Explainers

What is Dengue? – Mosquito Transmitted Diseases


Dengue is the most important viral disease transmitted by mosquitoes afflicting humans in a world context, with up to four hundred million new cases every year.

Dengue became nationally notifiable in Australia in 1991. From then until 2009, there were typically less than one hundred notifications a year, mainly imported from Papua New Guinea. Since then, there has been an upward trend to as many as twelve hundred imported cases a year, mainly in travellers to Indonesia.

North Queensland has also experienced a number of outbreaks in which dengue was brought in by a traveller and spread locally, resulting in more than one thousand cases in 2008-2009 and some two hundred in 2013. These outbreaks were contained, but constant surveillance is needed.

Causes and Transmittal

Dengue is caused by the dengue virus, of which there are at least four genotypes. Infection gives lifelong immunity to the infecting type but not to any of the others; infection with any of which may lead to more severe disease.

Humans are infected mainly through a bite from the Aedes aegypti mosquito. This mosquito feeds almost exclusively on humans, breeds in small man-made articles that contain water, rests inside buildings and is rarely found more than fifty metres from human habitation. In Australia, it is now found only in Queensland, but was previously found in New South Wales, Northern Territory and Western Australia. As well, Aedes scutellaris is known to carry the disease in Papua New Guinea and occurs in north Queensland. Aedes albopictus is also a threat to Australia, being an important vector in Asia and having been introduced from there to several countries in motor vehicle tyres and other containers.

The mosquito acquires the virus in its blood meal from an infected person, however the virus requires eight to twelve days development inside the mosquito before it can infect another human. The mosquito will carry it until it dies.

Dengue is also occasionally transmitted through blood transfusions, tissue or organ transplants, by needlestick injuries and by mucous membrane contact with blood infected with dengue virus.

Dengue may vary from mild or even unnoticed (more often in children), to fatal. Typically, three to fifteen days after a mosquito bite, the patient will have a sudden onset of fever, extreme tiredness and severe muscle pain. Most will have pain in ankle, knee and elbow joints, which can be so intense that dengue has also been called ‘break bone fever’. Intense headache is also usual, with pain behind the eyes in about three quarters of cases. About the same proportion experience nausea. Loss of appetite, vomiting, diarrhoea, abdominal pain, metallic taste and flushed skin on face and neck are also common. A fine skin rash develops on the arms and legs in about a third of patients as fever subsides, sometimes with itching and peeling of skin. Sometimes, there is hair loss.

What is Dengue?

Control of dengue is mainly by mosquito control by getting rid of breeding sites

Diagnosis

Depending on the clinical presentation and patient history, dengue may need to be differentiated from chikungunya, hantavirus, measles, rubella, enteroviruses, influenza, hepatitis A, meningococcal disease, scarlet fever, typhoid, leptospirosis, rickettsioses or malaria. Definitive diagnosis is by a variety of blood tests. In Australia, this will probably involve the use of commercial antibody kits; however, these are not specific and need to be confirmed by nucleic acid testing or a specific antigen test.

Treatment

Treatment is rest, plenty of fluids and paracetamol for pain (not aspirin or non-steroidal anti-inflammatory medicines (NSAIDs), which increase the risk of bleeding). Dengue usually resolves in one to two weeks, but lethargy and depression may persist for weeks or months.

However, especially in babies and young children, dengue may progress to dengue haemorrhagic fever. Some three to seven days after the first symptoms, there is a decrease in fever but onset of severe abdominal pain accompanied by: persistent vomiting with blood in the vomit, rapid breathing, bleeding into the skin (producing purple bruising) and from nose and gums, fatigue and restlessness. There may be liver and heart problems. Loss of plasma causes a concentration of haemoglobin in the blood, while deficiency of thrombocytes promotes bleeding.

In extreme cases, this may progress to dengue shock syndrome, in which severe bleeding leads to circulatory failure and hypotension progressing to profound shock (no measurable blood pressure), agitation, coma and death.

In these cases, rapid volume replacement through intravenous electrolyte solutions, plasma or plasma expanders lowers mortality from ten to twenty per cent, to about three per cent.

Control of dengue is mainly by mosquito control by getting rid of breeding sites (any open container that holds water around homes and workplaces) and by surface spray insecticides in homes. Personal protection is also important, especially when travelling to endemic areas. A number of live vaccines are currently being tested. A novel approach being tried is to infect mosquitoes with Wolbachia bacteria, which prevents the virus infecting the mosquito.

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