In vitro erythrophagocytosis by cultured macrophages stimulated with extraneous substances and those isolated from the blood, spleen and bone marrow of Boran and N'Dama cattle infected with Trypanosoma congolense and Trypanosoma vivax

A standard radioactive chromium (51Cr) release assay was used to assess the in vitro phagocytosis and lysis of bovine erythrocytes by cultured splenic, bone marrow and peripheral blood monocyte-derived (PBM) macrophages isolated from healthy and Trypanosoma congolense and T. vivax-infected cattle of the Boran and N'Dama breeds. Recombinant cytokines (rHuTNF-alpha and rBolFN-gamma) and non-acid-dialysed peripheral blood mononuclear cell (PBMNC) culture supernatants stimulated these PBM for enhanced activities. The stimulants caused increases in the rate of erythrocyte phagocytosis and lysis by cultured PBM in a concentration-dependent manner. But very high stimulant concentrations caused deceased in vitro erythrophagocytosis. However, bacterial lipopolysaccharide (LPS) and acid-dialysed PBMNC culture supernatants did not cause any increase in cultured PBM erythrophagocytosis. In vitro erythrocyte phagocytosis and lysis by splenic, bone marrow and peripheral blood monocyte (PBM)-derived macrophages of Boran breed of cattle infected with Trypanosoma congolense increased from 14 days post-infection (DPI) onwards and thereafter maintained at various levels above pre-infection. Cultured splenic macrophages showed the greatest erythrocyte destruction capability while PBM-derived macrophages was the least. The rates of in vitro erythrocyte phagocytosis and lysis were higher with the cultured PBM of the Boran than those of the N'Dama cattle during T. congolense infection. The rate of in vitro erythrocyte destruction was however, similar in both groups of cattle during T. vivax infection. These results correlated positively with the dynamics and degree of anaemia developed by these groups of animals during both T. congolense and T. vivax infections. Cattle infected with T. congolense and T. vivax developed varying degrees of normocytic normochromic anaemia during infection. Boran cattle developed a more severe anaemia, and had to be treated with diminazine aceturate, than N'Dama cattle during T. congolense infection. Both breeds of cattle developed a milder but similar degree of anaemia during T. vivax infection. None of the animals were treated. The results of this study indicated a role of in vivo macrophage stimulatory factors, notably cytokines such as TNF-alpha and IFN-gamma in host's serum, as well as parasite antigens, which may act singly or in concert, in the process of enhanced erythrocyte destruction, hence anaemia by the mononuclear phagocytic system (MPS) during bovine trypanosomosis.     

Analysis of host genetic factors influencing African trypanosome species infection in a cohort of Tanzanian Bos indicus cattle

Trypanosomosis caused by infection with protozoan parasites of the genus Trypanosoma is a major health constraint to cattle production in many African countries. One hundred and seventy one Bos indicus cattle from traditional pastoral Maasai (87) and more intensively managed Boran (84) animals in Tanzania were screened by PCR for the presence of African animal trypanosomes (Trypanosoma congolense, Trypanosoma vivax and Trypanosoma brucei), using blood samples archived on FTA cards. All cattle screened for trypanosomes were also genotyped at the highly polymorphic major histocompatibility complex (MHC) class II DRB3 locus to investigate possible associations between host MHC and trypanosome infection. Overall, 23.4% of the 171 cattle tested positive for at least one of the three trypanosome species. The prevalence of individual trypanosome species was 8.8% (T. congolense), 4.7% (T. vivax) and 15.8% (T. brucei). The high prevalence of T. brucei compared with T. congolense and T. vivax was unexpected as this species has previously been considered to be of lesser importance in terms of African bovine trypanosomosis. Significantly higher numbers of Maasai cattle were infected with T. brucei (23.0%, p=0.009) and T. congolense (13.8%, p=0.019) compared with Boran cattle (8.3% and 3.6%, respectively). Analysis of BoLA-DRB3 diversity in this cohort identified extensive allelic diversity. Thirty-three BoLA-DRB3 PCR-RFLP defined alleles were identified. One allele (DRB3*15) was significantly associated with an increased risk (odds ratio, OR=2.71, p=0.034) of T. brucei infection and three alleles (DRB3*35, *16 and *23) were associated with increased risk of T. congolense infection. While further work is required to dissect the role of these alleles in susceptibility to T. brucei and T. congolense infections, this study demonstrates the utility of FTA archived blood samples in combined molecular analyses of both host and pathogen.     

