Sporothrix brasiliensis: The Emerging Fungal Threat to Cats and Humans

Sporothrix brasiliensis is a highly contagious fungal infection spreading through South America

Sporothrix brasiliensis is a dimorphic fungus that causes severe skin ulcers and systemic infections in cats and humans. Since its emergence in Brazil in the 1990s, it has infected more than 11,000 people and thousands of cats, spreading from Brazil into Paraguay, Chile, Argentina, and Uruguay. While not yet officially documented in the United States, public health officials, including the CDC, warn that its arrival is likely due to the movement of infected pets.

High fatality rates and systemic progression

In cats, S. brasiliensis is often fatal without treatment. Even with medical intervention, the fungus maintains a high fatality rate. The infection typically begins as oozing skin ulcers and nodules with swollen lymph glands. If left untreated, the fungus can migrate to the lungs and spread throughout the body, becoming a respiratory disease that is 100% fatal in felines.

In humans, the fungus causes painful skin ulcers. While generally less lethal than in cats, the disease can be severe and potentially fatal for individuals with weakened immune systems.

Unique transmission mechanisms

Unlike other dimorphic fungi, S. brasiliensis can spread directly in its yeast form. Most fungal infections occur via the inhalation of spores or puncture wounds (such as the rose growers’ disease caused by Sporothrix schenckii). However, S. brasiliensis utilizes multiple pathways for contagion:

• Direct Contact: Cat bites and scratches inject the yeast directly into the host. This often occurs during grooming or fighting between cats, or when owners attempt to administer medication to infected pets.
• Respiratory Secretions: Infected cats can sneeze yeast-laden secretions, contaminating the immediate environment and the air.
• Environmental Persistence: The fungus is exceptionally hardy. Lab experiments show it can survive on stainless steel surfaces for up to 10 weeks, significantly longer than Candida albicans (48 hours) or Candida auris (one month).

Dimorphic nature and biological advantages

S. brasiliensis is dimorphic, meaning it exists in two distinct forms based on environment:

1. Hyphal Form: In soil and cold environments, it grows as long, stringy filaments called hyphae.
2. Yeast Form: Once it infects a host (the "beast"), it transforms into single-celled yeast.

This ability to switch forms allows the fungus to survive in the soil and then efficiently colonize animals and animals' tissues.

Challenges in detection and prevention

Public health officials are concerned about the global spread of the fungus because of several systemic vulnerabilities:

• Lack of Specialized Testing: There is currently no commercially available test specifically for S. brasiliensis.
• Lax Import Regulations: Pets entering the United States typically only require a health certificate stating they appear healthy, which is insufficient to detect an asymptomatic or early-stage infection.
• Latent Symptoms: Symptoms may not appear immediately. In one reported case, a family moving from Brazil to the UK developed the disease three years after their move, linked to an infected cat.

To mitigate the risk, the CDC recommends that veterinarians be vigilant and report any cases of sporotrichosis to local public health labs or the CDC, as veterinary professionals are the first line of defense in detecting the emergence of the fungus in new regions.