Association of pneumonia with concentrations of virulent Rhodococcus equi in fecal swabs of foals before and after intrabronchial infection with virulent R. equi

Abstract

Background: Intragastric administration of virulent Rhodococcus equi protects foals against subsequent experimental intrabronchial (IB) infection, but it is unknown whether R. equi naturally ingested by foals contributes to their susceptibility to pneumonia.

Hypothesis: Fecal concentration of virulent R. equi before IB infection with R. equi is positively associated with protection from pneumonia in foals.

Animals: Twenty-one university-owned foals.

Methods: Samples were collected from experimental studies. Five foals were gavaged with live, virulent R. equi (LVRE) at age 2 and 4 days; the remaining 16 foals were not gavaged with LVRE (controls). Fecal swabs were collected from foals at ages 28 days, immediately before IB infection. Foals were monitored for clinical signs of pneumonia, and fecal swabs were collected approximately 2 weeks after IB infection. Swabs were tested by quantitative PCR for concentration of virulent R. equi (ie, copy numbers of the virulence-associated protein A gene [vapA] per 100 ng fecal DNA).

Results: Fecal concentrations of virulent R. equi (vapA) before IB infection were significantly (P < .05) lower in control foals (25 copies/100 ng DNA [95% CI, 5 to 118 copies/100 ng DNA) that developed pneumonia (n = 8) than in healthy control foals (n = 8; 280 copies/100 ng DNA; 95% CI, 30 to 2552 copies/100 ng DNA) or those gavaged with LVRE (707 copies/100 ng DNA, 95% CI, 54 to 9207 copies/100 ng DNA).

Conclusions and clinical importance: Greater natural ingestion of LVRE might contribute to protection against pneumonia among foals.

Keywords: PCR assays; Rhodococcus; equid; neonatology; pneumonia.

FIGURE 1

Copies of vapA gene of virulent R. equi per 100 ng DNA from feces of 21 foals in 3 groups of foals infected intrabronchially (IB) with virulent R. equi at age 28 days: (1) control foals that developed pneumonia after IB infection (Pneumonia; n = 8); (2) control foals that remained healthy after IB infection (Healthy; n = 8); and (3) foals that were gavaged with live, virulent R. equi at ages 2 and 4 days (Gavaged; n = 5). The left panel (before) represents concentrations on the day of IB infection (age 28 days, before IB infection) and the right panel (after) represents samples collected after IB infection with R. equi (age 42 days for healthy and gavaged foals and collected between 12 and 19 days [median, 14 days]) for pneumonia foals). Groups with different maroon letters differ significantly (P < .05) after adjusting for multiple comparisons. Colors of dots represent different foals within treatment group. Concentrations of virulent R. equi were significantly lower before IB infection for foals in the pneumonia foal relative to the other 2 groups. Concentrations of virulent R. equi increased significantly after challenge for control foals that developed pneumonia but decreased significantly among foals in the gavaged group

FIGURE 2

Copies of vapA gene of virulent R. equi per 100 ng of fecal DNA from 21 foals at age 28 days (before intrabronchial [IB] infection) by birth‐month: JFM = January, February, or March; and AM = April or May. Dots are colored by study group: (1) triangles represent control foals that remained healthy after IB infection; (2) squares represent control foals that developed pneumonia after IB infection (n = 8); and 3) circles represent foals that were gavaged with live, virulent R. equi at ages 2 and 4 days (n = 5; none of these foals developed pneumonia). Fecal concentrations of virulent R. equi were significantly lower among foals born before April than for foals born in April or May