Lesson Notes By Weeks and Term v4 - SHS 3
DISEASES AND INFECTIONS
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DISEASES AND INFECTIONS
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Subject: Biology
Class: SHS 3
Term: 2nd Term
Week: 7
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 explores common diseases affecting plants and animals, which are crucial to Ghana's economy and food security. Agriculture, including cocoa farming and poultry rearing, forms the backbone of many communities. Understanding how diseases like Cocoa Black Pod or Newcastle Disease spread and how to control them is not just an academic exercise; it is essential knowledge for protecting our livelihoods, ensuring a stable food supply, and safeguarding public health. This lesson will equip you with the scientific knowledge to identify, understand, and propose solutions to these real-world challenges, following the NaCCA exemplar's focus on practical, research-based learning.
A. Foundational Terminology Disease: Any condition that impairs the normal functioning of an organism's body. It is a departure from a state of health. Pathogen: A disease-causing microorganism. Examples include viruses, bacteria, fungi, and protozoa. Infection: The invasion and multiplication of pathogens in a host's body. Host: The plant or animal in which a pathogen lives and multiplies. Vector: An organism that transmits a pathogen from one host to another but does not cause the disease itself. For example, the whitefly is a vector for the Cassava Mosaic Virus. Transmission: The process by which a pathogen is spread from an infected host to an uninfected one. This can be through air, water, direct contact, or vectors. Zoonotic Disease: A disease that can be transmitted from animals to humans (e.g., Brucellosis, Anthrax, Rabies). B. Common Plant Diseases in Ghana
We will focus on two major examples that affect our key cash and staple crops. Cocoa Black Pod Disease Causal Organism (Pathogen): A fungus-like organism (oomycete) called *Phytophthora palmivora* or *Phytophthora megakarya*. Host: Cocoa tree (*Theobroma cacao*). Symptoms: Starts as small, brownish-black spots on the cocoa pod. These spots rapidly enlarge and cover the entire pod within a few days, turning it completely black and rotten. A white mould (mycelium) may be visible on the surface of the black spots, especially in humid conditions. The beans inside the infected pod become unusable. Mode of Transmission: Rain Splash: Spores from infected pods or soil are splashed onto healthy pods by rain. Wind: Wind can carry the spores over short distances. Infected Tools: Tools like cutlasses used on infected pods can transfer spores to healthy plants. Prevention and Control Measures: Cultural Control: Frequent Harvesting: Harvest mature pods promptly to reduce the time they are exposed to infection. Sanitation: Regularly remove and destroy (by burying) all infected pods from the trees and the ground. This reduces the source of spores. Pruning: Prune the cocoa trees to allow for better air circulation, which reduces humidity and makes conditions less favourable for the fungus. Chemical Control: Fungicides: Regular application of copper-based fungicides (e.g., Bordeaux mixture) on the pods, especially during the rainy season. This is a common practice in Ghana. Cassava Mosaic Disease (CMD) Causal Organism (Pathogen): A group of viruses, specifically the *African cassava mosaic virus* (ACMV). Host: Cassava plant (*Manihot esculenta*). Symptoms: A distinct mosaic pattern on the leaves, with yellowish-green or pale patches mixed with the normal green colour. Leaves may be distorted, twisted, and smaller than normal. The entire plant can be stunted, leading to significantly reduced tuber yield. Mode of Transmission: Vector: The primary mode is through the bite of the whitefly (*Bemisia tabaci*), which feeds on an infected plant and then moves to a healthy one. Infected Cuttings: Planting infected stem cuttings is the most common way farmers spread the disease from one season to the next. The virus is present in the stem. Prevention and Control Measures: Cultural Control: Use of Clean Planting Material: Select cuttings only from healthy, disease-free cassava plants. This is the single most important control measure. Roguing: Uproot and destroy any infected plants as soon as they are identified to reduce the source of the virus for whiteflies. Biological Control: Resistant Varieties: Plant improved, disease-resistant varieties of cassava developed by institutions like the Crops Research Institute (CRI) of Ghana. Vector Control: Controlling whitefly populations can be difficult but may involve integrated pest management strategies. C. Common Animal Diseases in Ghana Newcastle Disease Causal Organism (Pathogen): A highly contagious virus called *Avian paramyxovirus type 1*. Host: Primarily affects domestic poultry (chickens, turkeys) and other birds. Symptoms: Respiratory: Gasping for air, coughing, sneezing. Nervous: Twisted neck (torticollis), paralysis of legs or wings, circling. Digestive: Greenish, watery diarrhoea. General: Depression, loss of appetite, swelling of the head and neck. Sudden death is common, with mortality rates reaching up to 100% in unvaccinated flocks. Mode of Transmission: Direct Contact: Contact with infected birds or their droppings, respiratory secretions. Indirect Contact: Contamination of feed, water, equipment, and clothing of farm workers. The virus can survive in the environment for some time. Prevention and Control Measures: Vaccination: This is the most effective method. A strict vaccination schedule (e.g., LaSota, I-2 vaccines) must be followed for chicks and adult birds. Biosecurity: Isolation: Keep new birds separate (quarantined) for at least two weeks before introducing them to the main flock. Traffic Control: Limit visitors to the farm and provide disinfectant footbaths at the entrance of poultry houses. Sanitation: Regularly clean and disinfect poultry houses and equipment. Culling: In an outbreak, immediate culling (humane killing) of the entire infected flock is often necessary to prevent further spread. Brucellosis (Contagious Abortion) Causal Organism (Pathogen): A bacterium of the genus *Brucella* (e.g., *Brucella abortus* in cattle). Host: Cattle, goats, sheep, and pigs. It is a zoonotic disease. Symptoms: In Animals: The most prominent sign is abortion or "contagious abortion," especially during the last trimester of pregnancy. Also causes retained placenta, reduced milk yield, and infertility. In males, it can cause inflammation of the testicles (orchitis). In Humans: Causes a disease called Undulant Fever, with symptoms like recurring fever, sweating, weakness, joint pain, and headaches. Humans get infected by consuming unpasteurised milk or dairy products from infected animals or through direct contact with infected animal fluids. Mode of Transmission: Contact with infected aborted foetuses, placentas, or uterine discharges. Consumption of contaminated milk or unpasteurised dairy products. Inhalation of the bacteria in contaminated environments. Prevention and Control Measures: Vaccination: Vaccinate young female calves (between 4-8 months) with an approved vaccine. Test and Slaughter: A national control program where herds are regularly tested for the disease, and any infected (positive) animals are culled to remove them from the population. Hygiene and Sanitation: Farmers and veterinarians should wear protective gloves when handling aborted materials or assisting with births. Public Health Education: Pasteurize all milk before consumption.
Guided Practice (With Solutions)
Question 1: A small-scale poultry farmer in Asamankese reports that her chickens suddenly stopped eating, developed greenish diarrhoea, and some started showing twisted necks. Within three days, over half the flock died. (a) Identify the most likely disease affecting the flock. (b) Name the type of pathogen responsible. (c) Suggest two immediate biosecurity measures the farmer should take to prevent the disease from spreading to a neighbouring farm.