Patho-physiology of ovine schistosomiasis. II. Some observations on the sequential changes in blood volume and water and electrolyte metabolism following a single experimental infection of Schistosoma mattheei

The pathogenesis of experimental Schistosoma bovis infections in Sudanese sheep and goats

The pathogenic effects of experimental Schistosoma bovis infection in Sudanese sheep and goats were investigated by a variety of clinical, parasitological, physiological and histopathological techniques; uninfected animals of each species were used as controls. Infected animals of both species lost or failed to gain weight and developed a haemorrhagic diarrhoea, inappetence, marked anaemia, hypoalbuminaemia, hyperglobulinaemia, hyperproteinaemia and eosinophilia. These changes first became noticeable around the time of onset of oviposition and their severity was generally related to faecal egg counts. Red cell breakdown and albumin catabolism were much higher in infected than in control animals of the same species, and it was concluded that these changes were due to haemorrhage resulting from the extrusion of large numbers of eggs through the intestinal mucosa. Although all the animals were infected with the same number of cercariae, both the number of worms reaching maturity and the tissue egg counts tended to be higher in sheep than in goats. On the other hand, goats had significantly higher faecal egg counts than sheep and it is suggested that this was the reason for the generally more severe disease in the former species.

Erythrokinetics and albumin metabolism in primary experimental Schistosoma bovis infections in Zebu calves

Red cell kinetics and albumin metabolism were studied in calves infected with either 100 or 200 Schistosoma bovis cercariae per kg body weight, by the use of 59Fe-labelled transferrin, 51Cr-labelled erythrocytes and 125I-labelled albumin; a third group of worm-free animals acted as controls. The anaemia which developed in the infected calves was shown to be due basically to an accelerated rate of red cell loss from the circulation, and became evident around the seventh week of infection, increased in severity during the following two months, and subsequently subsided. In view of its close similarity to the pattern of faecal egg excretion, it was concluded that haemorrhage into the intestine caused by the exit of eggs was the principal aetiological factor: haemolysis was excluded by the absence of both splenomegaly and hyperferraemia . Erythropoiesis was also accelerated in infected animals, but could not keep pace with the rate of red cell breakdown to which the animals were concurrently subjected. Haemodilution was involved, but not to a significant extent. The hypoalbuminaemia associated with infection was caused by an increased rate of albumin catabolism, and a plasma volume expansion, and was accompanied by marked depletion of all albumin pools, but particularly the extravascular pool. The pattern of albumin catabolism closely followed that of red cell loss, suggesting that passage of plasma as whole blood into the intestine was the basic cause of hypoalbuminaemia. Red cell losses and albumin hypercatabolism were more severe in the more heavily infected group, and although both subsided as egg counts fell, they remained evident even 1 year after infection. This may partly explain the failure of infected animals to regain the weight lost during earlier stages of disease.

Immunization of sheep against a virulent strain of Schistosoma mattheei using a strain of S. mattheei attenuated by hamster passage

We have previously described the characteristics of a relatively non-pathogenic laboratory strain of S. mattheei, attenuation of which was apparently caused by passage in hamsters. We now show that chronic infection with this avirulent strain largely protects sheep from the manifestations of acute schistosomiasis when challenged with a virulent strain of S. mattheei. Four sheep were each infected with 10 000 cercariae of the avirulent strain and, together with four wormfree sheep, challenged 63 weeks later with 10 000 S. mattheei cercariae of a pathogenic strain. Four more sheep acted as uninfected controls. Following challenge, the animals were weighed and bled weekly for PCV and serum protein determinations, and egg counts were carried out fortnightly on faeces taken from the rectum. Red cell and albumin turnover were monitored for two weeks immediately before challenge and for a similar period before necropsy, when the adult worms were recovered by perfusion and tissues sampled for histopathology and egg counting. The unvaccinated sheep developed severe disease 6-12 weeks after exposure characterised by marked anaemia, hypoalbuminaemia and hyper-gamma globulinaemia coinciding with the passage of blood-stained faeces and progressive inappetence. In the vaccinated sheep, there was an even earlier rise in gamma globulins, but the other clinico-pathological changes were generally slower to devleop and much milder in severity. The parasitological data showed that although this was partly due to a reduction in the establishment of the challenge worm population the main factor was probably a reduction in the fecundity of these worms.

