10.4 Zoonoses, Inflammation and Allergy

Many pathogens infect only a limited set of host species. Entry may require binding between a pathogen protein and a particular host-cell receptor; if the receptor is absent or shaped differently, infection fails at the first step. Successful replication also depends on compatible intracellular factors, nutrients, immune evasion and physical conditions. Host range is therefore a chain of requirements rather than a single universal lock-and-key rule.

Body temperature can restrict host range because pathogen enzymes and membranes function over limited ranges. A microorganism adapted to mammalian tissue may fail at a bird's higher core temperature, while another may not grow in a cooler ectotherm. Behaviour and anatomy also affect exposure: a pathogen cannot exploit a compatible receptor if it never reaches the relevant tissue.

A zoonosis is an infectious disease naturally transmitted between vertebrate animals and humans. Transmission may be direct through bites, saliva or contact; indirect through food, water or contaminated environments; or vector-borne through organisms such as mosquitoes. Rabies can enter through an infected bite, anthrax can be acquired from infected animals or products, and zoonotic influenza viruses circulate in animal hosts before occasional human infection.

Spillover into one person does not guarantee an epidemic. The pathogen must reproduce in the new host, exit in a form that reaches another susceptible person and maintain transmission across repeated cycles. Mutations or reassortment can alter receptor binding, replication or immune escape, but most variants do not acquire the complete set of traits needed for sustained human-to-human spread. Close contact among wildlife, livestock and people raises the number of opportunities for rare compatible events.

International movement can transport infected hosts or vectors faster than symptoms appear. Surveillance therefore combines animal, human and environmental information. Preventing spillover may involve vaccination, farm biosecurity, vector control, habitat management, food hygiene and rapid sequence sharing. The relevant unit of analysis is a connected system, which is why zoonotic control often follows a One Health approach.

Vector-borne transmission adds another compatibility filter. A mosquito must acquire the pathogen during feeding, support its development or replication, and deliver an infectious stage during a later bite. Changing vector abundance, biting behaviour or geographic range can therefore change disease risk even when the human pathogen and human immune response remain unchanged. Controlling the vector can interrupt transmission without treating every infected reservoir host.

An allergy is an excessive adaptive response to an antigen that is harmless to most people. During sensitization, exposure to an allergen can lead to production of a class of antibody that binds receptors on mast cells. The person may show little immediate reaction, but mast cells become coated with allergen-specific antibody. On later exposure, allergen cross-links adjacent antibodies and triggers rapid release of histamine and other mediators.

Histamine increases local blood flow and capillary permeability and stimulates sensory endings. In hay fever this produces swelling, mucus secretion and itching in nasal tissue. In asthma, inflammatory mediators contribute to airway narrowing, mucus production and difficulty breathing. The same vascular changes that help deliver immune components during infection become symptoms when triggered against harmless material.

Anaphylaxis is a severe systemic allergic response in which widespread mediator release can lower blood pressure and obstruct airways. Its danger illustrates the importance of scale: a local change in permeability can be useful, while the same response across much of the circulation can be life-threatening. Allergy is not immune deficiency; it is inappropriate specificity and magnitude.