One Health approach to vector biology and epidemiology of arboviruses, Rickettsia, and protozoa in smallholder livestock systems in western Kenya

Abstract

Vector-borne pathogens (VBP) present a serious threat to both animal and human health globally due to their zoonotic potential. ‘One Health’ epidemio-surveillance systems that integrate human, animal, and arthropod vector facets of VBP transmission enhance detection and can give rise to informed effective multi-sectoral vector-borne disease control options. The most important vectors are ticks and mosquitoes, which have a cosmopolitan distribution due to their ecological plasticity. Collectively, they transmit viral, bacterial, and protozoal pathogens in sub-Saharan Africa. The distributional ranges of VBP are expected to expand due to increased global travel and trade in goods and livestock. Arboviruses, such as Rift Valley fever (RVF), chikungunya, dengue, and Crimean-Congo haemorrhagic fever (CCHF), are endemic in East Africa with regular occurrence of outbreaks, while Rickettsia africae causes African tick bite fever in travellers with few clinical cases reported in local populations. Furthermore, the co-occurrence of these causes of febrile illness and malaria complicates the diagnosis of fevers in hospitals. Correspondingly, Theileria parva and Anaplasma marginale are serious constraints to cattle production in the region. The Lake Victoria basin that encompasses the East African community countries of Kenya, Uganda, and Tanzania has, an ecology that is conducive for the endemic proliferation of mosquitoes and ticks, high livestock density under subsistence production system and is an important trade corridor for the East African community. These factors are likely to enhance circulation of zoonotic VBPs in the area. Therefore, this surveillance project in western Kenya sought to establish the most important species of ticks and mosquitoes transmitting zoonotic pathogens among local livestock and human populations. It also sought to determine if livestock harbour these pathogens as asymptomatic carriers for human infection, in which they may cause zoonotic febrile illness. I sampled mosquitoes and ticks at hospitals, livestock markets (LMs), and slaughterhouses (SHs) in Kenya’s Busia, Bungoma and Kakamega counties. Ticks and blood were collected from livestock at LMs and SHs between the year 2017 and 2019. Mosquitoes were trapped using CDC light and BG sentinel traps at selected hospitals and LMs during the short and long rainy seasons of 2018/2019. Additionally, febrile patients were recruited at hospitals for blood-sample collection during the same period. Vectors were identified to their lowest taxa using identification keys and molecular markers. DNA and RNA were extracted from samples and subjected to PCR, high-resolution melting analysis and representatives sequenced to identify pathogens in mosquitoes, ticks, and blood samples from humans and cattle. In addition, blood meal analyses were performed and human and cattle demographic data were collected and analysed by logistic regression to identify risk factors for pathogen infection. I collected 486 ticks, lice, and fleas from livestock at LMs and SHs wherein the most abundant tick species were Rhipicephalus decoloratus (35%) in 108 pools and Amblyomma variegatum (30%) in 99 pools, while Ctenocephalides felis fleas and Haematopinus suis lice were also present. Overall, pools of Rh. decoloratus ticks were infected with A. marginale (estimated true prevalence: 2.44%), Anaplasma ovis (1.21%), Anaplasma platys (3.05%), Babesia bigemina (0.61%), Hepatozoon canis (0.61%), R. africae (1.83%), Theileria mutans (7.32%) and Theileria taurotragi (1.21%). Amblyomma variegatum tick pools were infected with B. bigemina (0.77%), Babesia caballi (6.28%), R. africae (78.95%), T. mutans (0.76%) and Theileria velifera (1.53%). I also detected CCHF virus in two pools of Rh. decoloratus (0.61%) and Rhipicephalus sp. (1.45%) ticks. No pathogen DNA was detected in C. felis, while a single pool of H. suis was positive for R. africae (3.71%). Concomitantly, 6,848 mosquitoes corresponding to 21 species were trapped with the most abundant being Aedes aegypti (59 %) and Culex pipiens (40 %). The majority of the blood-fed mosquitoes were Cx. pipiens with humans, chickens, and sparrows (Passer sp.) being identified from blood meal analyses. One Culex poicilipes pool was positive for sindbis virus, 30 pools of Ae. aegypti had cell fusing agent virus (CFAV; infection rate (IR) = 1.27%, 95% CI = 0.87%-1.78%), 11 