Transstadial transmission of Theileria annulata with Hyalomma anatolicum anatolicum, H. dromedarii and H. marginatum isaaci, and Haemaphysalis bispinosa, Rhipicephalus haemaphysaloides haema-physaloides and Boophilus microplus was determined.

It was found that the infection was successfully transmitted by H. a. anatolicum from larva to nymph and nymph to adult in all attempts. When larvae were fed on an infected calf the succeeding adults transmitted the infection when the intervening nymphs fed on a rabbit (nonsusceptible host) but not when fed on a calf (susceptible host). Infective adult ticks transmitted the parasite during the first 24 h of feeding on a calf. When the feeding was interrupted after 24 h, and the tick transferred to another calf, the infection was transmitted to the latter as well.

H. dromedarii successfully transmitted the infection from larva to nymph and from nymph to adult.

Larvae of H. marginatum isaaci did not feed on calves but the infection was successfully transmitted from nymph to adult.

Haemaphysalis bispinosa, Rhipicephalus h. haemaphysaloides and Boophilus microplus did not transmit Th. annulata from larva to nymph to adult.

The hind-body region of Schistosoma mansoni cercariae observed in the scanning electron microscope demonstrates various stages of contraction which may be compared with those of living larvae which are secreting the acetabular gland contents.

No evidence for an extensive lesion was found in cercarial bodies which had shed their tails under experimental conditions. Experiments on the permeability of the larvae to sodium fluoride, methylene blue and amino acids demonstrated that tail loss significantly affects the permeability of the bodies although the effect is greater immediately after decaudation than at later times. Subsequent increases in permeability may be correlated with a change in the general body surface.

The effect of different incubation media and of temperature on the induction of water sensitivity has been investigated in intact and tailless Schistosoma mansoni cercariae. Removal of the cercarial tail by vortex stirring and elevation of the temperature of the medium from 27 to 37°C resulted in the rapid onset of permeability changes in the larvae. The rate of change was greater in water than in TC-199 or Hanks' BSS media. Lowering the pH of the medium or increasing the concentration of Ca2+ ions decreased the rate of permeability change: raising the pH of the medium or the addition of 10−5 M EDTA enhanced the rate.

Raising the temperature of the medium also increased the rate of permeability change in intact cercariae although the rates obtained varied with the different media tested, being greatest in TC-199.

It is concluded that both temperature elevation and loss of the cercarial tail influence the onset and rate of permeability changes in cercarial bodies during the transformation to schistosomula.

Adult Rhipicephalus appendiculatus ticks, infected as nymphs by feeding them on an African buffalo harbouring Theileria lawrencei, were applied to the ears of rabbits.

Equal numbers of the feeding ticks were removed daily from rabbits and either dissected and processed so that parasites in their salivary glands could be examined morphologically and classified, or ground in Eagle's Minimal Essential Medium, to harvest parasites from the ticks in the resultant supernatant fluid. The infectivity of the supernatant fluid was tested by inoculation of aliquots into groups of susceptible cattle.

It was found that supernatant fluids produced from day 0 to day 2 fed ticks were uninfective to cattle but those produced from 3 to 9 days were infective. The most infective supernatant fluid was produced from 6 day fed ticks.

The reactions resulting in cattle were correlated with the number and morphology of parasites in sections of salivary glands and it was found that mature parasites coincided with infectivity of the supernatant fluid to cattle.

From the reactions in cattle inoculated with supernatant fluids from 5 and 6 day fed ticks it was concluded that this method of harvesting parasites was efficient.

Examination of the salivary glands of a representative sample of a T. lawrencei infected tick batch is a potential means of screening material for T. lawrencei stabilates.

Ectoparasite-free, SPF Italian Wistar rats were consistently found to carry a latent infection with Haemobartonella muris, activable by splen ectomy. In an inbred line this diminished and eventually ceased in six generations. Experimental infection from wild rats demonstrated that this was not apparently due to immunity.

The development of drug resistance by the present Houghton strain of Eimeria tenella to the quinolones, methyl benzoquate and buquinolate, was found to take place after a single experimental passage. The development of resistance was independent of drug selection pressure and showed cross resistance to other quinolones, but not to amprolium and robenidine. When the Weybridge, Beltsville and Elberfeld strains of E. tenella were compared under similar laboratory conditions, the Weybridge and Elberfeld strains developed resistance to methyl benzoquate after 6 passages and the Beltsville after 5. Studies on the response of the Houghton strain to methyl benzoquate and buquinolate revealed that the drugs did not completely control the infection as measured by weight gain and that oocyst production was not suppressed. These observations indicate that the strain had already acquired some resistance to these drugs. This was confirmed by examining the resistance to methyl benzoquate of a culture of the Houghton strain of E. tenella which had been stored frozen in liquid nitrogen since 1969. This showed full sensitivity to the drug and developed resistance after 8 passages. This suggests that drug tolerance has been acquired by the Houghton strain since 1969.