Hypothalamic-pituitary-adrenal axis responsiveness to insulin-induced hypoglycaemia is modified by trypanosome infection in Boran (Bos indicus) cattle.

Ten Boran (Bos indicus) cattle were used to study the stress responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis during trypanosome infection. Five cattle were infected with Trypanosoma congolense IL 1180 by tsetse challenge and five cattle served as controls. All infected animals developed acute trypanosomiasis. Insulin-induced hypoglycaemia (50 per cent of pre-insulin glucose concentration) was used as a stress factor. Acute hypoglycaemia was observed in three infected and three control animals after insulin challenge. Two animals from each group either did not respond or responded slowly. Hypoglycaemia in infected animals completely failed to induce an HPA axis response, while in control animals an HPA axis response was indicated by a significant increase in plasma adrenocorticotrophic hormone (ACTH) and cortisol concentrations (P less than 0.01). The results show that trypanosomiasis in Boran cattle can cause a decrease in the stress responsiveness of the HPA axis as indicated by a blunted ACTH/cortisol response to insulin-induced hypoglycaemia.     

Non-immune control of trypanosomosis: in vitro oxidative burst of PMA- and trypanosome-stimulated neutrophils of Boran and N'Dama cattle

An in vitro assay that measures the generation of superoxide anions (O2-) was used to assess the level of oxidative burst of phorbol myristate acetate (PMA)- and trypanosome-stimulated neutrophils isolated from healthy Boran and N'Dama cattle, and those infected with Trypanosoma congolense. PMA stimulation of healthy bovine neutrophils resulted in between 300-400 % increase in O2- generation. Neutrophils of Boran cattle exhibited slightly higher but insignificant O2- generation capacity than those of the N'Dama breed. In vitro stimulation by trypanosomes of neutrophils isolated from Trypanosoma congolense-infected cattle caused significant increases in O2- generation, especially on days 14, 28 and 42 post-infection, of both breeds of cattle. No significant differences were observed in O2- generation capacity of the neutrophils of both breeds of infected cattle throughout the period of assay. The results of this study have shown that PMA and trypanosomes do cause an enhanced in vitro oxidative burst, hence trypanosome phagocytosis and killing activity of neutrophils. Neutrophils have been shown to play very significant roles in parasite clearance, hence reduction of trypanosome parasitaemia. The rates of both in vitro generation of O2- and trypanosome phagocytosis over time did not differ significantly between Boran and N'Dama breeds of cattle, even during T congolense infection in this study. Hence, it may be inferred that sustained and higher parasitaemia, more pronounced neutropenia, inadequate bone marrow response and less effective trypanosome-specific immune response, rather than defective neutrophil trypanosome destruction, may be the problem of trypanosusceptible cattle breeds.     

Differential expression of surface membrane antigens on bovine monocytes activated with recombinant cytokines and during Trypanosoma congolense infection

The expression of surface membrane antigens on peripheral blood monocytes (PBM) of cattle of the Boran and N'Dama breeds activated with recombinant cytokines (TNF-alpha and IFN-gamma) and during experimental infection with Trypanosoma congolense was investigated using monoclonal antibodies (MoAbs) and fluorescein-activated cell sorter (FACS). The surface antigens investigated were C3bi receptor, major histocompartibility (MHC) II complex (Ia antigen) and two monocyte/macrophage (Mphi) differentiation antigens. The study revealed that both cytokines caused the enhancement of the expression of all the PBM surface antigens studied. rBolFN-gammaat low concentrations was more efficient in causing the activation of PBM. While the PBM of Boran cattle were more significantly activated to express the C3bi receptor vis-à-vis the Ia antigen than N'Dama cattle, the reverse was the case with the PBM of N'Dama cattle which expressed more Ia antigens than Boran PBM. Similar results were observed during T. congolense infection in the two breeds of cattle. The significantly higher expression of C3bi receptor and correspondingly lower Ia antigen expression by the PBM of Boran cattle, both during trypanosomosis and in vitro may be responsible for the higher rate of erythrocyte phagocytosis, hence the development of more severe anaemia by Boran cattle during trypanosomosis than N'Dama. In addition, the expression of significantly higher numbers of Ia antigen by N'Dama Mphi, hence are more able to process, present and initiate better trypanosome antigen-specific immune response than Boran cattle during infection. These two attributes are known genetic characteristics of trypanotolerance in cattle.     

