From plants to animals: Pythium, Lagenidium, Aphanomyces, and other oomycete pathogens of animals seen through a one-health lens.

Progress in treating oomycete infections of animals has been slow. Much relevant biological and therapeutic knowledge originates from plant pathology and agricultural science, while medical and veterinary literature develops independently. This review integrates these fields to highlight key knowledge gaps and translational opportunities in animal health. Oomycetes are eukaryotic Stramenopiles, distinct from true fungi; their hyphae are diploid and coenocytic, with cellulose and glucan cell walls. They reproduce via motile zoospores and durable oospores, enabling environmental persistence. Although best known as plant pathogens, Pythium insidiosum and Lagenidium spp. infect mammals, while Aphanomyces astaci and A. invadans cause serious disease in crustaceans and fish. Infection begins when zoospores enter damaged skin or mucosa, inducing eosinophil-rich pyogranulomatous inflammation before extending into deeper tissues and vessels. Host responses are often ineffective because oomycetes skew immunity toward a non-protective Th2 pathway. Standard antifungal agents have limited efficacy due to the absence of ergosterol. Management relies on surgical resection supported by antimicrobial and immunomodulatory therapy. Prognosis is guarded in dogs but more favourable in cats and horses, with immunotherapy providing benefit, especially in equine cases. Emerging strategies include agricultural biocides such as metalaxyl-M, nanoparticle drug delivery, hyperbaric oxygen therapy and the potential use of cyazofamid, which has yet to be evaluated in animals. Diagnosis involves histopathology, culture on selective media, MALDI-TOF testing and PCR with sequencing. Oomycete infections lie at the interface of plant pathology, veterinary medicine, aquaculture, immunology and mycology. Cross-disciplinary research is essential to improve diagnostic and therapeutic options for these neglected pathogens.