Newcastle disease (ND) is spread through direct contact with infected birds, their feces and secretions. Common insects associated with poultry production may readily feed upon these discharges. During the 1970s, seven species of flies were collected from ND-positive farms, but only the little house fly was incriminated in the transmission of ND. Remarkably, pools of house flies were not positive for ND virus. Like the lesser house fly, manure and excreted body fluids provide the house fly with much of its nutritional needs. Although the house fly has been implicated in the transmission of over 30 bacterial, protozoan, and viral diseases its role as vector for ND Virus transmission has not been examined.
The proposed study examined the potential of the house fly in the transmission of Newcastle disease virus. Specific objectives were 1) determine if NDV survives in crop and/or intestines of house flies fed virus-laden media; 2) determine longevity of virus survival in fly tissues; 3) determine if NDV actively replicates in fly tissues; and 4) determine if NDV-exposed flies transmit disease to chickens. These studies were conducted using the mesogenic vaccine strain of NDV (Roakin).
The NDV was propagated by inoculation of 10-day-old embryonating chicken eggs. Virus titer was determined by calculation of the 50 percent cell culture infectious dose (CCID50). An inoculum was prepared to contain approximately 107 CCID50/0.1 ml. Caged adult house flies were fed virus. Pools of 40 flies were removed 0.5, 1, 3, 6, 9, 12, 24 and 96 hrs postfeeding and immediately frozen to kill the flies and preserve the virus. Crops and guts were removed from the frozen flies and examined for the presence of virus using cell culture. Fly crops harbored live ND virus for 96 hours post feeding. Similarly, fly guts harbored live virus for 24 hours. The quantity of virus in the fly tissues decreased at each time interval indicating that the virus was not replicating. Virus titers were highest 30 minutes after feeding, with NDV logs of 103 and 107 observed in the crop and gut, respectively.
Transmission of NDV was studied by exposing 14-day old pathogen free chicks to virus-infected house flies. Ten chicks each were exposed to 0, 1, 10 and 25 infected flies per isolation unit. Chicks were observed eating the flies and few flies remained alive the next day. One group of flies (27) was held in reserve to confirm the presence of infectious virus. Infected flies carried a NDV titer of 3.75 X 104 CCID50. Twenty-one days after exposure to the infected flies, no chicks had developed clinical signs of NDV infection and no chicks were sero-positive for NDV.
Following a single feeding, the house fly harbored infectious NDV in both crop and gut tissues for 96 and 24 hours, respectively. NDV titers decreased, not replicating in fly tissues. In controlled experiments the house fly did not transmit as much as 104 logs of virus. The infectious dose (ID50) required to infect 50 percent of the chicks was determined to be 106 logs of virus administered by oral gavage. Intranasal administration of 104 logs of virus caused infection in all susceptible chicks. The house fly carried an infectious dose (oral) in the gut for three hours post feeding and might be important for the spread of NDV when fly populations are high, especially with highly virulent velogenic strains of NDV.
Recently completed research funded by the U.S. Poultry & Egg Association studies the role of the house fly, Musca domestica, in the transmission of Newcastle Disease Virus. The study is part of the association's extensive industry research program encompassing all segments of broiler, turkey, and commercial egg operations. More than $3 million is currently invested in research.
Additional information may be obtained from the authors on the date and place of publication of the data generated by this research project.
D. Wes Watson, Ph.D., Department of Entomology
James S. Guy, Ph.D., Department of Microbiology, Pathology, and Parasitology, North Carolina State University, Raleigh, N.C. 27695