Oocyst lines were established from the Houghton strain by infecting single birds with approximately 10 oocysts. Eleven of these lines were found to be sensitive to methyl benzoquate, and nine to give rise to resistant parasites. It is concluded that the Houghton strain is contaminated by a small number of resistant oocysts which can be eliminated from a culture by dilution of the challenge inoculum. One of these Houghton oocyst lines, sensitive to methyl benzoquate, developed resistance after 8 serial passages.

A comparison has been made of the responses of random-bred CFLP and inbred NIH mice to infection with Trichuris muris. Random-bred mice showed greater variation in worm burdens and less uniformity in worm expulsion. Irradiation prior to infection reduced variation, but did not increase the mean level of infection above that shown by the most susceptible unirradiated mice. In NIH mice, however, irradiation raised the level of infection in all mice. The factors responsible for variation between CFLP mice and for the level of infection in NIH mice came into play after the fifth day of infection and were inactivated by cortisone acetate. It is suggested that these factors are immunologically mediated and under direct genetic control. Uniformity of infection and expulsion in NIH mice is therefore seen as a consequence of genetic uniformity; variability in CFLP mice as a consequence of genetic variation.

The time of worm expulsion was found to differ markedly between inbred strains of mice. Hybrid progeny showed the expulsion time characteristic of the parental strain with the most rapid expulsion; greater resistance was therefore inherited as a dominant characteristic. The genetic control of immunity to T. muris is discussed in the context of the antibody- and cell-mediated components of the expulsion process.

In one (1c) and six (6c) cysticercoid primary infections of Hymenolepis diminuta in NIH (inbred) and CFLP (outbred) male mice 6±1 weeks old > 85% of the worms established but were rejected (destrobilated or expelled) subsequently. Rejection occurs more quickly in 6c infections than in 1c infections. Considerable worm growth occurs in 1c and 6c primary infections but worms from 6c infections weighed less than worms from 1c infections on all days studied.

Expulsion of H. diminuta does not occur more rapidly in secondary infections than in primary infections; loss of 6c secondary worms occurs at the same rate as 6c primary worms but 1c secondary worms survive longer than 1c primary worms. Although worms are not lost more quickly in secondary than in primary infections, they are affected at an early age by the immune response which stunts their growth. Increasing the intensity of primary and secondary infections increases the severity of stunting of secondary worms.

The results are discussed and it is suggested that immune responses to Hymenolepis spp. in rodents are common but that thresholds of worm numbers exist below which appreciable worm loss does not occur. Stunting due to crowding, which generally is attributed to inter-worm competition, may be in part immunologically mediated. For future immunological studies attempting to induce secondary responses to H. diminuta in mice, worm growth, not survival, is the criterion to evaluate.

Mice infected with Trichuris muris during lactation were unable to expel the infection at the normal time, but expulsion occurred when lactation was terminated. Suppression of expulsion was uniform in mice suckling more than five young but variable with smaller litters. Mice exposed to a primary infection while lactating were shown to have serum antibodies capable of passively transferring immunity to recipient mice and showed near normal immunity to a secondary infection given after lactation had ceased.

Acquired immunity to T. muris was also suppressed by lactation, but the worms which became established in lactating resistant mice were fewer and smaller than those in non-lactating, non-resistant controls.

It is suggested that the suppressive effect of lactation in this hostparasite relationship is exerted on the second, lymphoid cell-mediated phase of worm expulsion.

Natural leishmanial infections among feral mammals in the Republic of Panama were studied in relation to gross skin alterations. Three dermatotropic species of Leishmania, L. braziliensis, L. mexicana and L. hertigi, isolated from 230 sylvatic animals of 5 mammalian orders were involved. In 205 (89%) of the cases there were no gross skin alterations. Cryptic infections of all three leishmanial species among wild animals were confirmed by culture and hamster inoculation. This type of infection was always present in the sloths, Choloepus hoffmanni and Bradypus infusatcus, infected with L. braziliensis and in the porcupine, Coendou rothschildi, infected with L. hertigi. Suggestive skin alterations were encountered in 10 (4%) of infected animals. Only 15 (7%) of infected animals exhibited typical leishmanial lesions. It was possible to correlate skin depigmentation with the course of leishmanial infection in one animal, a young kinkajou, Potos flavus.

