Unistat : herpes virus
http://www.abc.net.au/health/library/herpes_ff.htm
Herpes Simplex Virus Fact File
The two strains of the herpes simplex virus cause both cold sores and genital herpes. Both cause a lifelong infection.
Background
Most people have heard of Shakespeare's 'Romeo and Juliet', and the majority know that it is a tragedy based on a love story. But you'd be forgiven if you missed its subtle reference to one of mankind's most common infections:
O'er ladies ' lips, who straight on kisses dream,
Which oft the angry Mab with blisters plagues,
Because their breaths with sweetmeats tainted are:
Act 1. Scene IV
The blisters that Shakespeare refers to are in fact cold sores produced by the herpes simplex virus (HSV).
There are two types of the virus. Type 1 herpes (HSV-1) is carried by over 80 per cent of the population and is the culprit behind recurrent cold sores. Most people pick it up in the first few years of life, usually in the form of a loving kiss from a parent or sibling.
HSV-2 affects five to 20 per cent of the population (including many individuals who are also infected with HSV-1). It is more often associated with genital herpes, although either virus can cause a similar disease at both anatomical sites.
Genital herpes is one of the commonest sexually transmitted infections, and the prevalence is increasing. The number of cases caused by HSV-1 has grown due to changing sexual practices, including oral sex.
Part of the reason that herpes infections are so common – and so easy to transmit – is that up to 30 per cent of infected people suffer no symptoms but periodically shed the virus in their saliva or genital secretions (depending on the site of infection).
The herpes virus itself is so small that 100 million of them could fit on a pinhead. Each consists of a core, which contains the viral DNA, wrapped in a protein-studded coat known as the envelope. The proteins are the viral equivalent of velcro and help HSV to lock on to, and invade, its target cells.
Shedding and recurrent lesions (sores) occur because once a person is infected with herpes, they carry the virus for life. A clever strategy, called latency, enables the virus to escape the immune system by hiding inside nerve cells. Then, in about 15 per cent of people carrying it, the virus periodically reawakens producing painful, infectious sores on the affected part of the body.
Infection
People often don't know that they have been infected with HSV because they don't always develop the telltale cold sore. Instead, most first cases of herpes affecting the face appear as a nasty sore throat, a sore mouth (which can occasionally ulcerate), swollen neck glands, and a temperature.
Initial infection in the genital region usually produces a painful, red, ulcerating crop of lesions that can spread over a wide area and may involve the perineum and anus. Genital infection can also be associated with temporary numbness in the affected area, swollen glands in the groin, difficulty passing urine, and a temperature. Occasionally, primary infections such as these can also trigger viral meningitis.
After the initial infection, HSV lies dormant in the body, but can be reawakened by various stimuli. These include stress, being run down, menstruation, use of drugs that suppress the immune system and skin damage from heat, sun or chemicals.
How often the virus reactivates varies from one person to the next. But, as a general rule, recurrences tend to occur most often during the year following infection, and then they tail off.
The virus escapes the usual immune response by penetrating nerve fibres. As the immune system moves in to control the infection, the virus conceals itself within nerve cells and slips away from the scene by hitching a ride on a special transport system that nerves use to move materials from one end to the other.
In this way the virus is carried to the nerve 'cell body' in a swelling called a ganglion located close to the spinal cord. When it reaches the cell body, the viral DNA is added alongside the nerve cell's own DNA in the nucleus. It remains there, hidden within the nerve cell and in an inactive state, for the lifetime of the infected individual.
Once reawakened, the virus reproduces itself using the viral DNA hidden in the nerve cell. These newly assembled viral particles are then shipped back down the nerve fibre to the region of the skin that it supplies – for instance around the lips. The virus 'buds off' from the nerve ending and infects the surrounding skin cells, producing a painful cluster of pale blisters that are crammed with herpes simplex virus and are highly infectious.