Isometamidium chloride prophylaxis against Trypanosoma congolense challenge and the development of immune responses in Boran cattle

Twenty-four Boran cattle were injected with isometamidium chloride (1 mg/kg bodyweight) to investigate the duration of drug-induced prophylaxis against infection by metacyclic forms of Trypanosoma congolense and to determine if specific antibody responses to the organism were mounted by animals under chemoprophylactic cover. Complete protection against either single challenge by five tsetse flies infected with T congolense, or repeated challenge at monthly intervals by five tsetse flies, lasted for five months. Six months after treatment, two-thirds of the cattle were resistant to challenge, irrespective of whether subjected to single or multiple challenge with trypanosome-infected tsetse flies, or titrated doses of in vitro-cultured metacyclic forms of T congolense (5 X 10(2) to 5 X 10(5) organisms), inoculated intradermally. No animal which resisted infection developed detectable skin reactions at the site of deposition of metacyclic trypanosomes or produced trypanosome-specific antibodies. It was concluded that drug residues effectively limited trypanosome multiplication at the site of deposition in the skin, thus preventing subsequent parasitaemia or priming of the host's immune response.     

Immunosuppression in bovine trypanosomiasis: studies with louping-ill vaccine.

The antibody response to louping-ill virus vaccine was examined in mice infected with Trypanosoma brucei and T congolense, and in Ethiopian cattle experimentally infected with T brucei, T congolense and T vivax. In mice the antibody response was completely suppressed, while in cattle infected with T congolense and T vivax the antibody response to the vaccine was only 10 per cent that of uninfected animals. In contrast, the response of cattle infected with T brucei was not significantly reduced, and this was attributed to their relatively light and transient parasitaemias. Trypanocidal chemotherapy (diminazine aceturate) administered on the same day as vaccination largely restored the competence of the immune response of both mice and cattle infected with T congolense. The use of such drugs should be considered when cattle are vaccinated in trypanosome endemic areas.     

Factors influencing the duration of isometamidium chloride (Samorin) prophylaxis against experimental challenge with metacyclic forms of Trypanosoma congolense

The duration of a single isometamidium chloride (Samorin) prophylactic treatment against Trypanosoma congolense ILNat. 3.1 and T. congolense IL 285 was examined in 24 Boran steers with regard to (1) the dose of drug, (2) the level of metacyclic challenge and (3) the influence of infection with an unrelated serodeme at the time of treatment. The cattle were repeatedly challenged at monthly intervals between 2 and 7 months following treatment, either by five infected Glossina morsitans centralis or by intradermal inoculation of 5 X 10(3) or 5 X 10(5) in vitro-derived metacyclic trypanosomes. A dose of 1 mg kg-1 afforded complete protection for 4 months and 0.5 mg kg-1 for 3 months against the two T. congolense serodemes examined, irrespective of the method or weight of challenge. In another group of cattle, which had an established infection at the time of treatment, the duration of chemoprophylaxis against an unrelated serodeme was the same as the other groups which had no previous experience of trypanosome infection. Antibodies to metacyclics did not appear in any of the cattle as long as the chemoprophylaxis was effective. An exception to this was the group challenged with 5 X 10(5) in vitro-derived metacyclic parasites, in which low antibody titres were detected. In all cases these proved to be non-protective. It was concluded that, under the experimental conditions employed, (1) there was a direct relationship between drug dosage and the duration of chemoprophylaxis, (2) the weight of metacyclic challenge did not affect the duration of chemoprophylaxis and (3) when used to treat an existing infection, isometamidium chloride exerted the same degree of chemoprophylactic activity.     

Trypanosome infection rate in cattle at Nguruman, Kenya

Trypanosome infection rate in cattle at Nguruman was investigated in a study conducted in 1984-1986. Shifting pastoralism significantly reduced trypanosome infections in cattle. The cattle were more heavily infected with Trypanosoma congolense (16.5%) than Trypanosoma vivax (4.95%) and Trypanosoma brucei (0.19%). Trypanosoma theileri was observed only once among the cattle examined. Mixed trypanosome infections in cattle were observed to be 2.75% and 0.014% for T. congolense/T. vivax and T. congolense/T. brucei, respectively. The duration of infection in the cattle was 55 days for T. congolense and 79 days for T. vivax. High infections in cattle were observed 2 months after the rains, which were concomitant with high tsetse densities.     

