East coast fever: 3. Chemoprophylactic immunization of cattle using oxytetracycline and a combination of theilerial strains

In the first of two experiments performed, 29 animals were immunized by chemoprophylaxis against either a combination of three theilerial strains, Theileria parva (Muguga), T. parva (Kiambu 5) and T. lawrencei (Serengeti transformed), or against one or two elements of the combination separately, and then challenged with either T. parva (Kiambu 1) or T. lawrencei (Solio KB1), both of which were known to be heterologous to T. parva (Muguga). The animals immunized against the combination had mild or inapparent reactions to challenge, whilst the susceptible control cattle and many of those immunized against one or two strains either had severe reactions of died.In the second experiment, 17 cattle immunized by chemoprophylaxis against the combination were challenged with lethal doses of three theilerial strains, T. parva (Entebbe 1), T. parva (Entebbe 2) and T. parva (Ukunda), recently isolated from two widely separated areas of East Africa. They had mild or inapparent reactions whilst 13 of 15 susceptible control cattle died.Chemoprophylaxis using a combination of theilerial strains and a limited oxytetracycline regimen was shown to be an effective means of immunizing cattle against East African theilerioses. The use of this technique as a method of immunizing cattle before exposure in the field is proposed.     

East coast fever: 1. Chemoprophylactic immunization of cattle against Theileria parva (Muguga) and five theilerial strains

In one experiment, groups of cattle were immunized by chemophylaxis or therapy against Theileria parva (Muguga), and were challenged with five recently-isolated strains of Theileria. Two of the strains caused mild or inapparent reactions in the immunized animals, and three caused significant reactions. None of 25 immunized animals died on challenge, whereas 22 of 25 susceptible (non-immunized) animals died.In a second experiment, groups of cattle were immunized against five field strains using chemoprophylaxis. They had minimal reactions and were solidly immune both to isologous challenge and to subsequent challenge with T. parva (Muguga). This demonstrated that cattle could be effectively immunized against lethal challenge with several theilerial strains by inoculating them with 1 ml of an infective stabilate of the appropriate strain and the concomitant daily inoculation of 5 mg/kg of a formulation of oxytetracycline on 4 consecutive days starting on the day of infection.It is suggested that T. parva (Muguga) may be a valuable component of a polyvalent East Coast fever (ECF) vaccine in East Africa.     

East coast fever: 4. Further studies on the protection of cattle immunized with a combination of theilerial strains

Four experiments were carried out in which cattle immunized with three cryopreserved stabilates, Theileria parva (Muguga), T. parva (Kiambu 5) and T. lawrencei (Serengeti transformed) and simultaneously treated with a single dose of long acting tetracycline were challenged with field isolates of T. parva. Three of the challenge isolates were from Tanzania and one from Kenya. The immunized cattle showed either mild or inapparent diease reations while susceptible control cattle suffered severe disease and 72% died. These experiments further demonstrated the efficacy of the immunization method against T. parva isolates from a wide distribution within East Africa. The value of such laboratory screening of field isolates prior to field exposures is discussed.     

Exposure of immunized cattle to prolonged natural challenge of Theileria lawrencei derived from African buffalo (Syncerus caffer)

Cattle were immunized with Theileria lawrencei stabilates either by fortuitous recovery or intentionally by chemoprophylaxis with oxytetracycline. The immunized cattle were exposed together with susceptible control cattle in a paddock where a lethal T. lawrencei challenge derived from two African buffaloes had been established.Theileria lawrencei stabilates used for immunization were of two types: one batch was prepared from Rhipicephalus appendiculatus fed on a buffalo in the paddock, and the other was from a number of tick batches fed on the two buffaloes over a period of 3 months.All the susceptible control cattle exposed in the paddock died of acute Theileria lawrencei infections. Cattle immunized with the composite stabilates survived T. lawrencei challenge for a prolonged period without showing clinical disease. The protection to the T. lawrencei challenge persisted for at least 1.5 years after the composite stabilates had been prepared. The stabilate prepared from ticks fed on a buffalo on one occasion failed to give effective protection since half of these immunized cattle died of T. lawrencei infection when exposed. These results suggest that different immunogenic types of T. lawrencei occur in buffalo which may hinder the effectiveness of a vaccine for T. lawrencei.     

