We are advised by Drs Sinden and Smalley that in their paper which appeared in Parasitology79, 277–96, the drug concentrations in Tables 1–5 were given as mg/ml but should read μg/ml.

The shore crabs, Hemigrapsus oregonensis and H. nudus, sometimes kill the female endoparasitic entoniscid isopod, Portunion conformis, a parasitic castrator. Studies of host populations from Baja California, Mexico to Vancouver Island, Canada, show that the incidence of parasitized hosts with dead parasites and the percentage of the parasite population found dead vary markedly with locality but only occasionally with season. Both higher incidences of hosts with dead P. conformis and higher proportions of the total parasite population found dead are associated with (1) high prevalence of parasitism, (2) female hosts and (3) large hosts. Within a host, the proportion of the parasites that are dead is not related to the degree of multiple infection. Typically, either all or none of the parasites in a multiple infection are dead. Supernumerary juvenile parasites do not suffer differential mortality. The developmental stage of the female parasite does not seem to influence the likelihood of death. The presence of dead parasites may not confer an acquired immunity to re-infection. These features suggest that parasite death is typically the result of activation of a successful host defensive process rather than indicative of a defect on the part of the parasites. Parasitized female hosts can regain their reproductive capabilities following death of the parasite. Post-parasitic broods are smaller than normal. Reproductive recovery is presumed to provide the selective pressure favouring evolution of a lethal host response. The host-produced sheath surrounding female parasites is a haemocytic response. Sheaths enclosing dead parasites are thicker and more electron dense than those containing healthy parasites. The sheath of a healthy P. conformis may actually protect the parasite from a more intense host response.

The response of Awassi sheep to primary infection with 10000 Haemonchus contortus larvae and subsequent challenge with 20000 larvae was studied. Two types of response to re-infection were observed. The first, characterized by a sharp decline in egg counts, progressive improvement in packed cell volume (PCV) and a marked loss of adult worms and challenge larvae, was considered as self-cure and protection. In the second type of response there was a temporary decline in egg counts, no improvement in PCV and necropsy findings indicated hyperinfection with challenge larvae.

Specimens of Notocotylus attenuatus were examined in life and their ventral papillae studied by scanning and transmission electron microscopy. It was found that the papillae are non-secretory, very mobile and similar in ultrastructure to the papillae of other notocotylids previously described. Worms were removed from the host's caecum and the caecum surface examined where it had been in contact with the papillae; this revealed that the crypts in this region had been distorted and enlarged. It is suggested that the papillae serve to attach the worms to the distinct surface of the distal caecum by acting as hydraulically operated anchors working in conjunction with the lateral margins of the worm and allowing the oral sucker of this monostome fluke to be used for feeding.

During the course of Schistosoma mansoni murine infection there is a dramatic increase of some immunoglobulins and S. mansoni-specinc antibodies. The most substantial response is initiated after 40 days of infection and results in a prolonged increase of total IgG1; IgM and IgA. The maximum increase is respectively 26, 14 and 3-fold the basic immunoglobulin level in control mice. Some anti-S. mansoni classes and sub-classes were studied by an original radio-immunoadsorbent test. Anti-S. mansoni IgG1 and IgM antibodies appear and increase at the same time as that of total IgG1 and IgM. Anti-S. mansoni IgA antibodies appear later (80th day) and correspond to a second peak of total IgA.

A living hamster cheek pouch preparation has been used to study the skin phase of schistosomulum migration. Between 0 and 16 h post-exposure schistosomula become increasingly difficult to see and lateral movement in the skin ceases. Up to 50 h post-exposure they remain poorly visible. They lie in fluid-filled lacunae within the epidermis, external to the basement membrane. The first schistosomula enter the dermis from. 40 h post-exposure, approximately half having done so by 52·5 h. Coincidentally, they can be more easily distinguished from surrounding tissue. Up to 91% of original penetrants have been observed alive in. pouch tissue at 65 h post-exposure. Since only 26% eventually mature in the hepatic portal system there would appear to be considerable attrition subsequent to the skin phase of migration. The mean diameter of venules penetrated was 22·7 μm and the mean time taken to penetrate was 8·01 h. The majority of schistosomula left the pouch between 60 and 80 h post-exposure.

