Diseases and Infections

What is a disease?

A disease is a condition in any organism that negatively affects the normal functioning of a living cell or the body.
Diseases also affect the homeostatic state of cells in the organism’s body.
They are accompanied by signs and symptoms.

Types of diseases

Diseases can be classified into two as;

Pathogenic disease

Pathogenic disease is a disease caused by organisms that are mostly microbes.
They are not caused by the pathogens themselves but the toxins they release following invasion of the cells.
Pathogens can enter the body of an organism through various means.
They feed on nutrients and other materials and may release toxins/poisons into the body, which often raises body temperature.
Pathogenic diseases can be transmitted (passed on) from an infected person to another person.
Pathogenic diseases are said to be infectious.

Mode of Transmission of Pathogens

This refers to how pathogens are transferred from a source (an organism) to a person (or another organism) that becomes infected.

Pathogens can be transmitted from one organism to another through;

A medium such as air, water or food,
Contact with an infected person or organism
A vector organism, such as a mosquito, housefly, tsetse fly, etc.

Infectious disease or pathogenic diseases can be classified or described according to the mode of transmission.

TRANSMISSION THROUGH A MEDIUM

AIRBORNE DISEASE

These diseases are acquired through breathing in contaminated air from infected individuals or organisms.

They include;

tuberculosis,
pneumonia,
chicken pox,
and flu

The airborne pathogens are released into the air in droplets produced through sneezing, coughing, spitting or mere talking.

They enter the body through the body orifices (mouth and nose).

Airborne diseases affect the infected person’s respiratory tract, throat, bronchi and lungs.

WATERBORNE DISEASES They are pathogenic or infectious diseases acquired through drinking contaminated water.

They include;

diarrhoea,
typhoid,
snail fever,
and cholera.

Waterborne pathogens infect the intestines and are released from the body through faeces, urine, which can contaminate water bodies.

FOODBORNE DISEASES

These diseases are acquired through eating contaminated food.

Food is contaminated through handling by an infected person and flies.

Food can also be contaminated when untreated water is used in preparing the food.

There are two types of foodborne diseases:

food intoxication
and food poisoning.

In food intoxication, the bacteria release toxins before food is ingested.

But in food poisoning, the microbe-contaminated food is ingested before the toxin is produced in the intestines.

Examples include

Salmonella infection,
coli infection,
Norovirus,
Listeria infection
Aflatoxin infection.

Foodborne diseases can be prevented through the following ways;

Safe handling of food,
Cooking food thoroughly before eating,
Refrigerating perishable foods,
Proper sewage disposal
Frequent hand hygiene practices before handling food.
Avoid eating cold food

SOILBORNE DISEASES

Soilborne diseases are caused by pathogens found in soil and can be acquired through contaminated soil.

They can infect plants, animals and humans through contact with contaminated soil.

Examples include

hookworm infection,
roundworm infection,
histoplasmosis
tetanus

Common signs and symptoms of soilborne diseases

gastrointestinal discomfort/pain,
skin problems,
fever and muscle stiffness.

Control /prevention;

Proper sanitation and waste disposal,
Wearing protective footwear in areas with contaminated soil
Vaccination

Transmission through contact

Through skin or body contact with an infected person, which includes;

kissing,
touching
sexual contact.
sharing clothing with infected persons

These diseases are also referred to as contagious diseases.

They include;

gonorrhoea,
syphilis,
leprosy,
HIV/AIDS,
Mpox

Transmission through a vector organism:

These diseases are not transmitted directly from person to person but from an infected person through a vector to a healthy person.

What is a vector?

A vector is an organism (usually an insect) that transmits/spreads pathogens from an infected to a healthy person who becomes infected and may suffer from the disease.

Example;

The female Anopheles mosquito, the tsetse fly and blackfly bite a carrier of the respective parasite and suck the pathogen into its system.
The vector organism becomes a secondary host for the pathogen (or parasite).
The pathogen-laden vector organism may bite another person, transmitting the pathogen in the process.
The housefly picks up the Vibrio cholerae mainly from faeces with its legs and body.
When it settles on uncovered food, the V. cholerae is released into the food, which becomes infected or contaminated.

