Infection of the testes may have a systemic or localized cause. Either will result in elevated testicular temperature and associated decline in spermato-genesis. The magnitude of the decline in fertility rates and the time period reflects the severity of the disease and the duration of the problem. Recovery, when it occurs, will be somewhat delayed, as the prime site of effect, as far as spermatogenesis is concerned, is the germinal cells. The spermatogenic cycle being 56 days, this period of time must be allowed after recovery for semen quality to return to normal (Varner et al., 1991b).
Systemic disease, causing orchitis, normally results in bilateral inflammation of the testes and epididymis. Bacterial agents causing such orchitis include Streptococcus equi (strangles), Streptococcuszooepidemicus, Klebsiella pneumoniae, Actinomyces bovis, Pseudomonas mallei (glanders) and possibly Salmonella abortus equi. Viral agents may also be a systemic cause of orchitis: these include equine viral arteritis, equine infectious anaemia and equine influenza (Rossdale and Ricketts, 1980; Ladd, 1985; Roberts, 1986a; De Vries, 1993; Slusher, 1997). Systemic infections cause chronic rather than acute orchitis and as such have more of a chance of causing low-grade testicular degeneration and, with it, permanently depressed semen quality.
Localized infections may be caused via a wound, often to the scrotum, but may also be caused by infection via the inguinal canal (Varner and Schumacher, 1991). Such infections tend to cause acute orchitis (De Vries, 1993) which may be unilateral or bilateral and presents itself initially as soft, flabby, swollen testes. If the condition persists, chronic orchitis may result. Semen quality will be poor, with a decline in spermatozoan concentrations and an increased incidence of abnormalities (Hurtgen, 1987). The major infective agents associated with localized orchitis are Staphylococcus species, Escherichia coli, Streptococcus zooepidemicus and Streptococcus equi. In cases of acute orchitis, rises in testicular temperature are also a potential hazard (Blanchard and Varner, 1993).
Orchitis may also be caused by trauma or parasites. The parasite most often associated is Strongylus edentatus larvae (Smith, 1973). These can migrate into the testicular tissue, causing orchitis or obstruction of the testicular artery within the pampiniform plexus. This will have an additional detrimental effect upon the efficiency of the counter-current heat exchange mechanism (Roberts, 1986a; Varner et al., 1993).
Finally, orchitis may be caused as a result of damage to Sertoli cells and hence the blood-testes barrier. This will result in an autoimmune response set up by the stallion's system (Zhang et al., 1990b; Papa et al., 1990).
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