Strangles

STRANGLES (adenitis equorum, or coryza contagiosa equorum), an acute infectious disease of horses; in its typical form it presents the picture of a diffuse inflammation of the nasal mucous membrane with purulent-slimy discharge; it is accompanied by fever, and often by swelling and suppuration of regional lymph glands (see figure). The causative agent of strangles, discovered by Schutz in 1888, is the streptococcus (Streptococcus equi), which stains easily with ordinary aniline dyes and also by Gram's method after careful decolorization with alcohol. In the organism and on serum nutrient media it forms long chains (of the Streptococcus longus type), consisting in the majority of segments not of a round but of a transversely oval shape, among which larger rounded formations stand out, which under appropriate staining appear as "tetrads." These, dividing along or across the axis of the chain, can give rise to branches located either parallel or at a right angle to the main filament. Quite often on preparations from pus, mucus, or from serum media, this streptococcus can be found to have a plasma sheath (capsule). On artificial media it grows aerobically and anaerobically, at a weakly alkaline reaction, better in an incubator, but generally very scantily, and after several subcultures loses its virulence and vitality. Relatively abundant growth is obtained on serum agar and especially on coagulated horse serum. Streptococcus equi possesses hemolytic properties, especially towards horse erythrocytes. From all other streptococci it is distinguished by the fact that it does not break down lactose and sorbitol. It withstands drying in the discharges of sick animals for several weeks. Strangles is distributed in almost all countries of the temperate zone, approximately from 40° to 60° north latitude. It affects horses primarily in the age range of 6 months to 5 years, as well as mules, donkeys, and hinnies. Artificial infection is easily achieved in domesticated gray and white mice, significantly more difficult in rabbits and guinea pigs; field mice are not susceptible. Horses that have undergone strangles in an obvious or latent form acquire a long-lasting immunity. In most cases strangles appears in the warm season, in the form of an epizootic or enzootic. Infection occurs almost exclusively through fresh virulent discharges (nasal discharge, pus) of sick animals, which get onto the tonsils and the mucous membrane of the pharynx. The portals of entry can also be the irritated or damaged mucous membrane of the nose, skin wounds, the healthy intestine (analogous to intestinal infection in tuberculosis and glanders), less frequently the genital organs (contagious strangles) and the udder (through sick sucklings). Intrauterine infection is not excluded.

From the site of entry the virus spreads by lymphatic or blood vessels, causing metastatic foci in the lymph glands and parenchymatous organs, which may turn into suppuration; there are also cases of pure bacteremia. Sometimes a rapidly transient diffuse vesicular rash appears on the delicate parts of the skin, and the vesicles and crusts contain virulent Streptococcus equi. In the typical form strangles begins after an incubation period of 4–8 days, and in weak foals of 1–2 days, with a rise in temperature to 40–41°. Almost simultaneously and no later than 2 days later, an acute catarrh of the nose develops, which spreads to the oral cavity and the connective tissue and takes on a mucopurulent or even purely purulent character. In most cases, painful swelling of the submandibular glands occurs from the very beginning with the formation of abscesses, which usually rupture and after abundant pus discharge heal quickly. At the same time, the temperature returns to normal, and the animal recovers in 2–4 weeks. In complicated cases, multiple lesions of the lymph glands (especially the retropharyngeal ones, which are particularly threatening), abscesses in parenchymatous organs, bronchopneumonia, peri- and endocarditis, meningitis, and septicopyemia are observed. Mortality rarely exceeds 3%, but under unfavorable conditions it can reach 12%; the economic damage is quite significant due to the delay in the development of foals and the prolonged reduction in working capacity of adult horses.

Specific therapy was first proposed by Delvos (1893), who used serum from horses that had recovered from strangles. Subsequently, anti-strangles serum was prepared by various authors by hyperimmunization (see). Evaluations of its therapeutic effect, as well as its significance as a means of passive immunization, vary greatly. Active immunization using the living causative agent is impossible because of its high infectivity for horses. The first investigations on the development of an anti-strangles vaccine belong to Gabrichevsky, who killed cultures with a 0.5% solution of carbolic acid. (Analogously, Gabrichevsky proposed vaccination against scarlet fever with fresh human isolates of streptococci.) Subsequently, various other means were proposed for this purpose: heating cultures to 53–55° (Kitt); shaking them for 4½ days in a 25% solution of urea (Maghreg); concentrating the serum-broth culture heated to 58° and adding carbolic acid to it (Jensen). "Druselymphe" of Schreiber, which found wide application, is an extract from various strains of Streptococcus equi after shaking them in some macerating fluid with the addition of diphtheria toxin for sterility. Baruchello, starting from Bail's theory of aggressins, mixed pleural exudate with a culture of Streptococcus equi. Reviews of the results of the practical application of all these preparations in the form of preventive vaccinations, as well as for vaccine therapy, are still very contradictory. Recently, "local immunization" of foals at risk according to Muller and Pfeiffer is recommended by subcutaneous injection of serovaccine on both sides of the larynx. A number of Russian authors (Sadovsky, Tsvetkov and Veller, Vinogradov) used various "filtrate-vaccines," autovaccines, and antiviruses (according to Bezredka) for the treatment of strangles with more or less success.