EXAMPLES OF VECTOR DISEASES, THE VECTOR INVOLVED, MODE OF TRANSMISSION, CONTROL AND PREVENTION.

MALARIA;

Malaria is a disease caused by a protozoan parasite called Plasmodium.

It is transmitted through bites from an infected female Anopheles mosquito, and other species of mosquitoes are; Aedes and Culex.

The common symptoms of malaria are;

Fever
Headache
Nausea
Chills
It may lead to anaemia
Enlarge the spleen

The causative organisms are species of Plasmodium, which are;

Plasmodium falciparum
Plasmodium malariae
Plasmodium ovale,
Plasmodium vivax

The most dangerous among them is the Plasmodium falciparum.

Transmission of the malaria parasite

The female Anopheles mosquito is the vector for the transmission of the Plasmodium parasite.

The parasites reside in the salivary glands of the infected female Anopheles mosquito.

They are transmitted when the mosquito bites a person.
Through blood transfusion.
When blood infected with the parasites is transfused to another person, malaria can result.
The Plasmodium parasite can also be transmitted from an infected pregnant mother’s blood to the foetus.
This is the main cause of stillborn and underweight babies.

Explain briefly how the Plasmodium parasite causes malaria.

The Plasmodium parasite circulates in the blood.
They infect the liver cells and the red blood cells and multiply asexually by fission.
An infected cell bursts to release more Plasmodium parasites into the blood, which infect more cells.
The bursting of the red blood cells to release the parasites means the red blood cells are destroyed in the process.
The destruction of the blood cells is accompanied by an increase in body temperature and anaemia, with other signs which may signify malaria infection.

Life cycle of the Plasmodium parasite

The life cycle starts in the female Anopheles mosquito and is completed in man.

The adult or sexually mature form occurs in man.

Man is thus called the definitive host.

The developmental stages or the immature form occur in the Anopheles mosquito (the intermediate host).

The Anopheles mosquito is the vector that disperses the parasites.

The stages involved are;

The Plasmodium parasites are injected into the bloodstream as sporozoites when the female Anopheles mosquito bites a person.
The sporozoites invade the liver cells, where they mature into forms called schizonts.
A schizont undergoes asexual reproduction by repeated fission to produce forms called merozoites in the liver cells.
The infected liver cell bursts to release numerous merozoites into the bloodstream, which invade the red blood cells.
Merozoites in red blood cells form schizonts and multiply as in a liver cell.
The infected red blood cell ruptures to release more merozoites.
Some merozoites in the red blood cells, however, grow (without dividing) into gametocytes, which are the sexual forms.
The gametocyte forms will only grow in the stomach of the Anopheles mosquito, as they cannot mature in the human body.
The cycle is completed when the Anopheles mosquito bites and sucks male and female gametocytes from a malaria-infected person.
The male and female gametocytes conjugate to form a zygote.
The zygote encysts and divides to form the numerous sporozoites, which migrate to the salivary glands of the mosquito.
The sporozoites are then injected into the bloodstream when the Anopheles mosquito bites another person.

(More information)

Life cycle of the Anopheles mosquito

The life cycle of the mosquito is a complete metamorphosis.

It has four stages: egg, larva, pupa, and adult.

The Anopheles mosquito lays eggs that float just above the water surface.
The eggs develop into larvae, which lie horizontally just beneath the surface of water.
They are kept in position by the surface tension of water.
The larvae have a pair of spiracles at the surface of the water for gaseous exchange.
The spiracle leads into a breathing system called the tracheal system.
Also present are mouth brushes sunk in the water, which filter the water for microscopic organisms for food.
The larvae develop into pupae.
A pupa has a fused, rounded head, thorax, and a curved abdomen, giving it a comma-shaped structure.
The head bears siphons (also called a breathing trumpet) at the surface of the water.
The pupa develops into an adult mosquito.

