Cases from an outbreak of avian metapneumovirus (aMPV) among broiler flocks in Mississippi are mostly coming with secondary infections, which can exacerbate disease severity.
Types A and B of aMPV, which affect both chickens and turkeys, have been spreading in the US since 2023, predominantly affecting the eastern states. An outbreak of type A was first detected in Mississippi in February 2025.
Previous research has pointed to a high prevalence in aMPV of diverse bacterial infections and from unusual infection sites. This prompted a research team led by Autumn Gregg, DVM, a poultry resident at Mississippi State University, to investigate the infections appearing in the outbreak.
Gregg presented the study results at the 2026 International Poultry Science Forum.
E. coli dominates; other threats lurk
The research team examined 68 cases from broilers and broiler breeders submitted to the university’s laboratory. They confirmed aMPV using PCR, which detects genetic material, and ELISA testing, which detects proteins. The researchers then swabbed lesions at necropsy for bacteria and used mass spectrometry, a rapid and sensitive technique, to identify the species found.
They found secondary bacterial coinfections in 58 of the cases. There were 177 individual bacterial isolates, with Escherichia coli accounting for 116, or over 65%, of them. Gregg noted that this dominance was “not a big surprise,” given E. coli’s common association with aMPV.
Enterococcus cecorum was the second most seen, occurring in 15 cases. Gallibacterium anatis, Staphylococcus aureus, S. agnetis, S. chromogenes, Salmonella spp. and Pasteurella multocida were also isolated in numerous cases.
Salmonella coinfections cause concern
Gregg pointed to the significance of three types of Salmonella of public health concern that her team isolated in the study: Infantis, Enteritidis and Kentucky.
“It’s important to remember the immunosuppressant effect of aMPV and the effect that that can have on public health when we send these birds for processing,” she said.
Disease links to infection sites
The research team observed severe signs of disease in the study, including respiratory and neurological symptoms. Some signs differed between broilers and broiler breeders, with neurological presentations more common in broiler breeders. Lesions also differed, with yolk peritonitis being the most common in broiler breeders and leg lesions being the most common in broilers.
Broilers also had higher mortality rates than the broiler breeders.
Given the range of clinical findings, where the positive samples were swabbed from in the birds’ bodies provided food for thought, Gregg noted.
“We isolated E. coli and G. anatis from the brain, head bone and subcutaneous tissue. Although we can’t confirm this, we think that this is a major contributing factor to all the neurological signs that we were observing in the broiler breeders.”
Gregg also drew attention to the isolates of S. agnetis from the innermost layer of the skin and from the ovaries of birds, and of S. chromogenes in the same skin tissue.
“When bacteria are isolated from these locations, we traditionally think of these as being less pathogenic or things we don’t have to worry about as much. But in some cases, these were the primary isolates that we had from those birds, so they could be causing the clinical signs of severe disease. This is concerning, and it shifted our thinking about these bacteria,” she said.
Thinking beyond the obvious
The study suggested that producers would be wise to put aside preconceptions about tackling aMPV outbreaks, Gregg explained.
“Both common and emerging bacteria were isolated from birds infected with aMPV, so it’s important to remember that when treating these birds, we’re not just dealing with E. coli. We may have a lot of other bacteria present as well, which can complicate treatment,” she added.
