Symptoms and Prevention Methods of Swollen Head Syndrome in Chickens

I. Disease Overview:

Swollen Head Syndrome in chickens is an infectious disease of chickens caused by avian pneumovirus with secondary infections such as pathogenic Escherichia coli, mainly characterized by swollen head and characteristic neurological symptoms; in laying hens it can also cause a drop in egg production and reduced hatchability.

II. Epidemiological Features:

SHS mainly affects chickens (broiler breeders, commercial laying hens, and broilers can all be affected) and turkeys. Chickens and turkeys are the known natural hosts.

SHS is mainly transmitted horizontally through contact; the digestive tract and nasal cavity secretions of sick or recovered chickens contaminate drinking water and the environment, becoming a source of infection. The disease spreads relatively slowly, and there is currently no evidence to indicate that the disease can be transmitted vertically.

For broilers, the onset age of SHS is 4-7 weeks, with a peak at 5-6 weeks; for broiler breeders and commercial laying hens it can occur at various ages. Without treatment, the disease course is about 10 days; if antibiotics are used and ventilation is improved, the course can generally be shortened to 3-5 days. In recent years, the disease has shown a chronic epidemic course. It often lasts 2-3 weeks. The incidence rate of the disease ranges from 1%-90%, broiler mortality ranges from 1%-20%, laying hen production drops range from 2%-40%, laying hen mortality ranges from 0%-20%, and broiler breeder mortality ranges from 0%-10%.

The occurrence of SHS is directly related to environmental factors; SPF chickens inoculated in isolators cannot reproduce SHS, and rearing in highly dense, high-ammonia, and poorly ventilated environments is one of the triggering factors.

III. Clinical Symptoms:

SHS is also called thick-head disease or facial cellulitis. The first symptom of the disease is sneezing; within one day, conjunctival redness and lacrimal gland swelling occur. In the following 12-24 hours, subcutaneous edema begins to appear on the head, first around the eyes, then spreading to the head, and then involving the subcutaneous tissue and wattles. In the early stage, chickens scratch their faces with their claws, showing local itching; then sick chickens become depressed, unwilling to move, and have reduced appetite. Mortality ranges from 1%-20%. For broiler breeders and commercial laying hens, in addition to the swollen head phenomenon, specific neurological symptoms are also shown. Besides some sick chickens showing only extreme depression and coma, most sick chickens show brain disorientation, with manifestations including persistent and repetitive head shaking, torticollis, ataxia, unstable gait, and opisthotonus. Some chickens tilt their heads upward, presenting a "star-gazing" appearance. Sick chickens are unwilling to move, and some die because they do not eat. The egg production rate and hatchability of laying hens also decrease, but return to normal after several days.

IV. Necropsy Symptoms:

The first lesions to appear are slight congestion of the nasal turbinate mucosa in chickens accompanying sneezing, later developing into less severe extensive changes from red to purple. Histopathologically, there is obvious flattening of the epithelium, gradual loss of cilia, subepithelial congestion, and lymphoid tissue hyperplasia. Subepithelial congestion and lymphoid tissue hyperplasia are also seen in the lacrimal gland with increased plasma cells. Although mild petechial congestion and reduced ciliary motility may be observed in the upper respiratory tract, the trachea is generally unaffected in the acute phase. The swollen head in sick chickens is caused by subcutaneous edema around the orbit and dilation. In some severe cases, cyanosis and swelling of the wattles also occur. In the enlarged head, after peeling off the skin, yellow edema to suppuration can be seen in the subcutaneous tissue. The gross lesions of the eyes remain normal, but the eyelids are closed due to edema and conjunctivitis. The disease can produce tracheitis, and sometimes pulmonary necrosis occurs. Histological examination reveals accumulation of lymphocytes and heterophils at the edematous sites.

V. Prevention and Control Measures:

1. Sanitary management measures: Changing the sanitary conditions of the chicken house, reducing stocking density, reducing ammonia concentration in the air, and increasing the ventilation rate of the chicken house all have good effects on preventing or reducing the occurrence of the disease and the degree of harm.

2. Preventing secondary bacterial infection: After poultry are infected with the virus, on the one hand, the immunity of the poultry can be reduced, leading to decreased resistance; on the other hand, viral infection of the upper respiratory epithelium disrupts the integrity of epithelial cells, making it very easy to develop secondary infections with various bacteria such as Escherichia coli and Pseudomonas aeruginosa. Therefore, furans, sulfonamides, and broad-spectrum antibiotics can be administered orally; adding vitamins and electrolytes at the same time will give better results.

3. Application of vaccines: Morley (1984) used a strain attenuated through 50 passages in chicken embryos to immunize 4-week-old chickens, which could resist challenge with the same virus. In South Africa, after extensive use of this vaccine for 1 year, SHS rarely occurred. Spray immunization of 1-day-old commercial broilers can achieve moderate economic benefits and immune response as well as a significant reduction in clinical incidence. At present, inactivated oil-emulsion vaccines and attenuated vaccines are already available in various European countries. There are no such reports in China yet. In addition, it is believed that the combined vaccine of NDV-EDX76 and SHS virus can effectively prevent the occurrence of the disease.