Feline herpesvirus (FHV) infection

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Feline herpesvirus (FHV, FHV-1) is a highly contagious virus that is one of the major causes of upper respiratory infections (URIs) or cat flu in cats.

This virus is ubiquitous and causes disease in cats all over the world. See also Upper respiratory infections (URIs, Cat flu) in cats. Together, FHV and feline calicivirus cause the vast majority of URIs in cats.

What is FHV and how is it spread?

Feline herpesvirus (FHV) is a virus that mainly causes acute upper respiratory infections (URIs) in cats, although it has been associated with some other diseases also (see below). The virus is readily transmitted between cats through:

  • Direct contact – through contact with saliva, ocular or nasal secretions
  • Inhalation of sneeze droplets
  • Sharing or food bowls and litter trays
  • A contaminated environment (including bedding and grooming aids) – this is less important with FHV than FCV as the virus is fragile can probably only survive for 1-2 days in the environment

With FHV, after infection, virtually all cats will remain latently infected (the virus persists in nerve cells). This means that infected cats effectively become life-long carriers of the virus. In many cats this causes no problems, and they do not continue to shed virus so are not a risk to others. However, some cats will intermittently shed virus again, and this is more common following episodes of stress or when the cat's immune system is suppressed (e.g., following the use of corticosteroids). When virus is shed again, some cats will also develop mild recrudescence of clinical signs. Additionally, persistent FHV infection can cause ocular problems (see below).

What are the clinical signs of FHV infection

  • Acute upper respiratory infection – acute URI is the most common manifestation of FHV infection. Typical signs include conjunctivitis, ocular discharge, sneezing, nasal discharge, salivation, pharyngitis, lethargy, inappetence, fever and sometimes coughing. Signs may last from a few days to a few weeks and shedding of virus typically continues for around 3 weeks. Clinical disease with FHV is generally more severe than that seen with FCV.
  • Keratitis – although relatively uncommon, one manifestation of chronic (long-term) FHV infection that is seen in a number of cats is conjunctivitis and keratitis (infection and inflammation of the cornea – the clear part at the front of the eye). Although keratitis can have a number of different causes, FHV infection causes the development of multiple small branching corneal ulcers (called ‘dendritic keratitis’) and this is considered diagnostic of FHV infection. .
  • FHV-associated dermatitis – a rare manifestation of chronic (long-term) FHV infection is the development of skin inflammation and ulceration. This is most commonly seen around the nose and mouth, but can affect other areas such as the front legs. This is only seen rarely.

How is FHV infection diagnosed?

In most cases, a specific diagnosis of FHV infection will not be required. The presence of typical signs of URI is enough for a presumptive diagnosis of FHV (and/or feline calicivirus – FCV) infection. If a specific diagnosis is required, ocular or oral swabs can be submitted to a veterinary laboratory where the virus can be grown in culture or, more commonly, detected by PCR (a molecular technique for detecting the genetic material of the virus). Evidence of the virus may also be present in biopsies and can be useful for the diagnosis of FHV-associated dermatitis (skin infection).

Treatment and management of FHV infections

FHV infections are frequently complicated by secondary bacterial infections, so supportive treatment with antibiotics is usually required. Good nursing care is critical and cats may need to be hospitalised for intravenous fluid therapy and nutritional support in severe cases. Steam inhalation or nebulisation may help in cases of severe nasal congestion and as the cat will not be able to smell food well, using tinned or sachet foods that are gently warmed will help.

Unlike FCV, with FHV infection certain anti-viral drugs are available and can be very helpful in managing the clinical manifestations of disease.

  • Systemic antiviral therapy: Famciclovir is a human anti-herpes virus drug that has been shown to be safe and effective in cats. It can be given by mouth and can be valuable in managing severe acute infections in particular.

  • Topical ocular antiviral therapy: idoxuridine, trifluridine and cidofovir are all human anti-herpes virus drugs that can be successfully used as topical ocular (eye drops) therapy for FHV-associated conjunctivitis and keratitis. Some of these drugs have to be given very frequently (several times daily) and they may be combined with topical interferon to enhance efficacy.

In colonies of cats, any cat showing clinical signs should be isolated if at all possible, and strict hygiene should be ensured with disinfection, and use of separate feeding bowls, litter trays, implements etc., careful washing of hands, use of separate (or disposable) apron etc.

Vaccination against FHV

Vaccination for FHV is important for all cats. Two or three injections are recommended in kittens, starting at around 8 weeks of age. Cats should receive a booster at a year of age, and after that should receive further booster vaccines every 1-3 years.

Vaccination does not necessarily prevent infection with FHV but will greatly reduce the severity of clinical disease. Unlike FCV, there is effectively only one strain of FHV, so vaccination is not complicated by the existence of different strains.

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