Non-specific immunization against bovine tropical theileriosis (Theileria annulata) using killed Corynebacterium parvum

Killed Corynebacterium parvum was used as an adjuvant for the production of non-specific resistance against Theileria annulata in cattle. Groups of cross-bred (Bos indicus X Bos taurus) calves were administered C. parvum adjuvant subcutaneously and were then challenged with T. annulata-infected ticks on 45, 60 or 90 days later. The challenge caused mild reactions in the protected calves. None of the 10 immunized calves died due to theileriosis, whereas all three paris of susceptible control calves died due to theileriosis. It appears from this pilot study that cattle can be protected non-specifically with C parvum adjuvant against T. annulata.     

A comparative study of the diseases in cattle caused by Theileria parva or T. lawrencei: II. Hematology,clinical chemistry,coagulation studies and complement

Theileria lawrencei tick-derived stabilate infection of 8 cattle resulted in the development of panleukopenia and hypoproteinemia. In addition to these changes, T. parva infection caused mild normocytic, normochromic, non-responsive anemia at either of two dose rates. Disseminated intravascular coagulation, as indicated by positive protamine paracoagulation tests, prolonged prothrombin and partial thromboplastin times, and thrombocytopenia, developed in cattle infected with either of the Theileria spp., and was probably an important intermediary mechanism leading to death. Infection of calves with a high dose of T. parva stabilate resulted in more rapid onset of clinico-pathologic changes than did the low dose infection. Theileria lawrencei infection produced a severe, acute syndrome, the clinico-pathologic alterations of which varied in time of onset and severity between those of the T. parva high dose and low dose groups.     

Theileriosis: The exposure of immunised cattle in aTheileria lawrencei enzootic area

Summary Three groups of steers were exposed to field challenge in aT. lawrencei-enzootic area of Kenya. Four out of five ECF-susceptible steers and four out of five steers immunised againstT. parva (Muguga) died of theileriosis, the surviving animals experiencing severe reactions. On the other hand, all five steers immunised againstT. parva (Muguga) and a strain of theileria probably homologous with the one present in the area of exposure, survived and only experienced mild reactions. These results indicated that immunisation of cattle withT. parva (Muguga) may not necessarily protect them against field challenge with certainT. lawrencei-type strains, but that immunisation with an apparently homologous strain of theileria may well do so.

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Sumario Tres groupos de novillos fueron expuestos al desafio de campo en un area de Kenya enzootica aT. lawrencei. Cuatro de cinco novillos susceptibles a la Fiebre de la Costa del Este y cuatro de cinco novillos inmunizados contraT. parva (Muguga) murieron de Theileriosis; los animales sobrevivientes experimentaron reacciones severas. De otra manera, todos los cinco novillos inmunizados contraTheileria parva (Muguga) y una cepa de theileria probablemente homologa con la presente en el area de exposición, sobrevivieron y solo experimentaron reacciones ligeras. Estos resultados indicaron de que la inmunización del ganado conT. parva (Muguga) puede no necesariamente proteger contra el desafio de campo con ciertas capas ó tipos deT. lawrencei, pero que la inmunizacion con una cepa de theileria aparentemente homóloga puede proteger.

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Résumé Trois groupes de bouvillons ont été soumis à l'épreuve sur le terrain dans une région du Kenya où sévit une enzootie àT. lawrencei. Quatre sur cinq bouvillons neufs et quatre sur cinq bouvillons immunisés contreT. parva (Muguga) sont morts, les survivants éprouvant des réactions graves. D'autre part, cinq bouvillons immunisés contreT. parva (Muguga) et une souche deTheileria probablement identique à celle présente dans la région d'exposition, ont survécu et n'ont présenté que de faibles réactions. Ces résultats montrent que l'immunisation de bovins avecT. parva (Muguga) peut ne pas les protéger nécessairement de l'épreuve sur le terrain avec certaines souches de typeT. lawrencei, mais que l'immunisation avec une souche deTheileria apparemment identique àT. lawrencei peut le faire.

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On O.D.A. secondment from the A.R.C. Institute for Research in Animal Diseases, Compton, nr. Newbury, Berks., U.K. (Research Project R 2396).

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Supported by a grant from Pfizer Corporation, New York.

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On U.S.D.A. secondment from Plum Island Animal Disease Laboratory, Box 848, Greenport, Long Island, N.Y. 11944, U.S.A.

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Project supported by the United Nations Development Programme, with the Food and Agriculture Organization of the United Nations as the Executing Agency, in co-operation with the East African Community. The Project is also supported by the Overseas Development Administration of the United Kingdom (Research Projects 2396 and 2494), the United States Department of Agriculture, the Rockefeller Foundation, the International Atomic Energy Agency and the Pfizer Corporation.     