Cytokine mRNA profiles in bovine macrophages stimulated with Trypanosoma congolense

It is known that different breeds of cattle display differential susceptibilities to Trypanosome congolense infections, and that N'Dama cattle remain more productive after infection than Boran cattle which are more susceptible to T. congolense. Macrophages from both breeds were cultured in vitro and the expressions of a number of cytokines and iNOS mRNA were analyzed using real time RT-PCR after stimulation with antibody-opsonized trypanosomes. No significant difference was seen between the responses of the two breeds. However, RNA levels of TNF-alpha in the IFN-gamma-primed macrophages were about 100-fold higher than those in the non-primed macrophages. A significant ten-fold decrease was seen for the anti-inflammatory cytokine IL-10. These results indicate that priming of the cells with IFN-gamma cause a serious shift toward an inflammatory response.     

Susceptibility of African buffalo and Boran cattle to Trypanosoma congolense transmitted by Glossina morsitans centralis

Four African buffalo (Syncerus caffer) and four Boran cattle (Bos indicus) were each exposed to the bites of 10 tsetse flies infected with Trypanosoma congolense. Although both groups of animals became infected, the buffalo showed no clinical signs of trypanosomiasis while the cattle suffered from the disease characterized by pronounced skin reactions, high parasitaemia and severe anaemia. The prepatent periods in the buffalo varied from 18 to 27 days in comparison with 11 to 14 days in the cattle. In the buffalo, skin reactions were only detectable by histological examination of skin biopsies, the peak of parasitaemia was at least a hundredfold below that in cattle and after 54 days parasites were no longer detected. In contrast, the cattle had a continuous high parasitaemia until they were treated with a trypanocidal drug 60 days after infection. Neutralizing antibody to metacyclic trypanosomes appeared in the buffalo during the prepatent period, 15-20 days after infection, whereas in cattle neutralizing antibody was not detected until 10 days after the first peak of the parasitaemia, 25-30 days after infection.     

Susceptibility of N'Dama cattle to experimental challenge and cross-species superchallenges with bloodstream forms of Trypanosoma congolense and T. vivax.

Susceptibility to Trypanosoma congolense, T. vivax challenge and cross species-superchallenges, and related effects on health and productivity were assessed in N'Dama cattle. Twenty-five N'Dama bulls aged 3-4 years and previously primed with trypanosome infections through natural tsetse exposure over more than one year were used. The experimental herd was divided in five groups each composed of five randomly selected animals. Group 1 was challenged with T. congolense, Group 2 with T. vivax, Group 3 was inoculated with T. congolense followed by a cross-superchallenge with T. vivax, Group 4 was inoculated with T. vivax followed by T. congolense cross-superchallenge. Animals in Group 5 were used as controls. Both T. vivax and T. congolense cross-superchallenges were carried out on Day 14 subsequent to respective initial T. congolense and T. vivax inoculations. All challenges were performed by intradermal needle inoculation of stocks of trypanosome bloodstream forms. In challenged animals (Group 1 to 4), parasitaemia profiles and packed red cell volumes (PCV) were measured for four months. Weight changes were recorded monthly and daily weight gain (DWG) computed. All cattle challenged with T. congolense became parasitaemic. Conversely, one animal in Group 2 and two in Group 3 never displayed patent T. vivax parasitaemia. Both in single (Group 1), initial (Group 3) and cross-superchallenged (Group 4) cattle higher percentage of positive blood samples and higher parasitaemia level were obtained following T. congolense than T. vivax inocula (Group 2, 3 and 4) (P<0.04 or greater). Overall the pre-challenge period, PCV values and DWGs were nearly identical in the five groups. Conversely, over the post-challenge period, cattle singly, initially and cross-superinoculated with T. congolense (Group 1, 3 and 4) displayed lower PCV values and DWGs in comparison with both control animals (Group 5) and with singly T. vivax challenged cattle (Group 2) (P<0.05 or greater). No difference in mean PCV levels and DWGs was found between animals in Group 2 and cattle in Group 5. It was concluded that trypanotolerant N'Dama cattle suffered more from T. congolense and mixed T. congolensel T. vivax infections, while pure T. vivax infection did not produce appreciable negative effects on their health and productivity. Therefore, considering that tsetse and trypanosomosis control campaigns are costly and are justified only when derived economic benefits exceed those of control, and also that an ample mosaic of farming systems exists in West Africa, species-specific trypanosome prevalence and relative impact should be assessed in various cattle populations and breeds differing in trypanosome susceptibility before advising any intervention. Moreover, virulence and related effects of T. congolense and T. vivax endemic stocks on health and productivity in local cattle populations should also be estimated in order to counsel appropriate economic protection measures against trypanosmosis, i.e. vector control and/or strategic use of trypanocidal drugs.     

