Study proves that domestic dogs play a key role in maintaining transmission of visceral leishmaniasis

In an article published in the journal PLOS ONE, Brazilian scientists show that the number of domestic dogs in an area influences the risk of transmission of the disease and that areas in which cases occur remain high-risk areas.

By Karina Ninni  |  Agência FAPESP – Leishmaniasis is a group of infectious diseases caused by protozoans of the genus Leishmania that affect humans and animals. Visceral leishmaniasis (VL), a particularly severe form, is caused by Leishmania infantum, among other species, and the main reservoir for this parasite found in close proximity to humans is the domestic dog.

An interdisciplinary group of Brazilian scientists set out to investigate the effects of the canine population on transmission of L. infantum. The study was conducted in Bauru, São Paulo state, and reported in an article published in PLOS ONE.

The number of dogs in a given area was indeed found to affect the risk of transmission of the disease. “The risk of transmission of L. infantum is not substantial when a household has only one dog, but increases as the number of dogs rises. This was a key factor in the study. The other was perimeter size. With ten dogs in a 100-meter radius, the risk of transmission is still low, but with 40 dogs it’s significantly higher – more than 700%,” said Patrícia Sayuri Silvestre Matsumoto, first author of the article. Matsumoto is a geographer and postdoctoral fellow at the Center for Parasitology and Mycology (CPM) at Adolfo Lutz Institute (IAL), São Paulo state’s main epidemiological surveillance laboratory.

The scientists collected 6,578 blood samples from dogs living in 3,916 households between November 2019 and March 2020. “Our analysis of the blood samples detected canine VL in 5.6%, while geospatial mapping of the city’s households found 8.7% to be positive for VL. The difference was due to separation of biological samples from variables on the scale of households, particularly cultural and socio-economic characteristics, which locally modify the prevalence rate for the disease,” Matsumoto said.

Areas in which VL had occurred in the past remained high-risk areas for the disease. “If the parasite is detected again some time later, where was it during the period in which there were no infected dogs or human cases, given that it requires a host? What’s going on in this environment? Is there a residue of some kind that can persist?” said biologist José Eduardo Tolezano, last author of the article and technical director of CPM-IAL.

The hypothesis considered by the scientists is that the parasite remains on the site because the conditions are favorable in some way. “It’s important to investigate households, analyze their interior and vicinity, undertake spatial readings, examine the characteristics of the neighborhood, and see if organic matter, water and vegetation are present. Our study covered all these variables,” Tolezano said.

Matsumoto stressed that many of the areas in which VL occurs are on the outskirts of the city. “Opossums and wild canines like crab-eating foxes may pass through and possibly play a role in transmission,” she said. “In addition, many infected domestic dogs are asymptomatic. They’re diagnosed with the disease but never manifest clinical symptoms. It may well be that they serve as a reservoir for the parasite over long periods.”

The study was conducted in partnership with the City of Bauru Health Department and was supported by FAPESP via two projects (19/22246-8 and 18/25889-4).

Transmission

Leishmaniasis is a natural-focal disease, in the sense that the pathogen is transmitted to humans from animals. In the case of VL, these include wild canids and marsupials, as well as domestic dogs.

According to Tolezano, there are more than 20 species of Leishmania, all of which are hosted by wild animals. “The protozoan originates in nature. We’ve found opossums infected with the very species that causes VL. We’ve also found it in primates,” he said.

The vector is the sandfly Lutzomyia longipalpis. In rural or semi-urban areas, a sandfly that bites an infected animal may then infect a domestic dog, which becomes a reservoir. “But a number of other factors are always involved in addition to the parasite, a wild animal and a nearby domestic dog that hosts L. infantum,” Tolezano explained. “The local conditions must be right for sandflies to breed, for example. Unlike the mosquito that transmits dengue, L. longipalpis has immature forms [larvae and pupae] that thrive not in stagnant water but in moist soil. The presence of decomposing organic matter helps. Shade is also required to protect the adult insect in very warm weather, and of course a source of blood. The insects are infected when they bite infected vertebrates, mainly foxes and other wild canids.”