In rare cases the herpes simplex viruses can also cause more serious infections. These include HSV meningitis, HSV encephalitis (HSV infection of the brain), neonatal herpes (HSV infection in the newborn acquired from the mother around the time of birth), and corneal ulceration, scarring and visual impairment following eye infection. HSV can also cause more severe generalised infections amongst those with weakened immunity. (See Severe infections below.)
Symptoms
Before the blisters appear, most people notice a mild tingling sensation on the patch of skin supplied by the affected nerve cell. Soon after the blisters form they break open, releasing their infectious cargo and leaving a red raw patch – the cold sore – which measures about half a centimetre across and takes seven to 10 days to heal.
Cold sores and genital lesions remain infectious until they have crusted over, so contact should be avoided during this time because the virus can be transmitted to other parts of the body, and to other people, especially between the mouth and the genitals.
Cold sores and genital lesions also create a breach in the skin's natural defences, creating portals for other infections, like HIV. So extra care should be taken to minimise any exposure.
Diagnosis
Diagnosis for herpes affecting the face is usually made solely on the basis of the symptoms. But in the case of suspected genital herpes, HSV affecting the eyes, or more severe skin outbreaks, it can be useful to confirm the diagnosis by laboratory tests.
Traditionally, tests involve taking a swab from an active lesion, growing the virus in the laboratory, and using colour-coded antibodies to pinpoint whether HSV-1 or HSV-2 is the culprit. This method can be useful in confirming the presence of asymptomatic shedding of the virus.
Another way, which is faster but yields less information, is to use an electron microscope to look for viral particles in fluid collected from the blisters.
More recently, many laboratories have moved to using a highly accurate DNA test that can rapidly pick up the virus in a sample, and at the same time tell whether it is HSV-1 or HSV-2. This approach is particularly useful in the diagnosis of HSV meningitis or encephalitis caused by herpes infection. In these instances a sample of cerebrospinal fluid (CSF), the watery substance that bathes the brain and spinal cord, is collected by lumbar puncture for analysis.
Doctors also sometimes take a blood sample, particularly in people with a history of possible herpes infection but no active lesions, to look for herpes antibodies. This can confirm whether a patient has been infected with herpes previously, and which type of virus they are carrying.
There are currently no treatments capable of removing herpes virus DNA from the nerve cells that carry it. Treatment focuses on reducing the intensity of a primary infection, and the frequency and severity of subsequent viral reactivations.
Cold sores
If you are prone to cold sores, take the following precautions:
- use high-factor sun creams to avoid sunburn
- try to avoid becoming run down
- eat a healthy diet, including plenty of fresh fruit and vegetables.
Aciclovir cream (brand name Zovirax) should be applied to the skin as soon as the telltale symptoms heralding a cold sore appear (that is, localised pain, or a tingling sensation on the lip). This cream is extremely safe (including during pregnancy) and can reduce the duration and severity of a recurrence.
Genital herpes
To avoid infection with HSV in the first place, take the following measures:
- always use barrier methods of contraception (condoms give the best protection)
- if you have a partner with known genital herpes, avoid intercourse whenever they experience a recurrence
- avoid oral sex when you, or your partner, have a cold sore – the virus is readily transmitted from the mouth to the genital area, and vice versa.
Like cold sores, genital lesions remain infectious until they have crusted over.
Genital infection with HSV also places partners and babies at risk, meaning that appropriate counseling and contact tracing play an important part in managing this condition.
For this reason, patients with genital herpes are often referred to a genitourinary medicine clinic. It's also important to exclude other infections. For example, chlamydia is very common, highly infectious, often symptom-free, and if left untreated can lead to infertility.
Initial infection with genital herpes can be extremely painful and very distressing, but prompt presentation to a doctor can help through the administration of antiviral drugs, and pain relief.
The following measures will reduce pain and discomfort:
- soaking the affected area in salt water
- simple painkillers such as paracetamol
- anaesthetics applied to the skin should only be used sparingly to avoid desensitisation.
