Tsetse-trypanosomiasis challenge to village N'Dama cattle in The Gambia: field assessments of spatial and temporal patterns of tsetse-cattle contact and the risk of trypanosomiasis infection

Factors associated with persistence of African animal trypanosomiasis in Lango subregion, northern Uganda

African animal trypanosomiasis (AAT) continues to inflict heavy losses on livestock production especially cattle in terms of decreased production and productivity in Uganda. AAT is a disease complex caused by tsetse fly-transmitted Trypanosoma brucei brucei, Trypanosoma brucei rhodesiense, Trypanosoma congolense, and Trypanosoma vivax. The disease is most important in cattle but also known to cause serious losses in pigs, camels, goats, and sheep. Several control measures including live bait technology, mass treatment of cattle with trypanocidal drugs, and deployment of tsetse traps have been used in the past 10 years, but the problem still persists in some areas. This necessitated an exploration of the factors associated with continued trypanosome infections in cattle, which are also known reservoirs for the zoonotic trypanosomiasis. A structured questionnaire was administered to 286 animal owners from 20 villages purposively selected from Lira, Kole, and Alebtong districts of Lango subregion to obtain information on the factors associated with persistence of infection. Over 50% of the respondents reported trypanosomiasis as a major challenge to their livestock. Land ownership (P = 0.029), type of livestock kept (P = 0.000), disease control strategy employed (P = 0.000), source of drugs (P = 0.046), and drug preparation (P = 0.017) were associated with persistent AAT infection. We recommend continued farmer sensitization on the threat of AAT and the available prevention and control options. The use of isometamidium chloride for prophylaxis against trypanosomiasis is highly recommended. There is also a need to foster qualified private veterinary drug supply in the region.

Improvements on restricted insecticide application protocol for control of Human and Animal African Trypanosomiasis in eastern Uganda

Background

African trypanosomes constrain livestock and human health in Sub-Saharan Africa, and aggravate poverty and hunger of these otherwise largely livestock-keeping communities. To solve this, there is need to develop and use effective and cheap tsetse control methods. To this end, we aimed at determining the smallest proportion of a cattle herd that needs to be sprayed on the legs, bellies and ears (RAP) for effective Human and Animal African Trypanosomiasis (HAT/AAT) control.

Methodology/Principal finding

Cattle in 20 villages were ear-tagged and injected with two doses of diminazene diaceturate (DA) forty days apart, and randomly allocated to one of five treatment regimens namely; no treatment, 25%, 50%, 75% monthly RAP and every 3 month Albendazole drench. Cattle trypanosome re-infection rate was determined by molecular techniques. ArcMap V10.3 was used to map apparent tsetse density (FTD) from trap catches. The effect of graded RAP on incidence risk ratios and trypanosome prevalence was determined using Poisson and logistic random effect models in R and STATA V12.1 respectively. Incidence was estimated at 9.8/100 years in RAP regimens, significantly lower compared to 25.7/100 years in the non-RAP regimens (incidence rate ratio: 0.37; 95% CI: 0.22–0.65; P<0.001). Likewise, trypanosome prevalence after one year of follow up was significantly lower in RAP animals than in non-RAP animals (4% vs 15%, OR: 0.20, 95% CI: 0.08–0.44; P<0.001). Contrary to our expectation, level of protection did not increase with increasing proportion of animals treated.

Conclusions/significance

Reduction in RAP coverage did not significantly affect efficacy of treatment. This is envisaged to improve RAP adaptability to low income livestock keepers but needs further evaluation in different tsetse challenge, HAT/AAT transmission rates and management systems before adopting it for routine tsetse control programs.

Poverty, hunger and human ill-health aggravated by trypanosomiasis in Sub-Saharan Africa can only be reduced by developing and using cheap and effective tsetse control methods. To further reduce the cost of tsetse control by restricting insecticides to the legs, belly and ears (RAP) we set out to determine the lowest RAP coverage that can effectively control tsetse. Cattle in 20 south-eastern Uganda villages were randomly allocated to 5 treatment groups, ear-tagged for ease of follow-up and treated twice forty days apart with a trypanocide at the beginning of the trial. Cattle in regimens 2–4 received monthly graded RAP (25%, 50% and 75% of village herd respectively), while those in regimens 1 and 5 received no more treatment and deworming once every three months respectively. Molecular techniques were used to check for trypanosome infections, while tsetse apparent density was determined by traps at 161 locations in the district. About 25% RAP coverage was effective at controlling T. brucei s.l. while 50–75% RAP coverage would need to be used for effective T.vivax and T.congolense nagana control. Use of RAP at lower herd coverage is envisaged to reduce its cost, damage to the environment and improve its uptake in resource poor communities.

Factors associated with acquisition of human infective and animal infective trypanosome infections in domestic livestock in Western Kenya

Background

Trypanosomiasis is regarded as a constraint on livestock production in Western Kenya where the responsibility for tsetse and trypanosomiasis control has increasingly shifted from the state to the individual livestock owner. To assess the sustainability of these localised control efforts, this study investigates biological and management risk factors associated with trypanosome infections detected by polymerase chain reaction (PCR), in a range of domestic livestock at the local scale in Busia, Kenya. Busia District also remains endemic for human sleeping sickness with sporadic cases of sleeping sickness reported.