The pattern of recovery of schistosomula from the lungs of mice, hamsters and rats is described following normal percutaneous infection via the abdominal skin. Peak numbers were found in hamsters and rats on day 5 and in mice on day 6 post-exposure. Schistosomula were recovered from the lungs up to approximately day 20 post-exposure in all 3 species. None were found in pleural washings and only 2 were recovered following mincing and incubation of host diaphragms. The pattern of recovery of schistosomula from the lungs of mice was also described following the injection of a pulse of parasites into the tail vein. Approximately 54% of these injected schistosomula eventually reached maturity in the hepatic portal system. The ability of schistosomula to migrate and mature following their injection into unusual locations was tested. Small numbers reached maturity in the hepatic portal system following injection into the pleural cavity or subcutaneously, more when injected intra-peritoneally and largest numbers following intravenous injection. Small numbers of schistosomula were recovered by mincing and incubation of systemic organs such as the kidney and spleen. Schistosomula injected into the left ventricle of hamsters were able to migrate via the systemic organs to the lungs and 47% eventually matured in the hepatic portal system. The pattern of schistosomulum accumulation in the hepatic portal system of mice, hamsters and rats is described. Following normal percutaneous infection, schistosomula were first detected in this site from day 6 to day 9 depending on the host species. In mice, migration was complete around day 20 but continued in hamsters up to day 40 post-exposure. Following injection of schistosomula into the tail veins of mice, the first arrivals in the hepatic portal system were detected 12 h later and were found to lose their ability to migrate shortly after arrival. However, a proportion of lung schistosomula injected directly into the hepatic portal system were able to traverse hepatic sinusoids and reach the lungs. It was concluded that the route of migration of schistosomula from lungs to liver was entirely intravascular, with potentially several passages round the pulmonary-systemic circulation, before chance entry into arteries leading to the hepatic portal system. The proportion of schistosomula exiting from the pulmonary-systemic circuit was estimated as 0·14/day. A computer simulation produced values of 11 h and 5 h for the duration of migration through pulmonary and systemic capillary beds. The time for 1 complete circuit would thus be 16 h and the proportion of schistosomula exiting per circuit would be 0·095. This approximates to the proportion of cardiac output going to splanchnic organs in the resting rat (0·128). The narrowness of hepatic sinusoids may be one factor contributing to the sequestration of schistosomula in the hepatic portal system.

New or improved techniques for recovering Schistosoma mansoni from the skin, lungs and liver have enabled us to trace the attrition of a challenge infection in naive (i.e. previously uninfected) and chronically infected mice. Within each experiment, the numbers of schistosomes recovered from the skin of naive mice on day 2 after challenge or from the skin and lungs on days 3, 4 or 5, did not differ significantly from the numbers recovered from the liver on days 14, 21, 28 or 35. Approximately 65% of cercariae which penetrated the skin failed to be recovered from naive mice by any of the assays and it appeared that these schistosomes had already died in the skin in the first 24 h. No further significant loss of the infection was detected in naive mice. In chronically infected mice a further attrition of the challenge infection was demonstrated in two distinct phases. An ‘early phase’ occurred within the first 3 days of exposure and accounted for the death of 30% of the remaining parasites. A ‘late phase’ occurred between days 6 and 14 and accounted for an additional 43% of deaths. Thus, the two phases of attrition accounted for a loss of approximately 73% of the infection that would have survived in naive mice. The late phase of attrition could be demonstrated before the primary infection had matured, in contrast to the early phase of attrition which was seen only after egg laying had commenced. We believe that the early phase of attrition takes place in the skin and the late phase occurs after the schistosomes have left the lungs, either en route for the liver or as soon as they arrive in that organ. The results suggest that there are two distinct mechanisms of immunity against re-infection with S. mansoni in mice.

A technique is described for the isolation of a pure suspension of second-generation schizonts of Eimeria necatrix and E. tenella. Most of the schizonts appeared viable as judged by their ability to exclude Trypan blue and the use of this criterion is examined. They were able to metabolize radio-isotope labelled glucose and accumulate thiamine against a concentration gradient. A formula allowing detailed consideration of the latter process is introduced. Crude extracts of these schizonts exhibited serine hydroxymethyl transferase activity which is significantly different from that of the host tissue in terms of optimal pH (8·3 for the parasite and 7·8 for the avian intestinal cells) and Michaelis constant (approximately 2·6 × 10−3m and 0·6 × 10−3m respectively). Differences in response to potential inhibitors are also shown.

The uptake of thiamine by isolated second-generation schizonts of Eimeria tenella and by host intestinal cells was found to consist of two components. One was passive and the other apparently an active process. The kinetic constants of the latter were compared in host (Km = 0·36 μm) and parasite (Km = 0·07 μm) and found to be significantly different. Both systems were competitively inhibited by amprolium but showed different affinities for the drug. (Host Ki = 326 μm; parasite Ki = 7·6 μm). Further differences were observed in schizonts of a drug-resistant line of E. tenella. These findings are discussed in terms of the inhibition of thiamine uptake being the basis of the anticoccidial activity of amprolium.

Extracts of rhesus monkey erythrocytes infected with Plasmodium knowlesi were fractionated by polyacrylamide gel electrophoresis (PAGE) and several zones of endopeptidase activity were demonstrated by an imprint-digest method. The enzymes were active only under acid conditions; activity was detected at pH 3·2 but not between pH 6·4 and 8·9 using haemoglobin, albumin or erythrocyte lysate as the substrate. Optimized PAGE conditions separated highly active parasite enzymes with Rf values of 73, 63 and 53 (± 7%), as well as a red cell endopeptidase, Rf44. Of two other minor bands of activity, one was associated with platelets.

