The first outbreak of human plague in Lushoto district, north-east Tanzania

Immunogenetics, sylvatic plague and its vectors: insights from the pathogen reservoir Mastomys natalensis in Tanzania

Yersinia pestis is a historically important vector-borne pathogen causing plague in humans and other mammals. Contemporary zoonotic infections with Y. pestis still occur in sub-Saharan Africa, including Tanzania and Madagascar, but receive relatively little attention. Thus, the role of wildlife reservoirs in maintaining sylvatic plague and spillover risks to humans is largely unknown. The multimammate rodent Mastomys natalensis is the most abundant and widespread rodent in peri-domestic areas in Tanzania, where it plays a major role as a Y. pestis reservoir in endemic foci. Yet, how M. natalensis' immunogenetics contributes to the maintenance of plague has not been investigated to date. Here, we surveyed wild M. natalensis for Y. pestis vectors, i.e., fleas, and tested for the presence of antibodies against Y. pestis using enzyme-linked immunosorbent assays (ELISA) in areas known to be endemic or without previous records of Y. pestis in Tanzania. We characterized the allelic and functional (i.e., supertype) diversity of the major histocompatibility complex (MHC class II) of M. natalensis and investigated links to Y. pestis vectors and infections. We detected antibodies against Y. pestis in rodents inhabiting both endemic areas and areas considered non-endemic. Of the 111 nucleotide MHC alleles, only DRB*016 was associated with an increased infestation with the flea Xenopsylla. Surprisingly, we found no link between MHC alleles or supertypes and antibodies of Y. pestis. Our findings hint, however, at local adaptations towards Y. pestis vectors, an observation that more exhaustive sampling could unwind in the future.

Plague in Tanzania: first report of sylvatic plague in Morogoro region, persistence in Mbulu focus, and ongoing quiescence in Lushoto and Iringa foci

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Six (0.93%) rodents carried antibodies against Yersinia pestis fraction 1 antigens.

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There is evidence of Y. pestis circulation in small mammals in Morogoro.

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Plague continues to persist in small mammals in Mbulu.

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There is ongoing quiescence in Lushoto and Iringa plague foci.

Objectives

Plague has been a threat to human health in Tanzania since 1886. This zoonotic disease has established several endemic foci in the country, posing a risk of outbreaks. This study was conducted to investigate the presence of Yersinia pestis in small mammals in five districts. These districts were selected because of recent (Mbulu), past (40-18 years ago: Lushoto) and historic (>100 years ago: Iringa and Kilolo) human cases of plague. In addition, one region that has not had any reported human cases of plague was included (Morogoro-Mvomero).

Methods

Blood from 645 captured small mammals was screened for antibodies against the fraction 1 (F1) antigen of Y. pestis using indirect enzyme-linked immunosorbent assay (ELISA) and competitive-blocking ELISA.

Results

Specific antibodies against Y. pestis F1 antigens were detected in six (0.93%) animals belonging to Mastomys natalensis. Of these, four animals were captured in the active focus in Mbulu, and two animals were captured from an area with no history of human plague (Morogoro-Mvomero).

Conclusion

These results provide evidence of the circulation of Y. pestis in small mammals in Tanzania. Furthermore, evidence of the circulation of Y. pestis in Morogoro-Mvomero highlights the importance of carrying out plague surveillance in areas with no history of human plague, which can help to predict areas where future outbreaks may occur.

Diversity and distribution of ectoparasite taxa associated with Micaelamys namaquensis (Rodentia: Muridae), an opportunistic commensal rodent species in South Africa

South Africa boasts a rich diversity of small mammals of which several are commensal and harbour parasites of zoonotic importance. However, limited information is available on the parasite diversity and distribution associated with rodents in South Africa. This is particularly relevant for Micaelamys namaquensis (Namaqua rock mouse), a regionally widespread and locally abundant species that is often commensal. To address the paucity of data, the aims of the study were to record the ectoparasite diversity associated with M. namaquensis and develop distribution maps of lice and mites associated with M. namaquensis and other rodents in South Africa. Micaelamys namaquensis individuals (n = 216) were obtained from 12 localities representing multiple biomes during 2017–2018. A total of 5591 ectoparasites representing 5 taxonomic groups – fleas, lice, mesostigmatid mites, chiggers and ticks was recorded. These consisted of at least 57 taxa of which ticks were the most speciose (20 taxa). Novel contributions include new host and locality data for several ectoparasite taxa and undescribed chigger species. Known vector species were recorded which included fleas (Ctenocephalides felis, Dinopsyllus ellobius and Xenopsylla brasiliensis) and ticks (Haemaphysalis elliptica, Rhipicephalus appendiculatus and Rhipicephalus simus). Locality records indicate within-taxon geographic differences between the 2 louse species and the 2 most abundant mite species. It is clear that M. namaquensis hosts a rich diversity of ectoparasite taxa and, as such, is an important rodent species to monitor in habitats where it occurs in close proximity to humans and domestic animals.