Results

In total, trypanosome infections were detected in 11.9% (329) out of the 2773 livestock sampled in Busia District. Multivariable logistic regression revealed that host species and cattle age affected overall trypanosome infection, with significantly increased odds of infection for cattle older than 18 months, and significantly lower odds of infection in pigs and small ruminants. Different grazing and watering management practices did not affect the odds of trypanosome infection, adjusted by host species. Neither anaemia nor condition score significantly affected the odds of trypanosome infection in cattle. Human infective Trypanosoma brucei rhodesiense were detected in 21.5% of animals infected with T. brucei s.l. (29/135) amounting to 1% (29/2773) of all sampled livestock, with significantly higher odds of T. brucei rhodesiense infections in T. brucei s.l. infected pigs (OR = 4.3, 95%CI 1.5-12.0) than in T. brucei s.l. infected cattle or small ruminants.

Conclusions

Although cattle are the dominant reservoir of trypanosome infection it is unlikely that targeted treatment of only visibly diseased cattle will achieve sustainable interruption of transmission for either animal infective or zoonotic human infective trypanosomiasis, since most infections were detected in cattle that did not exhibit classical clinical signs of trypanosomiasis. Pigs were also found to be reservoirs of infection for T. b. rhodesiense and present a risk to local communities.

Rhodesian sleeping sickness caused by Trypanosome brucei rhodesiense is a parasitic disease transmitted by tsetse flies which is fatal in humans if it is not treated. The parasites also infect a range of animal species in which they do not cause acute disease and may co-exist with other non human infective parasites. Busia District (Western Kenya) is a historic sleeping sickness focus. Human cases of this disease are still reported occasionally in Busia and neighbouring Teso District, most recently in 2008, showing that the human infective parasite species are still present in the area. However, trypanosomes in this region are mainly regarded as a threat to the productivity of domestic livestock and the responsibility for trypanosomiasis control has shifted from the state to livestock holders. To examine whether farmer-based control strategies can be successful, this study assessed the factors that influence trypanosomiasis in livestock at the local level.

The study showed that cattle are the livestock species most frequently affected by trypanosomes. However infection in cattle was not necessarily associated with signs of disease; furthermore pigs were shown to be important carriers of the human infective parasite. The treatment of only visibly diseased cattle to avoid losses in productivity will not successfully control the parasite in the long term. Keeping livestock in the vicinity of the homesteads also did not protect the animals from trypanosome infection. This indicated that the tsetse fly transmits the parasite in close proximity to human habitation, which could increase the risk of humans being infected.

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.

Methods for the rapid appraisal of African animal trypanosomosis in the Gambia

A technique for the rapid field assessment of African animal trypanosomosis (AAT) was developed during studies in the Gambia. This involved gathering indigenous information from rapid-appraisal questionnaires addressed to local informants, the results of single tsetse surveys and evaluations of the prevalence of trypanosome infections in village cattle. Local informants included livestock owners and herdsmen and trained personnel such as livestock assistants. The answers to the questionnaires were weighted in order to translate them into semi-quantitative ranked estimates (zero, low, medium, high or very severe) of the severity of AAT problems. A similar ranking was also defined for tsetse and prevalence data in the Gambia. The three assessment methods generally gave complementary results leading to similar conclusions about the severity of tsetse-trypanosomosis problems in a survey area; inconsistencies usually suggested that additional information was needed. The rankings of AAT intensity were used to develop management guidelines for minimising the impact of AAT at different levels through control interventions or improved livestock management. The methodology was designed to provide reliable, up-to-date and cost-effective assessments of AAT problems. Emphasis was placed on the importance of the involvement, priorities and perceptions of village livestock owners and herdsmen in making these assessments.

Trypanosome infections and tick infestations: susceptibility in N'Dama, Gobra zebu and Gobra x N'Dama crossbred cattle exposed to natural challenge and maintained under high and low surveillance of trypanosome infections

