2.1 Metabolic, Metazoal and Microbial Pests and Diseases

Key Knowledge: characteristics of metabolic, metazoal and microbial pests and diseases that threaten Victorian agricultural and horticultural plants and/or animals

In Victoria, our farmers and growers face constant threats from biological organisms. To manage these risks, we first have to understand what kind of enemy we are dealing with. Biologists and Agricultural scientists group these threats into three main categories: Metabolic, Metazoal, and Microbial.

Photo by Elijah Pilchard on Unsplash

1. Metabolic Diseases (Body System Failure)

Unlike a cold or the flu, metabolic diseases are not caught from another animal. They are internal breakdowns caused by an imbalance in the animal’s diet or body chemistry.

Characteristics: Usually related to a lack of specific nutrients (like minerals) or a sudden change in feed.
Example: Milk Fever in dairy cows.
- What happens: After calving, a cow's body sends so much calcium to her milk that her own blood calcium levels drop too low.
- The Impact: The cow's muscles stop working, she can't stand up, and it can be fatal if not treated quickly with calcium.

This Stock Sense Fact Sheet from the Victorian Farmer's Federation is a great resource and shows risk factors, prevention, treatment and a great explanation of what milk fever is.

PRACTICE: Using the site above and the row headings below, create a table to help you learn the symptoms, stages, prevention and treatment for this metabolic disease.

Animals affected (Cows are not the only animal that can get milk fever!)

Cause

Stage 1 symptoms

Stage 2 symptoms

Stage 3 symptoms

Prevention

Treatments for stage 1

Treatment for stage 2 and 3 (Note whether or not a particular treatment must be administered by a vet)

Note the glossary terms that have been added this week such as; down cow, bolus, condition score. They may help your understanding of this content.

2. Metazoal Pests

Metazoal refers to pests you can usually see with your eyes (or a simple magnifying glass).

Characteristics: These are complex organisms like insects, worms, or mites. They often have life cycles that involve different stages (egg, larva, adult).
Examples:
- Animal: Intestinal Worms in sheep. These live in the gut and steal nutrients, causing the sheep to lose weight and become weak.
- Plant: Aphids or Western Flower Thrips. These small insects suck the sap out of plants, causing leaves to wilt and often spreading viruses from plant to plant.

This is an aphid, do NOT get this guy mixed up with thrips!
Photo by Viktor Forgacs - click on Unsplash

NSW DPI photo: Wheat stripe rust alert 2018

Microbial Diseases

Microbial threats are caused by tiny organisms that you can only see under a microscope. These are infectious, meaning they can spread quickly from one plant or animal to another.

Characteristics: This category includes bacteria, fungi, and viruses. They reproduce very quickly and often thrive in warm, wet Victorian weather.
Victorian Examples:
- Fungal: Fungal Rusts (like Wheat Stem Rust). These look like orange "rusty" powder on leaves. The fungus eats the plant’s energy and stops it from growing grain.
- Bacterial: Footrot in sheep. Bacteria thrive in muddy paddocks, eating away at the soft tissue between the sheep's hooves, causing severe lameness.

Skill check:

PRACTICE: Work through the following case files and either complete the questions, or use your research for the questions to complete your notes.

Use the marking scheme to evaluate your work

Case File 1: The Shaking Cow

The Scene: It is early spring in the Gippsland dairy region. A high-producing Holstein cow has just had a calf within the last 24 hours. She is found lying down in the paddock, unable to stand. Her neck is tucked back against her shoulder in an 'S' shape, and her ears feel cold.

Identify the condition:
Category: (Metabolic / Metazoal / Microbial)
The fix (Action): What must the farmer inject to save the cow immediately and what precautions should the farmer take when choosing the injection site (under the skin or into a vein?)

Case File 2: The Silver Leaves

The Scene: A commercial flower grower in the Dandenong Ranges notices that her roses look "silvery" and scarred. She puts out Blue Sticky Traps and finds hundreds of tiny, slender insects with fringed wings. Some of the plants are starting to show signs of a virus.

Identify the pest:
The Monitoring Tool: Why did she use a blue trap instead of a yellow one?
IPM Strategy (Biological): What predatory insect could she buy to eat these pests?

Case File 3: The Orange Paddock

The Scene: After a week of warm, misty rain, a wheat farmer in the Mallee notices orange-brown, powdery spots appearing on the leaves of his crop. When he walks through the field, his boots turn orange.

Identify the disease:
Category: (Metabolic / Metazoal / Microbial)
Long-term Prevention: What should the farmer look for on the seed packet next year to stop this?
Resistance Level: (Low / High)

Case File 4: The "Tail-Wagging" Sheep

The Scene: It is a wet autumn. A mob of sheep is grazing on a lush green paddock. The farmer notices several sheep have "scoured" (runny manure) all over their back legs and are losing weight even though there is plenty of grass.

Identify the pest:
The Test: What is a WEC (Worm Egg Count) test used for? (Note: this is also known as a FWEC or Fecal Worm Egg Count)
IPM Strategy (Cultural): How can moving the sheep to a "clean" paddock help?

Summary:

To manage a farm effectively, you must know if a problem is caused by the environment, a bug you can see, or a germ you can’t.

1. Metabolic (The Nutrition/System Check)

What it is: A non-infectious breakdown of the body’s chemistry.
Key Example: Milk Fever (Calcium deficiency).
The Logic: You can't "catch" Milk Fever from another cow. It happens because the cow’s body can't keep up with the demand for minerals.

2. Metazoal (The Visible Attackers)

What it is: Multi-cellular parasites or insects.
Key Examples: Intestinal Worms (Sheep), Aphids, and Thrips (Crops).
The Logic: These pests have complex life cycles. To stop them, you often have to "break the cycle" (e.g., moving sheep to a different paddock so they don't eat worm larvae).

3. Microbial (The Microscopic Invaders)

What it is: Single-celled organisms like bacteria, fungi, or viruses.
Key Examples: Fungal Rust (Wheat) and Footrot (Sheep/Cattle).
The Logic: These are highly infectious and spread rapidly through wind, water, or touch. They love warm, damp Victorian weather.

Exam Tips:

Tip 1: Watch the Spelling

VCAA examiners are picky about these three "M" words. Make sure you can differentiate:

Metabolic: Think Metabolism (body chemistry/energy).
Metazoal: Think Zoo (animals/bugs).
Microbial: Think Microscope (germs).

Tip 2: Link Category to Management

Don't just know the category name, be able to explain how it changes the farmer's job:

For Metabolic, the farmer changes the Diet/Nutrition.
For Metazoal, the farmer manages the Life Cycle/Environment.
For Microbial, the farmer focuses on Biosecurity/Infection Control.

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