Serological and PCR investigation of Yersinia pestis in potential reservoir hosts from a plague outbreak focus in Zambia

Background

Plague is a bacterial zoonotic disease, caused by Yersinia pestis. Rodents are the natural hosts with fleas as the vehicle of disease transmission. Domestic and wild dogs and cats have also been identified as possible disease hosts. In Zambia, plague outbreaks have been reported in the Southern and Eastern regions in the last 20 years. Based on these observations, Y. pestis could possibly be endemically present in the area.

Methods

To substantiate such possibility, sera samples were collected from rodents, shrews, dogs and cats for detection of antibodies against Fraction 1 gene (Fra1) of Y. pestis while organs from rodents and shrews, and fleas from both dogs and rodents were collected to investigate plasminogen activator gene (pla gene) of Y. pestis using ELISA and PCR respectively.

Results

A total of 369 blood samples were collected from domestic carnivores, shrews and domestic and peri-domestic rodents while 199 organs were collected from the rodents and shrews. Blood samples were tested for antibodies against Fra1 antigen using ELISA and 3% (5/165) (95% CI 0.99–6.93%) dogs were positive while all cats were negative. Of 199 sera from rodents and shrews, 12.6% (95% CI 8.30–17.98%) were positive for antibodies against Fra1 using anti-rat IgG secondary antibody while using anti-mouse IgG secondary antibody, 17.6% (95% CI 12.57–23.60%) were positive. PCR was run on the organs and 2.5% (95% CI 0.82–5.77%) were positive for plasminogen activator gene of Y. pestis and the amplicons were sequenced and showed 99% identity with Y. pestis reference sequences. All 82 fleas collected from animals subjected to PCR, were negative for pla gene. The specific rat-flea and dog-flea indices were 0.19 and 0.27 respectively, which were lower than the level required to enhance chances of the disease outbreak.

Conclusions

We concluded that plague was still endemic in the area and the disease may infect human beings if contact is enhanced between reservoir hosts and flea vectors. The lower specific rodent-flea Indices and absence of Y. pestis in the potential vectors were considered to be partly responsible for the current absence of plague outbreaks despite its presence in the sylvatic cycle.

Predicting potential risk areas of human plague for the Western Usambara Mountains, Lushoto District, Tanzania

A natural focus of plague exists in the Western Usambara Mountains of Tanzania. Despite intense research, questions remain as to why and how plague emerges repeatedly in the same suite of villages. We used human plague incidence data for 1986-2003 in an ecological-niche modeling framework to explore the geographic distribution and ecology of human plague. Our analyses indicate that plague occurrence is related directly to landscape-scale environmental features, yielding a predictive understanding of one set of environmental factors affecting plague transmission in East Africa. Although many environmental variables contribute significantly to these models, the most important are elevation and Enhanced Vegetation Index derivatives. Projections of these models across broader regions predict only 15.5% (under a majority-rule threshold) or 31,997 km(2) of East Africa as suitable for plague transmission, but they successfully anticipate most known foci in the region, making possible the development of a risk map of plague.

Demographic and spatio-temporal variation in human plague at a persistent focus in Tanzania

Human plague in the Western Usambara Mountains in Tanzania has been a public health problem since the first outbreak in 1980. The wildlife reservoir is unknown and eradication measures that have proved effective elsewhere in Tanzania appear to fail in this region. We use census data from 2002 and hospital records kept since 1986 to describe the temporal, spatial and demographic variation in human plague. A seasonal peak in cases occurs from December to February with the numbers of cases during this peak varying between 0 and 1150. Variation in incidence, calculated for each village as the mean number of cases per thousand inhabitants per year, indicates that human plague is concentrated around a group of three neighbouring, relatively isolated, high-altitude villages; Nywelo, Madala and Gologolo. However, there was no evidence that these villages were acting as a source of infection for the remainder of the focus. The likelihood of becoming infected with plague is highest between the ages of 5 and 19 and lowest for adult men. This was most clear in the ward encompassing the three high-incidence villages where the risk of plague among children aged 10-14 was 2.2 times higher than for adults aged 30-34, and among adults aged 30-34, the risk was 2.4 times higher for women than men.