Immunization of cattle against Theileria parva bovis and their exposure to natural challenge

Theileria parva bovis isolates were tested for their immunizing capacity under natural field challenge on Willsbridge Farm in the highveld of Zimbabwe. Fifteen susceptible Sussex yearlings were immunized with the Boleni stock and 15 with a mixture of three isolates from the farm, using tick-derived sporozoite stabilates. No chemoprophylaxis was used. A dose of 0.1 ml of stabilate appeared to be safe in preliminary laboratory experiments, but the reactions were severe in the Sussex cattle and one died despite treatment. Twenty-nine immunized animals and 10 controls first experienced a mild infection, starting about 15 days after their arrival at the farm. Ten of the immunized animals and four controls had schizonts in peripheral lymph nodes for variable periods; one third of those had pyrexia. Nymphal Rhipicephalus appendiculatus ticks applied to three of the reacting immunized calves transmitted Theileria taurotragi to two animals and T. parva to a third. A second Theileria infection, due to T. parva bovis, was detected shortly after the first one. Schizonts were detected in seven out of 10 controls. Pyrexia was more severe and prolonged. Two of the controls died of theileriosis. At the same time schizonts were seen in three immune animals and eight of them had short periods of pyrexia. Intercurrent infections with Babesia bigemina, Borrelia theileri and Eperythrozoon were detected and may have contributed to the fever. Tick infestations were low during the exposure. In the second year of exposure, four out of eight new control animals had severe reactions, and one died. None of the immunized animals became ill, but one animal from the first year control group, which had not reacted previously, had clinical theileriosis. It is concluded that immunization provided an effective protection against field challenge.     

A comparative study of the diseases in cattle caused by Theileria Lawrencei and Theileria Parva. 1. Clinical signs and parasitological observations

A comparison was made between the clinical signs and blood and lymph node parasite levels in cattle reacting to the inoculation of tick-derived stabilates of Theileria lawrencei (TL), and T. parva at high dose (TPH) and low dose (TPL) levels. The TL and TPH infections produced acute diseases with statistically similar tines to death of 13.50 and 14.25 days, respectively, while TPL caused a more protacted but equally fatal disease of 20 days duration. In the TPL group, the local drainage lymph node (LDLN) became enlarged earlier and macroschizonts appeared later than in the TPH group. The characteristics of T. lawrencei infection which differentiated it from T. parva infection were: nervous signs; complete anorexia; low macroschizont index (MSI) levels of up to 5%; and lack of piroplasms. In T. parva groups, the high MSI levels of up to 60%, presence of piroplasms and dysentery were characteristic. In addition, the enlargement of lymph nodes in the TL group persisted to time of death, while in both T. parva infections the initial enlargement was followed by a decrease in size.The 100-fold dose difference between TPH and TPL influenced the times of onset of the majority of clinical signs, most of which occurred later in the TPL group. The MSI levels were 60% in the TPH group and 40% in the TPL group. The piroplasm parasitemia levels reached 5% in the TPH group, and 16% in the TPL group.Theileria lawrencei and T. parva caused diseases with significant differences in clinical manifestations and parasite levels in erythrocytes and lymphocytes. The size of the infecting dose influenced the time of onset of clinical signs as well as the duration of the disease.     

Exposure of cattle immunized with different stocks of Theileria parva to buffalo-associated Theileria challenge on two game parks in Zimbabwe.

Eight cattle immunized with cattle-derived Theileria parva Boleni stabilate together with six susceptible controls were released in Dombawera Game Park on the Highveld of Zimbabwe. This coincided with Rhipicephalus appendiculatus nymphal activity. The cattle grazed together with African buffaloes (Syncerus caffer) and were not treated against tick infestation. The nymphal tick infestation was high, and seven of the eight immunized cattle and three of the controls had severe and fatal reactions. Subsequently, two stocks of Theileria parva to be tested for their immunizing abilities were prepared-one from adult ticks which were fed as nymphs on one of the sick control animals (Dom 268) and the other from adult ticks collected from pastures grazed by buffaloes (Bv-1). Two groups of cattle were immunized with either the Dom 268-derived strain (eight animals) or the Bv-1-derived strain (four animals). These together with three non-immunized controls, were released in Bally Vaughaun Game Park in the Highveld, where buffaloes are present, during the season of nymphal tick activity. A third group of five cattle, immunized with stabilate Bv-1, and three non-immunized controls were released at the same site during the season of adult tick activity. The nymphal and adult tick infestations of the cattle were large and more than 2000 nymphs and 1000 adult ticks were counted per animal. Cattle were treated with a pyrethroid pour-on preparation to control the tick infestation and screw-worm strike. The immunized cattle in the three groups survived the theileriosis challenge for a period of 18 months, but the non-immunized control cattle suffered a severe and fatal theileriosis 19-23 days after being placed on the pasture.     

