Mesenteric lymphangitis and sepsis due to RTX toxin-producing Actinobacillus spp in 2 foals with hypothyroidism-dysmaturity syndrome.
Authors: Löhr C V, Polster U, Kuhnert P, Karger A, Rurangirwa F R, Teifke J P
Journal: Veterinary pathology
Summary
# Editorial Summary: Actinobacillus-Associated Mesenteric Lymphangitis in Dysmatric Foals Two foals presenting with congenital hypothyroidism-dysmaturity syndrome (CHDS) developed fatal mesenteric lymphangitis and sepsis caused by Actinobacillus species, prompting comprehensive genetic and toxin profiling of the causative organisms. Whilst Actinobacillus spp. are recognised commensals of the equine respiratory, digestive, and reproductive tracts, this case series demonstrates their pathogenic potential in immunocompromised neonates, with necropsy revealing thrombotic and necrotising mesenteric lymphangitis alongside characteristic CHDS features including contracted carpal flexor tendons, thyroid hyperplasia, and mandibular prognathism in one foal. Traditional biochemical and mass spectrometry identification proved insufficient; however, molecular genotyping based on RTX (repeat in toxin) toxin genes—specifically apxIA, apxIIA, and aqxA—successfully distinguished *Actinobacillus suis* sensu stricto from *A. equuli* subspecies haemolyticus, with one isolate later reclassified on comparative analysis. The findings underscore that CHDS foals warrant heightened vigilance for invasive actinobacillaemia and highlight the critical importance of toxin gene sequencing rather than phenotypic methods for accurate species identification in clinical and research settings. Practising veterinarians managing neonatal sepsis in dysmatric foals should consider actinobacilli in their diagnostic considerations, particularly when mesenteric involvement is present, and request appropriate molecular characterisation to guide treatment decisions.
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Practical Takeaways
- •Foals with hypothyroidism-dysmaturity syndrome appear predisposed to severe ASLO infections; clinicians should maintain high suspicion for bacterial sepsis in these dysmaturity cases
- •Accurate identification of Actinobacillus species requires toxin genotyping (RTX toxin genes), not just biochemical or conventional testing—request appropriate molecular diagnostics from your laboratory
- •Clinical signs of contracted tendons, thyroid abnormalities, and sepsis in neonatal foals warrant investigation for both endocrine dysmaturity and secondary bacterial infection
Key Findings
- •Two foals with congenital hypothyroidism-dysmaturity syndrome developed severe ASLO infection with mesenteric lymphangitis and sepsis
- •One isolate was identified as Actinobacillus suis sensu stricto and the other as A. equuli subsp. haemolyticus based on RTX toxin genotyping (apxIA, apxIIA, and aqxA genes)
- •3 of 3 equine ASLO isolates and 2 of 5 A. equuli isolates were reclassified as A. equuli subsp. haemolyticus, demonstrating the importance of toxin genotyping for accurate species identification