The focality is important as it refers to an area with the right conditions for transmission. “To say that VL occurs in Bauru doesn’t mean it occurs throughout the city, but in parts of it where the conditions are ideal,” he said. Until recently, Bauru was the main focus for human cases of VL, some 90,000-100,000 domestic dogs living there.

Matsumoto noted that biological factors are emphasized in the literature. “But it’s also important to study the context. For example, we already knew dogs were a major source of infection. When we examine infected dogs, we find the parasite in healthy skin and in the liver, spleen and most other organs. They carry very large parasite loads, and the sandflies are easily infected,” she said.

Scale

The main novelty in the article is the investigation of households as the context for dogs, Matsumoto noted. “The literature talks about rates at the level of a city or urban district, but doesn’t look at the household scale or analyze local differences,” she said. “Different characteristics in different parts of the urban environment are reflected in different rates of transmission between districts, so it’s a good idea to look at the canine population and these local differences.”

The group conducted two distinct types of analysis, she explained: one for households, taking into consideration the number of positive and negative dogs in each one; and a separate analysis that considered the number of dogs in a 100-meters radius, which is roughly the size of a city block. “We took a point, drew a 100-meter radius from the point, and counted the number of positive and negative dogs in that space. As noted, the analysis of both households and blocks showed that the number of dogs was important,” she said.

According to Tolezano, the risk analysis drilled down to the level of households with one, two, three and more than three dogs. “In addition, analysis of the perimeter proved to be well worthwhile,” he said. “If back yards communicate with each other, for example, you can’t say an infected dog is only in one household or the other. The dog may not go out, but even so it can be bitten by an infected fly and transmission happens. Even if the infected dog is removed, flies that have been infected by it may not have died yet. We arrived at very important results for an understanding of how and how fast the disease spreads in an urban environment.”

The habits of the people who live in the households analyzed are also important. “We’re doing another study on this, as a continuation of our research,” Matsumoto said.

The scientists used geospatial models to estimate and predict the risk of VL on the basis of the canine population, applying such statistical techniques as kernel density estimation, cluster analysis, geostatistics and generalized additive models.

“We designed the sample so as to be able to examine the city in terms of the ratio of dogs to people on the basis of data from IBGE [Brazil’s census bureau]. This enabled us to analyze areas that have important characteristics but had never been investigated before,” Tolezano said. 

Public policy

In 2019, 2,529 new cases of VL were confirmed in Brazil, according to the Health Ministry. Incidence reached 1.2 cases per 100,000 inhabitants, and the death rate was 9%, the highest in ten years.

Federal surveillance and control strategies consist of early detection and treatment of all human cases, control of the insects that transmit the disease using insecticide or by environmental management, and removal of canine reservoirs in urban areas.

“When city authorities conduct surveys with serological methods to identify infected dogs, public policy determines that dogs that test positive must be removed and put down. This creates difficulties in relations with the community, as transmission of the disease continues while dogs continue to be taken away from households to be put down, and at least half these animals are completely asymptomatic, if not more,” Tolezano said. “Many citizens refuse to let their dogs be examined, and when they do, and the dog tests positive, more than half won’t authorize impoundment, often leading to a court order. In any event, a large proportion of these positive animals remain in their neighborhoods, at least for a time. They’re asymptomatic but sandflies that bite them are infected.”

For Matsumoto, rethinking the canine population may be advisable from a public policy standpoint. “For example, when surveys are held, blood samples are taken from dogs and tested, but this is costly in terms of human resources, time and money. Now that our article has reinforced the importance of the canine population, it would be a good idea to conduct a census of domestic dogs and find out exactly where the surveys need to be done. Given that the number of dogs matters, different approaches could be used by the animal control authorities in different areas with higher or lower average numbers.”

The article “Impact of the dog population and household environment for the maintenance of natural foci of Leishmania infantum transmission to human and animal hosts in endemic areas for visceral leishmaniasis in Sao Paulo State, Brazil” is at: journals.plos.org/plosone/article?id=10.1371/journal.pone.0256534.