Leishmania incidence and prevalence in U.S. hunting hounds maintained via vertical transmission

Visceral Leishmaniasis and the Skin: Dermal Parasite Transmission to Sand Flies

Visceral leishmaniasis is a parasitic disease with significant dermal tropism. The skin is an important site of infection contributing to parasite transmission to naïve sand flies, but understanding how parasitism of host skin and the related immune microenvironment supports or prevents skin parasite replication is now the focus of major investigation in the field of leishmaniasis research. Here, we review dermatoimmunology during visceral leishmaniasis (VL), dermal Leishmania parasite burden, and the role of skin parasitism in transmissibility to sand fly vectors. First, we discuss the epidemiology of VL amongst dogs, the primary zoonotic reservoir for human infection. We explore the association between spatial distribution and the burden of parasites in the skin in driving outward transmission. Factors associated with parasite persistence in the skin are examined. We discuss systemic immunity during VL and what is known about immunological correlates in the skin microenvironment. Finally, we touch on factors egested into the skin during Leishmania inoculation by sand flies. Throughout, we discuss factors associated with the early and chronic establishment of Leishmania parasites in the skin and the role of the dermal immune response.

Epidemiologic, Clinical and Immunological Consequences of Co-Infections during Canine Leishmaniosis

Simple Summary

Canine leishmaniosis (CanL), the most severe, visceralizing form of disease caused by Leishmania infantum transmitted by phlebotomine sand flies. CanL is frequently diagnosed in the Mediterranean basin and South America, although it is also found in other regions, including the United States (U.S.). Dogs in these regions are at risk for co-infections, prominently tick-borne diseases. Our review examines epidemiologic, clinical, and immunologic mechanisms found during the most common eight CanL co-infections reported in published literature. Co-infections alter immunologic processes and disease progression impacting CanL diagnosis, therapeutic responses, and prognosis.

Abstract

Canine leishmaniosis (CanL) is a vector-borne, parasitic disease. CanL is endemic in the Mediterranean basin and South America but also found in Northern Africa, Asia, and the U.S. Regions with both competent sand fly vectors and L. infantum parasites are also endemic for additional infectious diseases that could cause co-infections in dogs. Growing evidence indicates that co-infections can impact immunologic responses and thus the clinical course of both CanL and the comorbid disease(s). The aim for this review is to summarize epidemiologic, clinical, and immunologic factors contributing to eight primary co-infections reported with CanL: Ehrlichia spp., Anaplasma spp., Borrelia spp., Babesia spp., Trypanosoma cruzi, Toxoplasma gondii, Dirofilaria immitis, Paracoccidioides braziliensis. Co-infection causes mechanistic differences in immunity which can alter diagnostics, therapeutic management, and prognosis of dogs with CanL. More research is needed to further explore immunomodulation during CanL co-infection(s) and their clinical impact.

Leishmania infantum xenodiagnosis from vertically infected dogs reveals significant skin tropism

BACKGROUND

Dogs are the primary reservoir for human visceral leishmaniasis due to Leishmania infantum. Phlebotomine sand flies maintain zoonotic transmission of parasites between dogs and humans. A subset of dogs is infected transplacentally during gestation, but at what stage of the clinical spectrum vertically infected dogs contribute to the infected sand fly pool is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

We examined infectiousness of dogs vertically infected with L. infantum from multiple clinical states to the vector Lutzomyia longipalpis using xenodiagnosis and found that vertically infected dogs were infectious to sand flies at differing rates. Dogs with mild to moderate disease showed significantly higher transmission to the vector than dogs with subclinical or severe disease. We documented a substantial parasite burden in the skin of vertically infected dogs by RT-qPCR, despite these dogs not having received intradermal parasites via sand flies. There was a highly significant correlation between skin parasite burden at the feeding site and sand fly parasite uptake. This suggests dogs with high skin parasite burden contribute the most to the infected sand fly pool. Although skin parasite load and parasitemia correlated with one another, the average parasite number detected in skin was significantly higher compared to blood in matched subjects. Thus, dermal resident parasites were infectious to sand flies from dogs without detectable parasitemia.

