Named Animal Diseases
OCR specification 4.1.1 (b) also requires you to know a list of named animal diseases, with the pathogen responsible and the cellular damage caused. These range from bacterial killers such as tuberculosis to fungal nuisances such as athlete's foot, via some of the world's most important global killers including HIV/AIDS, malaria and influenza. This lesson tackles all seven diseases on the OCR list, emphasising the link between cellular mechanism and clinical symptom.
Key Definitions:
- Infection — the colonisation of a host by a pathogen.
- Incubation period — the time between infection and the appearance of symptoms.
- Reservoir host — an organism in which a pathogen lives and multiplies, from which it can infect other species.
- Zoonosis — a disease transmitted from animals to humans.
1. Tuberculosis (TB) — Mycobacterium tuberculosis
TB is a bacterial disease caused primarily by Mycobacterium tuberculosis (and M. bovis in cattle, which can also infect humans). It remains one of the world's most lethal infections, killing approximately 1.3 million people each year.
How it causes damage
- Inhaled droplets carry the bacilli into the alveoli.
- Alveolar macrophages engulf the bacteria but cannot destroy them — M. tuberculosis has a waxy, mycolic acid-rich cell wall that resists lysozymes.
- The bacteria replicate inside macrophages and cause them to cluster into granulomas (tubercles).
- The lung tissue is damaged and forms fibrotic scars, reducing gas exchange.
- Classic symptoms: chronic cough, blood in sputum, weight loss, night sweats, fever.
Treatment
- A combination of antibiotics (isoniazid, rifampicin, pyrazinamide, ethambutol) taken for 6 months.
- BCG vaccine (live attenuated M. bovis) provides partial protection.
2. Bacterial Meningitis
Bacterial meningitis is inflammation of the meninges (membranes covering the brain and spinal cord). It is caused by several bacteria, most commonly:
- Neisseria meningitidis (meningococcus)
- Streptococcus pneumoniae (pneumococcus)
- Haemophilus influenzae type b (Hib)
How it causes damage
- Bacteria colonise the nasopharynx and cross into the bloodstream.
- They penetrate the blood–brain barrier and replicate in the cerebrospinal fluid.
- The immune response causes massive inflammation of the meninges, raising intracranial pressure and damaging brain tissue.
- Symptoms: severe headache, stiff neck, fever, photophobia, and a non-blanching petechial rash (meningococcal septicaemia).
- Can be fatal within hours without treatment.
Treatment
- Immediate antibiotics (usually intravenous ceftriaxone or benzylpenicillin).
- MenACWY and MenB vaccines provide protection in teenagers and infants.
3. HIV/AIDS
Human immunodeficiency virus (HIV) is a retrovirus that causes acquired immune deficiency syndrome (AIDS).
Structure
- Single-stranded RNA genome (two copies).
- Protein capsid surrounded by an envelope with gp120 and gp41 glycoproteins.
- Contains reverse transcriptase, which converts viral RNA to DNA.
How it causes damage
- HIV's gp120 binds to CD4 receptors on T helper cells and CCR5 co-receptors.
- Viral RNA enters the cell; reverse transcriptase makes complementary DNA.
- Viral DNA is inserted into the host genome by integrase.
- New virions are assembled and bud off, destroying the T helper cell.
- As CD4+ T cell numbers fall below 200 cells/μL, the immune system collapses and opportunistic infections (e.g., Pneumocystis pneumonia, Kaposi's sarcoma, oral thrush) take hold — this is AIDS.
Treatment
- Antiretroviral therapy (ART): combinations of reverse transcriptase inhibitors, protease inhibitors, integrase inhibitors. These do not cure HIV but suppress replication, allowing patients to live normal lifespans.
Exam Tip: HIV is transmitted by exchange of bodily fluids — blood, semen, vaginal secretions, breast milk. It is not transmitted by casual contact, saliva, or insects.
4. Influenza — Orthomyxoviridae
Influenza ("flu") is caused by influenza viruses A, B and C (type A is the most pathogenic).
Structure
- Enveloped virus with segmented single-stranded RNA genome.
- Surface proteins haemagglutinin (HA) and neuraminidase (NA) — these give strains their names, e.g., H1N1, H5N1.
How it causes damage
- HA binds to sialic acid receptors on respiratory epithelial cells.
- The virus enters, replicates, and destroys ciliated cells lining the airways.
- Loss of ciliated epithelium prevents mucus clearance, leaving the patient vulnerable to secondary bacterial pneumonia.
- Systemic symptoms (fever, myalgia) are driven by interferons and cytokines.