Stability of metacyclic variable antigen types (M-VATs) during the early stages of infection with Trypanosoma congolense

Local skin reaction (chancre) induced following inoculation of metacyclic trypanosomes in cattle by tsetse flies is dependent on CD4 T lymphocytes

The first visible response in livestock to the bite of a trypanosome-infected tsetse fly is the formation of a localized skin reaction, also known as a chancre. This is an inflammatory response in the skin associated with swelling and an influx of cells. It is thought to be associated with an acquired immune response to the injected metacyclic trypanosomes. In this study, we examined the role of T lymphocytes in the development of the inflammatory response, by depleting cattle of T cell subpopulations and monitoring the development of chancres. Depletion of CD4 cells, but not CD8 cells, resulted in a significant reduction in chancre formation, confirming that an acquired response mediates the inflammatory response. In addition, it was established that the CD4 T cells mediate the generation of memory for immunity to a homologueous re-challenge. The inflammatory response in the skin did not affect further progress of the 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.

Trypanosomosis research at the Centre for Tropical Veterinary Medicine (CTVM) 1970 to 1995

This review covers aspects of research work carried out on animal trypanosomes at the Centre for Tropical Veterinary Medicine (CTVM) during the last 25 years. The review covers work on antigenic variation, tissue culture, drug resistance, immunology, biochemistry and pathology of Trypanosoma brucei, T. congolense, T. gambiense and T. evansi. It is not intended as an exhaustive review of the subject but focuses on certain aspects of these areas which are presented in relation to work carried out within the broader scientific community.

Trypanosoma congolense infection in sheep: cellular phenotypes in lymph and lymph nodes associated with skin reactions

Intradermal inoculation of sheep with culture-derived metacyclic forms of Trypanosoma congolense resulted in the development of localized skin reactions (chancres) and enlargement of the draining lymph nodes 7 days after infection. Changes in the expression of surface antigens of lymphocytes in lymph leaving the affected skin reactions and in the associated lymph nodes were monitored by cannulating the afferent and efferent lymphatic ducts. Trypanosomes appeared in afferent and efferent lymph 3 to 5 days after infection and persisted even as the chancres regressed. The cellular output in both afferent and efferent lymph increased markedly after the onset of parasitosis. Sequential analysis of the phenotypes of lymphocytes by immunofluorescent staining and flow cytometry revealed that in afferent lymph draining the chancre there was an early response which was due to an increase in T cells, particularly CD4+ and CD8+ cells; however, as the chancres-regressed there was an increase in lymphoblasts and surface immunoglobulin-bearing cells. In contrast, in the efferent lymph, the increase in lymphocytes was due predominantly to a higher number of cells bearing surface immunoglobulins.

Early stages of infection with Trypanosoma congolense: parasite kinetics and expression of metacyclic variable antigen types

Trypanosoma congolense develops in the skin of sheep at the site of inoculation of metacyclic trypanosomes, forming a chancre containing large numbers of parasites. By cannulating the afferent and efferent lymphatic ducts draining the skin and regional lymph node, the progressive development and migration of trypanosomes from the chancre was monitored and the expression of metacyclic antigen types (M-VATs) was determined. The kinetics of development of parasitosis in the afferent and efferent lymph was similar. Trypanosomes were detected in lymph 5 to 6 days after the inoculation of cultured metacyclic trypanosomes, at the same time as the chancre first appeared in the skin. The numbers of trypanosomes in the lymph reached their peak levels 8 to 10 days post infection and thereafter numbers fell, although there were still parasites in the lymph after the chancre had regressed. Trypanosomes in the afferent lymph expressed mainly M-VATs and the absolute numbers of four M-VATs which were monitored increased up to 9 days post infection. There was a fall in numbers by day 10, but 92% of the trypanosomes in the afferent lymph continued to express M-VATs. In contrast, trypanosomes from the efferent lymph were found not to express M-VATs suggesting that a major switch in VAT expression occurs in the lymph node. Specific antibody responses, measured by neutralization tests, were evident 16 to 20 days after infection in afferent lymph but only low levels of antibodies were found in efferent lymph.

