Notes from the Field: Anthrax on a Sheep Farm in Winter — Texas, December 2023–January 2024

Investigation and Outcomes

On January 4, 2024, a male rancher aged 50–59 years was evaluated at hospital A for fever, leukocytosis, a black eschar on his right wrist, and extensive edema and blistered lesions on his right arm; he was febrile and had an elevated white blood cell count (Table); anthrax was suspected to be the etiology. Eleven days earlier, on December 24, 2023, he had butchered a lamb that had died suddenly on his ranch, located in a Texas county adjacent to a region with enzootic anthrax, known as the “Anthrax Triangle.”^§ Before its death, the lamb was healthy and showed no sign of disease. Five persons reported exposure to the lamb. The patient and another person seasoned and cooked the meat; the well-cooked meat was then consumed at a meal with three other persons. Among these five persons, only the index patient exhibited symptoms consistent with cutaneous anthrax, and none experienced symptoms consistent with gastrointestinal anthrax.^¶

The patient was initially seen by a general practitioner on January 1 and commenced a course of cephalexin for empiric treatment of soft tissue infection. Anthrax was not initially suspected as the etiology of his symptoms. After 3 days of empiric antibiotic therapy without response, the patient was evaluated at hospital A. A detailed clinical history and the patient’s clinical signs and symptoms raised the index of suspicion for anthrax, and wound swabs and blood were collected before initiation of antimicrobial monotherapy for presumed nonsystemic, cutaneous anthrax. The patient showed signs of systemic involvement and dual therapy for anthrax (ciprofloxacin and clindamycin) was initiated (3) the same day. The following morning, he was transferred to hospital B, a larger facility equipped for a more extensive evaluation of his severe edema and malaise. The Texas Department of State Health Services Laboratory performed real-time polymerase chain reaction (PCR) testing and culture from the patient’s wound swabs. Two wound swabs were positive for B. anthracis DNA** by real-time PCR; however, culture did not yield an organism consistent with B. anthracis. The patient recovered and was discharged after 1 week, on January 12.

The lamb was suspected to be the source of the patient’s illness and, in light of suspected anthrax, interviews were conducted with the patient and his family members. On January 6 and January 11, two ewes subsequently died on the farm with ocular and nasal hemorrhage. Nasal swabs were collected ≥12 hours after death and sent to the Texas A&M Veterinary Medical Diagnostic Laboratory for culture for B. anthracis. Test results from both animals were negative; however a high level of clinical and epidemiologic suspicion for anthrax remained. No other animal deaths occurred during the remaining winter season.

Paired sera from the patient were sent to CDC to measure anti-protective antigen (PA) antibodies and lethal factor (LF), a toxin produced by B. anthracis, using enzyme-linked immunosorbent assay (ELISA) and mass spectrometry, respectively. A more than fourfold increase in the concentration of anti-PA immunoglobulin G^†† was noted between serum specimens collected 11 days apart, indicating exposure to B. anthracis. LF concentration was 11.9 ng/mL in the acute serum sample,^§§ one of the highest LF levels ever measured in a patient with cutaneous anthrax at CDC or any other location (4). Cooked meat from the lamb was stored frozen for 2 weeks and sent to CDC for real-time PCR and culture. DNA extraction was performed on three separate sections of tissues; all were positive for B. anthracis by real-time PCR despite no culture growth.

Preliminary Conclusions and Actions

Nonculture testing through real-time PCR, ELISA, and mass spectrometry at CDC Laboratory Response Network sites was critical to confirming the diagnosis of anthrax considering of the unusual seasonality and inability to culture B. anthracis. Older evidence suggests that first-generation cephalosporins might be effective against B. anthracis (5) and might have prevented culture growth. However, treatment of naturally occurring B. anthracis with cephalosporins is contraindicated because of intrinsic resistance (3). This patient recovered only after receiving treatment with antimicrobials effective against anthrax (3).

The lack of culture growth from the two ewes could be attributed to factors including delayed sampling, handling, storing, or shipping swabs. B. anthracis DNA was detected in cooked meat from the lamb, and there was no culture evidence of viable bacteria from the meat. The infecting bacteria possibly were inactivated when the meat was cooked at high temperatures; however, there is no safe way to prepare meat for human consumption from an animal that has died of anthrax.

This outbreak occurred on a farm adjacent to the Anthrax Triangle in Texas and near the location of a 2019 human cutaneous anthrax case that was associated with an outbreak in animals, which included 25 culture-positive animal cases (2). In both the 2019 case and the current case, the patients reported direct skin exposure to animal carcasses, emphasizing the importance of avoiding processing carcasses of animals that unexpectedly die of unknown causes in this region regardless of the season. If animals must be moved, personal protective equipment should be worn. There was no clear history of routine vaccination against anthrax for this herd, or whether the remaining herd was vaccinated after the three animal deaths. Concerns about vaccine-associated adverse events among goats and horses were previously reported in this area (2), and routine animal vaccination remains essential in preventing anthrax in animals and subsequent spillover into humans (1).