Estimation of trypanosomal status by the buffy coat technique and an antibody ELISA for assessment of the impact of trypanosomosis on health and productivity of N'Dama cattle in The Gambia.

The buffy coat/dark ground phase contrast technique (BCT) and an indirect antibody enzyme immunoassay (ELISA) were employed to assess the trypanosomal status of 32 N'Dama cattle, aged 19-28 months, exposed to natural challenge of Glossina morsitans submorsitans and G. palpalis gambiensis. Prior to the start of the investigation animals experienced 9-16 months of tsetse challenge in the study area. Blood and corresponding serum samples were examined monthly for a period of 8 months for patent parasitaemia by BCT and presence of Trypanosoma vivax and T. congolense antibodies by ELISA. In the ELISA, the reactivity of sera to anti-trypanosomal antibodies was expressed in percent positivity (pp). Packed cell volumes (PCV) and body weights were also recorded monthly, and daily weight gain (DWG) computed to assess the impact of trypanosomal status on health and productivity. During the study period, the overall parasitaemic trypanosome prevalence was 3% (6/199), while the serological prevalence was 54.7% (109/199). Both diagnostic tests revealed a predominance of T. vivax over T. congolense infections in N'Dama cattle. Sensitivity of the immunoassay was 83.3%. In T. vivax-parasitaemic cattle, antibodies persisted for 4-6 months after the parasite was detected by BCT. A significantly higher overall mean PCV level was observed in blood samples obtained from cattle found, in any particular month, negative by BCT and ELISA, compared with those blood samples from animals responding serologically positively for anti-trypanosome antibodies. Likewise, mean DWG was significantly higher in cattle found negative for both tests in comparison to animals presenting detectable anti-trypanosome antibodies and those detected positive by both tests. A significant negative relationship was observed between pp values and PCV levels in animals seropositive for T. vivax and/or T. congolense. Similarly, a negative relationship was observed between DWGs and pp values. PCV levels were significantly positively correlated with DWGs. It was concluded that serological screening could provide useful information complementary to that obtained by the use of BCT not only to assess more accurately the trypanosomal status of cattle populations, but also to evaluate the effects of trypanosome infection on animal health and productivity and estimate the trypanosomosis risk.

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.

Seasonal prevalence of ticks and tick-transmitted haemoparasites in traditionally managed N'Dama cattle with reference to strategic tick control in the Gambia.

A survey of tick spatial and seasonal distribution in traditional managed N'Dama cattle over 1 year old was carried out in The Gambia over 16 months. Presence of Anaplasma marginale and Babesia spp. in the blood smears and their antibodies in the serum of same animals were also examined. Tick species, in decreasing order of abundance, were: Boophilus geigy, Rhipicephalus senegalensis, B.decoloratus, Hyalaomma truncatum, H.marginatum rufipes and Amblyomma variegatum. All tick species peaked during the rainy season. Additional B.geigy peaks occurred towards the end of the dry season. The most infested animal body areas were anogenital, udder and abdomen. A.variegatum, Hyalomma spp. and, to a lesser extent, Boophilus spp. showed strong preferences for these anatomical regions. Overall prevalence of A.marginale, B.bigemina and B.bovis haemoparasites were 3.2%, 0.9% and 0.1%, respectively. Peaks of A.marginale and B.bigemina occurred at the end of the rainy season-beginning of the dry season; an additional A.marginale peak was observed at mid-end dry season. Overall antibody seroprevalence were 29.6% for A.marginale, 44.7% for B.bigemina and 5.2% for B.bovis; monthly fluctuations in percentage of seroreactors were limited. Endemic stability for A.marginale and B.bigemina is postulated. Implications of the outcomes are discussed in relation to tick-control strategy in The Gambia.

A comparative study of the parasitological buffy coat technique and an antigen enzyme immunoassay for trypanosome diagnosis in sequential Trypanosoma congolense infections in N'Dama, Gobra zebu and N'Dama x Gobra crossbred cattle.

The buffy coat/dark ground techniques (BCT) and an antigen enzyme immunoassay (Ag-ELISA) were compared for the diagnosis of trypanosome infection in N'Dama, Gobra zebu and N'Dama x Gobra (F1) crossbred cattle following two sequential experimental Trypanosoma congolense infections. Both first and second challenge were performed by intradermal needle inoculation of trypanosome bloodstream forms. During the course of the first challenge, the overall trypanosome percentage of positive cases detected by BCT in blood samples was higher (P < 0.001) in comparison with that obtained by Ag-ELISA in tested serum samples of the three cattle breeds. Conversely, in the second infection the overall number of infections detected by BCT was lower in N'Dama (P < 0.005) and F1 (P < 0.001) than that obtained using Ag-ELISA; nearly identical positive rates were detected by the two diagnostic techniques in Gobra zebus. Both in the first and second T. congolense challenge, positive rate obtained using BCT significantly decreased as the infection progressed. On the other hand, the positive rate given by Ag-ELISA and its sensitivity manifested a significant opposite trend during the course of the first infection. No relationship between progression of infection and Ag-ELISA positive rate as well as Ag-ELISA sensitivity was found in the second T. congolense challenge. The Ag-ELISA was less than 50% sensitive in detecting circulating antigens during the first 2 months of the primary infection. However, it showed a high and stable sensitivity throughout the second trypanosome infection. It was concluded that the Ag-ELISA was suitable for use in detecting chronic or repeated infections but needs to be combined with BCT to provide reliable results.

Experimental Trypanosoma congolense infection on naturally occurring ticks in N'dama and Gobra zebu cattle.

The effects of experimental Trypanosoma congolense infection in Gambian N'dama and Gobra zebu cattle on number of naturally-occurring adult ticks attaching were studied. An indirect fluorescent antibody test was performed to detect serological prevalence of Cowdria ruminantium antibody. The intravenously imposed trypanosome infection did not result in significant (P > 0.05) differences in Amblyomma variegatum and Hyalomma spp. infestations between control and infected N'dama cattle. Control N'damas carried significantly (P < 0.001) lower numbers of A. variegatum and Hyalomma spp. than the control zebus. Serological frequency of C. ruminantium antibody was similar in both control or infected N'dama and in control or infected zebu cattle. No deaths occurred among N'dama cattle, while all six trypanosome infected zebus progressively died within nine weeks post-infection but trypanosomosis was excluded as the primary cause of death. Examined Giemsa-stained blood smears were negative for the presence of tick-borne micro-organisms. Four positive cases of cowdriosis were identified during post-mortem examination. It was concluded that N'damas, even when submitted to trypanosome infection, react consistently better than Gobra zebus to tick attachment. These results emphasize the benefits of rearing disease resistant cattle breeds, such as N'dama, in areas where risks of trypanosomosis and cowdriosis coexist.

Trypanosome infections in warthogs (Phacochoerus aethiopicus) in the Gambia.

The prevalence of trypanosome infections in warthogs (Phacochoerus aethiopicus) in The Gambia was found to be 11% of a sample of 62 animals. All isolates were identified as Trypanosoma simiae. Serological evidence indicated a higher level of exposure to T. simiae, but results were inconclusive for the presence of Trypanosoma congolense. The course of T. simiae infection in warthog piglets showed a rapidly rising parasitaemia, with a concomitant fall in packed cell volume, and resulted in a prolonged period of low-level parasitaemia. The same infections killed domestic piglets.