CONCLUSIONS/SIGNIFICANCE

Together, our data implicate skin parasite burden and earlier clinical status as stronger indicators of outward transmission potential than blood parasite burden. Our studies of a population of dogs without vector transmission highlights the need to consider canine vertical transmission in surveillance and prevention strategies.

Canine leishmaniasis in Northern California-A case report

A 3-year-old dog was referred to the Veterinary Medical Teaching Hospital of the University of California-Davis for further evaluation of episodes of epistaxis of 1-year duration and peripheral lymphadenopathy. The patient had a history of atopic dermatitis with no travel history outside of California. Hyperglobulinemia with a polyclonal gammopathy was noted on serum protein electrophoresis. Microscopic evaluation of a bone marrow aspirate sample revealed many free and intra-cellular amastigotes of Leishmania sp. that was further confirmed by qPCR as L infantum. This is, to the best of our knowledge, the first reported case of canine leishmaniasis in the state of California. The patient is believed to have been vertically infected from the dam who is from Serbia and remained subclinical until presentation. Because the clinical progression of leishmaniasis is variable, it is important that precautions be discussed with owners acquiring puppies with dams from endemic regions of leishmaniasis to prevent zoonotic exposure in states where competent vectors are present.

Demographics and travel history of imported and autochthonous cases of leishmaniosis in dogs in the United States and Canada, 2006 to 2019

BACKGROUND

Leishmania infantum infections are reported in foxhounds throughout the United States (US) and Canada, but only rarely in other dog breeds. A seroprevalence report from 2006 documented leishmaniosis in foxhounds (8.9%) tested in the US between 2000 and 2003. All other breeds were seronegative.

OBJECTIVE

To reexamine demographics and travel history of L. infantum-infected dogs in the US and Canada, we hypothesize detection of L. infantum in more foxhounds than nonfoxhounds and that infected nonfoxhounds will have traveled to endemic regions.

ANIMALS

A total of 125 dogs positive for L. infantum by immunofluorescent antibody, PCR, or both.

METHODS

Retrospective, descriptive study of L. infantum-infected dogs between 4 January 2006 and 22 May 2019. Travel history and known lineage to foxhounds was collected from questionnaires.

RESULTS

Leishmania infantum was detected in 125 (6.4%) of 1961 dogs tested between 4 January 2006 and 22 May 2019, of which 10 (8%) were foxhounds and 115 (92%) were nonfoxhound breeds. Travel history available for 69 (55%) dogs showed 60 (86.9%) dogs had traveled outside of the US or Canada. Nine (13%) dogs had not traveled outside of the US or Canada, 5 of which were nonfoxhounds.

CONCLUSIONS AND CLINICAL IMPORTANCE

The majority of L. infantum cases were detected in nonfoxhounds, many of which had traveled to L. infantum-endemic countries, and several nonfoxhound breeds had no travel history. Leishmania surveillance should be considered for dogs that return from L. infantum-endemic regions to monitor emergence of this zoonotic disease in the US and Canada.

Bayesian compartmental models and associated reproductive numbers for an infection with multiple transmission modes

