BackgroundVisceral leishmaniasis (VL) is a vector borne zoonotic disease endemic in humans and dogs in Brazil. Due to the increased risk of human infection secondary to the presence of infected dogs, public health measures in Brazil mandate testing and culling of infected dogs. Despite this important relationship between human and canine infection, little is known about what makes the dog reservoir progress to clinical illness, significantly tied to infectiousness to sand flies. Dogs in endemic areas of Brazil are exposed to many tick-borne pathogens, which are likely to alter the immune environment and thus control of L. infantum.ResultsA cross-sectional study of 223 dogs from an area of Natal, in the Rio Grande do Norte, Brazil, were studied to determine the association between comorbid tick-borne disease and Leishmania infection in this endemic area. The risk of Leishmania seropositivity was 1.68× greater in dogs with tick-borne disease seropositivity compared to those without (Adjusted RR: 1.68, 95% CI: 1.09–2.61, P = 0.019). A longitudinal study of 214 hunting dogs in the USA was conducted to determine the causal relationship between infection with tick-borne diseases and progression of VL. Hunting dogs were evaluated three times across a full tick season to detect incident infection with tick-borne diseases. A logistic regression model with generalized estimating equations to estimate the parameters was used to determine how exposure to tick-borne disease altered VL progression over these three time points when controlling for other variables. Dogs infected with three or more tick-borne diseases were 11× more likely to be associated with progression to clinical VL than dogs with no tick-borne disease (Adjusted RR: 11.64, 95% CI: 1.22–110.99, P = 0.03). Dogs with exposure to both Leishmania spp. and tick-borne diseases were five times more likely to die during the study period (RR: 4.85, 95% CI: 1.65–14.24, P = 0.0051).ConclusionsComorbid tick-borne diseases dramatically increased the likelihood that a dog had clinical L. infantum infection, making them more likely to transmit infection to sand flies and people. As an important consequence, reduction of tick-borne disease exposure through topical or oral insecticides may be an important way to reduce progression and transmissibility of Leishmania infection from the canine reservoir to people.Electronic supplementary materialThe online version of this article (10.1186/s13071-019-3312-3) contains supplementary material, which is available to authorized users.
Summary An investigation of the spring rise in strongyle egg output of grazing horses on two commercial horse farms in northern USA in 1981 and 1982 revealed two distinct spring and summer rises in faecal egg counts, with peaks in May and August/September. There was a marked rise in the concentration of infective larvae on pasture two to four weeks after the peaks in egg output, so that grazing horses were at serious risk from June onwards and pasture larval counts on one farm did not fall to low levels until June of the following year. The spring and summer rises in faecal egg counts appeared to be seasonal in nature, to be derived largely from worms developing from previously ingested larvae, rather than from newly ingested larvae, and to be unrelated to the date of foaling. An epidemiological approach to strongyle control based on prophylactic treatments in the spring successfully eliminated the spring rise in egg output but was inadequate to control the summer rise or subsequent escalation of pasture infectivity in September. It was, nevertheless, superior to a conventional treatment programme at eight week intervals, using the same drug, pyrantel pamoate. Prophylactic spring/summer treatments proved to be much more effective. Both pyrantel pamoate at four week intervals and ivermectin at eight week intervals kept faecal egg counts at low levels during spring and summer. As few as two ivermectin treatments (11 May, 6 July) resulted in a sixfold reduction in pasture larval counts on 9 November and 3 January for the treated group (8872, 8416 stage three larvae [L3]/kg) compared to the control group (52,824, 50,984 L3/kg). This strategy had far reaching prophylactic effects in preventing the late season rise in pasture infectivity and ensuring a low level of residual pasture infectivity at the start of the next grazing season.
Certain professionals have more exposure to animals and therefore an increased risk of zoonoses. Professional hunting dog caretakers work with upwards of 50 dogs and are exposed to zoonoses through exposure to multiple potentially infectious canine secretions or excretions, as well as to the ticks that dogs carry. Dog caretakers reported having found embedded ticks on their bodies 5.83 times more than environment-only controls. Zoonotic Lyme disease, first in the United States for morbidity due to a vector-borne infection, has dramatically expanded its geographic range over the last two decades. This finding emphasizes the increased risk of tick-borne diseases, including Lyme disease, based on dog exposure and in areas of disease emergence.
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