Abstract

Over the last 15–20 years, the population of Australian white ibis (Threskiornis molucca) on the Gold Coast of Queensland, Australia, has grown at an increasing rate, primarily due to the adaptation of these birds to urban scavenging. Access to suburban landfills has provided a virtually unlimited food resource for ibis in the southern coastal region of Queensland. With an increased presence of ibis in public recreational areas, local government and public health officials sought to determine the potential risk of transmission of zoonotic pathogens, such as Salmonella spp., Japanese encephalitis (JE) virus, and avian influenza virus between ibis and people. Because ibis also frequent farms, the potential for transmission of diseases such as Newcastle disease, salmonellosis, or avian influenza between ibis and food production animals, such as chickens and cows, was considered as well. In order to assess the potential risk of disease transmission, a two-part study was conducted, coupling disease surveillance with a behavioral study focused on the interaction between ibis and people, and ibis and food production animals.

One hundred ibis were captured at a suburban landfill for the disease surveillance component of the study. Blood samples and choanal and cloacal swabs were collected. Cloacal swabs were cultured for Salmonella spp. and blood serum was tested for antibodies to avian influenza, Newcastle disease virus, and generic flaviviruses, including JE virus, Murray Valley encephalitis (MVE), and Kunjin virus (KV). Fecal samples were cultured for Salmonella spp. and serotyped. Seven percent of the ibis samples grew Salmonella spp., with the majority (4/7) growing Salmonella virchow (Table 1). Avian influenza (AI) antibody tests were performed using a competition ELISA. The seroprevalence of AI was 30.7% (27/88), 20.5% (18/88) weak positive, 10.2% (9/88) strong positive. Of sera tested 32.9% (29/88) were positive for NDV antibodies using the hemagglutination inhibition test. Of samples tested 2.3% (2/88) were positive for JE or other generic flavivirus antibodies using a competition ELISA test. The two positive samples were then tested using a plaque reduction neutralization test and one sample tested positive for Kunjin virus and negative for JE and Murray Valley encephalitis virus, while the second sample tested negative for all three viruses.

Disease surveillance was coupled with a behavioral study of ibis during daily feeding activity to assess the potential for disease transmission to people and food production animals. Four locations were selected for observations: a public park, a landfill, a tidal mudflat, and a poultry farm. Five birds were randomly observed every 2 minutes for 5 hours over 4 days at each location (3000 bird observations per site). The park was chosen as a representative area where people and ibis frequently interact by means of people feeding ibis; ibis approaching people for food; and ibis having direct contact with food objects such as picnic tables, water fountains, and grills. Ibis may contaminate food objects with feces or mucosal secretions, which represents a potential means of transmission of zoonotic diseases. Young children may be at an increased risk of infection because they are more likely than adults to have contact with objects contaminated with bird feces and they require a smaller dose of pathogen to contract disease.⁴ Playing in an area where bird droppings were observed has been associated with an increased risk of Salmonella infection (OR=10.5, p=0.04).²

The public landfill represented a place where ibis aggregate in large numbers to feed. Because of the higher density of birds at landfills, there is a higher frequency of agonistic interactions between the birds (Table 2) relative to the other foraging sites included in this study. An increased level of interaction provides greater opportunity for communicable diseases to be amplified within an ibis population that feeds at a landfill. The tidal mudflat was included as a “wild-type” setting, where ibis feed and commingle at lower density, and forage for natural food such as invertebrates. At the mud flat, ibis interaction was less frequent than the other areas. A poultry farm was used for observation to assess the interaction between ibis and food production animals such as chickens (layers and broilers) and cows. Newcastle disease virus can be devastating to poultry operations, and as carriers of the virus, ibis may be a potential vector. Wild birds, in general, may be considered as a possible vector for transmitting Salmonella to cows, chickens, and other farm animals.³ On the contrary, cows and chickens may already carry salmonellosis, and salmonellas can survive for long periods of time in the environment (2 years or more) in feces, dust, and litter, thus farms may be a potential source of Salmonella infection for ibis.¹

Table 1. Salmonella spp. cultured from cloacal swabs of free-ranging Australian white ibis

Table 2. Comparative frequency data for agonistic behavior among Australian white ibis at different locations^a

^aAll behaviors were regarded as having been directed toward another conspecific.

Although wild birds have frequently been implicated as reservoirs for human disease, human activity usually plays a part in facilitating the perpetuation of diseases in wild bird populations. One study that reported on gulls as vectors of Salmonella spp., demonstrated that gulls that feed at landfills or sewage treatment areas become infected with the same species of Salmonella found in humans.⁵ While there have not yet been any reported associations between cases of Salmonella infection in people and ibis in the Gold Coast region of Queensland (J. Bates, personal communication), exclusion of ibis from landfill sites would decrease the opportunity for ibis to obtain human pathogens from putrescible waste.

This report demonstrates that ibis may carry zoonotic pathogens, however, the potential for transmission of disease between ibis and humans can be greatly reduced through public education and simple management strategies such as not feeding ibis; adopting proper personal hygienic practices; and protecting food and water resources from contamination at public recreational areas. Similar precautions undertaken by farm managers may help reduce risk of disease transmission from ibis to domestic animals, since ibis may be involved in outbreaks of significant diseases such as avian influenza, Newcastle disease, or salmonellosis. Restriction of ibis from direct contact with chickens or chicken feces as well as livestock and their water resources may reduce the potential for disease transmission between ibis and food production animals on farms.

Acknowledgments

This project was made possible thanks to the support of The Center for Conservation Medicine at Tufts University School of Veterinary Medicine and the Ibis Management Coordination Group, Queensland, Australia. Special thanks to Jeffrey McKee, Phil Shaw, Hume Field, Barry Rodwel, Craig Smith, Kell Irwin, Brad McCall, Wendy Paton, Vicki Hicks, Gino Micalizzo, John Bates, and Gretchen Kaufman—all whose guidance and support throughout this project was instrumental in its completion.

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