Explained: Major Vector-Borne Diseases, Symptoms & History in Numbers

While Kerala is witnessing a third outbreak of the Nipah virus, Uttar Pradesh is in the grip of a mysterious dengue-like fever that has killed at least 100 people, mostly children, in the past few weeks.

Mumbai too is seeing a rise in dengue cases from last year: 129 in 2020 to 138 in the past eight months of 2021. When it comes to malaria, 3,338 cases were reported in the financial capital till August 29 this year. As fresh cases of dengue and malaria rise amidst the COVID-19 pandemic, here is a look at major mosquito-borne diseases in numbers.

What are vector-borne diseases?

Vector-borne diseases are human illnesses caused by parasites, viruses and bacteria that are transmitted by vectors, according to the World Health Organization. These diseases account for more than 17% of all infectious diseases, causing over seven lakh deaths across the world annually.

Many of these vectors are bloodsucking insects, which take in disease-producing microorganisms during a blood meal from an infected host (human or animal) and later transmit it into a new host. When a vector becomes infectious, they are capable of transmitting the pathogen for the rest of their life during each subsequent bite/blood meal.

Mosquitos are only one type of vectors, who cause diseases such as Malaria, Dengue Chikungunya, etc. Other vector borne diseases are caused by vectors such as aquatic snails (Schistosomiasis), blackflies (river blindness), fleas (plague), lice (typhus), sandflies (Sandfly fever), ticks (Crimean-Congo haemorrhagic fever), Triatomine bugs (Chagas disease) and Tsetse flies (sleeping sickness).

Which mosquito causes which disease?

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The Aedes mosquito

There are various species in the aedes mosquito genus (group) but the primary vector is the Aedes aegypti. This species causes diseases such as dengue, chikungunya, Lymphatic filariasis, Rift Valley fever, Yellow fever and Zika. The Aedes aegypti can be identified by white markings on its legs and silver pattern of scales on its body. These mosquitoes mainly live in tropical subtropical and temperate climates, according to Centers for Disease Control and Prevention.

Dengue is the most prevalent disease spread by the Aedes mosquito. More than 3.9 billion people in over 129 countries are at risk of contracting dengue, with an estimated 96 million symptomatic cases and an estimated 40,000 deaths every year, according to the WHO. By June 30, 2021, India had reported 8,973 cases and four deaths caused by the virus, said Union Health Minister Mansukh Mandaviya in a response in the Lok Sabha on July 30, 2021. Tamil Nadu with 2062 cases, reports the highest number of dengue cases in India. This is followed by Kerala (1,596), Karnataka (1,203) and Maharashtra (936).

While Maharashtra reported two deaths, Kerala and Gujarat reported one death each. As reported by the states and Union territories, 99.82% people recovered from dengue in 2020, Mandaviya added. About one in four people infected with dengue fall ill, the CDC says.

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The most common symptom of dengue is fever with nausea, vomiting, rashes and aches and pains usually behind the eyes, muscles and joints. While symptoms of dengue typically last between two to seven days, most people recover after a week. While there is no specific treatment for dengue, pain relievers such as acetaminophen or paracetamol are generally prescribed to help with the recovery. The WHO specifically says to avoid medicines such as ibuprofen and aspirin because these are anti-inflammatory drugs that act by thinning the blood, and in a disease with risk of haemorrhage, blood thinners may exacerbate the prognosis.

The most common symptoms for Chikungunya are fever, often accompanied by joint pain. Other symptoms include muscle pain, headache, nausea, fatigue, and rash. Severe joint pain usually lasts a few days but can persist for months or even years, according to the CDC.

In India, 15,875 clinically suspected cases were reported till June 30, 2021. Karnataka (6,928) reported the most cases, followed by Gujarat (2,521), Puducherry (1,864) and Maharashtra (1,554), Mandaviya said in the LS response. There is currently no vaccine or specific drug against the virus. The treatment is focused on relieving the disease symptoms, says the WHO.

Rift Valley Fever is a viral disease usually seen in domesticated animals such as cattle, buffalo, sheep, goats, and camels. RVF is transmitted through an infected aedes mosquito and also through contact with blood, body fluids, or tissues of infected animals. Most people with RVF have either no symptoms or a mild illness with fever, weakness, back pain, and dizziness. Around 8%-10% develop severe symptoms such as eye disease, haemorrhage (excessive bleeding), and encephalitis (swelling of the brain).

In India, RVF virus in sheep and goats was first reported in Rajasthan in 1995. This was followed by an outbreak among sheep from Tamil Nadu. There is no specific treatment for this disease as well and mild symptoms are treated with standard over-the-counter medications.

The "yellow" in yellow fever refers to jaundice that affects people. It is an acute viral haemorrhagic disease transmitted by an aedes mosquito and is mostly found in sub-Saharan Africa (countries to the south of the Sahara Desert), South America and in parts of the Caribbean. Typical symptoms include high fever, chills, nausea, vomiting, loss of appetite, headache and muscle pain.

