Influenza-induced and subtype-dependent thrombocytopenia in humans and ferrets. (A) The blood platelet count and viral RNA load were inversely correlated in 2009 influenza A/H1N1 virus–infected patients (n = 34). Pearson’s r = −0.45; 95% CI, −0.68 to −0.14. (B) Experimental setup: ferrets inoculated with seasonal A/H3N2 (n = 24), pandemic A/H1N1 (n = 24), or A/H5N1 (n = 20) influenza virus with increasing disease severity in humans and ferrets.³⁹  Arrows: the virus replication sites in the URT and LRT of both humans and ferrets with similar α2,3- and α2,6-sialoglycan receptor distributions. (C) An inverse correlation is shown between platelet count and viral loads (PCR) in throat swabs of A/H5N1 virus–infected ferrets (n = 20). Pearson’s r = −0.69; 95% CI, −0.88 to −0.33. (D) Platelet counts and viral loads (PCR) were inversely correlated in nasal swabs of A/H5N1 virus-infected ferrets (n = 20). Pearson’s r = −0.49; 95% CI, −0.78 to −.03. (E) There was no significant correlation in A/H3N2 (n = 24) and A/H1N1 (n = 24) virus-infected animals. (F) Platelet counts at about the peak of virus infection (days 2-4; n = 12). Error bars, SD; Student t test (unpaired). (G) Top: mean lung virus titer in ferrets (n = 4). Error bars, SD. Bottom: blood platelet count. Mean (n = 4) platelet count. Error bars, SD; Student t test (unpaired). (C-G) Dotted lines indicate the lower limit of normal platelet levels in the ferret (729 × 10⁹/L ± 360 × 10⁹/L). *P < .05; **P < .01; ***P < .001, ****P < .0001.