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This time at Infection Landscapes we will explore one of the most widespread and prevalent zoonotic infections in the world: leptospirosis.
The Pathogen. Leptospirosis in humans is caused by one of five pathogenic bacteria species of the genus Leptospira in the the Leptospiraceae family. Leptospira spp. are spirochetes, displaying the characteristic spiraled cellular structure:
There are several species that are pathogenic to humans. These organisms have an outer membrane like gram negative bacteria, although the cell wall is not associated with this outer membrane but rather with the cytoplasmic membrane, which is structurally specific to spirochetes. Leptospira have two flagella used for motility.
Leptospira spp. bind to extracellular matrix, fibroblasts, endothelial cells, epithelial cells, and macrophages, ultimately making their way to the proximal renal tubules in the kidneys. The organisms are then shed in the urine of the infected host.
The Disease. Leptospirosis can range from asymptomatic infection to severe life threatening illness. Mild to moderate presentations may include fever, myalgia, headache, chills, diarrhea, vommiting, and petechial rash. Severe, complicated, leptospirosis, which is also referred to as Weil's disease, presents with multiple organ system involvement, most prominently the kidneys and liver. High concentrations of urea and creatinine in the blood, darkened and diminished urine output, and frank renal failure, and jaundice, abnormal liver enzymes, and frank liver failure characterized complicated leptospirosis in these two organ systems, respectively. Severe leptospirosis can also affect the lungs and heart, presenting with pulmonary hemorrhage, myocarditis, and pericarditis. Endothelial lesions and vascular injury are apparent in all involved organ systems. Depending on how extensive the vascular injury, complete cardiovascular collapse may follow. Finally, meningitis is another common complication in moderate to severe leptospirosis.
The Epidemiology and the Landscape. The spirochetes that cause leptospirosis are shed in the urine of infected mammals. The vast majority of human infections occur following contact with (most frequently) water or soil that has been contaminated by the urine containing the Leptospira. As such, the primary route of transmission is through a common vehicle, although most infections do not occur as a result of ingestion but rather by way of water or soil contact with the conjunctiva, mucous membranes, or contact with wounds or abrasions in the skin. Nevertheless, while this kind of common vehicle contact is typical, the more conventionally conceived mechanism for common vehicle transmission, i.e. the consumption of contaminated water (or food), can also play an important role in some epidemics. Indeed, consumption of a contaminated water source may be particularly relevant in some agricultural settings in the developing world where certain kinds of irrigation systems can combine with poor water and sanitation infrastructure to promote infection through the consumption of water.
Accurate estimates of the incidence of leptospirosis and associated morbidity and mortality are lacking in all areas of the world. This is in part due to the difficulty of diagnostic testing, and in part due to the lack of specific indicators of disease. As mentioned above, clinical leptospirosis can be quite similar to many infectious disease presentations, and there can be many asymptomatic cases as well. Nevertheless, cautious estimates suggest that the incidence ranges from approximately less than 1 case per million persons per year in the developed world to 1 case per thousand persons per year in parts of the developing world. The map below published by the International Livestock Research Institute displays the the distribution of leptospirosis in human and several other mammalian hosts in the geographic areas with the greatest burden of disease:
An important feature of the landscape epidemiology of leptospirosis is the pathogen's ability to occupy two very distinct domains. First, Leptospira can exist as free-living organisms in water and soil (provided the soils stays moist). Second, Leptospira can exist for extended periods of time in the kidneys of some sylvan and domestic hosts. Rodents are the likely primary natural reservoir for these spirochetes. However, other sylvan mammals including raccoons, rabbits, skunks, and deer also appear to be able to shed the organisms into the environment and transmit the infection to other mammals. Moreover, domestic livestock, such as cattle and sheep, and companion animals, such as dogs, are capable of shedding infectious Leptospira and passing infections on to human hosts. Indeed, domestic livestock can serve as the most significant source of infection to humans in many geographic settings.
The particular importance of these two features lies distinctly within their spaces of intersection in the landscape. Since the Leptospira can occupy both mammal hosts and water and soil environments, a cycle of pathogen movement from mammalian host to environment and back can be maintained efficiently in agricultural landscapes, which is aided by precipitation and the movement of water across such landscapes. In particular runoff from rainwater can directly spread the pathogen from one site of contamination to more distal sites, thus contaminating soil patches and bodies of water that may be distant from the original mammalian source of contamination. Indeed, in areas of both low and high endemicity, local outbreaks often follow periods of extensive rain. If such periods of rainfall follow seasonal patterns, then outbreaks of leptospirosis may follow seasonal patterns as well. Moreover, the use of canals and irrigation channels of varying size and integrity, which is very common in agricultural settings all over the world, can further enhance the transport of these pathogens across the landscape. Thus, the spatial range of this pathogen is a function of the presence and movement of water in specific landscapes:
Control and Prevention. Control and prevention of leptospirosis begins by following the usual guidelines: improving water infrastructure and sanitation in resource poor areas. In most settings in the world where leptospirosis constitutes a significant disease burden, improved infrastructure that can maintain adequate water resources is a first priority in its prevention. In addition, however, specific features of agricultural landscapes, particularly those that channel and pool water can be designed and constructed to reduce the potential contamination from livestock, or from other sylvan hosts. Unfortunately, in most places of the world that experience a significant burden of disease, specific engineering techniques or raw materials for expensive irrigation systems, or controlled placement and dispersal of livestock, are generally not available because of their high cost and resource demand. This is particularly true of subsistence farmers.
In addition, individuals should avoid recreational activities in specific high-risk contexts, for example swimming in bodies of water likely to be contaminated with urine from livestock.
