Health, parasites, pathogens & immunity

📚 Revision notes · External & internal parasites, pathogens, the immune system, vaccinations and health red-flags

These notes pull together the core disease-and-health topics you need for Level 2–3 animal care. Skim the headed sections, learn the key terms, then test yourself with the self-check questions at the end. Nothing here replaces a vet — it is study material.

1. External parasites (ectoparasites)

An ectoparasite is a parasite that lives on the outside of its host. They affect both warm-blooded (mammals, birds) and cold-blooded (reptiles, fish) animals. A useful link to remember: cats and dogs with fleas are also at risk of tapeworm, because fleas can carry tapeworm eggs and the animal swallows an infected flea while grooming.

Removing a tick: grasp the tick by the head, as close to the skin as possible, with fine tweezers (or a tick hook) and pull steadily until it lets go. Do not twist or squeeze the body — that can leave mouthparts behind or push contents back into the wound.

Common ectoparasites

• Fleas — tiny wingless insects that feed on blood. Signs: scratching, hair loss, redness and flea dirt (dark specks that turn red on wet paper). Spread from the environment or from other animals.
• Sarcoptic mange (scabies) — mites that burrow under the skin. Very itchy, with red/inflamed skin, sores and hair loss, often on the limbs and edges. Spread by direct contact or contaminated bedding; worse in stressed or immunocompromised animals. Zoonotic (can affect people).
• Demodectic mange — mites that live in the hair follicles. Tends to cause bald spots all over the body (contrast with sarcoptic, which is often on the limbs). Linked to a weak immune system, breed and poor nutrition; usually not very itchy unless infected.
• Lice — flightless insects that live in the hair or feathers. Signs: itchiness, scaly skin, dandruff and visible eggs (nits) glued to hairs. Host-specific.
• Cheyletiella — the “walking dandruff” mite. Causes scales, dandruff and mild itch; the moving scales are the mites themselves.
• Notoedric mange (feline scabies) — a highly contagious burrowing mite of cats; intense itching, especially on the head and ears.
• Ear mites (Otodectes cynotis) — live in the ear canal of cats and dogs; cause itching, pain, head-shaking and a dark, waxy discharge.

2. Pathogens & bacteria shapes

A pathogen is any organism that causes disease — bacteria, viruses, fungi, protozoa or prions. A zoonosis is a disease that can pass between animals and humans. Common zoonoses include rabies, salmonella, leptospirosis, tetanus, foot-and-mouth and avian influenza.

Bacteria shapes (morphology)

• Coccus — spherical.
• Bacillus — rod-shaped; typically gram-positive.
• Vibrio — curved (comma-shaped) rod; gram-negative.
• Spirillum — rigid curved / spiral shape; gram-negative.
• Spirochaete — flexible spiral with internal flagella; gram-negative.

Bacteria multiply by binary fission: a cell grows to roughly twice its size, copies its DNA, then splits into two identical cells. Under good conditions this happens very fast, which is why infections can flare quickly.

Key structures: pili are hair-like appendages on the surface (used to attach to surfaces and to swap genetic material); a flagellum is a tail-like structure used for movement.

3. The immune system

The immune system has a non-specific (innate) part that responds to anything, and a specific (adaptive) part that targets a particular pathogen.

• Macrophages — large white blood cells that engulf and digest microbes (phagocytosis), clear away dead cells and help stimulate other immune cells.
• White blood cell types — granulocytes, monocytes and lymphocytes (which include T cells and B cells).
• Red blood cell types — immature forms include normoblasts and megaloblasts.
• Antibodies — specialised proteins made by B-lymphocytes. They belong to the specific immune system and lock onto a particular pathogen.
• Lymph nodes — filter lymph and trap pathogens, so immune cells can act on them.
• The spleen — filters the blood, removing old or damaged red blood cells, and stores white blood cells.

4. Vaccinations

Vaccines prime the immune system so it can respond fast to a real infection. Many contain a tiny amount of the disease organism (killed or weakened) so the body makes antibodies that “remember” how to fight it.

• Puppies and kittens need a series of vaccinations in their first year. A standard dog initial course is at 8 and 12 weeks.
• Some breeds (Rottweiler, Dobermann, Pit bull types) are given a 3rd parvovirus vaccination at around 20 weeks.
• Vaccines usually do not give lifelong immunity, so booster shots are needed.
• Some vaccines are intranasal (nose drops), for example against kennel cough.
• Animal vaccines are usually not injected into a vein; cats are often vaccinated in the scruff.

5. Internal parasites (brief)

• Oral drenching — giving liquid wormer via a drenching gun into the side of the mouth, common in sheep and cattle.
• Worm grazing stock when a Faecal Egg Count (FEC) shows it is needed, rather than blanket-dosing, to slow resistance.
• Wormer dose in sheep is worked out by body weight — weigh, then dose to the heaviest in the group to avoid under-dosing.
• Heavy internal-parasite burdens cause weight loss, poor condition and sometimes anaemia or scour.
• The fluke (liver fluke) is usually found in the liver.

6. Health red-flags

• Yellow mucous membranes (jaundice / icterus) — possible liver complications.
• Capillary refill time (CRT) — press the gum, release, and count how long the colour takes to return. Normal is 1–2 seconds; slower can indicate poor circulation or shock.
• Rabies — notifiable, zoonotic and fatal. Report suspected cases immediately.

Self-check

Revision notes for study only. Always follow a qualified vet's advice for real animals, and your awarding body's specification for exams.

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