Recirculation of Trypanosoma brucei brucei in cattle after T. congolense challenge by tsetse flies

The effect of challenging cattle, chronically infected with Trypanosoma brucei brucei, with T. congolense on the development of the T. b. brucei infection was investigated. For this purpose, nine experimental animals were first infected with T. b. brucei through the bites of infected tsetse flies. Once the T. b. brucei had developed into a chronic infection, that was difficult to detect using routine parasitological diagnostic tools, seven of the experimental animals were challenged by tsetse flies infected with T. congolense. Two of the animals infected with T. b. brucei were kept as control. The infection with T. congolense resulted in a sudden increase in the parasitaemia of T. b. brucei. In the T. b. brucei control animals, on the other hand, the parasitaemia remained below the level of detection. The epidemiological repercussions of this increase in the parasitaemia of T. b. brucei after infection with T. congolense are discussed.     

Effect of chronic trypanosomiasis on immunization against East Coast fever

Two experiments were carried out in which uninfected cattle, or cattle chronically infected with Trypanosoma congolense, were immunized by the infection and treatment method against East Coast fever (ECF; Theileria parva infection). Chronic trypanosomiasis did not prevent cattle mounting an effective immunological response to ECF immunization and resisting subsequent lethal challenge. There appeared to be no difference in the level or quality of immunity between uninfected cattle and trypanosome-infected cattle. Thus, T. congolense infection on its own does not appear to provide a constraint to ECF immunization in the field.     

Application of PCR and DNA probes in the characterisation of trypanosomes in the blood of cattle in farms in Morogoro, Tanzania

Polymerase chain reaction (PCR) and deoxyribonucleic acid (DNA) probes were used to characterise trypanosomes from cattle in Morogoro region of Tanzania. Blood samples collected from 390 beef and dairy cattle in selected farms in Morogoro region were examined for presence of trypanosomes using the buffy coat technique (BCT) and blood smears (BSs). Fifty-two animals were found infected: 40 with Trypanosoma congolense, 10 with T. vivax and two with both T. congolense and T. vivax. DNA extracted from all the parasitologically positive and 62 randomly selected parasitologically negative samples were subjected to PCR amplification using primers specific for different trypanosome species. Using a set of seven specific-pairs of primers on the parasitologically positive samples, we detected only T. congolense, either the Savannah- or the Kilifi-type, as single or mixed infections. With the PCR, trypanosome DNA could be detected in 27 (43%) out of 62 samples that were parasitologically negative. DNA hybridisation using probes specific for Savannah- or Kilifi-types T. congolense, or T. vivax, confirmed the presence of these parasites in cattle kept on some farms in Morogoro region of Tanzania. From these studies, it is clear that there is a need to undertake molecular epidemiological studies to determine the distribution of trypanosome species and subspecies, and to assess the economic impact of these parasites in the productivity of livestock in Tanzania. In particular, it would be desirable to verify the assumed association between the different presentations of trypanosomosis on one hand and genotypes of T. congolense on the other.     

Therapeutic effect of Berenil and Samorin in mice infected with four trypanosome populations isolated from Zambian cattle

Four populations of Trypanosoma congolense and Trypanosoma brucei brucei were isolated from cattle under different management practices and environments in Zambia. All four isolates had varied responses to both diminazene aceturate (Berenil) and isometamidium chloride (Samorin) as curative drugs in infected mice. Trypanosomes from a traditionally managed herd in a high-tsetse-challenge area had the strains most resistant to Berenil, with maximum curative dose of 45 mg kg-1 body weight. Another isolate from a high-tsetse-challenge area was evidently resistant both to Berenil at 40 mg kg-1 and to Samorin at 4 mg kg-1. The strains most susceptible to both Berenil and Samorin were from a commercially managed herd of cattle under medium tsetse challenge. They responded to recommended cattle standard doses of 3.5 mg kg-1 or 7 mg kg-1 Berenil and to as little as 0.25 mg kg-1 Samorin. It is evident that trypanosome strains resistant to Berenil and/or partially resistant to Samorin exist, and that both T. congolense and T. b. brucei are implicated.     