The second stage is more severe and is referred to as the "toxic phase" of which symptoms include a yellow tinge to the skin and whites of the eyes caused by liver damage (jaundice), recurrent fever and kidney failure, according to the National Health Portal of India. Yellow fever can be prevented by vaccinations. A single dose provides lifelong protection for most people. Since the vaccination provides immunity only after ten days, it is recommended that people wait for two weeks before traveling to an endemic area.

When it comes to the Zika virus, symptoms are mainly mild. Fever, conjunctivitis, muscle and joint pain, malaise and headache are some of the main symptoms. A pregnant woman who is infected with the virus can pass it to the foetus. This can lead to microcephaly and other congenital malformations during infancy and it is known as congenital Zika syndrome. Since there is no specific treatment available for Zika, those infected with the virus are advised to take plenty of rest, drink fluids and treat pain and fever with pain relievers.

Anopheles mosquito

The adult Anopheles mosquito is dark brown to black in colour. While resting, the body of the female anopheles mosquito points upwards and is not parallel to the surface like most mosquitoes. The lifespan of the female Anopheles mosquito ranges for a few weeks to a month. During this time, she is known to produce thousands of eggs.

Malaria is the most common disease caused by this mosquito. In 2015, the Indian government developed The National Framework for Malaria Elimination (NFME) with the aim to eliminate the disease by 2030. In 2019, there were an estimated 229 million cases of malaria, and over four lakh deaths, worldwide, according to the WHO. Children under 5 years are the most vulnerable group affected by malaria, accounting for 67% of all malaria deaths. In the past eight months, 28,495 cases and 18 deaths were recorded in India alone, according to a Lok Sabha response. Malaria is mainly present in the eastern and central part of India. With 10,347 cases, Chhattisgarh has the highest number of malaria cases as of July 30, 2021. This is followed by Odisha with 6,260 cases and Jharkhand with 3,436 cases. Chhattisgarh (14) saw the highest number of deaths caused by malaria, followed by Maharashtra and Odisha with two deaths each.

"In 2020, 99.97% of malaria cases were reported to have recovered after treatment. The number of Malaria cases and deaths has been constantly declining in the country. There is 84.4% reduction in malaria cases and 83.6% reduction in malaria deaths in 2020 as compared to 2015," read the LS response.

The most common symptoms are fever, headache and chills. If not treated within 24 hours, it can progress to severe illness, often leading to death. But the disease can be treated with a combination of two or more drugs. Chloroquine phosphate is the preferred treatment for this parasite.

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Culex mosquito

Culex mosquitoes are grey in colour with white, silver or green scales and are most active during summer. West Nile Virus and Japanese encephalitis are the most common diseases caused by the Culex mosquito.

According to the World Health Organization, West Nile Virus is a member of the flavivirus genus and belongs to the Japanese encephalitis antigenic complex of the family Flaviviridae, FactChecker had explained this on September 1. While the virus can cause a fatal neurological disease in humans, about 80% of those infected do not show any symptoms. Recently, Russian authorities had alerted its public of a possible rise in WNV infections this autumn.

Japanese encephalitis is an extremely rare but severe disease found in Asia and the West Pacific with an estimated 68,000 clinical cases per year, worldwide, as per the WHO. It causes inflammation and swelling of the brain. The case-fatality rate among those with encephalitis can be as high as 30%. Permanent neurologic or psychiatric sequelae can occur in 30%-50% of those with JE. Although there is no cure for the disease, safe and effective vaccines are available for prevention.

Exception

Lymphatic filariasis (LF), commonly known as elephantiasis, is a parasitic neglected tropical disease caused by roundworms and transmitted by mosquitoes. It is the only disease where the vector can either be an Aedes, Anopheles or Culex mosquito. It is asymptomatic, acute and causes chronic conditions in the lymphatic system and kidneys, altering the body's immunity.

The disease generally disfigures the legs, arms, breasts and genitalia and is often very painful. Mosquito-transmitted larvae are deposited on the skin from where they can enter the body leading to chronic conditions such as lymphoedema (tissue swelling and elephantiasis (skin/tissue thickening).

While the infection may be acquired during childhood, its visible manifestations such as retention of fluid in limbs may occur later in life, causing temporary or permanent disability, according to the WHO.

In 1997, the health body and its member states made a commitment to eliminate LF as a public health problem by 2020. Further, The National Health Policy in 2002 had set a goal of Elimination of Lymphatic Filariasis in India by 2015. But it was extended to 2021.

In 2019, 859 million people in 50 countries worldwide were threatened by lymphatic filariasis and needed preventive chemotherapy to stop the spread of this parasitic infection. In

India, 256 districts in 21 states and UTs are endemic to LF. This disease can be eliminated by stopping the spread of infection through preventive chemotherapy.

For the elimination of LF, Mass Drug Administration (MDA) is provided to the endemic districts in 21 states. MDA was done with two drugs earlier but now a combination of three drugs has been introduced in selected districts, according to a 2019 Lok Sabha response.