm infection versus other underlying or concurrent allergy/hypersensitivity responses. Metastrongylus spp. are lung nematodes that can infect domestic and feral pigs worldwide, causing decreased growth, generalized unthriftiness, and coughing/dyspnea.^3-5 All ages of swine are susceptible, but heavy infections typically occur in young pigs over 6 weeks old.⁵ Ova containing first-stage larvae are passed in the feces of infected pigs and then hatch in the environment, with some larvae surviving in feces or moist soil for long periods of time. After the intermediate host (earthworm) ingests the larvae, they will continue development, reaching the infective third stage in about 10 days. However, these larvae may remain quiescent in the earthworm for up to 18 months, and multiple larvae can accumulate in a single earthworm. Pigs become infected when they ingest earthworms containing third-stage larvae, which then penetrate the intestinal mucosa and migrate via lymphatics and venous blood to the lungs, where they mature and produce larvated ova about 25 days after ingestion. These larvated ova are then coughed up, swallowed, and passed in the feces, and antemortem diagnosis can be made by identifying them in a fecal flotation.^{4, 5} A Baermann fecal technique, the preferred diagnostic assay for most lungworms, is unlikely to identify this parasite as it is passed as a larvated ovum (versus a larva) but can be used to supplement a negative fecal flotation procedure. White, threadlike, 14–66-mm long adult lungworms can be identified in the bronchial tree on postmortem examination.^{4, 5} Some larvae can migrate in the liver and cause gray to white areas of scarring, like ascarid migration.

To the authors' knowledge, this is the first report of the cytologic appearance and presence of Metastrongylus spp. ova in BALF in a pig. Lungworms are often overlooked as a cause of respiratory illness in pigs that are primarily raised indoors, as the incidence of disease has decreased with the development of confinement housing.^{4, 5} This case highlights the utility and importance of airway cytology and additional infectious disease testing, including fecal analysis, in any pig that presents with coughing.

The authors declare that they have no conflict of interest.