Lesson Notes By Weeks and Term v4 - SHS 3
DISEASES AND INFECTIONS
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DISEASES AND INFECTIONS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Biology
Class: SHS 3
Term: 2nd Term
Week: 9
Grade code: 3.3.3.LI.2
Strand code: 3
Sub-strand code: 3
Content standard code: 3.3.3.CS.1
Indicator code: 3.3.3.LI.2
Theme: DIVERSITY OF LIVING THINGS AND THEIR ENVIRONMENT
Subtheme: DISEASES AND INFECTIONS
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This lesson focuses on common and economically important diseases affecting plants and animals, with a special emphasis on the Ghanaian context. Agriculture is the backbone of Ghana's economy, and many of our families depend on farming cocoa, maize, cassava, or raising livestock like poultry and cattle. When diseases strike these plants and animals, it affects not only the farmer's income but also our nation's food security and public health. By understanding these diseases—what causes them, how they spread, and how to control them—we become better future scientists, farmers, veterinarians, and informed citizens who can contribute to a healthier and more prosperous Ghana.
Before we dive into specific diseases, let's clarify some fundamental terms. Disease: Any condition that impairs the normal functioning of an organism's body. Pathogen: A disease-causing microorganism, such as a virus, bacterium, fungus, or protozoan. Host: The plant or animal that is infected by a pathogen. Vector: An organism (usually an insect or arthropod) that transmits a pathogen from one host to another but does not cause the disease itself. For example, a mosquito is a vector for malaria. Transmission: The way a pathogen spreads from one host to another. This can be through direct contact, air, water, food, or vectors. Symptom: A physical or functional sign of a disease. For example, a yellowing leaf or a coughing animal. Zoonotic Disease: A disease that can be transmitted from animals to humans. A. Common Diseases of Plants in Ghana
We will focus on three diseases that have a significant impact on Ghanaian agriculture. Cocoa Swollen Shoot Disease (CSSVD) Causative Agent: A virus, specifically the *Cacao swollen shoot virus* (CSSV). Mode of Transmission: The virus is transmitted by vectors, primarily small insects called mealybugs. These mealybugs feed on the sap of an infected cocoa tree and then move to a healthy tree, carrying the virus with them. Key Symptoms: Swellings on the shoots and roots of the cocoa plant. Red vein-banding on young leaves (the veins turn red). Chlorosis (yellowing) of the leaves. Reduced pod size and eventual death of the tree within 2-5 years. Economic/Health Effects: CSSVD is a major threat to Ghana's cocoa industry, our most important cash crop. It has destroyed millions of cocoa trees, leading to massive financial losses for farmers and the country. Prevention & Control Measures: Eradication (Cutting Out): The most effective method is to identify and cut down infected trees and any surrounding trees that may be infected. This is a policy enforced by COCOBOD. Vector Control: Using insecticides to control the mealybug population, though this can be expensive and have environmental impacts. Development of Resistant Varieties: Scientists at the Cocoa Research Institute of Ghana (CRIG) are working to breed cocoa varieties that are resistant or tolerant to the virus. Sanitation: Keeping farms clear of weeds and alternative host plants for the mealybugs. Cassava Mosaic Disease (CMD) Causative Agent: A virus, the *African cassava mosaic virus* (ACMV). Mode of Transmission: Vector: Primarily spread by the whitefly (*Bemisia tabaci*). Infected Cuttings: Planting stem cuttings from an infected cassava plant will result in a new, diseased plant. This is the most common way farmers spread the disease unknowingly. Key Symptoms: A distinct "mosaic" pattern on the leaves, with patches of yellow or pale green mixed with normal green. Distorted and stunted leaf growth. Overall stunting of the plant, leading to significantly reduced tuber (cassava root) yield. Economic/Health Effects: Cassava is a staple food for millions of Ghanaians. CMD can reduce yields by up to 90%, threatening food security and income for small-scale farmers. Prevention & Control Measures: