Yellow fever

Vector: mosquitoes

Organism: virus, Flavivirus

Yellow fever is a haemorrhagic fever that originated in the central belt of Africa but spread to South America in the 17th century with the slave trade. It is endemic in 34 countries in Africa where the majority of the estimated 30,000 deaths from 200,000 infections a year occur. It has three patterns of transmission:

urban, transmitted by A. aegypti, which breeds in fresh water in the human environment such as tin cans, water drums, gutters, etc. This is the main mode of infection in Africa;
forest pattern, transmitted by mainly A. africanus, A. bromeliae, where monkeys are the primary host. This is the main pattern in South America; and
an intermediate or ‘savannah’ pattern, which has become more prominent in Africa recently.
Symptoms show 3-6 days after infection, including fever, headache, backache, general muscle pain, nausea, and vomiting. About 15% of cases develop into an acute, toxic phase with jaundice due to liver damage and haemorrhagic signs — bleeding in the mouth, eyes, nose, and gastrointestinal tract giving ‘black vomit’. This stage is fatal for 20% or more of patients.

A safe vaccine is available and gives protection for at least 10 years.

Lyme disease

Vector: deer tick

Organism: bacteria, Borrelia spp, Borrelia burgdorferi sensu lato

Lyme disease is the most common tick-borne disease in the northern hemisphere, maintained by many species of mammal that the ticks feed on becoming reservoirs of the disease. Infections are reported to be 7.9 cases per 100,000 population in the USA (WHO) and there are over 6,000 cases reported in England and Wales annually.

Symptoms of Lyme disease include: a pink or red rash which in most cases has a central red spot and outer red circle — like a ‘bull’s eye’; temperature of 38°C/100.4°F or above; flu like symptoms such as headache and joint pain; swollen lymph nodes. It can be treated with antibiotics, but if untreated, symptoms can last for years, including arthritis, numbness, paralysis and around 10% of people infected develop central nervous system disease.

Zika virus

Vector: mosquitoes, Aedes spp.

Organism: virus: Zika virus

The Zika virus is a Flavivirus closely related to Yellow Fever and West Nile disease. It was first isolated from a monkey in Uganda in 1947 in the Zika forest and described in 1952. The first known human infection was in 1954 and since then it has been rare until the last few years. It was reported to occur on a small scale in African and tropical Asian countries.

Recently, however, it has spread to SE Asia and through Polynesia (2012-14), then to Central and South America and the Caribbean (2015), causing a large outbreak in French Polynesia in 2013 and Brazil in 2015. Researchers in Brazil suspected that the outbreak was linked to the large number of visitors during the World Cup in 2014, according to

In December 2015, the Pan American Health Organization (WHO) reported cases in Colombia, El Salvador, Guatemala, Mexico, Paraguay, Suriname, Venezuela, Easter Island (Chile) and Puerto Rico. Confirmed cases in 2015 were also reported in Thailand, Maldives, Cape Verde Islands and Pacific Islands by the European Centre for Disease Prevention and Control (ECDC). Between January 2015 and February 2016, 34 new territories and countries in the Pacific, Central and South America and the Caribbean reported cases of the Zika virus, according to WHO.


The symptoms are usually a mild fever, skin rash, joint pain and conjunctivitis, and can include muscle pain and headache. These are similar to dengue and Chikungunya, which are also transmitted by Aedes mosquitoes, so could be confused with them. Only one in five infected people develop symptoms and no fatalities have been recorded, so the majority of people infected will not realise they have the virus.


An increase in cases of microcephaly in new-born babies has been recorded in Brazil, leading to a suspected association to the virus. It is also suspected of causing Guillain-Barré syndrome and other neurological complications. The US CDC issued a warning to pregnant women in January 2016 to postpone travelling to 13 countries in South and Central America and the Caribbean to avoid the risk to babies. This is updated as more countries report outbreaks of the virus.

On 1 Feb 2016 the World Health Organization declared Zika a Public Health Emergency of International Concern (PHEIC) and recommended countries take “aggressive measures” to reduce infection with the Zika virus especially among pregnant women and women of child bearing age.


The virus is spread through bites of infected Aedes aegypti and Aedes albopictus mosquitoes, the same types of mosquito that transmit dengue, chikunguya and yellow fever. It therefore has the potential to spread anywhere that these mosquitoes are present. A. aegypti occurs in tropical and subtropical climates, while A. albopictus (tiger mosquito) can survive in cooler climates.

The mosquitoes bite in the daytime, unlike most mosquito species, so it is important to take precautions when going out in infected areas during the daytime. Only female mosquitoes bite, and prefer to feed off more than one person, after which they rest for 3 days before laying eggs. The eggs can survive up to one year without water and need only a small quantity of standing water to develop into larvae and mature into adults. The female adults then catch the virus by feeding on infected people.

Person-to-person transmission of the virus has been recorded through sexual contact and from mother to foetus, crossing the placental membrane. It is suspected that infection by a mother in early pregnancy increases the risk of microcephaly and brain damage in the child.

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