South Central Idaho recorded 59 quarantined dairy herds and 86 individual cattle confirmations across four counties by 12 May , compared with one confirmed herd twelve days earlier. 1 The national US dairy total crossed 1,047 confirmed cases across 17 states since the outbreak began in March 2024. Idaho moved in twelve days to second place nationally behind California, which has led cumulative totals throughout.
In the same period, H5N1 was confirmed in domestic cats in Washington (King and Snohomish counties) and Oregon, and in alpacas in Idaho. The mammalian host list now spans dairy cattle, domestic cats, and camelids. Domestic cats are not milked; their route to infection is more likely rodent prey or raw milk consumption, and they spend more time in household environments than dairy cows. The alpaca confirmations inside Idaho reinforce that transmission is happening across species sharing farm premises rather than confining itself to the dairy production chain.
The CDC human case counter remained at 71 cases since 6 March. The NIH/NIAID nasal epithelium study published on 12 May in CDC Emerging Infectious Diseases identifies the likely partial explanation: 66% of exposed dairy workers carry pre-existing pdm09 H1N1 cross-immunity. The four human H5N1 cases confirmed across Asia by February were all tied to poultry contact, not dairy cattle , underscoring that the livestock-to-human transmission pathway remains less well-characterised than the avian route.
The only concurrent defensive advance against this expanding picture is the CEPI and Moderna Phase 3 mRNA H5N1 vaccine trial that began on 22 April . A Phase 3 trial measures efficacy, not production capacity; regulatory approval and deployment at scale would take years beyond trial completion. The surveillance contraction and the outbreak geography are both moving faster than any defensive countermeasure.
