Equine herpesviruses at times use indirect pathways to infect horses, review finds

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Most known EHVs are transmitted directly between individuals as a result of direct exposure to body fluids and aerosols
Most known EHVs are transmitted directly between individuals as a result of direct exposure to body fluids and aerosols.

Indirect pathways may play an important role in the transmission of equine herpesviruses in both captivity and nature, according to the authors of a just-published review.

Equine herpesviruses (EHVs) represent one of the most economically important disease agents of horses.

Most known EHVs are transmitted directly between individuals as a result of direct exposure to body fluids and aerosols.

However, there is accumulating evidence that environmental transmission may play a role, including air, water, and contaminated objects.

Researchers Anisha Dayaram, Peter Seeber and Alex Greenwood, in their review published in the journal Pathogens, examined previously published studies on EHV’s environmental stability and transmission to obtain a clearer picture of potential routes of transmission.

“There is a growing body of evidence suggesting that different pathways may affect the transmission of EHV within and among species,” concluded the trio, who are with the Leibniz Institute for Zoo and Wildlife Research in Berlin.

Several EHV species are commonly found in the environment (for example, EHV-1, EHV-5, EHV-7, and asinine herpesvirus 5), whereas others such as EHV-4, EHV-6, and EHV-8 have not been detected in environmental samples so far.

“This is surprising for species such as EHV-4 that are highly prevalent, with over 90% of domestic and wild equid populations infected and with infectious virions being shed through nasal exudate,” they said.

Detection in the environment likely depends on virus and host species-specific factors, they said.

EHV species that occur frequently in the likes of nasal fluid or systemically in the blood may be expected to be more likely to be shed directly into the environment, compared to EHVs that mostly occur in nervous tissue.

By contrast, species such as EHV-3, which is a strictly venereal pathogen, would not be expected to occur in samples that are not contaminated by genital fluids or urine.

“Direct contact with infected animals and exposure to aerosols, infected respiratory secretions, placenta and uterine fluids, and fetal tissue from aborted fetuses remain the most common and direct pathways for EHV transmission between hosts,” the review team concluded.

“However, indirect pathways may play an important role in viral transmission in both captivity and nature.”

EHV transmission via contaminated objects, known as fomites, is of particular concern, with the viruses capable of remaining infectious on many different materials for extended lengths of time.

This, they said, poses a particular risk for cross-species transmission in captive animal facilities where animals that would not be exposed to EHVs in their natural habitat may come into contact through an indirect pathway.

The review team said studies are needed to fully understand how pathogenic EHV-1 strains might transmit through aerosol droplets to infect equids in nearby enclosures.

“The equine industry would benefit from precise guidelines for distances in which equids can be safely housed without the risk of disease transmission.

“In addition, fecal matter now needs to be treated as a possible fomite for all EHV transmission until further studies are performed to show if fecal matter contains intact virions or just naked viral DNA.

“Better understanding of these different transmission pathways is of particular interest to captive animal facilities such as zoos, where exotic animals face high likelihoods of exposure and limited immunity to viruses such as EHVs.”

Zookeepers and veterinarians working with equids need to be aware and understand how to isolate sick animals and prevent further spread of infection throughout facilities via indirect pathways, including fomites, aerosols, water, and feces, they said.

“In natural settings, researchers need to better understand how global climate changes and destruction of habitat will increase direct and indirect contact rates among wildlife.”

Understanding all transmission pathways for EHV may allow for better prediction of viral transmission, both within species and between them.

Dayaram, A.; Seeber, P.A.; Greenwood, A.D. Environmental Detection and Potential Transmission of Equine Herpesviruses. Pathogens 2021, 10, 423. https://doi.org/10.3390/pathogens10040423

The review, published under a Creative Commons License, can be read here.

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