pools of Ae. aegypti had Aedes flavivirus (AeFV; IR = 0.43%, 95% CI = 0.23%-0.74%), and seven pools of Cx. pipiens had Culex flavivirus (CxFV; IR = 0.23%, 95% CI = 0.1%-0.45%). None of the 422 blood samples collected from cattle at LMs and SHs were positive for arboviruses or Rickettsia. However, Anaplasma spp. (19.67 %), Theileria spp. (12.32 %), Ehrlichia spp. (6.64 %), and Babesia spp. (0.24 %) were detected in 39% of cattle samples. The most prevalent species were T. velifera (7.35 %), A. marginale (4.98 %), Theileria mutans (3.08 %), although A. platys (2.84 %), T. parva (1.60%) and B. bigemina (0.24 %) were also detected. Cattle breed and tick infestation were risk factors for Anaplasma spp. and Ehrlichia spp. infection. Exotic breeds of cattle had significantly higher infection rates with Ehrlichia spp. (OR: 2.39, 95 % CI: 0.98-5.63, p = 0.049) and A. marginale (OR: 4.50, 95 % CI: 1.75-11.91, p = 0.002) compared to local breeds. Tick infestation was a significant predictor for Anaplasma spp. (OR: 2.01, 95 % CI: 1.27-3.51, p = 0.004) and Ehrlichia spp. (OR: 2.83, 95 % CI: 1.22-7.38, p = 0.021) infection. None of the 336 febrile patients recruited into the study were positive for arboviruses or Rickettsia; however, 18.5% were positive for Plasmodium falciparum. Patients living in Busia (OR: 5.478; 95% CI: 2.509-13.055; p = 0.000) and Bungoma counties (OR: 3.027; 95 % CI: 1.358-7.316; p = 0.009) had 5.5 and 3 times higher odds of being PCR-positive for malaria, respectively, compared to those living in Kakamega County. Patients from a household in which the female household head/spouse did not receive any level of formal education had higher odds of being infected (OR: 4.446, CI: 1.402-14.044, p = 0.010). The presence of CCHF and sindbis viruses in ticks and mosquitoes, respectively, suggest risk of transmission to humans and livestock. Crimean-Congo haemorrhagic fever virus is harboured asymptomatically in cattle and causes serious haemorrhagic fever in humans, while sindbis causes febrile illness and arthralgia also in humans. The relatively high infection rates of insect-specific flaviviruses (ISFs) in the sampled mosquitoes in a region where few arbovirus outbreaks have been reported points to the possibility that ISFs may be acting as a natural control mechanism preventing transmission of the pathogenic arboviruses. East Coast fever and anaplasmosis caused by T. parva and A. marginale, respectively, are major constraints to cattle production. Both were detected in this study. Correspondingly, risk factor analysis showed that cattle breed improvement in the study region by introducing exotic breeds can be hampered by tick-borne pathogens. Accordingly, the livestock markets can be utilised as control points for ticks using acaricides and other biological control mechanisms. Despite not finding CCHF virus, sindbis virus, and R. africae in livestock and human blood as detected in the vectors, clinicians should be made aware of their existential risk and include them in the differential diagnostics of febrile illnesses. A combination of IgM serology and RT-PCR could also improve detection sensitivity given the fragility of viruses in field samples. Only a small percentage of fevers in the recruited patients could be attributed to malaria (P. falciparum), suggesting undetected circulation of zoonotic VBPs or other pathogens causing fever not investigated in this study. Expectedly the malaria prevalence was not uniformly distributed due to the ecological differences in the three counties. Female household heads/spouses are in charge of households and most likely to impart their knowledge on mosquito control to other members of the households, highlighting gender aspects in malaria control. This knowledge can also be extrapolated to the control of arbovirus-transmitting mosquitoes. Anaplasma platys and genetically-related strains found in high prevalence in cattle blood have zoonotic potential even though they are yet to be detected in febrile patients in East Africa. Further studies are therefore needed to confirm their ability to cause disease in humans. In spite of the high infection rate of Am. variegatum ticks with R. africae, this zoonotic pathogen was not detected in cattle or humans, indicating either low transmissibility or low/diminishing titers in infected vertebrates. This surveillance study generated data that can inform vector control strategies, raise clinical awareness, and serve as a baseline for further studies on vector-borne diseases of livestock and humans, including zoonoses.