Zoonotic visceral leishmaniasis (ZVL) is a serious neglected tropical disease that is endemic in 98 countries. ZVL is primarily transmitted via a sand fly vector. In the United States, it is enzootic in some canine populations; it is transmitted from infectious mother to pup transplacentally, and vector-borne transmission is absent. This absence affords a unique opportunity to study (1) vertical transmission dynamics in dogs and (2) the importance of vertical transmission in maintaining an infectious reservoir in the presence of a vector. In this paper, we present Bayesian compartmental models and reproductive number formulations to examine (1) and (2), providing a mechanism to plan and evaluate interventions in regions where both transmission modes are present. First, we propose an individual-level susceptible, infectious, removed (SIR) model to study the effect of maternal infection status during pregnancy on pup infection progression. We provide evidence that pups born to diagnostically positive mothers during pregnancy are more likely to become diagnostically positive both earlier in life, and at some point during their lifetime, than those born to diagnostically negative mothers. Second, we propose a population-level SIR model to study the impact of a vertically maintained reservoir on propagating infection in a naive canine population through emergent vector transmission using simulation studies. We also present reproductive numbers to quantify contributions of vertically infected and vector-infected dogs to maintaining infection in the population. We show that a vertically maintained canine reservoir can propagate infection in a theoretical naive population in the presence of a vector.

Frequency, active infection and load of Leishmania infantum and associated histological alterations in the genital tract of male and female dogs

Visceral leishmaniasis caused by the protozoan Leishmania infantum is a zoonosis. The domestic dog is the primary reservoir in urban areas. This study aimed to evaluate the frequency, active infection and load of L. infantum in the genital tract of male and female dogs seropositive for this parasite, as well as to identify histological genital alterations associated with this protozoan. We studied 45 male and 25 female L. infantum-seropositive noncastrated dogs from the same endemic area in Brazil. Tissue samples from the testis, epididymis, prostate, vulva, vagina, and uterus were examined by singleplex qPCR and parasitological tests (histopathology, immunohistochemistry, and parasitological culture). The latter were performed for the detection of active infection (parasites able to multiply and to induce lesions). Forty-four (98%) males and 25 (100%) females were positive for L. infantum in the genital tract (epididymis: 98%; vulva: 92%; vagina: 92%; testis: 91%; uterus: 84%; prostate: 66%). Active infection in the genital tract was confirmed in 69% of males and 64% of females (32% in the uterus). Parasite loads were similar in the testis, vulva, epididymis and vagina and lower in the prostate. Only the parasite load in the vagina was significantly associated with the number of clinical signs. Granulomatous inflammation predominated in all organs, except for the prostate. Only in the testis and epididymis was the inflammatory infiltrate significantly more intense among dogs with a higher parasite load in these organs. The high frequency, detection of active infection and similarity of L. infantum loads in the genital tract of infected males and females suggest the potential of venereal transmission of this parasite by both sexes and of vertical transmission by females in the area studied. Additionally, vertical transmission may be frequent since active L. infantum infection was a common observation in the uterus.

The balancing act: Immunology of leishmaniosis

Immune control of Leishmania infantum, the causative agent of most canine leishmaniosis (CanL), requires a balancing act between inflammatory and regulatory responses. This balance is specifically between the proinflammatory T helper 1 type (Th1) CD4+ T cells that are responsible for controlling parasite replication and T regulatory 1 cells which mediate an immunosuppressive, regulatory, response needed to dampen overabundant inflammation but if predominant, result in CanL progression. How this delicate immune cell interaction occurs in the dog will be highlighted in this review, focusing on the progressive changes observed within myeloid lineage cells (predominantly macrophages), B cells and T cells. After exposure to parasites, macrophages should become activated, eliminating L. infantum through release of reactive oxygen species. Unfortunately, multiple parasite and host factors can prevent macrophage activation allowing parasites to persist within them. T cells balance between a productive T_H1 type CD4+ response capable of producing IFN-γ which aids macrophage activation versus T cell exhaustion which reduces T cell proliferation, IFN-γ production and allows parasite expansion within macrophages. Neutrophils and Th17 cells add to the inflammatory state, aiding in parasite removal, but also leading to pathology. A regulatory B cell population increases IL-10 production and down regulates the T_H1 response allowing parasite growth. All of these immune challenges affect the balance between progression to clinical disease and maintaining sub-clinical disease. Vaccines and immunotherapies targeted at recovering or maintaining T and B cell function can be important factors in mending the immune balance required to survive CanL.