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Anhydrosis (also spelled anhidrosis) is the failure of a horse to sweat normally, either partially or completely, despite heat load and exercise. Because evaporative sweating is a horse’s primary heat dissipation mechanism, anhydrosis is a serious thermoregulatory disorder that can cause dangerous hyperthermia during work in warm weather.

The condition most commonly develops in horses moved from cooler climates to hot, humid environments, Florida, the Gulf Coast, and tropical regions see the highest prevalence. The working theory is that chronic overstimulation of sweat gland beta-adrenergic receptors leads to receptor downregulation, eventually causing the glands to stop responding to epinephrine. Once a horse becomes anhidrotic it may not recover fully even when moved to a cooler climate, though partial recovery occurs in some cases.

Signs include a dry, rough coat after work that should have produced sweat, elevated rectal temperature (above 103°F) following moderate exercise, rapid breathing as the horse compensates by panting, and reduced performance. Diagnosis is clinical, sometimes confirmed by intradermal terbutaline testing. Management focuses on reducing heat load: working only in the early morning or evening, providing access to shade and cool water, and in some cases relocating to a cooler climate. One supplement, One AC (a beer-based supplement), has anecdotal support and some veterinary backing, though evidence remains limited. Any horse that stops sweating in hot conditions warrants veterinary attention; year-round hydration management is critical for affected horses.

Further Reading

• Anhidrosis: Wikipedia article on the condition of impaired or absent sweating and its physiological basis.
• Anhidrosis in athletic horses: peer-reviewed review of thermoregulatory failure in performance horses in hot climates (PubMed Central).

A larva is the immature, post-embryonic stage of an insect that has hatched from an egg but has not yet undergone metamorphosis into the pupal or adult form. In entomological terms, larvae are the feeding stage of holometabolous insects, those undergoing complete metamorphosis, and differ from the adult in body form, diet, and habitat. In the context of equine health, the larval stages of several parasites are clinically significant because they are the forms responsible for tissue migration, organ damage, and the pathological effects associated with parasitic infections in horses.

Botfly larvae (genus Gasterophilus) are among the most visible equine parasites in larval form. The adult botfly deposits eggs on the horse’s coat, particularly on the legs and belly; the horse ingests the eggs during grooming, and the larvae migrate through oral and gastric tissues before attaching to the stomach lining, where they develop over winter. Strongyle larvae, both large and small strongyles, are ingested during grazing, migrate through intestinal and mesenteric arterial tissue causing vascular damage, and undergo developmental arrest (hypobiosis) as encysted larvae in the gut wall, making them resistant to some anthelmintic drugs. This is a central reason why targeted deworming based on fecal egg counts is now the preferred management protocol over calendar-based rotation.

The larvae of horse lice (nits attached to hair shafts) and mange mites also affect horses, though mites are arachnids rather than insects and their immature stages are technically nymphs rather than larvae. Mucking out and harrowing managed pasture disrupts larval survival by exposing strongyle larvae on the ground to UV and desiccation, reducing pasture infectivity as part of an integrated parasite control program.

Further Reading

• larva (Wikipedia)
• For the most economically significant larval parasite of horses: bot fly larvae in horses (Wikipedia): Wikipedia

A parasite is any organism that lives on or within a host organism and derives nutrients or shelter at the host’s expense, typically causing some degree of harm. Equine parasites are divided into ectoparasites (living on the body surface) and endoparasites (living internally). The most clinically significant internal parasites of horses are strongyles (large and small), ascarids (Parascaris equorum), tapeworms (Anoplocephala perfoliata), pinworms (Oxyuris equi), and bots (Gasterophilus spp. larvae). Large strongyles, particularly Strongylus vulgaris, were historically the leading cause of parasite-associated colic through larval migration in mesenteric arteries, but their prevalence has declined substantially in populations with strategic deworming programs. Small strongyles (cyathostomins) have emerged as the primary clinical concern because of their ability to encyst in the intestinal wall and develop resistance to benzimidazole anthelmintics. Fecal egg counts (FECs) and fecal egg count reduction tests (FECRTs) guide targeted selective treatment, replacing calendar-based deworming. Ectoparasites of importance include skin-burrowing ectoparasites, ticks, and lice, each capable of causing dermatitis, anemia, or acting as vectors for other pathogens. Regular fecal-egg-count-guided anthelmintic strategy based on fecal egg counts is the current best-practice approach. For pasture management that reduces larval exposure see the reducing larval exposure through pasture management.

