Animals can be hazardous to human health. When asked to name the most dangerous animal, many people will give the wrong answer. Sharks have captured the attention of the public, and other animals that kill people may come to mind, like black widow spiders, rabid dogs, and venomous snakes. And don’t forget the human animal: gun control is a hot topic because when humans murder other humans with guns it gets a lot of press. We are subject to the availability heuristic: if we can think of examples or have recently read a news story about it, we tend to overestimate its prevalence. An article on BBC Focus magazine reports the actual number of people killed by the top ten animals that kill people. Sharks didn’t even make the list. The actual number of deaths recorded for each deadly animal around the world may surprise you. Here they are:
- Lions kill 200 humans each year
9. Hippos kill 500
8. Elephants: 600
7. Crocodiles: 1000
6. Scorpions: 3300
5. Assassin bugs (by transmitting Chagas disease): 10,000
Note: Chagas disease occurs only in the Americas. It is particularly icky: the so-called “kissing bug” poops while feeding. The poop gets rubbed into wounds and mucous membranes, and the disease can damage the heart muscle leading to death decades later. It has been suggested that Chagas disease contracted during the voyage of the Beagle might have contributed to Charles Darwin’s lifelong ill health and to his death.
4. Dogs (especially those that carry rabies): 59,000
3. Snakes: 138,000
2. Other humans (only counting homicides): 400,000
1. Mosquitos: 725,000
Note: seven of these kill directly, and 3 kill indirectly by infecting the human with a virus or a parasite. Either way, you’re just as dead.
Other animals can also be done in by mosquitos; your pet dog may succumb to the deadly heartworm disease, where masses of worms up to 12 inches long accumulate inside the heart. Heartworm is only transmitted by mosquitos. The disease is treatable but sometimes dogs die even with treatment. It’s better to prevent it. Protect your puppy from those deadly mosquitos!
Mosquitos can transmit several viral and parasitic diseases to humans. Aedes mosquitos transmit Chikungunya, dengue, lymphatic filariasis, Rift Valley fever, yellow fever, and Zika. Culex mosquitos transmit Japanese encephalitis, lymphatic filariasis, and West Nile fever. Anopheles mosquitos transmit lymphatic filariasis and malaria.
Dogs really do bite people, and we talk of mosquito “bites,” but that’s inaccurate. Mosquitos don’t exactly bite, at least not in the way that dogs do. Dogs have teeth; mosquitos don’t. The male mosquito is harmless. The female mosquito inserts her proboscis through the skin and uses it to suck up blood and inject saliva. The itchy bump that annoys us is our body’s reaction to her saliva. Some people are more attractive to mosquitos than others. I used to come home from family camping trips with 20-30 itchy bumps, while other family members had only a few or none at all. You might say I was a sucker for blood-suckers; I just thought “That sucks.” I didn’t appreciate being “in good taste.”
The most serious disease transmitted by mosquitos is malaria, but it’s not automatic. To get malaria from an encounter with the deadly mosquito,
- The mosquito must be female
- She must belong to the genus Anopheles. Culex and Aedes species do not transmit malaria, although they transmit a number of other diseases.
- She must be one of the 30-40 species of Anopheles mosquitos (out of a total of 340) capable of transmitting the disease.
- She must have previously sucked blood from an infected human or animal.
- She must inject viable malaria organisms through her proboscis.
The Center for Disease Control (CDC) has a handy map showing areas of the world where malaria is endemic.If there is no malaria in the area, there is no risk.
Malaria is one of the world’s most serious public health problems The World Health Organization (WHO) tells us 409,000 people died of malaria in 2019, 67% of them children under the age of 5. Most cases and deaths (94%) were in Africa. Babies and pregnant women are particularly susceptible. In World War II, 500,00 American troops were infected in the South Pacific and 60,000 died.
Malaria parasites are protozoa belonging to the Plasmodium genus. At least four species are known to infect humans: P. falciparum, P. vivax, P. ovale, and P. malariae. The parasite’s life cycle is complicated and requires two hosts. The terminology can be confusing. When a mosquito takes a blood meal from an infected host, it ingests male and female gametocytes. These mate in the mosquito’s gut, producing sporozoites. After 10-18 days the sporozoites migrate to the mosquito’s salivary glands and can be transferred to a human host. In the human, the sporozoites mature and multiply in the liver, producing schizonts. When schizonts rupture, they produce merozoites which then infect red blood cells. There they develop into ring-stage trophozoites which develop into schizonts, which can rupture and release merozoites.
After an incubation period of 7-30 days, the infected human develops symptoms such as fever, chills, sweats, headache, nausea and vomiting, body aches, and general malaise. Attacks of fever can be tertian (recurring every third day) or quartan (recurring every fourth day). Or the parasite may enter a dormant stage, unexpectedly relapsing many years later. In severe cases, every part of human physiology may be affected. Cerebral malaria can cause abnormal behavior, impairment of consciousness, seizures, coma, and neurologic abnormalities. Destruction of red blood cells can cause serious anemia. The lungs, kidneys, blood pressure, and coagulation system can be affected. The patient may develop jaundice. In rare cases, the spleen can rupture. There may be long-term consequences, especially in children, including deafness, blindness, and palsies.
