Equine influenza: a comprehensive review from etiology to treatment

Understanding the divergent evolution and epidemiology of H3N8 influenza viruses in dogs and horses

Cross-species virus transmission events can lead to dire public health emergencies in the form of epidemics and pandemics. One example in animals is the emergence of the H3N8 equine influenza virus (EIV), first isolated in 1963 in Miami, FL, USA, after emerging among horses in South America. In the early 21st century, the American lineage of EIV diverged into two 'Florida' clades that persist today, while an EIV transferred to dogs around 1999 and gave rise to the H3N8 canine influenza virus (CIV), first reported in 2004. Here, we compare CIV in dogs and EIV in horses to reveal their host-specific evolution, to determine the sources and connections between significant outbreaks, and to gain insight into the factors controlling their different evolutionary fates. H3N8 CIV only circulated in North America, was geographically restricted after the first few years, and went extinct in 2016. Of the two EIV Florida clades, clade 1 circulates widely and shows frequent transfers between the USA and South America, Europe and elsewhere, while clade 2 was globally distributed early after it emerged, but since about 2018 has only been detected in Central Asia. Any potential zoonotic threat of these viruses to humans can only be determined with an understanding of its natural history and evolution. Our comparative analysis of these three viral lineages reveals distinct patterns and rates of sequence variation yet with similar overall evolution between clades, suggesting epidemiological intervention strategies for possible eradication of H3N8 EIV.

Equine Influenza Virus: An Old Known Enemy in the Americas

Equine influenza is a highly contagious disease caused by the H3N8 equine influenza virus (EIV), which is endemically distributed throughout the world. It infects equids, and interspecies transmission to dogs has been reported. The H3N8 Florida lineage, which is divided into clades 1 and 2, is the most representative lineage in the Americas. The EIV infects the respiratory system, affecting the ciliated epithelial cells and preventing the elimination of foreign bodies and substances. Certain factors related to the disease, such as an outdated vaccination plan, age, training, and close contact with other animals, favor the presentation of equine influenza. This review focuses on the molecular, pathophysiological, and epidemiological characteristics of EIV in the Americas to present updated information to achieve prevention and control of the virus. We also discuss the need for monitoring the disease, the use of vaccines, and the appropriate application of those biologicals, among other biosecurity measures that are important for the control of the virus.

Experimental Infection of Horses with Influenza D Virus

Antibodies to influenza D virus (IDV) have been detected in horses, but no evidence of disease in the field has been reported. To determine whether IDV is infectious, immunogenic, and pathogenic in horses, four 2-year-old horses seronegative for both influenza A (H3N8) and D viruses were intranasally inoculated with 6.25 × 10⁷ TCID₅₀/animal of D/bovine/California/0363/2019 (D/CA2019) virus, using a portable equine nebulizer system. Horses were observed daily for clinical signs including rectal temperature, nasal discharge, coughing, lung sounds, tachycardia, and tachypnea. No horses exhibited clinical signs of disease. Nasopharyngeal swabs collected from 1–8 days post-infection demonstrated virus shedding by qRT-PCR. The horses showed evidence of seroconversion as early as 13 days post-infection (dpi) and the geometric mean of the antibody titers (GMT) of all four horses ranged from 16.82–160 as demonstrated by the microneutralization assay. Further, deep RNA sequencing of the virus isolated in embryonated chicken eggs revealed no adaptive mutations indicating that IDV can replicate in horses, suggesting the possibility of interspecies transmission of IDV with bovine reservoir into equids in nature.