Further Reading

• parasite (Wikipedia)
• For the clinical overview of equine internal parasite control: gastrointestinal parasites of horses: Merck Veterinary Manual

A progenitor is a founding ancestor whose genetic contribution persists substantially in the animals that follow. In horse breeding, the term refers to early individuals, stallions or mares, whose descendants define a breed, family, or bloodline. Three imported Arabian stallions (the Byerley Turk, Darley Arabian, and Godolphin Arabian) are the progenitors of the Thoroughbred breed; every modern Thoroughbred traces its male line to one of these three horses.

The term differs from sire or dam (the immediate father or mother of an offspring) in that a progenitor need not be a direct parent, it is a founding ancestor of a broader genetic pool. The distinction matters in population genetics: a progenitor’s alleles may appear in a high proportion of a breed’s current individuals, creating a bottleneck effect that limits ongoing breed diversity.

Foundation sires and mares recorded in a breed studbook are often the breed’s progenitors. Identifying them is central to understanding a purebred lineage and predicting heritable traits across the wider descendant population.

Further Reading: The three founding progenitors of the Thoroughbred are individually documented: the Byerley Turk, the Darley Arabian, and the Godolphin Arabian. Every modern Thoroughbred’s male tail-line traces to one of these three stallions, making them the canonical case study in equine progenitor influence.

To infest is to attack, overrun, or establish a large-scale parasitic presence on or within a host organism or its environment. The term is applied to macroscopic parasites, ectoparasites living on the body surface and endoparasites residing in body cavities or tissues, whereas the parallel term microscopic pathogens that infect is reserved for microscopic pathogens such as bacteria, viruses, and fungi. In horses, common infestations include bot fly larvae (Gasterophilus spp.) in the stomach, strongyle worms in the large intestine, pinworms (Oxyuris equi) in the rectum, and ectoparasites such as ticks, lice (Damalinia and Haematopinus spp.), mites (mange), and culicoid midges on the skin and coat.

The clinical impact of an infestation depends on parasite species, load, and host immune status. Heavy strongyle burdens in young horses produce protein-losing enteropathy, poor body condition, and colic; cyathostome larvae mass-emerging from the colon wall (larval cyathostominosis) cause acute diarrhea and rapid deterioration that can be fatal. Bot larvae attach to the gastric mucosa and, at high loads, can cause ulceration and impaired digestion. Ectoparasite infestations cause pruritus, skin excoriation, mane and tail hair loss (sweet itch from Culicoides hypersensitivity is one of the most common equine skin problems in temperate climates), and, in heavy tick infestations, anemia and transmission of tick-borne infections such as equine piroplasmosis and Lyme disease.

Fecal egg count (FEC) monitoring is the evidence-based standard for managing internal parasite infestations; it replaces calendar-based universal targeted anthelmintic treatment guided by egg counts protocols that accelerated anthelmintic resistance by treating horses that did not carry significant parasite loads. Horses shedding more than 500 eggs per gram are considered high shedders and dewormed strategically; horses consistently below 200 eggs per gram receive treatment only at peak seasonal risk periods. Pasture hygiene, regular manure removal from the field, rotational grazing, and avoiding stocking densities that concentrate fecal contamination, is the non-pharmacological complement to drug treatment because it reduces the infective larval population that horses ingest while grazing. A clean pasture with low contamination pressure is the single most effective tool for keeping parasite burdens at subclinical levels.