Fixation of carbon dioxide has been demonstrated for extracts from Crithidia fasciculata, Trypanosoma mega and Trypanosoma brucei brucei bloodstream and culture forms. The enzymes involved in this fixation were found to be ADP-stimulated phosphoenolpyruvate carboxykinase (E.C. 4. 1. 1. 32), ‘malic’ enzyme (E.C. 1. 1. 138–40) and pyruvate carboxylase (E. 0. 6. 4. 1. 1). The subcellular localization of these enzymes has been investigated in all three organisms. Products of short and long term fixation experiments were separated and identified.

The importance of carboxylation reactions is discussed in relation to the maintenance of oxidized and reduced coenzyme levels.

Hydrocortisone treated male and female mice, given a primary infection with Aspiculuris tetraptera, did not reject the worms during the third week of infection. Mice given hydrocortisone during the first week of infection had elevated worm burdens on day 10, suggesting that some worm loss was encountered during the anterior migration in control mice. Furthermore, this temporary period of treatment was sufficient totally to suppress rejection and to allow the parasite to persist until day 28. Methotrexate also significantly delayed rejection, but larval growth was retarded in treated mice. These results, it is suggested, add strength to the hypothesis that the loss of A. tetraptera in a primary infection in mice, is an immunological phenomenon.

Effect of gamma radiation on oocysts of Eimeria necatrix was investigated. It was observed that oocysts exposed to 200 kR or above did not sporulate. Irradiation at 10–150 kR caused a progressive decrease in sporulation. Irradiation affected normal development of unsporulated oocysts as the zygote protoplasm divided into unequal masses or was shattered into granules.

Increase in the intensity of irradiation of sporulated oocysts resulted in the progressive decrease in severity of the resultant infections in chicks and their effects – mortality, type of lesions developed, total oocyst production and immunity produced – were comparable with infections induced by decreasing the number of unirradiated oocysts. Infection produced by 1000 unirradiated oocysts was comparable with that resulting from 50000 oocysts irradiated at 25 kR. Infection obtained with 20000 unexposed oocysts approximated to that produced by 50000 ooeysts irradiated at 2·5 kR. It was concluded that irradiation abolished infectivity of the oocysts/sporozoites rather than bringing about attenuation of the parasite.

When protoscoleces of Echinococcus multilocularis were cultured in vitro, under axenic conditions in either monophasic or diphasic media, segmentation was suppressed in most organisms, some 70–80% of which developed into unsegmented, monozoic forms with a complete set of sexually mature male and female genitalia. The most striking feature of monozoic worms was the large lateral swelling produced by the cirrus sac the effect being to produce organisms with an unusual asymmetric shape. Worms which did not become monozoic either (a) underwent some somatic growth, developed two sets of genitalia and became ‘pseudosogmented’, i.e. with the inter-proglottid membranes absent or poorly defined, or (b) became vesicular or abnormal. The mechanisms which could be involved in the suppression of somatic growth and the induction of the monozoic condition, are examined in terms of cell lineage. The possible significance of these results in understanding the evolution of the cestodes is discussed.

Similar survival characteristics were demonstrated for hexacanths of Hymenolepis diminuta incubated in Tyrode's solution with or without a glucose supplement (0–50 mg/ml).

The survival rate of hexacanths in all media tested was shown to be age-dependent and led to a maximum life-span of approximately 11 h.

The amount of energy reserves, as measured by microdensitometric determinations of PAS + material, declined rapidly in time to a plateau at approximately 8 h. Residual PAS + matter present beyond that period was interpreted as structural and thus non-utilizable material.

The rate of activity as measured by hook movements declined more rapidly, and continuous hook cycles were rarely observed after 2 h and ceased after 4 h. A close correlation was demonstrated between the decline in PAS + material and the total number of hook cycles completed per unit of time.

The quantitative results on survival, energy reserves and activity are discussed in relation to the penetration of hexacanths into the haemocoele of the intermediate host.

The development of strains of Eimeria maxima resistant to buquinolate, methyl benzoquate, clopidol, sulphaquinoxaline and robenidine is described. It was not possible to standardize a schedule of inoculations and drug administration, which would enable the development of resistance to the different drugs to be compared directly. Resistance developed most readily to the quinolones. One robenidine-resistant strain proved to be drug-dependent. Dinitolmide showed unusual effects upon sporogony and three attempts to develop resistance against this activity failed. Chicks previously immunized with the parent strain were completely protected against infection with the drug-resistant strains.