A set of n-(9-anthroyloxy) fatty acids (n = 2, 6, 9, 12, 16) have been used as fluorescent probes to examine the lipid environment at different depths in the outer membrane of normal mouse erythrocytes and red blood cells from Plasmodium berghei-infected blood. Fluorescent polarization experiments with normal mouse erythrocytes have demonstrated a typical gradient in microviscosity from the surface to the centre of the bilayer as a consequence of the motional properties of the C-atoms of the phospholipid acyl chains. The fluorescent probes rotate faster in the membrane of purified pluriparasitized cells (> 90% purity) than with the remaining fraction of red blood cells from infected blood (20–40% immature, infected red cells, and uninfected red cells), or normal mouse erythrocytes. This increase in fluidity with heavily infected cells occurs predominantly at the centre of the lipid bilayer, rather than at the membrane surface. A comparison of the polarization values of intact and lysed infected cells indicates that the fluorescent fatty acids preferentially label the plasma membrane rather than the internal membranes of infected cells. The results suggest that P. berghei infection causes a change in the composition and/or organization of the outer membrane of pluriparasitized cells which produces a decrease in membrane microviscosity.

African trypanosomes can undergo antigenic variation and evade the host immune response. Whether the antigenic variants arise in an ordered sequence or randomly has been in dispute but has not been statistically tested. The coefficient of concordance (W), a statistic designed to detect similarities between sequences of objects, was applied to the literature data. The tendency towards a reproducible order of variants was strong, although in several of the studies the number of experimental animals was so low that no conclusions could be drawn. A computer model was used to determine whether this degree of order could arise with random generation of variants followed by selection. The model simulated a trypanosome clone with 90 possible variants, widely differing variant-specific growth rates, random variant origin and variant eradication by an anamnestic host immune response. Parameters varied were maximum parasitaemia, growth rate differential between ‘fast’ and ‘slow’ variants, and parasitologist ability to detect minor variants. Random generation and selection by growth rate alone could not produce the degree of variant orderliness reported in the literature. However, experiments with larger numbers of host animals and direct investigation of variant growth rates and competitive interactions are necessary before the random generation-selection hypothesis can be proven or disproven.

Eighteen clones of variable antigen type 052 of Trypanosoma brucei stock S. 427 were derived and maintained as animal-infective bloodstream forms in vitro for up to 60 days of cultivation. The antigenic composition of such clones was monitored weekly by immunofluorescent analysis of viable trypanosomes, using antisera raised to isolated variant-specific surface glycoproteins of both 052 and a variable antigen type (221) which consistently appeared in the first relapse population of type 052 in vitro. The appearance of new variants was detected in 9 of the 18 clones 18–46 days following initiation of the clone and variable antigen type 221 was found in all 9 clones. On one or more occasions in 8 of such clones, viable trypanosomes were found which did not react with either antiserum but were mouse-infective on the 4 occasions tested and probably represent other variable antigen types. The process of antigen, variation in vitro appears to resemble the process in vivo except that new variant types are detected earlier in vivo. This possibly results from different growth rates of the trypanosomes in vivo and in vitro, together with the fact that elimination of the initial variant population by the host's immune response facilitates the detection of newly arising variable antigen types in vivo.

Clones of animal-infective bloodstream forms of Trypanosoma brucei (stocks S.427 and LUMP 227) were made by transferring a single organism from bloodstream-form cultures into each well of Microtest II Tissue Culture Plates containing bovine fibroblast-like feeder cells. When the number of trypanosomes increased to 102–103/well on days 4–16, they were transferred into plastic T-25 culture flasks also containing feeder cells and fresh medium. Cultures were thereafter maintained by partially replacing the trypanosome suspension with the same volume of fresh medium (diluting the density to 2–5 × 105 trypanosomes/ml) every 24 h. Sub-cultivations could be made by transferring 1–2 ml of the trypanosome suspension to a new culture flask at 4–5 day intervals. A total of 42 clones in the 3 series TC221, TC52 and TC227, carrying variable antigen types (VATs) 221, 052 and ILTat 1·4, respectively, have been established. Average population doubling times for clones of TC221, TC52 and TC227 were 8·7, 14·5 and 15·5 h respectively. Of 35 populations examined, 34 clones retained the original specificity of their VATs for at least 8–32 days from cloning. One cloned population of TC52 consisted of 99·8% VAT 052 and 0·2% VAT 221 at the time when the first VAT test was made on day 18.

Forms of Trypanosoma brucei isolated from lymph nodes of cyclically or intravenously infected rats, at different stages of infection, were morphologically compared with the corresponding bloodstream forms. Unlike the pleomorphic bloodstream trypanosome population, the lymph node trypanosome population was essentially monomorphic and resembled the slender forms of the bloodstream. Values for the morphometric parameters indicated a position between the slender and the stumpy forms. The number of trypanosomes/ml of blood showed the well-known periodic fluctuations while the number of trypanosomes/g of lymph node remained fairly constant. Serology revealed that lymph node trypanosome populations differed antigenically from the trypanosome populations isolated simultaneously from the bloodstream.