Periodic fevers have been recognized throughout recorded history and the parasites that cause malaria have been detected in fossils millions of years old. The name malaria comes from the Italian words for “bad air.” In the pre-scientific era before germ theory, malaria was believed to be caused by fumes emanating from swamps. It came from the swamps, all right, but it was brought by mosquitos that lived there, not by fumes. Malaria may have contributed to the fall of the Roman empire.
Tertian and quartan fevers were recognized long before their cause was known. Spanish Jesuit missionaries in South America learned of an effective treatment, cinchona bark, from the Quechuas and introduced it to Europe in the early 1600s. It worked for malarial fevers because it contained the effective anti-malarial drug quinine. Fevers were indistinguishable from each other in the pre-scientific era, before the discovery of microorganisms and the germ theory of disease, so quinine was used to treat all fevers. Sometimes it seemed to work.
An interesting evolutionary dilemma
An inherited enzyme deficiency, G6PD deficiency, affects 600,000 people worldwide. It can be asymptomatic or can cause anemia and death. It killed 33,000 people in 2015. Hemolytic crises are triggered by several factors including illnesses and certain medications such as antimalarial drugs. A common trigger is eating fava beans, which are a common dietary component in some areas of the world and can cause favism. Favism can be triggered by just walking through a field where the plant is in flower. Since G6PD deficiency tends to impair survival, one might wonder why evolution hasn’t been able to eliminate it. The answer: it offers a survival advantage by reducing the risk of malaria and sickle cell disease.
Drugs to treat malaria
Quinine was the first successful use of a chemical compound to treat an infectious disease. Up until World War II, quinine was the only effective drug for malaria, but it did not prevent relapses and could not be synthesized in commercial quantities. Synthetic derivatives were developed. Chloroquine was more effective than quinine but did not prevent relapses. Primaquine produced complete cures without relapses. As resistance to newer anti-malarial drugs developed, quinine resurfaced in some parts of the world as the drug of choice, despite its risk of toxic side effects, which can include hearing loss and visual disturbances.
Malaria itself was used as a treatment for tertiary syphilis in the early 20th century before the germ theory of disease was universally accepted and before antibiotics were available.”
Patients were intentionally infected with malaria; the resulting fever killed the heat-sensitive spirochetes, and then quinine could be used to cure the malaria. Unfortunately, malariotherapy killed 15% of those treated. More recently, Henry Heimlich advocated malariotherapy as a cure for AIDS. Some small studies in China found that it reduced HIV viral loads, but it can’t be recommended.
A Chinese scientist, Youyou Tu, won a Nobel prize in 2015 for her work on artemisin, a derivative of wormwood. It proved very effective against malaria but resistance to the drug has developed, and it doesn’t prevent relapses, so it is commonly combined with other drugs.
It’s always preferable to prevent a disease in the first place than to play catch-up by treating it after it develops. Persons travelling to areas where malaria is endemic are advised to take a prophylactic drug before, during, and after travel. The specific drugs recommended vary by country. Drugs are not 100% effective and must be combined with personal protective measures such as long sleeves, insect repellants, and bed nets. For people living in endemic areas, mosquito control measures can reduce the risk. Indoor spraying, eliminating standing water that harbors mosquito larvae, and housing modifications can be helpful. Insecticide-impregnated bed nets are estimated to have saved the lives of 250,000 infants in Sub-Saharan Africa between 2000 and 2008. Bed nets are inexpensive ($5 each) and are provided free by several organizations but they have only reached a minority of those at risk. 33% of households owned bed nets in 2008.
A small phase 1 trial of a monoclonal antibody was promising, apparently preventing malaria in a few subjects who were exposed to a controlled infection after subcutaneous or intravenous treatment with the antibodies. It’s too early to know how effective it will be; further studies will be needed.
A vaccine for malaria has finally been developed after a decades-long effort. The RTS,S vaccine (trade name Mosquirix) was recommended for children in sub-Saharan Africa by the World Health Organization (WHO) in October, 2021. A real breakthrough, it is the first vaccine ever for a parasitic disease. Unfortunately, it is only modestly effective, cutting the risk of infection by 40% and the risk of severe infection by 30%. And getting it to those who most need it will be problematic; it requires a series of four injections for infants who live in an area with poor access to medical care.
Counterintuitively, the most deadly animal is the smallest: mosquitos that transmit malaria. Despite their bad press, sharks kill only 10 people a year. Those annoying little mosquitos do far more damage to humans than big scary sharks.
This article was originally published in Skeptical Inquirer.