Hepatozoon in grey squirrels (Sciurus carolinensis) trapped near Reading, Berkshire

Hepatozoon infections of Sciurus carolinensis were investigated by a 30-month capture/recapture trapping programme. Details of trapping methods, squirrel husbandry and blood sampling techniques are discussed.
Hepatozoon gametocytes infected blood monocytes and could be detected in blood smears or by concentration of leucocytes. From blood smears, 71% (154/218) of the squirrels were infected. Prevalence appeared to be influenced by host hormonal and breeding patterns. Significantly more adult males than adult females were infected (P<0.025). Infections were significantly more prevalent in adults overall and in adult males than in juvenile males (P< 0.001 in both cases), but not significantly different between female adults and juveniles (P>0.05). Prevalence rates were generally higher: (i) in summer and winter, when animals mate, compared to spring and autumn; and (ii) in 1984 than in 1983, possibly relating to differences of squirrel breeding success and juvenile recruitment in the two years. Parasitaemias were overdispersed in the sampled host population and significantly lower in females (38%) than in males (48%) (P<0.05), although there was no significant difference between the age classes. Animals, either recaptured or laboratory-maintained, showed chronic fluctuating parasitaemias with no obvious pattern. Squirrels with overt parasitaemias showed trophozoites and schizogonic stages of Hepatozoon in the lung and rarely in liver and spleen. Three out of 16 animals with no obvious parasitaemias had lung tissue stages of the parasite. Results suggest that Hepatozoon is more prevalent in grey squirrel populations than blood smears suggest.