Modification of the pathogenicity of Schistosoma mattheei for sheep by passage of the parasite in hamsters

Border Leicester X Suffolk sheep infected with a strain of S. mattheei maintained in hamsters do not develop the same pathological changes as Romney Marsh sheep infected with the same strain of parasite before hamster passage. To determine the cause of this reduced pathogenicity, five Romney Marsh sheep were each infected with 10 000 cercariae of the hamster-passaged parasite and five with 10 000 cercariae of a S. mattheei strain from Onderstepoort, South Africa, passaged exclusively through sheep. Striking pathological and parasitological differences were found between the two strains. Infection with the "sheep" strain was lethal, whereas infection with the "hamster" strain produced little evidence of clinical disease. By 13 weeks post-infection the mean body weight of the sheep infected with the sheep strain had declined by 15% compared with both the uninfected controls and the sheep infected with the hamster strain, and the mean PCV was lowered to 20% in the sheep strain infected animals. Egg production began at seven weeks with the sheep strain, faecal counts rising to more than 300 e.p.g., whereas only two of the sheep infected with the hamster strain passed eggs in the faeces (at nine weeks) and the maximum egg count was 50 e.p.g. Twice as many adult worms of the sheep strain were recovered, and, although the number of eggs found in the tissues "per worm pair" was not significantly different, overall egg production was higher for the sheep strain; also more of the sheep strain eggs were deposited in the intestines. Similar parasite differences were seen in a supplementary study in mice and it seemed that "attenuation" of the parasite had occurred, presumably due to its maintenance in hamsters. Histopathological observations and faecal egg counts both indicated an inability of hamster strain eggs to penetrate the intestinal lumen; this was probably important in reducing the pathogenicity of the hamster strain.

Studies on the pathogenesis of a strain of Schistosoma mattheei maintained in hamsters

As a continuation of our studies on the pathogenesis of domestic animal schistosomiasis, eleven Suffolk X Border Leicester sheep were each infected with 10 000 S. mattheei cercariae, sixteen with 5000 cercariae, and thirteen acted as worm-free controls. Blood samples were taken weekly for packed cell volume determination, serum samples were collected weekly during the first 30 weeks and thereafter fortnightly for protein analysis, and samples from the rectum were obtained at similar intervals for faecal egg counts. At intervals, 4–8 animals from each group were placed in metabolism cages for periods of 2–3 weeks to monitor albumin and red cell turnover. The animals were necropsied at 13, 35 or 57 weeks after infection, their worms recovered by perfusion and various tissues were collected for histological examination and egg counting.

A most unexpected finding was the almost complete absence of clinical signs in most of the infected animals, the only manifestations of infection being a slight retardation in growth and a mild anaemia between the 2nd and 5th months. These findings were quite unlike previous ones and were remarkable in view of the large cercarial exposure doses used and the heavy worm and tissue egg counts these produced. However, the present infections differed in two respects from those described previously: there was an absence of eggs from the faeces with associated lack of intestinal bleeding, and also large numbers of worms died in the later stages of the infections. In our previous studies with S. mattheei we used the same parasite but the sheep used in the current study were infected after the parasite had undergone several passages in laboratory hamsters. This fortuitous reduction in the pathogenicity of the parasite provided comparative haematological and parasitological data indicating that the major factor in the aetiology of severe S. mattheei infection in sheep is intestinal bleeding caused by the passage of eggs through the bowel wall.

The pathology of naturally-occurring bovine schistosomiasis in the Sudan

The gross and histopathological lesions of naturally-occurring bovine schistosomiasis in the Sudan are detailed. Out of 61 animals examined, 53 were found infected with Schistosoma bovis, showing lesions in their livers, intestinal tracts and other organs. Apart from granulomas, periportal inflammatory infiltration, fibrosis and pigmentation of Kupffer cells, livers of infected cattle revealed peculiar lesions in the form of "nodular sclerosis", medial hypertrophy of portal veins and lymphoid nodule and follicle formation around parasites. In the intestines, variable manifestations of catarrh, with eosinophilic infiltration, granulomatous reaction and vascular changes, were observed. Pancreatic schistosomiasis was common and some animals had lesions in their lungs and lymph nodes. Urogenital lesions, rectal and colonic polyps, congestive splenomegaly, gastro-oesophageal variaces and Symmer's clay-pipe stem fibrosis of the liver were not seen in cattle. The haematological and serum protein findings in infected animals were included. The findings were related to the lesions observed and the need for detailed study of functional disturbances in bovine schistosomiasis is indicated.