To infect is to introduce a viable microorganism—bacterium, virus, fungus, or parasite—into a host in sufficient quantity that it establishes itself, replicates, and produces a pathological response. The process begins with entry: respiratory pathogens such as equine influenza virus and Streptococcus equi (strangles) enter via inhalation of aerosolized droplets; gastrointestinal organisms such as Salmonella spp. and internal parasites are acquired by ingesting contaminated feed, water, or forage; skin pathogens including dermatophytes (ringworm) gain entry through abrasions or maceration of wet skin. Wound established infectious disease occurs when environmental or commensal bacteria contaminate a penetrating injury before the tissue can be cleansed and treated.
Whether infection proceeds to clinical disease depends on the balance between the pathogen's virulence and dose and the host's host immune competence competence. A horse with robust adaptive immunity from prior exposure or vaccination may harbor a pathogen briefly and eliminate it without showing signs—a subclinical or inapparent infection. Stressed, immunocompromised, or naive horses are more likely to develop overt disease because their innate and adaptive defenses are slower to contain the replicating organism. The incubation period—the interval between initial infection and the appearance of clinical signs—varies from hours (endotoxemia) to weeks (equine herpesvirus neurological disease) depending on the organism, route, and dose.
Management practices aimed at preventing infection focus on reducing both pathogen load in the environment and the routes by which organisms reach susceptible tissues. Hygienic stall management, disinfection of shared equipment, quarantine of newly arriving horses for twenty-one days, and current vaccination records all reduce the probability that a pathogen encounters a naive host at a sufficient dose to infect it. When inflammatory signs indicating an established infection are observed, veterinary diagnosis should precede antibiotic use to confirm that the organism is bacterial and susceptible to the chosen drug, because antifungal, antiparasitic, and antiviral agents each target different biological mechanisms.
Further Reading
- infection (Wikipedia)
- Further reading on the transmission and management of equine infections: overview of equine infectious diseases — MSD Veterinary Manual