dengue fever

introduction

flaviviral mosquito-borne (Aedes albopictus and A. aegypti) illness of the tropics including northern Queensland
these mosquitoes usually live between the latitudes of 35° North and 35° South below an elevation of 1,000 metres (although in Mexico and Sth American tropics have been found up 2200m but the mosquitoes are rare higher than 1700m) where average winter isotherms are above 10degC
50–100 million people infected yearly globally and this is increasing
virus has four different types
infection with one type usually gives lifelong immunity to that type, but only short-term immunity to the others.
there is no vaccine or treatment but most resolve spontaneously
dengue can also be transmitted via infected blood products and through organ donation
mortality of symptomatic cases overall is 1-6% untreated, and < 1% if treated

Take care to avoid mosquito bites at all times. Be extra cautious during early mornings and late afternoons. This is when mosquitoes are most active.
Avoid areas with a lot of mosquitos. For example, near stagnant water.
Wear long, loose, light-coloured clothing and closed shoes.
Use insect repellent. If you use both sunscreen and repellent, apply the sunscreen first.
Sleep under a bed net treated with insecticide.
Keep doors and windows closed if they don't have insect screens.
Ask for hotel rooms to be sprayed if you notice mosquitos inside.
second infection can be prevented with Dengvaxia® vaccine
- however it is only recommended to receive Dengvaxia® if certain clinical conditions are met and the benefit outweighs any risks.

short incubation period 4-7 days
80% are asymptomatic
children may have a URTI-like illness or a gastro-like illness but then are more at risk of complications
fever often over 40degC, classically biphasic but is variable
headache - typically retro-orbital
muscle and joint pains hence its other name “break-bone fever”
characteristic maculopapular rash in 50-80% of those with symptoms
- initially just flushed skin appearance on day 1-2
- then on days 4-7 similar to measles rash +/- petechiae +/- mild bleeding from mouth/nose
leukopenia
thrombocytopenia
metabolic acidosis

severe disease is more common in babies and young children, and in contrast to many other infections it is more common in children that are relatively well nourished
can be life-threatening in people with chronic diseases such as diabetes and asthma.
G6PD deficiency
certain gene polymorphisms involving genes coding for TNFα, mannan-binding lectin, CTLA4, TGFβ, DC-SIGN, and particular forms of human leukocyte antigen.
subsequent infection with a different type increases the risk of severe complications
a study on blood tests on day one of when children presented with dengue appears to show that those who go on to develop severe disease had dysfunctional immune cells:¹⁾
- antigen presenting cells failed to take up dengue virus particles, cut them up into smaller pieces, and show these virus fragments to the rest of the body
  - whilst uptake of virus was intact, there was impaired interferon response and antigen processing and presentation signatures
- B cells became heavily infected with dengue virus and had impaired ability to produce antibodies
- natural killer cells, and T cells, seemed to be in a strange state of quiescence or exhaustion

occurs in 5% of symptomatic patients and follows resolution of high fever and typically lasts 1-2 days
severe disease is marked by two problems:
- dysfunction of endothelium
- disordered blood clotting
dengue shock syndrome and dengue haemorrhagic fever
severe bleeding from GIT
pleural effusions and ascites may develop from capillary leakage
organ failure and death in up to 26% who develop severe disease
this is followed by a recovery phase lasting 2-3 days:
- fluid is resorbed
- striking recovery
- severe pruritis
- slow heart rate
- risk of fluid overload, cerebral oedema with seizures and reduced conscious state

clinical
tourniquet test - count number of petechiae distally after 5 minutes application of BP cuff
PCR from blood
serology

supportive care only
watch for severe depression in the recovery phase
avoid invasive procedures, aspirin (acetylsalicylic acid) and non-steroidal anti-inflammatory drugs (NSAIDs) as bleeding risk

¹⁾