Treatment of theileriosis with parvaquone in Zambia

Parvaquone was used to treat 126 cattle with theileriosis. Theileria species schizonts were present in their lymph node biopsy smears and the majority of the animals had clinical signs of theileriosis. One hundred and fifteen treated and one untreated cattle survived the infection while 11 treated and 12 untreated animals died of the disease. Despite serological evidence of a parasite challenge during the subsequent rainy season, recovered cattle did not develop clinical signs of theileriosis but untreated cattle in the area continued to die from the disease. An intermittent low piroplasm parasitaemia (less than 1 per cent) was observed in recovered cattle for up to 14 months after detailed monitoring of cattle in the trial; this could be evidence for a carrier status for the Theileria species or strains involved.     

Immunisation of cattle against theileriosis in Coast Province, Kenya: laboratory evaluation of a Theileria parva parva stabilate for use in 'infection and treatment' immunisation in the field

Theileria parva parva Marikebuni stock, previously shown to give good protection to immunised cattle in Kilifi District, Coast Province of Kenya, was chosen for large scale immunisation in the district. A large sporozoite stabilate was prepared and evaluated for efficacy and safety in the 'infection and treatment' method, using a long or short acting formulation of oxytetracycline. Susceptible cattle were infected with selected doses of stabilate (10(0), 10(-1), 10(-1.7) and left either as untreated controls, or treated with one of the two oxytetracycline formulations. It was concluded that stabilate dilution at 10(-0.7) or 10(-1) in combination with either formulation of oxytetracycline would effect satisfactory immunisation. The short acting oxytetracycline treatment was judged to be the most efficacious in protecting cattle against homologous challenge. On heterologous challenge it was found that T p parva Marikebuni immune cattle were protected against seven T p parva stocks from Kilifi District and also against four stocks of T p parva from other areas of Kenya. In addition, the Marikebuni stock provided partial protection against challenge by T p lawrencei stocks. Furthermore, cattle immune to T p parva and T p lawrencei were protected against lethal challenge of T p parva Marikebuni stock. Thus, it appears that large scale immunisation of cattle against theileriosis in Kilifi District could be undertaken using the Marikebuni stock. With continued assessment, this stock could provide a master theilerial stock for immunisation against cattle theileriosis in areas free of buffaloes elsewhere in Kenya.     

Susceptibility to Tropical Theileriosis of Calves Born to Dams Immunized with Theileria annulata (Hisar) Cell Culture Vaccine

The susceptibility/immune status to tropical theileriosis of calves born of immunized dams was evaluated. Six cows were vaccinated with the Theileria annulata cell culture vaccine in the eighth month of pregnancy. Sera from the immunized dams exhibited very high post-vaccination antibody titres as determined by the indirect fluorescent antibody (IFA) test. The calves born to these dams did not show antibodies against T. annulata at the time of birth (IFA titres of <1:20). The new-born calves were fed colostrum from their mothers and were challenged with T. annulata-infected ground tick supernate at 5–7 days of age. All the calves developed fever (from day 5–6 onwards) and parasitological reactions (from day 8–9 onwards) after challenge. There was a significant decrease in the haemoglobin and packed cell volume of the calves after challenge. All the calves showed signs of acute theileriosis by day 9–10 after challenge and had to be treated with buparvaquone in order to save their lives. The study indicated that detectable levels of anti-theilerial antibodies were not transferred from immune dams to their offspring. All the calves born to immunized dams were fully susceptible to theileriosis and thus themselves needed vaccination.     

Laboratory and field investigations into the Theileria parva carrier-state in cattle in Zimbabwe.