Use of Clean Planting Material: The single most important control measure is to plant cuttings only from healthy, disease-free plants. Resistant Varieties: Planting cassava varieties that have been bred for resistance to CMD. Sanitation (Roguing): Regularly inspecting the farm and removing (roguing) and destroying any plants that show symptoms of the disease. Maize Streak Virus (MSV) Causative Agent: A virus, the *Maize streak virus*. Mode of Transmission: Spread by a vector, the leafhopper insect (*Cicadulina* species). The leafhopper feeds on an infected plant and transmits the virus to healthy plants as it continues to feed. Key Symptoms: Long, narrow, broken yellow or whiteish streaks along the veins of the leaves. Stunted growth of the plant. Poor or no cob development, leading to massive yield loss. Economic/Health Effects: Maize is another key staple food in Ghana. MSV can cause complete crop failure, especially if infection occurs at an early stage of growth. This impacts both food availability and the poultry industry, which relies on maize for feed. Prevention & Control Measures: Planting Resistant Varieties: The most effective and sustainable method. Early Planting: Planting maize at the very beginning of the rainy season can help the crop to be well-established before the leafhopper population builds up. Vector Control: Using appropriate insecticides to control leafhoppers, especially during the early growth stages. B. Common Diseases of Animals in Ghana
Let's examine two diseases that affect livestock farming. Newcastle Disease (in Poultry) Causative Agent: A virus, the *Avian paramyxovirus 1*. Mode of Transmission: Highly contagious. Direct Contact: Contact with infected birds' droppings, nasal discharge, or breath. Indirect Contact: Contaminated feed, water, equipment, and clothing of farm workers. It can also be airborne over short distances. Key Symptoms: Respiratory: Gasping for air, coughing, sneezing. Nervous: Twisted neck (torticollis), paralysis of wings and legs, tremors. Digestive: Greenish, watery diarrhoea. Sudden drop in egg production in laying hens. High mortality rate, often reaching 90-100% in an unvaccinated flock. Economic/Health Effects: Newcastle disease can wipe out an entire poultry farm in days, causing devastating economic loss to farmers. This affects the supply of chicken and eggs in our markets. It is not considered a major threat to human health. Prevention & Control Measures: Vaccination: This is the cornerstone of prevention. A strict vaccination schedule (e.g., Lasota vaccine in drinking water) must be followed. Biosecurity: Implementing strict hygiene measures on the farm. This includes: Restricting visitor access. Using footbaths with disinfectant at the entrance of poultry houses. Quarantining new birds before introducing them to the flock. Proper disposal of dead birds and manure. Brucellosis (in Cattle, Goats, and Sheep) Causative Agent: Bacteria of the genus *Brucella* (e.g., *Brucella abortus* in cattle). Mode of Transmission: Contact with infected birthing materials (placenta, aborted foetuses, vaginal fluids). Ingestion of contaminated feed or water. Through milk from an infected animal. Key Symptoms: The most prominent sign is "abortion storms," where many pregnant animals in a herd lose their babies (abort), typically in the last trimester. Reduced milk production. Inflammation of the testes in males (orchitis). Infertility. Economic/Health Effects: Brucellosis causes significant economic losses through loss of calves and reduced milk yields. Crucially, it is a zoonotic disease. Humans can get infected (called Undulant Fever) by consuming unpasteurised milk or dairy products, or through direct contact with infected animals (a risk for farmers and veterinarians). Symptoms in humans include fever, sweats, and muscle pain. Prevention & Control Measures: Test and Slaughter: Regular testing of herds and culling (removing and slaughtering) of infected animals. Vaccination: Vaccinating young female animals (calves) can provide long-term protection. Hygiene: Proper disposal of afterbirth and aborted foetuses. Farmers should wear protective clothing (gloves) when assisting with births. Public Health: Pasteurising milk before consumption to kill the bacteria.
Guided Practice (With Solutions)