During poultry farming, many diseases are easily confused clinically. Chicken viral arthritis (VA) and synovitis caused by Mycoplasma (MS) have very similar clinical presentations and are easily misdiagnosed. Laying hens, broiler breeders, white-feathered broilers, yellow-feathered broilers, and other types of chickens are susceptible to both diseases. When people see swollen joints or paralyzed chickens, they often feel helpless. Today, we will provide a brief analysis and discussion of this issue.
I. Differences in Pathogens and Epidemiology
1. Chicken viral arthritis is an infectious disease in chickens caused by avian reovirus (VAV); it is transmitted both vertically and horizontally. Clinically, it mostly occurs at 4-6 days of age. Heavy breeds such as broiler breeders and broilers are more susceptible, while other lighter breeds are less susceptible. The older the chicken, the lower its susceptibility to avian reovirus; susceptibility decreases significantly after 10 weeks of age. Chicks at 2 weeks of age are more susceptible than chickens aged 5—20 weeks.
2. Mycoplasma synoviae disease is caused by Mycoplasma synoviae (MS) and can also be transmitted vertically and horizontally. Clinically, it mostly occurs at 4—16 weeks of age. In adult chickens, it is usually a continuation of disease onset during the rearing period.
II. Differences in Clinical Signs
1, VA symptoms
The length of the incubation period varies depending on the virulence of the strain, the route of inoculation, and the susceptibility of the chickens. The disease occurs most often in broilers and in larger-bodied chickens, such as meat-type or dual-purpose birds. Chickens of any age may develop the disease, but clinically it is most commonly seen at 4–6 weeks of age. Most infected chickens have a subclinical course, showing only serological and histological changes without clinical signs. Among infected flocks, symptomatic cases generally account for 1%–5% of the flock, although reports also describe rates of 10% or above 10%.
Affected chickens have reduced appetite and activity and are reluctant to move. When driven, they may move with difficulty, but their gait is unsteady. This is followed by lameness or hopping on one leg. Because they do not get enough water and feed, affected chickens gradually become emaciated and anemic, and their growth is retarded; a small number gradually become debilitated and die. Examination may reveal swelling of one or both hocks and knee joints. In chronic cases, the ribs are twisted and the toes are bent backward.
After infection in breeder or layer flocks, egg production may decrease by 10%–15%. Some reports state that infection in breeder flocks reduces the fertility rate of hatching eggs. This may be due to impaired mobility in affected birds interfering with normal mating. Lesions are mainly found in the hock joints, toe joints, digital flexor tendons, and extensor tendons.
2, MS symptoms
The initial signs in affected chickens are a pale comb, slow growth, ruffled feathers, and a shrunken comb. This is followed by lameness and swelling of the knee joints and toe pads. The swollen areas feel warm and fluctuant, although some chickens occasionally have systemic infection without obvious joint swelling. Affected chickens are restless, dehydrated, and emaciated, and often have breast blisters. If feed and water are placed near them, they still show appetite and thirst. After these acute signs, the disease enters a slow recovery phase, but synovitis may persist for 4-5 years.
III. Differences in pathological changes
1, VA pathological changes
In the acute stage of the disease, the joint capsule and tendon sheath are edematous and congested, or show petechial hemorrhages. The joint cavity contains a small amount of pale yellow or blood-tinged exudate; in a few cases, the exudate is purulent. This may be associated with concurrent infection by certain bacteria. In chronic cases, there is less exudate in the joint cavity, the joint becomes rigid and cannot be extended to its normal position, the articular cartilage is eroded, the synovium hemorrhages, the muscle fascia ruptures, hemorrhages, and becomes necrotic, and the tendon sheath becomes adhered. Pericarditis may also sometimes be observed. Small necrotic foci are present on the liver, spleen, and myocardium.
2, MS pathological changes
In the early stage of infection, the joint synovial bursae and tendon sheaths of affected chickens contain a thick, milky-white exudate. As the disease progresses, the exudate becomes caseous. Caseous material is occasionally found under the skin of the neck, but rarely extends to the muscles and air sacs. If an affected chicken becomes extremely emaciated and dehydrated before caseous material forms, there may be no fluid around the joints. In chronic cases, the articular surfaces are often yellow or orange-yellow. In about 50 percent of early cases, the spleen, liver, and kidneys are enlarged. These lesions become more pronounced as the disease worsens, while some severely affected chickens show no visible lesions in the internal organs.
Four. AV and MS prevention and control
At present, there is no specific treatment for viral arthritis (AV), but vaccination can provide some degree of prevention. Several inactivated or attenuated vaccines are currently available for use domestically, and vaccination schedules vary. Avian reovirus has multiple serotypes, which must be taken into account when selecting a vaccine. Before the local viral serotype has been identified, it is generally advisable to choose a vaccine with broad antigenic coverage. For breeder flocks, attenuated vaccines are generally administered at 1—7 days of age and at 4 weeks of age, with each vaccination given 1 time(s); an inactivated vaccine is administered 1 time(s) before the onset of lay. For broiler flocks, an attenuated vaccine is usually administered 1 time(s) at 1 days of age. Once disease occurs, it is recommended that affected birds be buried deeply. The flock can be given interferon in drinking water at 2-3 times the dose, together with highly effective polysaccharide drugs + broad-spectrum antiviral drugs in drinking water for 4-5 days. Disinfection of the flocks in the farm and poultry houses should also be carried out.
Treatment of MG. According to some reports, adding 100—200g of tetracycline antibiotics per ton of feed and administering continuously is effective in controlling the disease. If the calcium content of the feed is reduced to 0.8%, the drug efficacy can be increased 1 times. In addition, intramuscular injection of streptomycin in the early stage of infection (100mg per kilogram of body weight), drinking water containing 0.01% lincomycin, or drinking water containing azithromycin may also be effective.