In fact the present Zika outbreak came on the back of two other viral epidemics, that of dengue and chikungunya. During the early phase of the Zika outbreak, before the devastating effect of the virus on foetal brains was realised, the three diseases were circulating in the Pacific islands at the same time, spread by the same mosquito: the Aedes mosquito.
Of course the illness chiefly associated with mosquitoes is malaria. This worldwide killer continues to infect millions of individuals each year, resulting in hundreds of thousands of fatalities despite there being both effective prophylaxis and treatment available. There are many other current diseases able to be transmitted by biting insects, however. This month’s Healix World Report highlights lesser-known vector-borne diseases active around the globe at present. It also introduces a novel life-threatening epidemic recently started in South Sudan, the causative agent of which is presently unknown.
Yellow fever, presently resurging in Angola and spreading to countries as distant as China, has infected 2,000 people in the last 4 months, killing 325. Initially producing the flu-like symptoms common to many viral illnesses, yellow fever infection can cause a patient to develop uncontrolled bleeding, or haemorrhage. One of the reasons for this is explained by the virus’s particular predilection for the liver. As clotting factors are made in the liver, their production can be disrupted when the liver is infected by the yellow fever virus.
The liver is also important with the removal of the highly coloured products that result when old red blood cells are broken-up and removed from the circulation: when liver function is disturbed, these pigments build-up in the body, resulting in jaundice and giving the virus its name of yellow fever.
At least there is an effective vaccine for yellow fever - although there are serious global production issues at present severely restricting its availability. Other more recherché viruses spread by biting insects, such as the oropouche virus, are less studied and no vaccine has as yet been developed.
The disease due to the oropouche virus, discovered in 1960, is transmitted by culicoides paraensis. As this illness is strictly speaking spread by a midge rather than a mosquito, oropouche disproves the mantra that midges don’t spread diseases.
There is currently an outbreak of oropouche virus in Peru, the majority of cases occurring in towns in the northern part of the famous Cusco region – 57 people have been infected so far. The particular importance of the disease is that it is difficult to differentiate its non-specific ‘flu-like’ symptoms from those of other vector-borne viruses such as dengue, yellow fever, chikungunya or Zika. As all of these diseases are present in South America at present, the WHO is now urging physicians to consider the possibility of an oropouche infection when a patient presents with fever, headache, muscle ache and joint pain. Oropouche fever is not yet thought to be associated with a common serious complication, so the consequences of missing this diagnosis are few. This may not be the case if a yellow fever infection is missed, however, or if the patient is pregnant and actually has the Zika virus. The consequences in these circumstances can be devastating for the patient, or their unborn child.
Although being responsible for one of the largest epidemics of a mosquito-borne disease ever recorded, the illness O’nyong-nyong is equally unknown as the oropouche virus.
First discovered in 1959 at the Ugandan Virus Research Institute, the same facility that first discovered the Zika virus, O’nyong-nyong infected over 2 million people in East Africa alone between 1959 and 1962. A further outbreak occurred in Uganda in 1996. It is genetically similar to the more prevalent chikungunya virus, although spread by a different species of mosquito. The viruses’ similarity goes further, however.
Both viruses cause a disease which is noted for a terrible joint pain that gives rise to both their names: O’nyong-nyong is from the Nilotic language of the Acholi people from Uganda and simply means ‘severe joint pain’, whereas chikungunya from the kimakonde language of Tanzania references how a sufferer with joint pain might look: it translates as ‘to become contorted’.
Few further cases of O’nyong-nyong have been diagnosed, until recently. In South Sudan an outbreak of an undiagnosed fever has infected 51 people in the last few months, killing 10. The characteristic features of this infection are uncontrolled bleeding (haemorrhage), fever, headache and vomiting. The novel disease is therefore being described as a ‘haemorrhagic fever syndrome’ – but the causative virus is unknown. Blood samples of infected patients have been sent to reference laboratories in several countries, including the Ugandan Virus Research Institute, where they found that 5 samples were positive for O’nyong-nyong, 3 for chikungunya and 1 for dengue fever. The rest of the samples had no known virus isolated.
Much is not clear at present.
The severity of the Sudanese cases seems to not fit easily with cases previously described for both O’nyong-nyong and chikungunya. These may have been co-incidental infections, with an as yet unknown organism being responsible for the severe infection. As this disease is so new, it is also not known how the illness is transmitted. If the unknown haemorrhagic virus turns out to be spread by mosquitoes, like O’nyong-nyong, chikungunya and dengue, then many more cases are to be expected.
What is clear is that O’nyong-nyong, chikungunya, yellow fever and of course malaria, are all currently circulating in Africa. Many predict that it is only a matter of time before Zika will be added to their number. For those working and travelling in tropical countries the need to follow mosquito bite prevention measures has never been greater. For those living in affected countries, mosquito eradication methods are urgently required. Such expensive measures are unlikely to be implemented, however, and less likely to be applied regularly in order to prevent mosquito populations from resurging and spreading disease anew.