The helminth community of urban slum rats
The latest Paper of the Month from Parasitology is ‘The helminth community of a population of Rattus norvegicus from an urban Brazilian slum and the threat of zoonotic diseases’ by Ticiana Carvalho-Pereira, Fábio N. Souza, Luana R. N. Santos et al.
In urban slums, humans typically live in crowded conditions, with inadequate infrastructure and poor sanitation, including the presence of open sewers and a lack of rubbish collection. These characteristics provide favourable conditions for the presence of rats, which establish close contact with human domiciles. Slums constitute, therefore, a perfect scenario for the transmission of parasites from rats to humans, directly, or indirectly through the environment. Rats of the species, Rattus norevegicus are known to harbour and shed numerous parasites, helminths amongst them, that can cause disease in humans. For example, Hymenolepis cestodes are associated with human diarrhoea, whereas the rat lungworm Angiostrongylus cantonensis can cause eosinophilic meningitis – an endemic human disease in southeast Asia, that has been considered emergent worldwide. Although descriptive studies have been extensively conducted to characterize the helminth community of rats (e.g., species identification and prevalence), there is a need to understand which factors structure such communities in the urban rat population. This is particularly important, since individuals in natural populations will vary in their ability to acquire necessary resources, which can affect their body condition. Variation in body condition can further influence individuals’ vulnerability to infection and, probably, parasite shedding rates, which, ultimately, can affect humans.
In this study, therefore, we combined considerations of environmental drivers with variables of rat demography and body condition, the latter of which have never previously been acknowledged, to identify potential risk factors that are associated with the presence and intensity of helminth species in an urban slum population of R. norvegicus from Salvador, Brazil. Among 299 rats, we identified 11 species/groups, of which Strongyloides sp., Nippostrongylus brasiliensis and, the human pathogen, A. cantonensis were the most frequent (97%, 41% and 39%, respectively). Interactions of other factors with sex highlighted behavioural differences between males and females, as, for example, males were more likely to be infected with N. brasiliensis where rat signs were present, and males presented more intense infections of Strongyloides sp. Moreover, rats in poor body condition had higher intensities of N. brasiliensis. We describe a high global richness of parasites in R. norvegicus, including five species known to cause disease in humans. Apart from A. cantonensis, which was ubiquitous and of high prevalence, four other human pathogens were identified in rats’ faeces, including the roundworms Toxocara sp. and Ascaris sp. These parasites, despite being found in rats’ excreta, do not complete their life cycle in rat tissues. However, this finding shows that both roundworm species are circulating in the studied slum – knowledge which, together with A. cantonensis, is of public health importance. Furthermore, a variety of environmental, demographic and body condition variables were associated with helminth species infection of rats suggesting, perhaps, a comparable variety of risk factors for human infection.
Read the full article ‘The helminth community of a population of Rattus norvegicus from an urban Brazilian slum and the threat of zoonotic diseases’ in full for free until 31st January 2018.