Infectious Diseases

Predator-prey transmission of encysted larvae within prey animals (vertebrate or invertebrate) being eaten by predators where adult worms develop and produce eggs (e.g. Dracunculus larvae in copepods ingested by humans leading to guinea worm infection, Taenia cysticerci in beef and pork being eaten by humans, Echinococcus hydatid cysts in offal being eaten by dogs).

Rhabditid ‘threadworms’ have tiny bodies which become embedded in the host mucosa. Their life-cycle includes parasitic parthenogenetic females producing eggs which may hatch internally (leading to auto-infection) or externally (leading to transmission of infection or formation of free-living male and female adults). Super-infections by Strongyloides may cause severe haemorrhagic enteritis in humans.

Schistosoma japonicum is the only human blood fluke that occurs in China. It is the cause of schistosomiasis japonica, a disease that still remains a significant health problem especially in the lake and marshland regions.

Filariasis is a parasitic disease that is caused by thread-like roundworms belonging to the Filarioidea type. These are spread by blood-feeding black flies and mosquitoes.

Adult worms have a life-span of 1–2 years which means that individuals may be infected all their lives as worms die and new worms are acquired.

Onchocerciasis is caused by infection from the parasitic worm Onchocerca volvulus.

Elephantiasis results when the parasites lodge in the lymphatic system.

There is no evidence that this leads to excess Candida infections.

There is a large variation in the amount of eggs produced by the different worms in one event; this figure varies in the range of 3,000 to 700,000.

Rheumatic fever usually affects young children and adolescents.

This is a relatively benign form of leukaemia in elderly people.

Zygomycosis in the debilitated patient is the most acute and fulminate fungal infection known. The disease typically involves the rhino-facial-cranial area, lungs, gastrointestinal tract and skin. It is often associated with acidosis, diabetes, malnourished children, severely burned patients and other diseases such as leukemia and lymphoma.

Patients with primary immunodeficiencies are susceptible to Aspergillus species infections particularly when cell-mediated immunity is compromised.

Candida norvegensis is a very rare clinical isolate that has been reported as a causative agent of peritonitis and disseminated candidiasis in a patient on CAPD.

Pythium insidiosum is the causative agent of swamp cancer, especially in horses, mainly in wet tropical environments. Infections are characterised by tumour-like masses which develop mostly on the legs.

Mortierella wolfii is an important casual agent of bovine mycotic abortion, pneumonia and systemic mycosis. It has also been isolated from soil, rotten silage, hay and coal spoil tips.

It is a rare cause of subcutaneous, sinusitis, endocarditis, peritonitis and disseminated infection in immunosuppressed patients.

Dermatophytosis of the scalp, glabrous skin and nails is caused by dermatophytes which have the ability to utilise keratin as a nutrient source. No living tissue is invaded the keratinised stratum corneum is simply colonised.

Folliculitis is the name given to a group of skin conditions in which there are inflamed hair follicles. The result is a tender red spot, often with a surface pustule. Acne and its variants are also types of folliculitis.

This fungi is usually isolated as a saprophytic contaminant. World-wide, it is known as an insect parasite. Sympodial development of single-celled conidia on a narrow geniculate or zig-zag rachis emanating from a flask-shaped conidiophore.

Parasitic worms are all parthenogenetic females, measuring from 2-3mm in length and characterised by the presence of an extremely long filariform pharynx (one third of body length) and a blunt pointed tail.

The microfilaria migrate to the skin during the day.

It is a common human pathogen causing pulmonary, rhinocerebral, disseminated, CNS zygomycosis. It is a small pyriform shaped sporangia with a characteristic conical shaped columellae and pronounced apophysis.

It is one of the most common yeast species to be found on the body surface and is often isolated as an incidental finding from skin and urine.

It is a mycotic infection of humans and animals caused by a number of different fungi and actinomycetes characterised by draining sinuses, granules and tumefaction. The disease results from the traumatic implantation of the aetiologic agent and usually involves the cutaneous and subcutaneous tissue.