This time at Infection Landscapes we will explore one of the most widespread and prevalent zoonotic infections in the world: leptospirosis.
The Pathogen. Leptospirosis in humans is caused by one of five pathogenic bacteria species of the genus Leptospira in the the Leptospiraceae family. Leptospira spp. are spirochetes, displaying the characteristic spiraled cellular structure:
There are several species that are pathogenic to humans. These organisms have an outer membrane like gram negative bacteria, although the cell wall is not associated with this outer membrane but rather with the cytoplasmic membrane, which is structurally specific to spirochetes. Leptospira have two flagella used for motility.
Leptospira spp. bind to extracellular matrix, fibroblasts, endothelial cells, epithelial cells, and macrophages, ultimately making their way to the proximal renal tubules in the kidneys. The organisms are then shed in the urine of the infected host.
The Disease. Leptospirosis can range from asymptomatic infection to severe life threatening illness. Mild to moderate presentations may include fever, myalgia, headache, chills, diarrhea, vommiting, and petechial rash. Severe, complicated, leptospirosis, which is also referred to as Weil's disease, presents with multiple organ system involvement, most prominently the kidneys and liver. High concentrations of urea and creatinine in the blood, darkened and diminished urine output, and frank renal failure, and jaundice, abnormal liver enzymes, and frank liver failure characterized complicated leptospirosis in these two organ systems, respectively. Severe leptospirosis can also affect the lungs and heart, presenting with pulmonary hemorrhage, myocarditis, and pericarditis. Endothelial lesions and vascular injury are apparent in all involved organ systems. Depending on how extensive the vascular injury, complete cardiovascular collapse may follow. Finally, meningitis is another common complication in moderate to severe leptospirosis.
The Epidemiology and the Landscape. The spirochetes that cause leptospirosis are shed in the urine of infected mammals. The vast majority of human infections occur following contact with (most frequently) water or soil that has been contaminated by the urine containing the Leptospira. As such, the primary route of transmission is through a common vehicle, although most infections do not occur as a result of ingestion but rather by way of water or soil contact with the conjunctiva, mucous membranes, or contact with wounds or abrasions in the skin. Nevertheless, while this kind of common vehicle contact is typical, the more conventionally conceived mechanism for common vehicle transmission, i.e. the consumption of contaminated water (or food), can also play an important role in some epidemics. Indeed, consumption of a contaminated water source may be particularly relevant in some agricultural settings in the developing world where certain kinds of irrigation systems can combine with poor water and sanitation infrastructure to promote infection through the consumption of water.
Accurate estimates of the incidence of leptospirosis and associated morbidity and mortality are lacking in all areas of the world. This is in part due to the difficulty of diagnostic testing, and in part due to the lack of specific indicators of disease. As mentioned above, clinical leptospirosis can be quite similar to many infectious disease presentations, and there can be many asymptomatic cases as well. Nevertheless, cautious estimates suggest that the incidence ranges from approximately less than 1 case per million persons per year in the developed world to 1 case per thousand persons per year in parts of the developing world. The map below published by the International Livestock Research Institute displays the the distribution of leptospirosis in human and several other mammalian hosts in the geographic areas with the greatest burden of disease:
An important feature of the landscape epidemiology of leptospirosis is the pathogen's ability to occupy two very distinct domains. First, Leptospira can exist as free-living organisms in water and soil (provided the soils stays moist). Second, Leptospira can exist for extended periods of time in the kidneys of some sylvan and domestic hosts. Rodents are the likely primary natural reservoir for these spirochetes. However, other sylvan mammals including raccoons, rabbits, skunks, and deer also appear to be able to shed the organisms into the environment and transmit the infection to other mammals. Moreover, domestic livestock, such as cattle and sheep, and companion animals, such as dogs, are capable of shedding infectious Leptospira and passing infections on to human hosts. Indeed, domestic livestock can serve as the most significant source of infection to humans in many geographic settings.
The particular importance of these two features lies distinctly within their spaces of intersection in the landscape. Since the Leptospira can occupy both mammal hosts and water and soil environments, a cycle of pathogen movement from mammalian host to environment and back can be maintained efficiently in agricultural landscapes, which is aided by precipitation and the movement of water across such landscapes. In particular runoff from rainwater can directly spread the pathogen from one site of contamination to more distal sites, thus contaminating soil patches and bodies of water that may be distant from the original mammalian source of contamination. Indeed, in areas of both low and high endemicity, local outbreaks often follow periods of extensive rain. If such periods of rainfall follow seasonal patterns, then outbreaks of leptospirosis may follow seasonal patterns as well. Moreover, the use of canals and irrigation channels of varying size and integrity, which is very common in agricultural settings all over the world, can further enhance the transport of these pathogens across the landscape. Thus, the spatial range of this pathogen is a function of the presence and movement of water in specific landscapes:
Control and Prevention. Control and prevention of leptospirosis begins by following the usual guidelines: improving water infrastructure and sanitation in resource poor areas. In most settings in the world where leptospirosis constitutes a significant disease burden, improved infrastructure that can maintain adequate water resources is a first priority in its prevention. In addition, however, specific features of agricultural landscapes, particularly those that channel and pool water can be designed and constructed to reduce the potential contamination from livestock, or from other sylvan hosts. Unfortunately, in most places of the world that experience a significant burden of disease, specific engineering techniques or raw materials for expensive irrigation systems, or controlled placement and dispersal of livestock, are generally not available because of their high cost and resource demand. This is particularly true of subsistence farmers.
In addition, individuals should avoid recreational activities in specific high-risk contexts, for example swimming in bodies of water likely to be contaminated with urine from livestock.