Avian metapneumovirus subtype B: First cases confirmed in Canada
Emily Martin
Animal Health Laboratory, University of Guelph, Guelph, ON
AHL Newsletter 2024;28(2):17.
In recent months, multiple states throughout the US have documented increased cases of avian metapneumovirus infection (aMPV) with subtypes A and B. Although subtype C has been identified in wild birds previously, we have recently identified subtype B in commercial poultry flocks in Canada. Therefore, the poultry industry needs to be aware of this disease and the associated clinical presentations.
Avian metapneumovirus is in the family Pneumoviridae, genus Metapneumovirus. It is an enveloped, single-stranded negative-sense RNA virus, and has 4 identified antigenic subtypes (A to D). Subtypes A and B are identified in chickens and turkeys, whereas subtype C is identified primarily in turkeys as well as ducks. Other birds at risk include pheasants, game birds, and guinea fowl. Clinically healthy wild birds are considered a reservoir for this organism; e.g., waterfowl, sparrows, swallows, pigeons, and falcons. Wild birds and game birds have been found to be seropositive.
Avian metapneumovirus infection results in respiratory and reproductive disorders. High density poultry populations tend to have a higher incidence of disease. The organism is spread primarily through direct contact or fomites. The incubation period is 3-7 days, and the disease spreads rapidly within and between flocks. An entire flock can become clinically affected within a day. The birds shed virus for only a few days, and there is no latency or carrier state. However, there are species differences in the onset and development of lesions. Unfortunately, clinical signs and lesions are non-specific.
TURKEYS:
Clinical disease caused by aMPV in turkeys has been called turkey rhinotracheitis (TRT) and avian pneumovirus infection of turkeys (APV).
Regardless of age, turkey morbidity ranges from 40 to 100%, and mortality ranges from 0.4% to 50%. Clinical signs include snicking, rales, nasal discharge, foamy conjunctivitis, swollen infraorbital sinuses (Fig. 1A), submandibular edema, coughing, open mouth breathing, and head shaking. Severe disease can be identified in 3 to 12-week-old turkeys. In breeders, uterine prolapse can occur secondary to coughing. In layers, there can be up to 70% drop in egg production (range 10-40%), including increased occurrence of poor shell quality and peritonitis. Recovery can take up to 3 weeks.
CHICKENS:
Clinical disease caused by aMPV in chickens, guinea fowl and pheasants has been called swollen head syndrome (SHS).
The disease is not as well defined in chickens, and can be subclinical. Less than 4% of the flock may be affected. Mortality is rarely >2%. Egg production in broiler breeders and egg quality in egg layers are affected. Clinical signs can include swelling of the periorbital and infraorbital sinuses, torticollis, disorientation, and opisthotonus (Fig. 1B).
DUCKS:
Clinical signs can include respiratory symptoms (subtype C), decreased egg production (40-85%), and poor shell quality (i.e., soft, thin shelled, cracked).
There is variable pathogenicity between strains. Uncomplicated aMPV infection may have mild clinical signs, and can clear in 10-14 days. However, secondary infection(s) can increase the severity of disease. The virus affects the function of the cilia of the respiratory and reproductive epithelial cells, increasing susceptibility to secondary infections. Secondary infections include bacteria such as E. coli, ORT, Pasteurella spp., B. avium, R. anatipestifer, and Mycoplasma (MG), in addition to aspergillosis and viral infections (i.e., IBV), resulting in potential development of airsacculitis and pneumonia.
On postmortem examination, the follow lesions can be observed:
Turkeys: Mucoid exudates in turbinates and trachea. Catarrhal inflammation of the upper respiratory tract such as rhinitis, laryngitis, and tracheitis. Various reproductive tract abnormalities in mature birds, e.g., egg peritonitis, folded shell membranes, misshapen eggs. Secondary infections can result in lesions of airsacculitis, pericarditis, pneumonia, perihepatitis, and in severe cases, subcutaneous exudate and osteomyelitis in cranial bones (Fig. 1C).
Chickens: Severe edema of the subcutaneous tissues of the head, neck and wattles. Variable swelling of the infraorbital sinuses.
When deciding on diagnostic testing, it is important to realize that this virus does not persist within affected birds. The virus is cleared quickly, and may only be detectable for 6-7 days post infection; therefore, by the time clinical signs are recognized, virus may be undetectable by PCR testing. Combining PCR and ELISA testing will aid in diagnosing and tracking disease within and between flocks. Antibody titres may be detectable 7 days post infection. Recommended samples include nasal secretions and sinus or tracheal swabs of mildly affected birds to test for aMPV and secondary infections.
Histopathology requires collection of fresh tissues to attempt detection of intracytoplasmic inclusion bodies in upper respiratory ciliated epithelial cells otherwise lesions are not considered pathognomonic.
There is no treatment for uncomplicated aMPV infection. Suggested interventions include general recommendations for disease management, including: biosecurity (preventing exposure to wild birds or other infected poultry), disinfection, and dedicated barn clothing. Good barn management involves providing optimal ventilation, controlling temperature, preventing overcrowding, maintaining litter quality, and avoiding multiage facilities.
Since this is an enveloped virus, it is sensitive to multiple disinfectants, including quaternary ammonia products and bleach. It is stable at pH 3.0-9.0, and is inactivated at 56 °C for 30 minutes. However, it has longer survival times (i.e., weeks) at lower temperatures, and that could explain some seasonal occurrences.
AHL has recently validated PCR testing that includes detection of multiple subtypes of aMPV, including A, B, and C. The ELISA available at the AHL can also detect antibodies against multiple subtypes of aMPV. If you have questions about this disease or diagnostic testing, we recommend veterinarians contact the AHL. AHL
Figure 1. Lesions caused by aMPV infection. A: Swollen infraorbital sinuses in a turkey (Photo: Select Genetics). B: Chickens displaying torticollis and opisthotonus (Photo: US poultry integrator). C1: Normal turkey cranial bones. C2. Cranial bones with opaque fluid. C3. Cranial bones with osteomyelitis (Photos: Dr. Jenny Nicholds and Dr. Jason Sousa, PDRC, Georgia).
References
1.Rautenschlein S. Avian metapneumovirus. In: Diseases of Poultry, 14th ed. Swayne DE, ed. Wiley Blackwell, 2020; vol 1:135-143.
2. Jones RC, Rautenschlein S. Avian metapneumovirus. In: Diseases of Poultry, 13th ed. Swayne DE, ed. Wiley Blackwell, 2013:112-119.
3. Kaboudi K, Lachheb J. Avian metapneumovirus infection in turkeys: A review on turkey rhinotracheitis. J Appl Poult Res
2021;30:100211. https://doi.org/10.1016/j.japr.2021.100211