Module 107. Plant, Animal and Human Diseases

Diseases are disorders or malfunctions that affect the normal functioning of living organisms. They may be caused by infectious agents such as bacteria, viruses, fungi, and parasites, or by non-infectious factors including genetic defects, nutritional imbalances, and environmental conditions. The study of diseases in plants, animals, and humans is central to biology and medicine, as it helps in understanding pathology, prevention, and control measures essential for health and survival across ecosystems.

Classification of Diseases

Diseases can broadly be divided into infectious and non-infectious categories:

• Infectious diseases: Caused by pathogens (microorganisms such as bacteria, viruses, fungi, or protozoa) that can spread from one host to another.
• Non-infectious diseases: Arise from genetic, physiological, environmental, or lifestyle factors and are generally not transmissible.

In both plants and animals, diseases disrupt metabolic functions and may lead to reduced growth, productivity, or death if untreated.

Plant Diseases

Plant diseases significantly affect agricultural productivity and global food security. They may result from pathogenic or environmental causes.
1. Bacterial Diseases: Caused by bacterial pathogens that invade plant tissues through wounds or natural openings.

• Bacterial blight of rice (Xanthomonas oryzae): Leads to leaf wilting and reduced yield.
• Citrus canker (Xanthomonas citri): Produces lesions on leaves, stems, and fruits, making them unmarketable.

2. Fungal Diseases: Fungi are among the most common plant pathogens. They reproduce via spores that spread through air, water, or soil.

• Late blight of potato (Phytophthora infestans): Causes leaf spots and tuber rot, leading to major crop losses.
• Rust of wheat (Puccinia graminis tritici): Affects cereal crops, producing rust-coloured pustules on leaves and stems.
• Powdery mildew (Erysiphe graminis): Forms white powdery coatings on plant surfaces.

3. Viral Diseases: Transmitted mainly through insect vectors such as aphids and whiteflies.

• Tobacco mosaic virus (TMV): Causes mottling and curling of tobacco leaves.
• Banana bunchy top virus (BBTV): Results in stunted growth and clustered leaf formation.

4. Parasitic Plant Diseases: Caused by parasitic plants that extract nutrients from host plants.

• Cuscuta (dodder): A parasitic vine that affects crops like clover and lucerne.

Control Measures:

• Use of disease-resistant plant varieties.
• Crop rotation and proper field sanitation.
• Application of fungicides, bactericides, and biological control agents.
• Vector management to prevent viral transmission.

Animal Diseases

Animals, both domestic and wild, are susceptible to numerous infectious and non-infectious diseases that impact health, productivity, and food safety.
1. Bacterial Diseases:

• Anthrax (Bacillus anthracis): Affects cattle, sheep, and goats; characterised by sudden death and bleeding from natural openings.
• Brucellosis (Brucella abortus): Causes abortions in livestock and can infect humans (zoonosis).
• Bovine tuberculosis (Mycobacterium bovis): Leads to respiratory infection and weight loss in cattle.

2. Viral Diseases:

• Foot-and-mouth disease (FMD): A highly contagious viral disease of cloven-hoofed animals causing blisters in the mouth and feet.
• Rabies: Caused by the Rabies virus; affects the nervous system and is transmissible to humans via animal bites.
• Rinderpest (Cattle plague): A viral disease eradicated globally by vaccination campaigns.

3. Parasitic Diseases:

• Trypanosomiasis (Sleeping sickness in animals): Transmitted by tsetse flies, affecting cattle in tropical regions.
• Tick fever: Caused by protozoans such as Babesia and transmitted through tick bites.
• Helminthic infestations: Parasitic worms (roundworms, tapeworms, flukes) cause malnutrition and anaemia.

4. Fungal and Other Conditions:

• Ringworm: A contagious fungal infection affecting skin and hair in livestock.
• Nutritional deficiencies and metabolic disorders (e.g., milk fever in cows due to calcium deficiency) are examples of non-infectious diseases.

Prevention and Control:

• Vaccination programmes for major infectious diseases.
• Quarantine and culling of infected animals.
• Vector control using acaricides and insecticides.
• Proper nutrition and hygienic housing.

Human Diseases

Humans are affected by a wide range of diseases caused by various biological agents and physiological imbalances.
1. Bacterial Diseases:

• Tuberculosis (TB): Caused by Mycobacterium tuberculosis, primarily affecting the lungs.
• Cholera: Caused by Vibrio cholerae, leading to severe diarrhoea and dehydration.
• Typhoid fever: Caused by Salmonella typhi, transmitted through contaminated food and water.
• Pneumonia: Caused by Streptococcus pneumoniae; affects the respiratory system.

2. Viral Diseases:

• Influenza: Caused by the influenza virus; highly contagious respiratory illness.
• COVID-19: Caused by the novel coronavirus (SARS-CoV-2), leading to global pandemic impact since 2019.
• Hepatitis: Inflammation of the liver caused by various viruses (A, B, C, D, E).
• HIV/AIDS: Caused by the Human Immunodeficiency Virus (HIV); weakens the immune system.
• Dengue and Zika: Mosquito-borne viral diseases common in tropical regions.

3. Protozoan Diseases:

• Malaria: Caused by Plasmodium species and transmitted by Anopheles mosquitoes.
• Amoebiasis: Caused by Entamoeba histolytica, leading to intestinal infection.
• Sleeping sickness: Caused by Trypanosoma brucei, transmitted by tsetse flies.

4. Helminthic Diseases:

• Ascariasis: Roundworm infection of the intestines.
• Filariasis: Caused by Wuchereria bancrofti; results in swelling of limbs (elephantiasis).

5. Fungal Diseases:

• Candidiasis: Infection by Candida albicans, affecting mucous membranes.
• Ringworm and athlete’s foot: Caused by dermatophytic fungi.

6. Non-infectious Human Diseases:

• Genetic disorders: Such as Down syndrome, haemophilia, and sickle-cell anaemia.
• Nutritional deficiencies: Including scurvy (vitamin C deficiency), rickets (vitamin D deficiency), and anaemia (iron deficiency).
• Metabolic disorders: Such as diabetes mellitus and gout.
• Cardiovascular diseases: Including hypertension, coronary artery disease, and stroke.
• Cancer: Uncontrolled cell division forming malignant tumours.

Preventive Measures:

• Maintenance of hygiene and sanitation.
• Immunisation and vaccination.
• Safe food and water supply.
• Early diagnosis and medical intervention.
• Health education and awareness programmes.

Zoonotic and Emerging Diseases

Zoonotic diseases are those transmitted between animals and humans. Examples include rabies, avian influenza, brucellosis, and COVID-19 (believed to have originated from animal reservoirs). With increased human–animal interaction and globalisation, emerging and re-emerging diseases have become major public health concerns.
The role of One Health, an integrated approach linking human, animal, and environmental health, is increasingly recognised as vital in managing such diseases.

Significance of Disease Study

Understanding plant, animal, and human diseases is essential for improving health, food production, and ecological balance. Research in pathology, microbiology, and epidemiology enables the development of vaccines, antibiotics, and genetic resistance in crops and livestock. The global coordination of disease surveillance, vaccination campaigns, and sustainable health practices ensures long-term biological stability and welfare for all forms of life.