The Theileria parva carrier-state in cattle on commercial farms on Zimbabwe was investigated using parasitological and serological methods. The proportion of cattle showing Theileria piroplasms on two farms, which had recent histories of disease outbreaks, were 64% (n = 106, total of heifers and weaned calves examined) and 71.5% (n = 60) while the proportion of T. parva antibodies for the same animals were 59% and 98.5%, respectively. On four farms where no cases of the disease occurred for over 10 years, the average proportion of animals showing piroplasms and antibodies were 55.4% (range 32-82, n = 223) and 73% (range 47-91, n = 223), respectively. However, on another three farms which had no history of theileriosis outbreaks these proportions were very low, being 11.4% (0-24, n = 157) for piroplasms and 12.2% (5-23, n = 157) for antibodies. The mean infection rate in unfed Rhipicephalus appendiculatus adults collected from farms with a high prevalence of cattle which were carriers of Theileria piroplasms during the tick activity season was 29% (range 12-60%) with 9.3 (range 2-18.7) mean infected acini per infected tick. The infectivity of different tick batches to susceptible cattle produced a wide spectrum of theileriosis reactions. Laboratory controlled experiments were carried out to study the persistence of T. parva (Boleni) piroplasms in cattle immunized with this strain as well as its infectivity for ticks and its subsequent transmissibility to cattle. Examination of the salivary glands of 15 batches of ticks collected from six immunized cattle on three different occasions over 18 months showed that none were infected with Theileria parasites. However, the infectivity of other ticks in the same batches to susceptible animals was demonstrated 6, 10 and 18 months after cattle had been immunized with Boleni stabilate.     

Immunization of cattle with nymphal Hyalomma anatolicum anatolicum extracts: effects on tick biology

Antigens derived from partially engorged nymphs of Hyalomma anatolicum anatolicum were used in immunizing crossbred (Bos indicus×Bos taurus) cattle against larval, nymphal and adult H. a. anatolicum and H. dromedarii. The cattle were either infected with Theileria annulata at low parasitaemia or were uninfected. Whole nymphal extract (WNE), nymphal membrane antigens (NMA) and nymphal soluble antigens (NSA) were used for immunization. The group immunized with WNE showed significant and better rejection of H. a. anatolicum ticks as compared to calves immunized with either NMA or NSA. The moulting rates of both engorged larvae and nymphs remained unaffected. Nymphs which engorged on the immunized calves were fully susceptible to infection by T. annulata as indicated by the intensity and abundance of Theileria infections in the resulting adult ticks from immunized and unimmunized Theileria infected cattle. These ticks also transmitted fatal theileriosis to susceptible calves.     

Four p67 alleles identified in South African Theileria parva field samples

Previous studies characterizing the Theileria parva p67 gene in East Africa revealed two alleles. Cattle-derived isolates associated with East Coast fever (ECF) have a 129 bp deletion in the central region of the p67 gene (allele 1), compared to buffalo-derived isolates with no deletion (allele 2). In South Africa, Corridor disease outbreaks occur if there is contact between infected buffalo and susceptible cattle in the presence of vector ticks. Although ECF was introduced into South Africa in the early 20th century, it has been eradicated and it is thought that there has been no cattle to cattle transmission of T. parva since. The variable region of the p67 gene was amplified and the gene sequences analyzed to characterize South African T. parva parasites that occur in buffalo, in cattle from farms where Corridor disease outbreaks were diagnosed and in experimentally infected cattle. Four p67 alleles were identified, including alleles 1 and 2 previously detected in East African cattle and buffalo, respectively, as well as two novel alleles, one with a different 174 bp deletion (allele 3), the other with a similar sequence to allele 3 but with no deletion (allele 4). Sequence variants of allele 1 were obtained from field samples originating from both cattle and buffalo. Allele 1 was also obtained from a bovine that tested T. parva positive from a farm near Ladysmith in the KwaZulu-Natal Province. East Coast fever was not diagnosed on this farm, but the p67 sequence was identical to that of T. parva Muguga, an isolate that causes ECF in Kenya. Variants of allele 2 were obtained from all T. parva samples from both buffalo and cattle, except Lad 10 and Zam 5. Phylogenetic analysis revealed that alleles 3 and 4 are monophyletic and diverged early from the other alleles. These novel alleles were not identified from South African field samples collected from cattle; however allele 3, with a p67 sequence identical to those obtained in South African field samples from buffalo, was obtained from a Zambian field isolate of a naturally infected bovine diagnosed with ECF. The p67 genetic profiles appear to be more complex than previously thought and cannot be used to distinguish between cattle- and buffalo-derived T. parva isolates in South Africa. The significance of the different p67 alleles, particularly the novel variants, in the epidemiology of theileriosis in South Africa still needs to be determined.     