Multiplication of Trypanosoma evansi at the site of infection in skin of rabbits and cattle

Local skin reactions (chancres) developed at the sites of inoculation with Trypanosoma evansi in rabbits and calves. Trypanosomes multipled in the dermal collagen and, in the rabbit, were present in large numbers by 7 days after infection. Thereafter, however, numbers decreased and few parasites were observed by 11 days after infection. The presence of trypanosomes in the skin caused an extensive inflammatory reaction with disruption of collagen, oedema, necrosis of the skin and increases principally in neutrophils and lymphocytes. In calves, similar changes were observed although there were fewer trypanosomes present in the chancre and the cellular involvement was less extensive than that seen in the rabbit. This early extracellular proliferative phase of development of T. evansi may be of importance in naturally transmitted infections both in the initial establishment of the parasite in the mammalian host and in enabling the parasite to increase the numbers of antigenic variants expressed before the parasites invade the general circulation.

Trypanosoma congolense: re-expression of a deleted metacyclic variable antigen type in vivo and in vitro

The expression of variable antigen types (VATs) was determined among dividing populations of T. congolense growing in vivo in rabbit chancres and in vitro on bovine aorta endothelial cell monolayers. Experiments were performed in which a single metacyclic VAT (M-VAT) was deleted from a cultured metacyclic population by neutralisation with a monoclonal antibody and complement. Subsequent expression of the deleted M-VAT and two unrelated M-VATs was determined by an indirect immunofluorescent antibody test. The deleted M-VAT was re-expressed both in vivo and in vitro and the proportions of unrelated M-VATs were not markedly affected by the neutralisation of this single M-VAT. In addition, an overall similarity was observed between M-VAT expression and re-expression in vivo and in vitro.

Susceptibility of N'Dama and Boran cattle to tsetse-transmitted primary and rechallenge infections with a homologous serodeme of Trypanosoma congolense

Eight trypanotolerant N'Dama cattle controlled an infection of Trypanosoma congolense ILNat 3.1 transmitted by Glossina morsitans centralis, more efficiently than a group of similarly infected trypanosusceptible Boran cattle. All eight N'Damas maintained their PCV above 15% throughout the primary infection whereas the PCV of six of the eight Borans dropped below 15%; these latter animals were treated with diminazene aceturate to prevent possible death. Lymphocyte, neutrophil and platelet counts also decreased in the Boran during the primary infection. In contrast, a lymphocytosis was observed in the N'Dama; and although the neutrophil and platelet counts decreased, the drop was less severe than in the Boran. Two years after the primary infection and immediately prior to a homologous rechallenge infection, all eight N'Damas had neutralizing anti-metacyclic trypanosome variant-specific antibodies present in their sera compared to five of the eight Borans. Following the homologous rechallenge infection the eight N'Damas became parasitaemic but there were no alterations in their erythrocyte or leukocyte counts. The Borans became highly parasitaemic and developed severe, chronic anaemia and leukopaenia. Thus, the trypanotolerant N'Damas controlled a primary infection of T. congolense more efficiently than trypanosusceptible Boran cattle and eliminated a homologous rechallenge infection without the pathology associated with the disease.

Immunohistology of lymph nodes draining local skin reactions (chancres) in sheep infected with Trypanosoma congolense

Marked enlargement of lymph nodes draining local skin reactions (chancres) occurred in sheep following intradermal inoculation of cultured metacyclic forms of Trypanosoma congolense. Histologically, these lymph nodes were characterized by follicular hypertrophy and hyperplasia, compression and relative reduction of the paracortical areas and expansion of the medullary regions. Immunohistochemical staining with monoclonal antibodies to ovine lymphocyte subsets and Fc receptor (FcR) bearing macrophages, revealed increased expression of B cells (CD45R+), major histocompatibility complex (MHC) Class II, FcR+ macrophages, and CD1+ cells in the cortical and paracortical areas. The paracortical areas were found to be sparsely populated by CD5+, CD4+ and CD8+ cells, while the medullary areas contained numerous CD8+ cells and FcR+ macrophages. FcR+ macrophages were also present in cortical trabecular and subcapsular sinuses. As the chancre regressed, lymph node reactivity also subsided and fewer B cell follicles were observed and there was decreased expression of CD45R+ and MHC Class II+ cells.