comparisons of infected and non-infected lizards may be misleading for
two reasons. First, such comparisons may obscure actual cause-and-effect
relationships, i.e. perhaps sickly lizards may simply be more prone to
infection with parasites. Foufopoulos (1999) has manipulated infections
ofP. chiricahuae in mountain spiny lizards and observed changes in
health status similar to some of those shown in Fig. 14.3, which argues
that malaria infection is the cause of the pathologies to be described here.
Secondly, some kinds of pathology may be dependent on parasitaemia,
and density-dependent virulence would be missed by simple com-
parisons of infected vs. non-infected hosts. Our studies have found that
some effects of parasitism are more severe with increasing parasitaemia
(change in social behaviour appears related to parasitaemia (Schall and
Dearing, 1987)), but other effects appear to be more or less independent of
parasitaemia (effects on blood haemoglobin levels (Schall, 1982)). Some
other measures are difficult to relate to parasitaemia – clutch size,
for example, is determined by long-term events and parasitaemia is
determined from a single blood smear (Schall, 1983).Parasite Virulence 297
Fig. 14.3. Summary of costs suffered by western fence lizards (Sceloporusoccidentalis)
when infected with the malaria parasitePlasmodiummexicanum.