Outbreak of highly pathogenic avian influenza a(H5N1) among house cats: A case series involving oseltamivir treatment

Abstract

We present a highly pathogenic avian influenza A(H5N1) outbreak among four domestic cats from the same household within close proximity to a dairy farm in Tulare, California – the epicenter of the H5N1 dairy cattle outbreaks in California, USA. We demonstrate that with early supportive care and treatment with oseltamivir, H5N1 is survivable in domestic cats, and that survivor cats may maintain high titers of neutralizing antibodies against H5N1 at least 3–4 months post recovery.

Highlights

• •We report a highly pathogenic avian influenza A(H5N1) outbreak among four domestic cats from the same household.
• •With early supportive care and antiviral treatment, H5N1 is survivable in domestic cats.
• •Survivor cats may maintain high titers of neutralizing antibodies against H5N1 at least 3–4 months post recovery.

1. Case series

Highly pathogenic avian influenza A(H5N1) is emerging in mammals [[1], [2], [3]], including dairy cattle [[4], [5], [6]] and domestic cats [5,7,8]. An outbreak of H5N1 among four domestic cats from the same household occurred in late October to mid-November 2024 in Tulare, California (Fig. 1).

Fig. 1.

Timeline of a Highly Pathogenic Avian Influenza A(H5N1) Outbreak Among House Cats in Tulare, California.

1.1. Cats 1 and 2

On October 31, 2024, a family in Tulare, California brought their 1-year 7-month-old male neutered domestic long-haired pet cat (Cat 2) to a local veterinary hospital. The owner reported that the patient was ADR (“ain't doing right”), lethargic and refusing to ambulate. The owner reported no known exposure to toxins and that Cat 2 had indoor/outdoor access. Cat 2 was up to date on core feline vaccines in California (Feline herpesvirus 1 (FHV1), Feline calicivirus (FCV), Feline panleukopenia virus (FPV), and rabies) in addition to Feline leukemia virus (FeLV) vaccination. Physical examination of Cat 2 confirmed lethargy. The pet was afebrile and weighed 4.68 kg. Bloodwork and a urinalysis were offered but declined by the owner. The patient was given supportive treatment with subcutaneous fluids (200 mL Lactated Ringers Solution) and a broad-spectrum antibiotic, cefovecin sodium (Convenia 8 mg/kg SC, Zoetis). Cefovecin sodium is a single-injection long-acting cephalosporin that lasts 14 days. It is often used for compliance reasons and to treat feline patients that are not eating. Cat 2 returned 24 h later with minimal signs of improvement. An appetite stimulant (Mirtazapine ∼2 mg transdermally once daily) was provided but Cat 2 passed away two days later.

During this initial visit, the family reported that prior to symptom onset of Cat 2, another one of their cats (Cat 1) was acutely ill and taken to a different veterinary clinic. Cat 1, a 1-year 5-month-old male neutered domestic shorthair, was an indoor-only cat with no outdoor access and was up to date on vaccines (FHV1, FCV, FPV, rabies, and FeLV). Cat 1 suddenly became lethargic and was rushed to a local emergency facility where imaging of the thorax was performed along with bloodwork. Cat 1 was suspected of having an upper respiratory infection (URI) by the attending veterinarian and was given a broad-spectrum antibiotic injection (Convenia) and oxygen therapy but passed suddenly/acutely after being taken home.

The attending veterinarian in charge of care for Cat 2 is a mixed-animal veterinarian who cares for just under 100,000 cattle in California. At the time of these feline patient visits, multiple local dairy herds and poultry operations were experiencing outbreaks of HPAI and were being investigated by the United States Department of Agriculture (USDA) and the California Department of Food and Agriculture (CDFA). Considering the acute mortality of Cat 1, the veterinarian became concerned about HPAI. The owner family lives less than 1 mile from the nearest cluster of dairy farms and the husband works within the dairy sector as a traveling hay salesman who frequently visits farms. Precautions and isolation techniques were discussed with the owner, and they were encouraged to immediately bring in the remaining cats at home if any started showing signs. The attending veterinarian reached out to contacts within the USDA and CDFA regarding H5N1 testing and possible necropsy. Due to the current demand from the food animal sector, feline outbreak calls were not returned and no treatment or testing options were provided. The attending veterinarian later placed a phone call to the California Animal Health and Food Safety Laboratory (CAHFS) where they offered to potentially test future feline cases but could not guarantee accuracy of their testing method.

1.2. Cats 3 and 4

On November 6, the family returned with another pet (Cat 3). Cat 3 is a 1-year 7-month-old male neutered domestic longhair. The pet was current on vaccines (FHV1, FCV, FPV, rabies, and FeLV) and lived an indoor-only lifestyle. Cat 3 presented with lethargy and anorexia and an initial weight of 5.18 kg. Physical examination revealed a dull mentation as well as 5 % dehydration. Aside from the dull mentation, neurologic examination was otherwise unremarkable. The patient was febrile with a temperature of 104 °F (40 °C). An injectable NSAID (Meloxicam 0.1 mg/kg subcutaneously) was started alongside subcutaneous fluid therapy (200 mL Lactated Ringers Solution). With the previous suspicion of HPAI, the patient was started on 15 mg oseltamivir (Tamiflu) by mouth every 12 h (PO q12h) for 10 days. Subcutaneous fluids (200 mL LRS) were repeated 3 days later, and the pet was started on oral NSAIDs (Meloxicam 0.05 mg/kg PO q24h). On November 13, the pet was rechecked. His appetite had improved as noted by an increase in weight to 5.66 kg. He was drinking normally and his activity returned to normal. A physical exam confirmed evidence of hypopyon within the right eye and that the fever had resolved. The pet was started on a broad-spectrum topical antibiotic/steroid eye drop (Neopoly Dex Ophthalmic Suspension, 1 drop OD q12h). The pet was then lost for follow-up.