The drugs aciclovir, valaciclovir and famciclovir have all been shown to be effective at reducing the severity and duration of a genital infection. Ideally they should be introduced within five days of the start of the episode and continued for at least five days, or while new lesions are forming, whichever is the longer.
In general, oral medications (those taken by mouth) are extremely well tolerated, have few side effects, and are more effective for genital herpes than topical medications (those applied to the skin). Trials have also shown that combining oral and topical medications is no better than oral medication alone.
Lesions that do not respond to medication might be due to drug-resistant forms of herpes. Under these circumstances it may be necessary to switch to another class of antiviral drugs, and to collect samples of the virus for drug susceptibility testing.
For patients with infrequent herpes reactivations the condition can be controlled effectively by using oral antiviral medications, for example aciclovir, whenever they experience a recurrence. When used in this way, known as episodic therapy, antivirals have been shown to reduce the duration of symptoms by one to two days as well as the severity of the outbreak.
Patients with more frequent recurrences (six or more per year) often benefit from 'suppressive therapy'. This involves taking antiviral medications every day to prevent the virus from reactivating. Again, aciclovir, valaciclovir, and famciclovir have all been shown to be effective at preventing recurrences, but this course of action must be balanced against the inconvenience and costs of taking regular medication.
Most doctors advise stopping suppressive therapy after a year in order to re-assess the activity of the disease, and to reduce the risk of developing viral resistance to the antiviral drugs.
Severe infections
Patients with HIV, or other immune-disabling conditions such as organ-transplant recipients, are at increased risk of developing severe (and sometimes life-threatening) infections. They should receive prompt antiviral therapy, which should be continued until fresh lesions have stopped appearing and the existing lesions have crusted over.
Neonatal herpes: Genital herpes can be dangerous for the newborn, but the risks can be minimised by careful medical management. All women who develop new genital herpes during pregnancy should be referred to the genitourinary medicine clinic. Aciclovir has been used extensively during pregnancy, and in over 20 years of use there have never been any reports of foetal toxicity or birth defects.
All women with first-episode genital herpes lesions at the time of delivery are advised to deliver by caesarean section because the risk of transmitting the infection to the newborn under these circumstances is 40 per cent.
However, caesarean is not recommended for women who contract HSV during the first or second trimesters, or for women with a past history of genital herpes but without any signs of recurrence. The local infection will have cleared by the time of delivery and protective antibodies will have been produced against the virus.
These antibodies will be passed to the developing baby before it is born, greatly reducing the risk of transmission. The baby should, however, be monitored closely when it is first born for any signs that it may have picked up the infection, in which case treatment with intravenous aciclovir should be started immediately.
Eye infections: Herpes infection involving the eye usually presents as a single, sore, red eye which is extremely sensitive to the light. Often the virus attacks the cornea, which can lead to corneal scarring, opacity, and blindness. It should be treated aggressively with pain relief and aciclovir.
HSV meningitis: HSV can occasionally trigger aseptic (viral) meningitis. Patients usually complain of a severe headache, neck stiffness, nausea, fever, and a dislike of bright lights. Unlike meningitis caused by a bacterial infection there is usually no skin rash. Anyone with these symptoms should see a doctor urgently. Cases of HSV meningitis usually resolve rapidly with intravenous aciclovir therapy, without long-term consequences.
HSV encephalitis: A very rare manifestation of HSV is encephalitis in which the brain tissue itself becomes infected by the virus. There are very few warning signs, but patients with encephalitis tend to become confused and drowsy. Without prompt treatment with intravenous antivirals (usually aciclovir) the condition is often fatal, and even when treated rapidly often causes long-term neurological problems including memory loss and epilepsy.
Scientists are currently trying to identify the chemical signals responsible for reactivating the latent (inactive) viral DNA and producing recurrent disease. This would provide scientists and drug manufacturers with new targets for antiviral drugs capable of preventing reactivation, shedding and transmission of herpes infections.
But with no solution presently on the horizon, we will just have to cross our fingers, and possibly our legs, and hope that the answer is not too far away.