Both methods were only able to identify a small number of parasites. (sciencedaily.com)
Clinical H. canis infection has a much higher number of parasites in the blood (parsitemia), so it is easier to identify with a blood test. (wagwalking.com)
So cats that have not been exposed previously may react as ill as kittens if they are suddenly confined to shelter and group - home situations where large numbers of parasite eggs (ova) are found. (2ndchance.info)