How Contagious is Mono

How contagious is mono you ask?  The short answer is only moderately contagious. Unlike many common illnesses infectious mononucleosis (mono) is spread by ingesting the causative Epstein Barr Virus (EBV) shed in another person’s saliva, hence the “kissing disease” nickname. Other than kissing you can spread mono by sharing eating or drinking utensils or by touching something contaminated by mono and then putting your hand in your mouth. Also limiting spread is the fact that many infants and young children contact mono as an asymptomatic minor illness and so already have immunity. This lends a degree of “herd immunity” to the population and holds down rapid spread of the disease.

The Epstein-Barr Virus is a member of the Herpes family of viruses. The typical presentation of mono is in a teenager who develops a severe sore throat and fever. Keep in mind that the usual office blood test for mono, the “mono spot” test, is often negative until up to 10 days of symptoms. The presentation can be clinically indistinguishable from strep throat initially, and up to 50% of patients with mono have positive throat cultures or rapid strep tests. Pair this with the 20-30% false negative rates of the office rapid strep tests, and the initial diagnosis is often not made until a second or third office visit when it becomes apparent that the child is not responding to treatment for strep throat and the mono test becomes positive. If treated with amoxicillin a patient with mono will often develop a reddish macular rash all over, and this can be mistaken as a penicillin allergy; see amoxicillin side effects for a nice resource. Still the classic thick white exudate on the tonsils of a child or teen with mono can be very suspicious to an experienced examiner, and if paired with really big 3-5 cm anterior or posterior cervical lymph nodes or an enlarged spleen we can often suspect the diagnosis on the initial visit even if it’s too early to confirm the diagnosis with lab tests.

Because of the wide variety of symptoms that mono can cause, and the wide variety of severity of illness, it is the disease that comes to my mind first when I hear the old med school adage, “Uncommon manifestations of common problems are much more common than common manifestations of uncommon problems.” Despite its peak incidence in children and young adults, mono is seen at all ages. Presentation in young children is often quite subtle, and not diagnosed. In older adults it often presents with sore throat, enlarged tonsils with a thick white exudate, and swollen cervical nodes like in teens, but can also present with florid hepatitis, simple fatigue, or abdominal pain from hepatosplenomegaly. Rare presentations include aseptic meningitis and leukocytosis found on a routine CBC.

One of the frustrating aspects of mono is the long duration of fatigue some patients get. This can be very debilitating, and mono is one of the few diagnoses where rest is especially important. Listening to your body and not trying to push yourself to your limit and allow the symptoms to resolve more quickly than if you try to push yourself to exhaustion every day. Half days of school, with naps and focus on the key subjects can allow students to stay on course throughout the course of the illness. Antiviral medications are not helpful in mono. In severe acute presentations a short course of prednisone can quickly shrink the tonsils and allow the patient to eat and drink enough to stay hydrated and nourished. Fever can persist for several weeks in some cases.  (See prednisone side effects)

The relationship between EBV and chronic fatigue syndrome is not clear, and although I believe chronic fatigue syndrome is a real condition, I don’t believe it has been shown to be related to EBV. Some recent research suggests a virus called XMRV as a cause of chronic fatigue syndrome, although this is very controversial. Most patients with EBV can get back to school or work within a few weeks. Management of athletes in contact sports has been better defined in recent years. Most sports medicine experts allow participation in contact sports after the fatigue has resolved if absence of hepatosplenomegaly can be documented with ultrasound. In past years we insisted on at least 8 weeks time after diagnosis prior to contact sports. The newer approach seems more appropriate, and allows both individualization of advice, and gives confidence to the physician who does need to keep a young athlete out of competition.

For a graphic look at a goofy teen with mono and a flashlight see this video. Note the very white exudate on the very large tonsils. Thanks kid.

12 Responses to How Contagious is Mono

  1. Madeline: I know of no data on sexual transmission of EBV the mono virus. It is generally felt that after a few weeks it is no longer transmitted by saliva. DrP.

  2. Embarrassing question, but any idea how long until I can safely have sex without infecting my husband? Some sites say this sticks around for a year, even more ….!

  3. I can’t help but trying too figure out how or when I contacted this’s so frustrating, it was also difficult for me to except all the side effects, because I live my life with most of these symptoms.
    The fatigue is tough for me to except.

  4. Lori O: Mono is felt to be only contagious thru direct oral to oral contact, or from sharing drinking or eating utinsils. We allow kids to go to school as soon as they feel well enough. Have her avoid kissing and take some care with sharing drinks etc and she should be fine. DrP.

  5. My 20 daughter has recently been diagnosed with mono but is showing very little signs of illness. We are planning to travel to see family for Christmas. I have an elderly mother and very young nephews, one (7 months old) just hospitalized with a severe respiratory virus. Is it safe for all involved for us to still make the trip? I’m worried about one of them catching it from her.

  6. If I broke up with my boyfriend about four weeks ago and he just is now showing symptoms- how likely is it that I have also contracted mono?

  7. Jan: I don’t know what you mean by the Epstein Barr stage. Epstein Barr is just the name of the virus that causes mono. The exact incubation period of mono is not well understood, but is probably 1-4 weeks. How long is mono contagious? Again poorly understood, likely from prior to symptoms for a few weeks. Sorry no more specific help. DrP.

  8. My wife has mono and is in the Epstein Barr stage. I have never had mono and had a test last week and show no symptoms. What is the incubation period and how long should we stay apart? They think she contracted Mono 6-8 weeks ago and showed no true symptoms until fever one week ago. She was tired and had stomach and sleep problem.s

  9. The bummer is, Michael, that with the incubation period of the virus being several weeks before you even knew you were sick, your girlfriend has already been exposed 🙁

  10. I have mono (for the 3rd time) and I was wondering how long I should stay out of work? I work as a sign language interpreter in a college but my situation is different from the normal interpreter. I work with a deaf-blind student so I tactile interpret (the student’s hand are on top of mine while I interpret into American Sign Language what is being said).

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