Chapter 36 Diseases of the Urogenital System
The urinary system of the bird is composed of two kidneys located along the dorsal body wall, two ureters, and the cloaca. There are two types of nephrons in birds: the cortical, which is reptilian in form, and the medullary, which is mammalian in form. The cortical form produces urates, and the medullary form produces urine. There is a renal portal system that allows blood to be shunted from the caudal abdominal structures directly to the kidney, bypassing the rest of the body. As a result of the renal portal system, any medications injected in the caudal portion of the bird will be eliminated by the kidney before distribution to the body. The ureters empty into the cloaca.
The reproductive system of the bird is quite different from that of mammals. The female system is composed of two ovaries (although the right one is usually inactive in adult birds), the oviduct, and the cloaca. The oviduct is divided into segments: the infundibulum, the magnum, the isthmus, and the shell gland. The male reproductive system consists of two testicles located internally, the ductus deferens, the seminal glomus, and the ejaculatory duct that opens into the cloaca. A phallus may be present in some species but is absent in psittacines. In both the female and male bird, the cloaca is divided into the coprodeum, the urodeum, and the proctodeum. The coprodeum, the most cranial portion, is a continuation of the rectum or large intestine. The urodeum contains the openings from the ureters and genital ducts. The proctodeum contains the bursa of Fabricius, or the cloacal bursa, the site of B-lymphocyte production. The vent is the opening of the cloaca to the external surface of the bird. It is located under the tail.
Seasonal changes in the photoperiod affect the reproductive system of the bird. Breeding and laying of eggs increase during periods of increased light in the spring. After breeding, shortening periods of light seen in the autumn stimulate molting. When housing birds under artificial light, owners can influence the breeding periods, sometimes to the detriment of the pet bird.
Renal disease, usually as a consequence of another systemic disease, is seen in pet birds. Signs of renal disease are similar to those in mammals, but lameness may be the only sign of renal enlargement seen in some birds. It is important to recognize the early signs of renal disease, but this is often difficult because the symptoms are nonspecific and obtaining a urine sample is not easy.
Reproductive disorders are common in pet birds. Egg binding and dystocia, prolapse of the oviduct, excessive egg laying, ectopic eggs, and egg yolk peritonitis are diseases that affect the female bird, whereas neoplasia, papillomas, and infertility are seen in male birds.
The failure of an egg to pass through the oviduct at a normal rate is defined as egg binding, whereas dystocia is related to mechanical obstruction of the cloaca related to the presence of the egg (egg is stuck in cloaca). Many factors predispose birds to egg binding and dystocia: obesity, excessive egg laying (oviduct fatigue), inadequate nutrition, inadequate exercise, genetic predisposition, and other stressors.