Determination of duration of immunity of calves vaccinated with the Theileria annulata schizont cell culture vaccine

Bovine tropical theileriosis caused by Theileria annulata is a serious haemoprotozoan disease of cattle affecting exotic cattle, their crossbreeds and young indigenous calves. Cell culture vaccines have been developed and used effectively in various countries for the control of this disease. However, the duration of immunity provided by these vaccines is poorly understood. The present experiments were planned to study the duration of immunity in animals after vaccination with the T. annulata (Hisar) schizont cell culture vaccine. Two groups of calves were vaccinated and challenged after a period of 3 and 6 months, respectively. There was no fever in any of the vaccinated calves after challenge. However, the vaccinated animals exhibited mild to moderate enlargement of lymph nodes and parasitological reactions. The parasitological reactions were very mild in calves challenged after 3 months and moderate in calves challenged after 6 months. There was a mild but significant decrease in the haematological values of calves after challenge. A significant rise in the anti-theilerial antibody titres was observed in all calves after vaccination, which increased further, by many folds after challenge. On the other hand, all the challenge control calves showed symptoms of acute theileriosis and died. The observations suggested that the T. annulata (Hisar) schizont cell culture vaccine provided immunity in vaccinated animals for at least 6 months in the absence of field tick challenge. However, there was some decline in immunity after 6 months, if the animals are not exposed to ticks during this period.     

Prophylactic efficacy of buparvaquone in experimentally induced Theileria annulata infection in calves

The antitheilerial activity of buparvaquone (BW 720C) was evaluated in experimentally induced Theileria annulata infections in cross-bred male calves. T. annulata infections were induced by injecting a suspension of infected ground tick tissue suspension (GUTTS) equivalent to two ticks subcutaneously into each calf. Buparvaquone at a dose of 2.5 mg kg-1 body weight was given as a single injection (intramuscularly) on Day 0 (Group 1), Day 8 (Group 2) and Day 12 (Group 3) post-infection. The animals in Groups 4 and 5 were untreated and challenged controls, respectively. All of the recovered animals from Groups 1-4 were challenged with a lethal dose of T. annulata at 6 weeks post-infection. The immunized animals were resistant to the homologous challenge, which killed three of four control animals (Group 5); the controls showed typical antemortem and post-mortem lesions of theileriosis.     

Progress in research on tick‐borne diseases: Theileriosis and heartwater

Summary

The rapid population growth in subsaharan Africa necessitates a great increase in animal production in the more humid zones. Vector‐borne diseases occurring in these zones will assume more importance, but are difficult to control. They include theileriosis and heartwater. Recent developments in research on these diseases are presented. Indigenous animal populations in endemic areas, subjected to natural selection, are far less susceptible than exotic stock.

Heartwater, caused by the rickettsia Cowdria ruminantium, transmitted by Amblyomma ticks, causes high mortality in exotic ruminants. It has received much attention in recent years, partly because the disease has been introduced from Africa into the Caribbean and threatens the American mainland. Since the recent success of in vitro culture, much progress in research has been made, but so far prevention still relies mainly on acaricidal tick control; an infection and treatment method is used on a limited scale. Antigenic diversity is a complication for immunization procedures.

Theileria parva (East Coast fever, Corridor disease and January disease) and T.annulata (Mediterranean or tropical theileriosis) are the most pathogenic of the 6 species of this protozoan genus that infect cattle. Great progress has been made in recent years in knowledge on the immunology, the epidemiology, the taxonomy and the chemotherapy of theileriosis. Intensive acaricidal tick control can now be supplemented by an attenuated schizont vaccine against T.annulata, while immunization against East Coast fever is carried out on a limited scale using virulent sporozoite infection and treatment. Research on recombinant vaccines is promising. Antigenic diversity in T.parva is a serious complication.     

Immunizing efficacy of Toxoplasma gondii strains isolated from different hostsEfficacite d'immunisation par differentes souches de Toxoplasma gondii isolees de differents hotes

Studies were carried out to determine the fate of virulent Toxoplasma gondii challenge in immune animals. Toxoplasma strains isolated from human, swine, rabbit and cat hosts were used for the primary immunization of mice. Brains were removed at various intervals after the challenge and subinoculated into normal mice. Prior immunization with one of the nine toxoplasma strains enabled the mice either to eliminate or harbour the challenge organisms without loss of virulence. The outcome of challenge infection was dependent on the parasite strain used for the immunization and the time interval between challenge and subinoculation. Mice immunized with strain KSU isolated from a cat eliminated repeatedly administered challenge from 80% of animals. Strain S 162 isolated from a swine eliminated similar challenge from only 20% of animals. The results indicate that the extent of protection against virulent T. gondii can vary widely and is related to the immunogenicity of the original immunizing strain.