VACCINATION IN POULTRY
After intestinal replication, the virus spreads throughout the entire body via the bloodstream. The most pathogenic viruses are located in the tibiotarsal joint, where they cause arthritis. The liver may also be affected. One-day-old chicks are more sensitive than older birds: the younger the infected bird the more likely they are to develop disease.
Affected parameters and clinical signs The symptoms most commonly produced by reoviruses are viral arthritis and tenosynovitis. These symptoms are usually observed in birds of 3-4 weeks of age; between 5 % and 10 % of the population of the farm are affected, and show varying degrees of lameness and stiffness. Affected birds exhibit inflammation of the hock joint (Fig. 1), limb paralysis (Fig. 2) and clenched feet in cases of bilateral paralysis. Inflammation of the digital flexor tendons and the metatarsal extensors is observed (Fig. 3), and to a lesser extent, of the footpad and tarsal joint (Figs. 4 and 5). The inflammation of tendinous areas subsequently progresses to the formation of chronic lesions characterised by the hardening and fusion of tendon sheaths. The gastrocnemius tendon may rupture (Fig. 6) in older birds (Jones et al., 1975). A significant percentage of infected birds show stunted growth (Fig. 7). Mortality ranges from 2-10 %. MAS is characterised by continuous lesions of the mucosa of the small intestine, stunting (Fig. 8), prostration, pallor of the mucous membranes and appendages of the head, poor pigmentation of the shanks (Fig. 9), abnormal feathering (helicopter disease; Fig. 10), diarrhoea and bloating.
Figure 1. Open hock joint with marked inflammation. Image courtesy of Dr Rafael Fernández.
Figure 2. Paralysis of the legs in a chicken affected by avian reovirus. Image courtesy of CESAC.
Lesions Necropsy reveals inflammation of the gastrocnemius and the flexor tendon (Fig. 11); in severe cases, rupture of the gastrocnemius tendon may be observed. Marked oedema of the tarsal and metatarsal tendon sheaths is observed at the onset of infection. The affected tarsus or hock shows bloody or yellowish exudate (Fig. 12). MAS may be accompanied by disorders of the digestive system such as proventriculitis, enteritis, pancreatitis and pancreatic atrophy. Examination of the intestines may reveal undigested food or water, peeling of the intestinal mucosa, and in some cases hypertrophy of the proventriculus. Lesions typical of viral arthritis are also quite common, and some authors have described necrosis of the humeral head (Fig. 13).
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Figure 3. Swollen tarsus and joint in chicken with viral arthritis. Image courtesy of Dr Rafael Fernández.