Correlations between host phenotype and vulnerability to parasites can clarify the processes that enhance rates of parasitism, and the effects of parasites on their hosts. We studied an invasive parasite (the pentastome Raillietiella frenatus, subclass Pentastomida, order Cephalobaenida) infecting a new host (the invasive cane toad Rhinella marina), in tropical Australia. We dissected toads over a 27-month period to investigate seasonal changes in pentastome population dynamics and establish which aspects of host phenotype are related to infection. Pentastome prevalence and intensity varied seasonally; male toads were 4 times more likely to be infected than were females; and prevalence was highest in hosts of intermediate body size. The strong sex effect may reflect habitat or dietary divergence between the sexes, resulting in males encountering parasites more often. The relationship between pentastome prevalence and host size likely reflects a role for acquired immunity in preventing re-infection. Infection did not influence host body condition (fatbody size), suggesting that R. frenatus does not impose high energy costs in cane toads. Infected toads had heavier spleens (likely an immune response to infection) and larger testes (perhaps since reproductively active hosts have altered microhabitat use and/or immunocompetence) than did uninfected conspecifics. Although experimental studies are required to identify the causal bases of such patterns, our data confirm that infection status within a population can be strongly linked to host phenotypic traits.

Adults of the porocephalid pentastomid Porocephalus crotali infect the lung of rattlesnake definitive hosts and larvae develop in rat intermediate hosts. In the latter, nymphs encyst within a variety of tissue sites (commonly abdominal fat bodies and lungs) and each becomes the focus of an eosinophilic granuloma. From an early stage in infections, granulomas become increasingly infiltrated by mast cells which, using conventional histology and paired immunofluorescence against mast cell proteases, appear to be exclusively of the mucosal phenotype. Mucosal mast cells are concentrated along the dorsal region of the parasite and in a plug of tissue containing degenerating cuticles within independent granulomas, which is located between its head and tail. ELISAs against the rat mast cell proteases I and II (RMCP I and II), extracted from abdominal fat, lung, spleen, liver and kidney granulomas at various intervals post-infection, reveal a substantially elevated concentration of RMCP II in all lesions. In fat, concentrations increase up to about 100 days post-infection, at which time moulting ceases and inflammatory responses subside. RMCP II was scarcely detectable in matched control tissues. Unlike infections with certain nematode parasites, where enteric mucosal mast cells secrete RMCP II systemically, concentrations of RMCP II in the serum of infected rats were significantly reduced when compared with age-matched uninfected controls. These results confirm that P. crotali can selectively recruit mucosal mast cells to a variety of tissue sites, most of which are non-mucosal. Possible mechanisms are discussed.

A specific enzyme-linked immunosorbent assay, based on a 48 kDa metallo-proteinase isolated from the frontal glands of a porocephalid pentastomid (Porocephalus crotali) in rat intermediate hosts, is described. SDS–PAGE analysis of whole nymphal homogenate, followed by Western blotting and probing with immune rat serum, revealed that only 2 protein bands label consistently; these are the 48 kDa metallo-proteinase and a 150 kDa metallo-proteinase from the body fluid, but only the former labels strongly. Furthermore this response is detected earlier and persists much longer. These characteristics uniquely qualify the 48 kDa metallo-proteinase as a diagnostic antigen. We review the literature concerning the related porocephalid Linguatula serrata from the nasal cavities of domestic dogs, which can zoonotically infect man principally through the ingestion of eggs. We conclude that the high prevalences in dogs in certain areas, most notably the Middle East, must inevitably signal significant human infections. Our ELISA can be readily adapted for the serodiagnosis of linguatulosis and this should establish whether our prediction is true.

All instars of the pentastomid Porocephalus crotali, in the tissues of rat intermediate hosts and the lung of rattlesnake definitive hosts, are covered by a vesicular or stacked membranous secretory product which is synthesized in sub-parietal cells (SPC) and channelled to the cuticle via multitudinous chitin-lined ducts. In rats this enveloping foam of vesicles survives for the duration of a cuticle. We have purified and partially characterized the lipid and protein composition of lamellate droplets from SPC of infective nymphs. Lipids in the droplets comprised a mixture of neutral and polar lipid classes with cholesterol being the major neutral lipid and phosphatidylcholine the dominant polar lipid. In addition, Triton X-114 phase separation of the protein component of droplets partitioned these into an aqueous phase (with a major band at 60 kDa), a detergent phase with two hydrophobic polypeptides (24 and 16 kDa) and an insoluble pellet containing several minor proteins and major bands at 31 and 107 kDa. Western blotting, with rabbit anti-lamellate droplet antiserum, strongly label only the 60 kDa and the two hydrophobic proteins. Significantly no proteins at all label with serum from infected rats from 50–150 days post-infection, a finding endorsed by indirect fluorescent antibody tests on sectioned V–VII stage nymphs. Possible immunomodulatory functions of SPC-derived surface membranes are discussed and, in this regard, they are compared with pulmonary surfactant, a product of alveolar Type II cells which lines all vertebrate lungs.

The in vitro development of the pentastomid Porocephalus crotali from the infective, seventh (VII) instar, dissected from the tissues of rat intermediate hosts, to the adult male (X) and female (XI) instars, normally resident in the lung of rattlesnake definitive hosts, is described. The culture medium comprised washed human blood cells, resuspended in bovine serum (50[ratio ]50, v/v), with 20% minimum essential medium and antibiotics. Two batches of approximately 100 pentastomids, maintained at a density of 2 worms/ml, were cultured in 500 ml bottles in an atmosphere of 5% CO2 in air at 28°C. Culture bottles were rotated slowly on a Rollacell system and the medium was replenished every 2–3 days. Growth was monitored at various intervals by weighing worms, measuring cast cuticles with attached moulted hooks recovered from the medium, and by dissecting worms for measurements of various organ systems. Three moults separated the infective and adult male instar, whereas 4 moults were necessary in the case of females. In both cultures natural mortality was about 10% over a 160-day period, by which time 33–51% of females and 62–70% of males had reached the terminal instar. Some males attained full sexual maturity with seminal vesicles loaded with viable sperm: such males were indistinguishable from males recovered from naturally infected rattlesnakes. However, females never achieved full size, even after 200 days, and although copulation did not occur in vitro, some females became patent.