Early Release – Probable Transmission of SARS-CoV-2 from African Lion to Zoo Employees, Indiana, USA, 2021 – Volume 29, Number 6—June 2023 – Emerging Infectious Diseases journal

Setting

The sentinel case occurred in December 2021, when the zoo was closed for the season. The lion was housed alone in a building with an indoor/outdoor enclosure located >30 feet from other animal enclosures. Feeding and veterinary procedures were conducted indoors by dedicated staff with assigned key access. Susceptible species elsewhere in the zoo included a snow leopard (Panthera uncia), Amur tigers (Panthera tigris altaica), Amur leopards (Panthera pardus orientalis), North American river otters (Lontra canadensis), and several species of nonhuman primates. All susceptible animals, including the lion, had received 2 doses of Zoetis mink coronavirus vaccine (https://www.zoetis.com) during September–November 2021. Animal health was overseen by a full-time onsite veterinarian.

Zoo practice before the outbreak included SARS-CoV-2 prevention measures from the Zoo and Aquarium All Hazards Partnership (20,21), such as suspending behind-the-scenes tours and close-distance behavioral training for susceptible species. Employees were required to complete the COVID-19 vaccination series, practice social distancing, monitor for COVID-19 symptoms, and, when ill, exclude themselves from work. Disinfectant foot baths were placed at enclosure entrances, and high-pressure cleaning methods were replaced by methods less likely to aerosolize infectious material. Employees were required to wear surgical masks at all times and nitrile gloves during feedings.

Sentinel Case

The lion was a geriatric (20-year-old) male with chronic renal insufficiency and severe degenerative intervertebral disc disease prohibiting normal range of motion for self-grooming of the caudal body. He was vaccinated with the Zoetis vaccine on September 14 and October 7, 2021. Routine care included hand feeding twice daily, therapeutic laser treatments of the spine and hindquarters, corticosteroids, gabapentin, renal supplements, and mesenchymal stem cell treatments. On December 18, employees observed coughing, dyspnea, shivering, lethargy, sneezing, nasal discharge, anorexia, and exposed nictitans. Marbofloxacin treatment was initiated. Nasal swabs were collected on December 18 and 23. He was euthanized on December 23 because of declining mobility associated with intervertebral disc disease. The acute clinical signs attributed to SARS-CoV-2 had largely resolved and were not a factor in the euthanasia decision.

Investigation

Figure 1

Figure 1. Traceback and forward contact tracing investigations of SARS-CoV-2 transmission between an African lion and zoo employees, Indiana, USA, 2021–2022. The traceback investigation narrowed the potential source of the lion’s SARS-CoV-2…

We initiated an investigation on December 18 to identify the source of the lion’s exposure and the potential for forward transmission (Figure 1). We defined a confirmed cluster-associated case as illness in a human or animal at the zoo with laboratory evidence for the same strain as the sentinel case, based on genomic sequencing of a clinical sample. We defined a probable cluster-associated case as illness in a human or animal at the zoo with laboratory evidence for SARS-CoV-2 based on reverse transcription PCR (RT-PCR) or rapid antigen test of a clinical sample but for which no genomic sequencing information was available.

For both traceback and forward contact tracing investigations, we assumed the exposure period to begin 10 days before illness onset, with probable acquisition within 5 days prior. We assumed the infectious period to begin 2 days before illness and extend 10 days after illness onset. We considered any person who entered the enclosure during the lion’s exposure or infectious period to be a lion contact. We classified the care provided by lion contacts as cranial (e.g., feeding and nasal swabbing with <2 feet between human and lion heads) or caudal (e.g., arthritis care and injections performed ≈9 feet from the lion’s head). We defined close contact between persons as proximity of <6 feet for >15 minutes in 1 day.

Infection Control

Upon detection of SARS-CoV-2 in the lion, employees providing care for susceptible species at the zoo were required to wear N95 respirators. Those entering the lion building also used face shields, disposable gowns, shoe covers, and gloves. Lion building access was restricted further, and laser treatments were discontinued. Work assignments were altered so that employees providing care to the lion would not come in contact with other susceptible animals (22).

Sentinel Case Investigation

We detected evidence of SARS-CoV-2 in the lion through onsite screening on December 18. We detected further evidence of SARS-CoV-2 by RT-PCR in nasal swab samples from December 18 and December 23, and in nasal turbinate, lung tissue, and intestinal tissues obtained at necropsy on December 23. Extracted RNA from both nasal swab samples produced high-quality 29,707-nt sequences for genomic sequencing (SARS-CoV-2 Delta variant, AY.103 lineage). Necropsy confirmed intervertebral disc degeneration, chronic renal disease, chronic lower airway disease, and severe rhinitis.

Traceback Investigation

Figure 2

Figure 2. Timeline of probable transmission of SARS-CoV-2 from an African lion to zoo employees, Indiana, USA, 2021–2022. The lion’s likely exposure period was December 8–17, and his infectious period was December…

Nine people (employees Z1–Z9) entered the lion building at some time during the lion’s potential exposure period during December 8–18 (Figure 1). All 9 reported being asymptomatic on December 18. Eight had screening tests on December 18, and 1 had a screening test on December 19; all were negative. Employees Z1, Z2, and Z3 had onset of COVID-19 during December 21–24, with infectious periods that began on or after December 19 (Figure 2). Employee Z5 had cranial contact during the lion’s 5-day probable acquisition period. Employees Z4, Z6, Z7, Z8, and Z9 had lion contact 6–10 days before onset of the lion’s illness, performed caudal activities exclusively, or both.

Forward Contact Tracing Investigation

No additional animals at the zoo had illness suggestive of SARS-CoV-2 infection (Figure 1). Eight persons entered the lion building during the lion’s infectious period (December 16–23). Employees Z1, Z2, and Z3 were fully vaccinated and previously healthy, each of whom was involved in cranial care 3 days before having onset of upper respiratory symptoms. Each of those 3 employees was positive on rapid antigen test within 24 hours after symptom onset and had subsequent RT-PCR confirmation of SARS-CoV-2 infection. Samples from patients Z1 and Z2 were suitable for sequencing; the sample from Z3 was of insufficient quality for sequencing.

Figure 3

Figure 3. Phylogenetic tree of SARS-CoV-2 Delta variant, AY.103 lineage, genome sequences from an African lion (day 1 and day 5) and 2 zoo employees (Z1 and Z2) shown in comparison with…

The sequences generated from lion samples on December 18 and 23 were identical to those from Z1 and Z2 samples. The 10 community comparators varied from the cluster sequences by 12–24 nt (Figure 3).

Employees Z5, Z8, and Z9 had no close contact with Z1, Z2, or Z3 during their exposure periods. Z4 had close contact with Z2 on December 18, the day that both had negative screening tests. Z6 entered the lion enclosure at the same time as Z3, but was maintaining >6 feet distance, 9 days before Z3’s symptom onset. Z7 had close contact with Z1 on December 11, 12, and 18. Z7 worked in the lion enclosure with Z2, but that contact took place a full 10 days before Z2’s onset of symptoms and was at >6 feet for <15 minutes. Z7 also worked in the lion enclosure with Z3 for <15 minutes and at a distance of >6 feet 8 days before Z3’s onset of symptoms. Employee Z3 had no known exposures outside of the zoo but did participate in community social activities with risk for unrecognized transmission of SARS-CoV-2 on several occasions during the 10 days before onset of symptoms.