Further Reading

• infestation (Wikipedia)
• For parasite identification and control protocols: horse parasites and flies: MSD Veterinary Manual

A lesion is any localized region of tissue that has undergone pathological change, in structure, composition, or function, as a result of disease, injury, or a developmental abnormality. The term is used broadly across veterinary and medical disciplines to describe virtually any discrete abnormality found during physical examination, diagnostic imaging, biopsy, or post-mortem evaluation. In equine medicine, identifying the location, character, and extent of a lesion is the central task of clinical examination, because it determines the differential diagnosis list and guides the choice of further investigation or treatment.

Lesions are classified by their physical characteristics: macules (flat discolored areas), papules (raised solid elevations), vesicles (fluid-filled blisters), pustules (pus-filled elevations), ulcers (breaks in epithelial continuity), erosions (superficial loss without full-thickness involvement), and nodules (solid masses within tissue). In the context of equine anatomy, a skin lesion at the fetlock or pastern might indicate pastern dermatitis (scratches/mud fever), while a lesion within the navicular bone identified on radiograph indicates degenerative joint disease. Internal lesions, such as those in the intestinal mucosa associated with strongyle migration, or in the gastric mucosa associated with equine gastric ulcer syndrome, require endoscopy or necropsy to characterize fully.

The term does not imply a specific cause or severity; a lesion may be acute, chronic, primary (the direct result of the causative agent), or secondary (resulting from a complication or host response). Clinicians describe lesions in terms of size, shape, depth, distribution, and any associated discharge or odor. Accurate lesion description is the foundation of veterinary communication, ensuring that a second examiner or specialist reviewing notes can reconstruct what was observed. For owners, recognizing when new skin lesions appear and reporting them promptly allows earlier veterinary intervention and better outcomes across most dermatological and musculoskeletal conditions.

Further Reading

• lesion (Wikipedia)
• For a clinical overview of equine skin lesion types and management: dermatitis and dermatologic problems of horses: Merck Veterinary Manual

Immunity is the physiological state in which an organism resists or overcomes infection by a specific pathogen. In horses it is divided into two broad systems that operate in parallel. Innate immunity is the rapid, non-specific first line of defense: physical barriers such as intact skin and mucous membranes, phagocytic cells (neutrophils and macrophages) that engulf and destroy invaders, and soluble proteins of the complement system that tag pathogens for destruction or directly lyse bacterial membranes. Innate responses activate within minutes to hours and do not require prior exposure to the pathogen.

Adaptive immunity develops over days to weeks after exposure and is distinguished by specificity and memory. B lymphocytes produce antibodies that bind to a particular pathogen's antigens, neutralizing it or marking it for phagocytosis. T lymphocytes coordinate the cellular response: helper T cells amplify the antibody response and cytotoxic T cells directly kill virus-infected host cells. After the pathogen is cleared, long-lived memory cells persist in the circulation; re-exposure to the same antigen triggers a faster and larger secondary response that typically eliminates the pathogen before clinical disease develops. This immunological memory is the biological basis on which priming the memory pool through vaccination programs are designed, by presenting antigens without live disease, vaccines prime the memory pool.

Passive immunity is the temporary transfer of pre-formed antibodies from one individual to another. In horses the most critical instance is the transfer of immunoglobulins from the mare's colostrum to the neonatal foal dependent on colostral transfer in the first twelve to eighteen hours after birth. The equine placenta does not allow immunoglobulin passage in utero, so the newborn foal is entirely dependent on colostral antibody absorption through the intestinal wall during this narrow window; failure of passive transfer (FPT), defined as serum IgG below 400 mg/dL at twenty-four hours of age, leaves the foal highly susceptible to septicemia, pneumonia, and joint infections. Testing IgG at eighteen to twenty-four hours is standard neonatal care, and plasma transfusion from a healthy donor corrects FPT when detected early. The mare should receive her booster vaccinations four to six weeks before foaling to maximize the immunoglobulin concentration of the colostrum she produces.

Further Reading

• immunity (Wikipedia)
• For a clinical overview of equine immune function and disorders: the immune system of horses: MSD Veterinary Manual

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