Jeno Reiczigel

Quantifying parasites in samples of hosts.

Whereas terminological recommendations require authors to use mean intensity or mean abundance to quantify parasites in a sample of hosts, awkward statistical limitations also force them to use either the median or the geometric mean of these measures when making comparisons across different samples. Here, we propose to reconsider this inconsistent practice by giving priority to biological realism in the interpretation of different statistical descriptors and choosing the statistical tools appropriate to our decisions. Prevalence, mean intensity, and indices of parasite distribution (such as median intensity) are suitable descriptors to quantify parasites in a sample of hosts. These measures have different biological interpretations and need different statistical methods to be compared between samples.

HOST-MEDIATED SITE SEGREGATION OF ECTOPARASITES: AN INDIVIDUAL-BASED SIMULATION STUDY

Site segregation of coexisting ectoparasite species may result either from a direct interaction such as resource competition between them or from a host-mediated interaction. Here we present an individual-based model for the coevolution of 1 host and 2 parasite species to study this latter hypothesis. Parasite species are generalists at the start of the simulation and develop site specificities under the following assumptions. Parasite populations are not subject to resource limitations but are limited directly by host defense as predation. Hosts have 2 sites that need different defensive abilities to reduce their parasite burden. Parasites need to exhibit different evasive abilities to survive on different sites. Host grooming selects parasites for an increasing capability for evasion, whereas parasites select hosts for an increasing efficiency of grooming. Two trade-offs are incorporated into the model: one between host defensive abilities on the 2 sites, and another between parasite evasive abilities on the 2 sites. We conclude that, under these assumptions, the optimization of host defense and parasite evasion strategies may select ectoparasites for site segregation and this may stabilize the coexistence of parasite species.

PRESSURE–VOLUME INDEX-BASED VOLUME CALCULATION OF CONTRAST MEDIUM FOR ATLANTO-OCCIPITAL MYELOGRAPHY IN DOGS

Subarachnoid pressure recordings were made during atlanto-occipital myelography in 45 dogs with clinical signs of spinal disease. Iohexol was injected at a dosage of 0.3 ml/kg body weight and simultaneous pressure values were recorded in the cerebellomedullary cistern. The mean subarachnoid pressure was 9 ± 3 mmHg before and 70 ± 32 mmHg at the end of administration. From the pressure change induced by the volume load, the pressure-volume index (PVI) of the subarachnoid space was calculated and found to be in close correlation with body weight and the crown-rump length (r = 0.94 and 0.87). Using the estimated PVI values, the appropriate volume of contrast medium can be calculated for an animal according to body weight. Dogs of a large body size require relatively less contrast medium than small-sized dogs (range 0.17-0.35 ml/kg). This calculated volume is unlikely to increase the subarachnoid pressure above 40 mmHg as a specific pressure limit. Using these data, simplified recommendations for the choice of contrast medium volumes have been generated.