Comparison of production losses caused by chronic Fasciola gigantica infection in yearling Friesian and Boran cattle

Yearling Friesian and Boran cattle were given a standard dose of Fasciola gigantica metacercariae designed to produce chronic infection. Their liveweights were then monitored for 23 weeks post-infection. Following standard meat inspection procedures, all the livers from the infected cattle were condemned. At 22 kg per animal, the mean reduction in liveweight gain in infected Borans was significantly greater than in infected Friesians. Compared to the control cattle and based on current sale prices in Kenya, the production losses from the combination of liver condemnations and reduction in liveweight gain constituted a loss in value of US$12.11 (4.92%) and US$23.41 (10.34%) for Friesians and Borans respectively. These data indicate that productivity losses and the economic consequencies of fasciolosis in yearling Boran cattle are much more severe than in Friesians. These findings have implications for assessing the likely benefits of control measures against this parasite.     

Implications of the re-invasion of Southeast Uganda by Glossina pallidipes on the epidemiology of bovine trypanosomosis

A study to assess the influence of re-invasion of Glossina pallidipes on the epidemiology of bovine trypanosomosis was conducted in Southeast Uganda. A total of 1,992 cattle were screened in villages, with (949) and without G. pallidipes (1043) for trypanosomosis using a combination of the BCT and HCT methods. The prevalence of trypanosomosis (15.5%), Trypanosoma brucei infection (1.4%), T. congolense infection (7.2%), T. vivax infection (5.3%) and mixed infection (1.6%) in cattle in villages with was significantly higher than in those without G. pallidipes: trypanosomosis (7.1%), T. brucei infection (0.6%), T. congolense infection (2.0%), T. vivax infection (3.3%) and mixed infection (1.2%) (overall trypanosome infection, chi2=35.5, d.f.=1, P<0.05; T. brucei infection, chi2=8.06, d.f.=1, P<0.05; T. congolense infection, chi2=22.8, d.f.=1, P<0.05 and T. vivax infection, chi2=6.4, d.f.=1, P<0.05). Infections of Trypanosoma congolense were predominant in cattle in villages with G. pallidipes, while T. vivax infections were predominant in cattle in villages without. In all villages, T. brucei infections were fewer than either T. congolense or T. vivax infections. The risk of transmission of T. brucei, T. congolense and T. vivax infections was 3, 2.7 and 1.6 times, respectively, higher in villages with G. pallidipes than in those without, despite the presence of G. f. fuscipes in either set of villages. The mean PCV (28.27+/-0.41, 95% CI) and mean herd size (3+/-0.46) of cattle in villages with G. pallidipes were significantly (P<0.05) lower than in those in villages without (mean PCV, 29.48+/-0.34; mean herd size, 4+/-0.72). It is evident that presence of G. pallidipes brings about an increase in the prevalence of T. congolense, which causes a more severe disease in cattle than other species of trypanosomes. This is a rare case of a re-invasion of a tsetse species whose disease transmission capability calls for refocusing of the traditional national tsetse and trypanosomosis control strategies to contain it.     

Some preliminary observations on the susceptibility and resistance of different cattle breeds to Theileria parva infection

Theileria parva-na?ve Friesian (Bos taurus), Boran (Bos indicus) and Maasai Zebu steers (B. indicus) were infected with a T. parva sporozoite stabilate dose which had previously been shown to induce an estimated 50% mortality rate in Boran cattle. All the cattle developed patent infections with no significant differences in the length of the prepatent period to development of macroschizonts (P > 0.05) between the three groups. Clinical theileriosis occurred in all eight the Friesians (100%), five out of nine Borans (55.6%) and two out of five Zebus (40%). Three of the Friesians (37.5%), and two of the Borans (22.2%) died of theileriosis. The different cattle types were equally susceptible to the infective dose used as indicated by the length of the prepatent periods, but there was a marked difference in their development of clinical theileriosis. The gradation in resistance to disease confirms the findings of earlier less critical studies and identifies these cattle breeds as suitable for investigations into the mechanisms of resistance to theileriosis.