Swollen Head Syndrome

Avian pneumoviruses have been implicated in the upper respiratory tract disease of turkeys and chickens known as turkey rhinotracheitis. The virus has also been associated with swollen head syndrome of chickens. First described in south Africa in the late 1970s, avian pneumovirus soon appeared in Europe and the Middle East. The disease has now been reported from all the major poultry-producing areas in the world except for Australasia. The virus has also been detected in pheasants and guinea fowl, and serologic evidence suggests other avian species are susceptible. Recent studies have indicated an antigenic relationship with a newly discovered human pneumovirus isolated from children with respiratory tract disease.

Etiology:

Avian pneumoviruses are members of the subfamily Pneumovirinae, belonging to the family Paramyxoviridae. The subfamily consists of 2 genera: Pneumovirus, consisting of mammalian respiratory syncytial viruses and mouse pneumovirus, and Metapneumovirus, in which avian pneumoviruses are placed. Based on observed differences following sequence analysis of the viral genes, at least 3 subtypes of the virus have been described (A, B, C). Subtype C viruses appear to be the only subtype found in North America and have not been reported in other parts of the world.

Transmission and Epidemiology:

Following infection, the virus is shed from the nares and trachea but not in the feces. After initial introduction of the virus, the disease spreads rapidly within a geographic area or country. The methods by which the virus is spread are unclear and often unpredictable. Direct contact attributed to movement of infected birds, personnel, equipment, and vehicles have all been implicated, while airborne transmission has also been reported to occur. There is no published evidence of vertical transmission via the egg, even though the virus has, on occasion, been detected in the reproductive tract of laying birds. Persistence of the virus in turkeys and chickens has not been demonstrated and, following experimental infection, virus was detected for 6-7 days only after inoculation. Wild birds have been implicated in the spread of avian pneumovirus, particularly waterfowl and gulls.

Clinical Findings:

Clinical signs in turkey poults include snicking, rales, sneezing, nasal discharge, foamy conjunctivitis, swollen infraorbital sinuses, and submandibular edema. Coughing and head shaking are frequently observed in older poults. In laying birds, egg production may drop up to 70% with an increased incidence of poor shell quality and peritonitis. Coughing associated with lower respiratory tract involvement may lead to prolapses of the uterus in laying turkeys. Morbidity in birds of all ages is usually described as up to 100% with mortality ranging from 0.4% to as high as 50%, particularly in fully susceptible young poults. Secondary pathogens and management factors significantly influence the levels of morbidity and mortality. Infection in chickens and pheasants is less clearly defined and may not always be associated with clinical signs. Avian pneumovirus is associated with swollen head syndrome in chickens. This condition is characterized by swelling of the peri- and infraorbital sinuses, torticollis, cerebral disorientation, and opisthotonos. Typically, <4% of the flock is affected, although respiratory signs may be widespread. Mortality is rarely >2%. In broiler breeders and commercial layers, egg production and quality are frequently affected. Evidence suggests that infectious bronchitis virus (Infectious Bronchitis: Introduction) and Escherichia coli may also be associated with swollen head syndrome. Lesions: In turkeys, excess mucus found in the nares, sinuses, and trachea is clear at first but rapidly becomes mucopurulent. When bacteria are involved, typical lesions of colisepticemia are frequently seen in various organs. The oviducts of affected breeders may contain inspissated albumen and solid yolk. Egg peritonitis may be associated with oviduct regression. Microscopic examination of the upper respiratory tract 1-2 days after infection reveals localized lesions, including loss of cilia, increased glandular activity, congestion, and mild mononuclear infiltration of the submucosa. Inflammatory infiltration of the submucosa can be observed with some mild lesions in the trachea between days 3 and 5. Similar but milder lesions can be observed in affected chickens.

Diagnosis:

Taking samples from the upper respiratory tract of birds in the very early stages of the disease is extremely important when attempting virus isolation. Early investigators found that tracheal organ cultures prepared from turkey or chicken embryos, or 1- to 2-day-old chicks, were the most sensitive for primary isolation of avian pneumoviruses. Ciliostasis may occur within 7 days of inoculation or on passage. The virus has also been isolated following the inoculation of 6- to 8-day-old embryonated chicken or turkey eggs via the yolk sac route and identified by electron microscopy, virus neutralization, and molecular techniques. Cell cultures have not proved successful for the primary isolation of the virus. However, once the virus has been isolated and adapted in the systems above, it will grow in a variety of avian and mammalian cultures. PCR tests have been developed and are widely used to detect the virus in clinical material, particularly respiratory swabs. Some PCR tests have been constructed so that the subtype as well as the identity of virus can be determined from the clinical sample. Antigen detection tests have also been developed, including immunofluorescence and immunoperoxidase assays on both fixed and unfixed tissues. Due to difficulties in isolating and identifying pneumoviruses, serologic assays have been developed to confirm infection in commercial chickens and turkeys. A number of commercially prepared ELISA kits are available and are used most commonly, but other techniques including virus neutralization and indirect immunofluorescence have also been used. As with all serologic tests, both acute and convalescent samples should be submitted for analysis. The sera should be heated to 56°C for 30 min and stored at -20°C if delays in testing are unavoidable. While ELISA that use either subgroup A or B strains as antigens detect antibodies to both of these subgroups, the homologous antigen should be used for the efficient detection of subgroup C. Differential Diagnosis: Paramyxoviruses (particularly Newcastle disease and paramyxovirus 3), infectious bronchitis virus, and influenza viruses may cause respiratory disease and egg production problems in chickens and turkeys that closely resemble pneumovirus infection. These viruses can be differentiated on the basis of morphology, hemagglutinating and neuraminidase activity, and molecular characteristics. A wide range of bacteria and Mycoplasma spp can cause clinical signs very similar to those of avian pneumovirus. These agents are frequently present as secondary opportunistic pathogens and may mask the presence of the pneumovirus.

Prevention and Treatment:

Good management practices can significantly reduce the severity of infection, especially in turkeys; in particular, optimal ventilation, stocking densities, temperature control, litter quality, and biosecurity all have a positive influence on the disease. Some success in reducing disease severity by controlling secondary adventitious bacteria with antibiotics has also been reported. Both live and inactivated vaccines are available for immunization of chickens and turkeys and are widely used in countries where the disease is endemic. Live vaccines stimulate both local and systemic immunity in the respiratory tract, and cross-protection between subtypes can occur. To produce complete protection in adult birds, oil-adjuvanted inactivated vaccines are administered to birds previously primed with live vaccines.