No more silent spread: learn how advanced surveillance methods protect vaccinated duck flocks from avian flu outbreaks.
Research: Surveillance strategy for flocks of ducks vaccinated against the highly pathogenic avian influenza virus. Image credit: Pablesku/Shutterstock
In a recent article in the journal Emerging infectious diseasesResearchers used mathematical models to evaluate the effectiveness of highly pathogenic avian influenza (HPAI) detection strategies among vaccinated duck flocks in France.
Their results indicate that enhanced passive surveillance by testing dead birds weekly was the most effective surveillance method, while monthly testing of live birds was the least effective method.
Background
HPAI viruses have caused significant outbreaks globally, particularly since 2016, when HPAI clade 2.3.4.4b emerged. These viruses primarily affect birds, posing a major challenge to the poultry industry due to direct losses due to high disease and mortality rates and indirect economic impacts due to market disruptions and management and prevention efforts. Notably, in unvaccinated herds, HPAI outbreaks result in a 93% probability of virus transmission after initial infection. Additionally, there has been an increase in transmission of HPAI to mammals, raising concerns about its zoonotic potential despite the limited number of human cases.
France has been strongly affected by HPAI, particularly during the 2021-2022 epizootic, which saw nearly 1,400 outbreaks on poultry farms. The recurrence and scale of these epidemics have revealed the inadequacy of traditional biosecurity measures.
Therefore, the European Union (EU) authorized the vaccination of poultry in 2023. In October 2023, France launched a national duck vaccination campaign, focusing on ducks due to their significant contribution to transmission of HPAI and their high risk of viral spread.
However, vaccination introduces the risk of silent circulation of the virus, requiring strict surveillance protocols. France has launched a comprehensive surveillance program in accordance with EU regulations, involving active, passive and enhanced passive surveillance methods to detect HPAI early. Enhanced passive monitoring involves weekly testing of dead ducks, while active monitoring involves monthly testing of live ducks. This approach aims to control the virus and reassure international trading partners about the safety of vaccinated herds.
About the study
Researchers used a mechanistic model to evaluate the applicability of different surveillance strategies to detect HPAI. They simulated the transmission of HPAI within a typical group of 6,400 ducks raised for the production of foie gras. Ducks were vaccinated at 10 and 28 days of age, with the study focusing on the breeding phase of production.
The researchers used a stochastic sensitive, exposed, infectious, recovered or dead (SEIRD) model, classifying ducks into compartments. They modeled the introduction and transmission of the virus, testing different levels of effective vaccination coverage (70%, 80%, 90%). The model took into account various parameters, including transmission rate, mortality, case fatality risk and development of immunity.
The effectiveness of surveillance strategies was evaluated by simulating outbreaks and testing different surveillance protocols. These included passive surveillance (based on mortality thresholds), enhanced passive surveillance (regular testing of dead ducks) and active surveillance (testing of live ducks). Each strategy was evaluated for its sensitivity (ability to detect outbreaks) and timeliness (speed of detection).
Ultimately, the researchers conducted thousands of simulations to determine the best surveillance approach, comparing strategies based on the number of undetected outbreaks, outbreak size, and speed of detection. For example, in vaccinated herds with 90% effective coverage, the reproduction number dropped significantly from 16 in unvaccinated herds to 1.7 during the immunity phase. Enhanced passive surveillance appeared to be the most effective strategy.
Results
The study found that vaccinating duck flocks significantly reduced the likelihood of avian flu outbreaks. Without vaccination, the probability of an epidemic was 93%. With an effective vaccination coverage of 90%, this probability dropped to 38% during the transition phase (days 28 to 35) and to 8% during the immunity phase (after day 35). If the first infected duck was immune, the probability of an outbreak was further reduced to 3%.
Almost all ducks (99%) became contagious in unvaccinated flocks within 14 days. However, in flocks vaccinated with 90% coverage, only 0.3% of ducks became contagious during the immunity phase. Enhanced passive surveillance, particularly sampling five to seven dead ducks per week, was most effective, detecting 81 to 88 percent of outbreaks. Passive surveillance based on daily mortality showed lower sensitivity (28% to 64%), and active surveillance involving live bird sampling was the least sensitive, with delays of up to 28 days.
Overall, only 7% of outbreaks were not detected in vaccinated flocks, with a median of eight infectious ducks and a minimal number of infection-related deaths. Undetected outbreaks were generally characterized by very low herd prevalence and short outbreak duration.
Conclusions
Vaccination against HPAI in poultry significantly reduces outbreaks and the spread of the virus. In France (2023-2024), only two outbreaks occurred in vaccinated duck farms, reflecting a reduction of 92 to 98% in the expected size of the epizootic.
However, vaccinated herds show fewer signs of disease, making passive surveillance less effective. Enhanced passive surveillance, involving virological testing of dead ducks, was found to be the most sensitive and timely. The study found a 10-fold decrease in outbreaks and a 100-fold reduction in the number of infectious ducks during the immunity phase in vaccinated flocks. Although no single surveillance strategy has been fully effective, combining enhanced passive and active strategies could improve detection. Future research should explore alternative methods and consider factors such as cost and workload.