Preliminary observations on factors responsible for long persistence and continued outbreaks of plague in Lushoto district, Tanzania

Human plague has been an important public health problem in Tanzania for over a century. Recorded outbreaks of the disease have been reported from various parts of Tanzania, including Iringa, Kagera, Singida, Mbulu, Arusha and Kilimanjaro since 1886. Since 1980 however, only Lushoto, Singida and Karatu districts have experienced outbreaks of the disease. Of these areas, Lushoto has disproportionately high incidences of the disease and this has persisted for nearly 17 years. Efforts to curb the disease through conventional methods, including control of vectors and reservoirs, chemotherapy and chemoprophylaxis, enforcement of sanitation improvement as well as health education, have been applied every year, but plague cases and deaths continue to occur in the area to date. During the period April 1980 to December 1996, a total of 6599 cases with 580 (8.8%) deaths were recorded. Biological factors, such as the presence of suitable rodent reservoirs, efficient flea vectors and plague bacillus, could be partly responsible for the long persistence and recurrence of the disease. Since such factors are also common in other plague foci where the disease has never been persistently recurrent, and where indigenous people are culturally different from those in Lushoto, it is assumed that socio-cultural factors play an important role as determinants of the disease in the latter district. This paper reports preliminary observations on socio-cultural, biological and environmental factors which are thought to be, at least partly, responsible for the long persistence and repeated outbreaks of plague in the district. These include traditional beliefs on the cause and health seeking behaviour for treatment of plague, sleeping and food storage habits, large populations of rodents and fleas, and status of the immediate environment.

Plague in Lushoto district, Tanzania, 1980-1988

Rodents were live-trapped in selected plague-inflicted villages from June 1980 to March 1988. Flea infestation rates were determined and the animals were serologically tested for plague. Clinically suspected and clinically healthy people in the affected areas were similarly tested for plague antibodies. Of 1596 rodent sera tested, 91 (5.7%) were positive for plague. These were mostly from Rattus rattus, Mastomys natalensis, Otomys spp. and Pelomys fallax. A total of 1772 fleas, of which Dinopsyllus lypusus, Xenopsylla brasiliensis and Ctenophthalmus calceatus comprised the largest proportion, was collected from the captured rodents. Total flea indices ranged from 0.67 to 1.12 fleas per rodent. A total of 2809 human cases and a mortality rate of 10.2% were recorded in 1980-1988. It was concluded that most rodent species in the area were suitable reservoirs of plague and that D. lypusus, X. brasiliensis and C. calceatus were probably responsible for transmitting the pathogen. Lack of effective quarantine measures during outbreaks was partly responsible for the spread of the disease to many villages, while inadequate rodent and flea control and poor sanitary measures could be responsible for continued outbreaks of plague in the area.

A decade of plague epidemiology and control in the Western Usambara mountains, north-east Tanzania

Outbreaks of human plague have been occurring in the Western Usambara mountains since 1980, involving many cases and deaths. Epidemiological surveys and control activities were carried out from June 1980 to May 1990. Rodents were trapped live, identified and serologically tested for plague, using the passive haemagglutination and passive haemagglutination inhibition tests. Rodent fleas were collected, processed, identified and counted. House fleas were caught with light traps and similarly treated. People and domestic carnivores were serologically tested for the disease. Various plague control measures were undertaken. A total of 2433 animals, 2254 rodent fleas and 1366 house fleas were collected. Average indices of rodent and house fleas were 0.93 and 5.38 respectively. Rattus rattus and Mastomys natalensis were the most abundant rodent species. Xenopsylla brasiliensis and Dinopsyllus lypusus were their commonest flea ectoparasites while Pulex irritans was the major house flea. Of 2044 rodent, 1880 human and 176 dog sera tested, 5.5%, 0.5% and 6.3% respectively were positive. It was concluded that plague was active in the focus despite the control measures and that common reservoirs and efficient vectors were present. It was suggested that dogs were probably involved in the epidemiology of the disease, that P. irritans was not susceptible to the insecticide used and that the flea was probably involved in murine plague transmission. Bacteriological research on the causative agent to establish the nature of its long persistence in the area and maintenance of a surveillance service are recommended.