Assessing the Relative Contribution of Pre‐Deployment Abroad Versus In‐Mission Dirofilaria immitis Infections in French Military Working Dogs Screened Post‐Mission: A Quantitative Risk Assessment

Dirofilaria immitis (D. immitis) is a mosquito-borne nematode responsible for canine cardiopulmonary dirofilariasis, a parasitic infection widely distributed worldwide and established in mainland France, particularly in southern regions. Military working dogs (MWDs) may be at increased risk of exposure to vector-borne pathogens because of their predominantly outdoor lifestyle and frequent missions abroad. Nevertheless, D. immitis infection remains sporadic in this population, likely due to well-implemented medical prophylaxis. The aim of this study was to investigate the likely origin of D. immitis infections detected in MWD during post-mission screening using a quantitative risk analysis approach. Scenario trees were developed to describe the sequences of events leading to the return of an infected dog from a mission abroad, accounting for parasite biology, prophylactic measures, and compliance. The model estimated a mean annual number of 4.25 infected MWD returning from missions out of an average annual population of 466 returning dogs, of which 79% were detected through post-mission screening. Most infections were likely acquired before deployment, either during previous missions, at the breeding facility before purchase, or within mainland France. Only 7.9% of infections acquired during missions were expected to be detected by testing performed at the end of the post-mission quarantine period. Alternative scenarios highlighted the effectiveness of chemoprophylaxis, as its absence was associated with a 3.2-fold increase in the number of infected dogs. Simulations also showed that extending the interval between return and testing would substantially improve the detection of mission-acquired infections. These findings support the continued use of preventive measures while questioning the adequacy of current post-mission screening alone for identifying D. immitis infections acquired during deployment.