(Further information)

Prevention and control of malaria

Rooms can be sprayed with insecticides to kill mosquitoes, kept clean, dry, and airy
Biological control: This is the introduction of fish like Tilapia and Gambusia into stagnant water bodies like ditches, ponds, etc, to feed on mosquito larvae.
Stationary (stagnant) water bodies should also be drained to prevent mosquitoes from laying eggs.
Containers, which can collect water, e.g., empty cans, should be buried or disposed of properly.
Grass and shrubs are also breeding places for mosquitoes and should be cut down.
Oils like kerosene can be sprayed on the surface of stagnant waters to break the surface tension of water. The larvae suffocate and die.
Irradiation of the male mosquito to induce mutation in them, i.e., to sterilize the males. Only a few can mate to reproduce. This will eventually reduce the population density of mosquitoes.
Use mosquito nets, treated mosquito nets, and mosquito repellents. These prevent the mosquito from landing on the body to bite.
Anti-malarial or prophylactic drugs can be taken regularly to prevent the occurrence of malaria.
Pregnant women are advised to sleep under treated mosquito nets and to take prophylactic drugs.
Blood for transfusion must be free of parasites.

Why is malaria prevalent in the tropics?

Mosquitoes reproduce under warm conditions.
The warm conditions needed are satisfied in the equatorial regions and the tropics.

State the economic importance of mosquitoes

Malaria is one of the major causes of death in tropical Africa
As a result, there is a reduction in the labour force and work rate.
Sickness decreases productivity, hence loss of income, as sick people cannot work.
Source of food for aquatic and some terrestrial organisms

Do mosquitoes feed on blood only?

Both male and female mosquitoes suck juices like nectar and sap from plants as their normal food.
But the female Anopheles mosquito also sucks blood to enable it lay eggs

Worm Diseases

These include onchocerciasis, elephantiasis, schistosomiasis, ascariasis, hookworm, and tapeworm infection.

River blindness/Onchocerciasis (human)

It is caused by the filarial worm Onchocerca species and transmitted by the blackfly, Simulium fly

Symptoms include;

itchy skin,
lumps or nodes at the site of the bite,
and finally, blindness.

Treatment:

Clear the bush around the banks of river bodies and treat breeding places with insecticides.

Elephantiasis (Filariasis in humans)

Another filarial worm, Wuchereria bancrofti, causes elephantiasis.
They are transmitted from person to person by mosquitoes.

Symptoms:

Abnormally large swelling of the infested parts, usually the limbs.

Rickettsial diseases (in ruminants)

Rickettsia are microorganisms formally regarded as viruses.
They are now referred to as a special type of bacteria, even though they are far smaller than bacteria.
Like viruses, they reproduce only in living cells.

As a parasite, they cause several epidemic diseases such as;

epidemic typhus (caused by Rickettsia prowazekii),
spotted fever (Rickettsia rickettsia)
and Q fever (Coxiella burnetii).

Symptoms of reckettsial disease include;

high fever,
body muscle convulsions,
rashes
and sudden death.

Tapeworms

Tapeworms are long, parasitic flatworms that live in the gut of humans.
Several species of tapeworms infect humans.

Example;

Taenia solium, the pork tapeworm,
Taenia saginata, the beef tapeworm

Structure

Each tapeworm has a knob-like head known as the scolex and a large number of segments known as proglottids arranged in a long row.
The scolex bears hooks and suckers, with which the worm clings to the wall of the host’s intestine.
This prevents the worm from being carried away with the food.
Internally, the tapeworm has become simplified as a result of adaptation to its parasitic way of life.
It has no mouth or alimentary canal.
Hence, it absorbs food digested by its host over its whole body surface.
It has no organs of respiration and circulation.
Its nervous system consists only of a ring of nervous material in the head, joined to a pair of longitudinal nerve cords which pass down the whole length of the body.
Its excretory system consists of a pair of tubes, the excretory ducts, which also pass down the whole length of the body.
Each proglottis, when mature, has both male and female reproductive parts, i.e., testes and ovaries in it.
The tapeworm is, therefore, a hermaphrodite.