On November 7, the same family brought in Cat 4, a 4-year 11-month-old male neutered domestic shorthair. Cat 4 was current on vaccines (FHV1, FCV, FPV, rabies, and FeLV) and lived a strictly indoor lifestyle. This pet presented with concerns of lethargy, nasal discharge, anorexia, and an unkempt hair coat. The owner reported that she had been keeping all of her cats isolated due to the suspected contagious nature of the illness. At intake, Cat 4 was febrile with a temperature of 103.4 °F (39.7 °C), a weight of 5.36 kg, and signs of URI (purulent nasal discharge bilaterally, serous discharge from both eyes, and increased bronchovesicular sounds bilaterally). This pet was started on subcutaneous fluids (200 mL LRS), injectable NSAIDs (Meloxicam 0.1 mg/kg subcutaneously), and oseltamivir (15 mg (Tamiflu) PO q12h for 10 days). After 24 h, the pet was started on oral NSAIDs (Meloxicam 0.05 mg/kg PO q24h). The pet was lost for follow-up at that time.

On February 26, 2025, the owner was contacted to evaluate the status of Cat 3 and Cat 4 since they initially were lost to follow-up. The owner reported that following treatment, both pets recovered and were back to normal. No information on human illness in the household was provided. On March 13, blood samples were collected from the recovered cats at a follow-up visit.

2. Serological methods

Aliquots of sera were shipped on ice to the University of Maryland Department of Veterinary Medicine and the University of Texas Medical Branch One Health Laboratory for influenza serology research. Samples were stored in −80 °C freezers prior to serological testing.

We screened sera samples from Cat 3 and Cat 4 for influenza A virus antibodies using a commercial ELISA kit: IDEXX AI MultiS-Screen Ab Test (IDEXX, Westbrook, ME), and an Agilent BioTek Epoch plate reader (Agilent Technologies, Inc., Wilmington, DE) at the University of Maryland School of Public Health (UMD SPH). For the ELISA assay, we used a sample-to-negative (S/N) ratio of ≤0.5 as a cutoff for influenza A virus antibody positivity. We tested sera for neutralizing antibodies against influenza A virus subtype H5N1 (A/H5N1) at the University of Texas Medical Branch (UTMB), using a recombinant H5 microneutralization (MN) assay described in Shittu et al. (2025) [6]. We used a minimum dilution of 1:40 as a cutoff for H5N1 seropositivity. The results of both assays were blinded to the other research group until shared concurrently.

3. Results and conclusion

Sera from Cat 3 and Cat 4 tested positive for influenza A virus antibodies via ELISA, and positive for neutralizing antibodies against influenza H5N1 2.3.4.4b (Table 1). Blinded results were in agreement.

Table 1.

House Cats with Highly Pathogenic Avian Influenza A(H5N1) – Tulare, California, USA, October to November 2024.

Cat	Age, Sex, and Breed	Lifestyle	Symptoms	Treatment	Outcome	Influenza A ELISA	H5N1 microneutralization
1	1 year 5 months, neutered male, domestic shorthair	Indoor-only	Respiratory	Antibiotics, oxygen therapy	Sudden death	Not tested	Not tested
2	1 year 7 months, neutered male, domestic longhair	Indoor/Outdoor	Severe lethargy	Subcutaneous fluids, antibiotics, appetite stimulant	Death	Not tested	Not tested
3	1 year 7 months, neutered male, domestic longhair	Indoor-only	Lethargy, anorexia, dehydration, fever, hypopyon	Subcutaneous fluids, NSAIDs, oseltamivir	Full recovery	+	1:640 (+++)
4	4 years 11 months, neutered male, domestic shorthair	Indoor-only	Lethargy, nasal discharge, anorexia, unkempt hair, fever	Subcutaneous fluids, NSAIDs, oseltamivir	Full recovery	+	1:160 (++)

This outbreak of H5N1 among house cats demonstrates how quickly H5N1 can spread through multiple animals in a home, posing an unexpected spillover risk to family members and veterinarians caring for them. Most importantly, we demonstrate that with early supportive care and early treatment with oseltamivir, H5N1 is a survivable disease in domestic cats. Our results support the continued prioritization of neuraminidase inhibitors in the treatment of H5N1 in mammals and highlight the importance of early identification and targeted treatment. Furthermore, we demonstrate that survivor cats can regain full health and maintain high titers of neutralizing antibodies against H5N1 at least 3–4 months post recovery. These findings suggest that cats and other mammalian species may sustain strong protection from reinfection of H5N1 after natural infection and continue to thrive.

CRediT authorship contribution statement

Jacob F. Gomez: Writing – review & editing, Writing – original draft, Resources, Investigation, Formal analysis. Ian G. Bemis: Writing – review & editing, Validation, Investigation, Data curation. Ismaila Shittu: Writing – review & editing, Validation, Methodology, Formal analysis, Data curation, Conceptualization. Gregory C. Gray: Writing – review & editing, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. Kristen K. Coleman: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Ethics statement

Ethics approval was not required as this is a case series reporting the outcomes of clinical veterinary care provided to ill feline patients.

Funding statement

We acknowledge the support of the University of Maryland Baltimore (UMB) Institute for Clinical & Translational Research (ICTR) and the University of Maryland Strategic Partnership: MPowering the State (MPower) to KKC, as well as discretionary funding from the University of Maryland School of Public Health, Department of Global, Environmental, and Occupational Health to KKC. This project was supported in part by the USDA-ARS Agreement 58–3022–4-048 to GCG.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Raw data are available through and may be provided by the authors upon reasonable request.