Susceptibility to trypanosome infections and tick infestations was assessed in 51 N'Dama, 48 Gobra zebu and 37 Gobra x N'Dama crossbred (F1) cattle exposed to field-tick infestations and natural high tsetse challenge over more than one year. From these cattle, 12 animals of each breed were randomly selected and examined parasitologically for trypanosome infections and packed cell volume (PCV) twice a week (high surveillance, group HS). In the remaining 100 cattle trypanosome infection and PCV were monitored monthly (low surveillance, group LS). Mortality rates were recorded in both groups. Tick infestation was quantified fortnightly from all animals in group HS and from four to seven randomly selected animals of each breed in group LS. In both HS and LS groups, trypanocidal drug treatment was administered to trypanosome positive animals with PCV equal to or less than 20% or when they showed clinical evidence (dullness, weight loss) of trypanosomosis. In both groups, N'Dama cattle exhibited a superior capacity to control trypanosome infections and limit tick burdens. Particularly, in group HS N'Dama cattle showed lower trypanosome infection rate, higher mean PCV value, lower requirement for trypanocide treatments and lower tick load than crossbred and Gobra cattle in the corresponding group (P < 0.05 or greater). This was also true in N'Damas in comparison with crossbreds in group LS. Unfortunately, the high mortality occurring in Gobra cattle in group LS did not allow within group comparative analysis between N'Dama, Gobra and crossbred cattle overall the study period. No death occurred in N'Dama cattle maintained under high surveillance of trypanosome infection, while approximately 8% of crossbreds and 50% of Gobras died of trypanosomosis. In group LS, all Gobra and more than 70% of crossbred cattle died. In this group, mortality in N'Dama was limited to less than 16%. In both groups, differences in mortality were significantly higher (P < 0.01) in Gobras than in N'Damas. Within breed, animals of the three breeds maintained under high surveillance of trypanosomal infection showed higher mean PCV values, lower tick burdens and required proportionally less trypanocide treatments than corresponding cattle in group LS. The infection rate in N'Dama under high surveillance was lower in comparison with N'Dama cattle in group LS. There was no significant difference in mortality between groups within the N'Dama breed. Conversely, mortality rates were lower in crossbred and Gobra in group HS than in respective cattle in group LS. It was concluded that cattle of the three breeds suffered from trypanosomosis and that trypanosome infections affected tick susceptibility. However, N'Dama showed a superior ability to limit both the pathological effects of trypanosomosis and the level of tick infestations. Therefore, considering the cost of labour and consumable equipment required for a high surveillance, use of multi-disease resistant N'Dama cattle is recommended for the low-input traditional African farming systems in areas where trypanosomosis, ticks and tick-borne diseases are constraints to livestock production. Additional comparative investigations are needed to assess the impact of high surveillance scheme of trypanosome infection in different production systems in trypanotolerant and trypanosusceptible cattle exposed to different gradients of tsetse challenge.

Effect of controlling natural field-tick infestation on the growth of N'Dama and Gobra zebu cattle in the Gambia

The effect of tick infestations on liveweight gain (LWG) was assessed by comparison of weight changes in flumethrin-treated N'Dama and Gobra zebu cattle (16-20 months old) with respective control groups submitted to natural tick challenge over 1 year. Flumethrin was applied monthly, fortnightly or weekly. Preventive treatments against anaplasmosis, babesiosis and trypanosomosis were given. Mortality rate was recorded and post-mortem examinations carried out. In both treated and control animals, significantly fewer Hyalomma spp. and Amblyomma variegatum were found on N'Dama than on Gobra zebu cattle. Both breeds are equally susceptible to Rhipicephalus senegalensis infestation. Total annual tick burdens did not cause significant differences in LWG between acaricide-treated and control cattle in either breeds. LWG was also not affected during or after the annual peak of tick infestation (composed mainly by A. variegatum and R. senegalensis). Equally-high mortality (35%), due to unidentified causes, was recorded in acaricide-treated and control Gobra cattle; mortality in N'Dama cattle was 7.5%. In both breeds, about the 90% of mortality occurred at the end of the dry season. Breed differences in tick burden confirm previous results. If tick-borne infections do not influence LWG or mortality, then it is concluded that intensive tick control is not justifiable in Gambian livestock.

Heterogeneities in the transmission of infectious agents: implications for the design of control programs

From an analysis of the distributions of measures of transmission rates among hosts, we identify an empirical relationship suggesting that, typically, 20% of the host population contributes at least 80% of the net transmission potential, as measured by the basic reproduction number, R0. This is an example of a statistical pattern known as the 20/80 rule. The rule applies to a variety of disease systems, including vector-borne parasites and sexually transmitted pathogens. The rule implies that control programs targeted at the "core" 20% group are potentially highly effective and, conversely, that programs that fail to reach all of this group will be much less effective than expected in reducing levels of infection in the population as a whole.

Observations on the prevalence of trypanosomosis in small ruminants, equines and cattle, in relation to tsetse challenge, in The Gambia

The prevalence of trypanosome infections in Djallonké sheep and West African Dwarf goats at different sites in The Gambia showed a significant, positive correlation with contemporary assessments of tsetse challenge. A similar correlation was observed in village N'Dama cattle which showed comparable prevalence values in the same areas. Trypanosome prevalences also tended to be higher in horses and donkeys in areas with high tsetse challenge compared with sites with relatively few flies. A ranking of the numbers of tsetse blood-meals from cattle, small ruminants and equines (1:0.06: > 0.03) corresponded with the estimated biomass of these livestock groups (1:0.09:0.05). Observations on the grazing ranges of livestock showed that, while cattle foraged widely into tsetse-infested habitat, sheep, goats and donkeys remained closer to the villages. This difference indicated that, under the management system practised in The Gambia, small ruminants and equines were probably exposed to a lower level of tsetse attack than cattle.