Adaptation of the tapeworm to its mode of life.

Tapeworms exhibit adaptations to their parasitic lifestyle.
This is seen in the loss or poor development of unwanted organs such as the digestive system, sense organs, and muscles;
Long, flattened body that provides a large surface area for the absorption of nutrients.
Presence of hooks and suckers for attachment;
Ability to live in the absence of oxygen (anaerobic conditions);
The ability to produce large numbers of eggs and their hermaphroditic nature.

Life cycle

The life cycle of the tapeworm involves two hosts: a human being, regarded as the primary host, and another animal, such as a cow or pig, which is known as the secondary host.
Tapeworm has a flat, long, tape-like or ribbon-like body.
It is a hermaphrodite (bears male and female sex organs).
Adult tapeworms live in the intestinal tract of adult vertebrates.
Its head is attached to the host intestine with suckers and hooks.
Mature proglottids contain ripe fertilised eggs, which break off and are passed out with faeces of the primary host.
Pig or cow is the secondary host that feeds on the faeces of man or foliage contaminated with infested human faeces.
The eggs hatch into hexacanth embryos with six hooks.
The eggs are protected by an oncosphere.
The hexacanth embryo bores through the alimentary canal and moves into the bloodstream, from where it moves into the muscles of the cow or pig to develop into a bladder worm, which is dormant
If the infected pork or beef is eaten by a human, the head is pushed out of the bladder and hooks itself to the intestinal wall of the human, where it grows into an adult

(Further information https://m.youtube.com/watch?v=NYeIoqLy55E&pp=ygUYTGlmZSBjeWNsZSBvZiBUYXBld29ybXMg0gcJCbAJAYcqIYzv and https://m.youtube.com/watch?v=57T2eoPAwTs&pp=ygUYTGlmZSBjeWNsZSBvZiBUYXBld29ybXMg0gcJCbAJAYcqIYzv)

Economic importance

Tapeworms do not cause any severe disease in humans, but because they feed on the digested food present in the intestines, they draw away much of the nutritive substances and cause anaemia.
If the anaemia is severe, the infected person will be susceptible to many other diseases as his resistance will be poor.
Tapeworms will also irritate the walls of the intestine, which results in pain and physical damage.
Tapeworm infection may give rise to more severe symptoms, which include indigestion, vomiting, loss of appetite, and nervous disorders.

Non-pathogenic diseases / Non-infectious diseases

Diseases that are not caused by pathogens are described as non-pathogenic diseases.

They are broadly caused by:

Lack of one or more nutrients in the diet, i.e., poor nutrition
Genetic disorder.
Stress condition
Cancer

A disease caused by a lack of one or more nutrients in one’s diet is called nutritional deficiency disease. Examples of nutritional deficiency diseases are:

Anaemia, caused by a deficiency of iron in the diet.
Goitre, caused by a lack of iodine in the diet.
Scurvy, caused by a lack of vitamin C.
Osteoporosis, caused deficiency of calcium.
Night-blindness, caused by a lack of vitamin A.

A genetic disease is a disease caused by changes in the chemical structure of the DNA or inheritance of defective DNA.

A parent may transfer a defective gene to an offspring (the child), who inherits it.

The offspring may then express the disease caused by the defective gene.

Examples of genetic diseases are;

Haemophilia,
Red-green colour blindness
And sickle cell anaemia.
Some cancers, such as breast cancer, can run through a family.

Stress condition is a condition that occurs during feelings of anxiety as a result of having too many tasks to perform at a set standard and deadline.

The anxious moments put the victim under pressure/tension and may lead to some stress conditions.

Some of the diseases arising from stress conditions are high blood pressure (hypertension) and a heart attack.

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