Feline sporotrichosis in Rio de Janeiro: Space-time analysis of the main enzootic region in Brazil (1998–2018)

In the bustling streets of Rio de Janeiro, a quiet but persistent health crisis has been unfolding for two decades, involving a fungus, a cat, and the people who live alongside them. This story centers on a disease called sporotrichosis, an infection caused by a microscopic fungus that typically enters the body through a scratch or a bite. While this fungus can affect many animals, in Brazil, it has found a particularly potent host in the domestic cat. Unlike other regions where the disease might spread through soil or plants, here the primary engine of transmission is the cat itself. When an infected cat scratches or bites a human, or even another animal, it can pass the infection along, creating a chain of illness that links the health of pets directly to the well-being of their owners. This connection is a core part of a concept known as "One Health," which recognizes that the health of people, animals, and the environment are deeply intertwined. Understanding where and when these infections happen is not just an academic exercise; it is a vital tool for stopping the spread of a disease that causes painful, long-lasting sores and can be difficult to treat.

For twenty years, from 1998 to 2018, researchers at a major medical institute in Rio de Janeiro, the Instituto Nacional de Infectologia Evandro Chagas, have been collecting data on every cat diagnosed with this fungal infection. They gathered the medical records of more than five thousand cats, creating a massive archive of the disease's history. The scientists wanted to see if they could find patterns in this data that would reveal how the disease moved through the city over time. To do this, they took the home addresses of the infected cats and mapped them onto a digital map of the region. They used computer tools to look for clusters, or groups of cases that appeared close together in both space and time, which would suggest that the disease was spreading in specific neighborhoods rather than appearing randomly. They also looked at the characteristics of the cats themselves to see if certain types were more likely to get sick.

The results of this twenty-year review tell a clear story of how the disease expanded and where it took hold. The researchers found that the number of cases grew steadily over time, reaching a peak in 2010 with over five hundred new cases in a single year. The cats most likely to be infected were young, male, and of mixed breeds. Most of them had not been neutered, a detail that suggests their behavior played a role in their exposure. Because unneutered male cats often roam freely and fight with other cats, they are more likely to get scratched or bitten, which provides an easy entry point for the fungus. The data showed that these infections were not spread evenly across the city. Instead, they were heavily concentrated in lower-income neighborhoods in the northern part of Rio de Janeiro and in the surrounding municipalities of the Baixada Fluminense region. In contrast, the wealthier areas of the city, where cats are more likely to stay indoors, saw very few cases.

When the researchers looked at the map, they saw the disease spreading outward like a ripple, moving from the city center into neighboring towns like Duque de Caxias and Nova Iguaçu. They identified thirteen distinct hotspots where the disease appeared in clusters. The earliest of these hotspots appeared in the late 1990s in a neighborhood called Penha. Over the years, new clusters formed in other areas, including Manguinhos and Bonsucesso, creating a belt of transmission that followed the path of the city's expansion. These areas shared common features: they were densely populated, and the homes were often single-story houses that allowed cats to move freely in and out. The study also revealed a striking pattern of repetition. In many of these locations, the same address was home to multiple infected cats, sometimes as many as forty-nine different cases over the two decades. In most of these instances, the infections happened within a single year of each other, suggesting that once the fungus arrived in a household, it stayed there and spread quickly among the animals living there.

Interestingly, the pattern of these clusters changed after 2012. The study found no significant new clusters in the years following 2013. The researchers believe this shift was not because the disease disappeared, but because the way people sought help changed. For many years, the research institute was the only place in the state offering free diagnosis and treatment, so people from nearby neighborhoods naturally brought their sick cats there. However, in 2014, the city government began offering free veterinary services directly in local neighborhoods. This change likely meant that many infected cats were being treated closer to home, rather than being referred to the central institute. As a result, the data collected at the institute no longer showed the same tight clusters, even though the disease was likely still present in those communities.

The study concludes that the fight against this disease requires a strategy that understands where it lives. The fungus thrives in specific environments where cats have access to the outdoors and where many animals live in close quarters. The high number of cases at the same addresses suggests that transmission is often local and intense, with a few households acting as major hubs for the spread of the infection. By identifying these high-risk areas, health officials can focus their efforts where they are needed most, rather than trying to treat the entire city at once. The findings reinforce the idea that controlling this zoonotic disease, which jumps from animals to humans, depends on managing the health of the cat population in the specific neighborhoods where the risk is highest. The work provides a clear map of the past two decades, showing that while the disease has moved and changed, its roots remain deep in the urban landscape of Rio de Janeiro.