Cortisol depresses a range of immune responses.

Aspergillus parasiticus is a mold known to produce aflatoxins which is a potent liver carcinogen. It is closely related to Aspergillus flavus.

T-lymphocytes are specialised for recognising peptides derived from intracellular infectious agents which are exhibited on the surface of an infected cell together with MHC class I & II molecules.

There are at least 93 serotypes in the world. Though there are 23 prevalent serotypes that cause disease in different parts of the world, but no switching of the bacterium between serotypes is there. Infections with these organisms are common and often infect those with antibody, or innate or combined deficiencies.

T-cells are important for immunity to M. tuberculosis through the provision of Th1 cytokines such as IFN-gamma to activate macrophages containing the bacilli and perhaps also the participation of cytotoxic T-cells. The latter may kill macrophages which cannot be rescued by cytokines thereby releasing the bacteria to be taken up by fresh young macrophages which can be activated to become killers.

IL-8 produced by endothelial cells in the inflammatory response is a powerful chemotactic agent for neutrophils.

Antibodies may block the binding to target cell and would increase the rate of removal of toxin from the circulation by immune complex formation.

Capsules do not adhere readily to phagocytic cells and may cover carbohydrate molecules which could be recognised by phagocyte receptors.

Mastitis is the inflammation of breast tissue. S. aureus is the most common etiological organism responsible, but S. epidermidis and streptococci are occasionally isolated as well. Mastitis can be classified as milk stasis, non-infectious or infectious inflammation and abscess.

The superantigens combine with the framework regions of certain TCR V beta families irrespective of their complementarity determining regions.

The ability to interchange large segments of the genome with a viral reservoir in other animal species is the basis of antigenic shift leading to radical change in antigen, particularly crucial antigens giving protective immunity such as the viral hemagglutinin.

The poor T-cell responses specific for leprosy bacillus prevent the host from mounting a killing response and the bacilli grow profusely.

It thereby prevents effective presentation of antigen peptides to the T-cell receptor.

Interferon inhibits the intracellular replication of viruses.

Th1 cells produce cytokines which activate the killing macrophages. Where Th2 cells predominate, antibodies are produced and the individual responds poorly to the infection.

The killing early on, in the infection of a virally-infected cell prevents the virus from utilising that cell for replication.

Merozoite is a form of the malaria parasite that invades red blood cells.

FDCs naturally express high levels of the normal prion protein which then becomes abnormal following exposure to the transmissible spongiform encephalopathy (TSE) agent.

The membrane attack complex containing C8 and C9 is important for destroying the Neisseria bacterium.

Leprosy is caused by a slow-growing type of bacteria called Mycobacterium leprae (M. leprae). Leprosy is also known as Hansen's disease, after the scientist who discovered M. leprae in 1873.

Up-regulation of P-selectin leads to binding to sialyl Lewisx on the neutrophil and causes rolling along the endothelium. Upregulation of PAF leads to firm binding to the receptor on the neutrophil and the cell is then subject to chemotactic attraction to pass through the endothelial cells.

Trypanosomes are susceptible to phagocytosis, although some of the evasion mechanisms they employ will limit the ability of the phagocyte to initially bind the parasite and to subsequently kill the engulfed organism.

IgA with secretory piece is dimeric and is the main Ig which prevents adherence of bacteria to the mucosal surface and opsonizes bacteria for adherence to macrophages with Fc alpha receptors.

These mechanisms come into play when the macrophage is activated. Other mechanisms, e.g. peptide defensins and cationic proteins may also contribute to killing.

Major killing of extracellular bacteria is through phagocytosis and intracellular killing. Complement aids opsonisation of bacteria to facilitate phagocytosis but may also be responsible for the bactericidal action of fresh serum containing antibody on Gram-negative bacteria.

These are delayed type hypersensitivity skin tests which are mediated by T-cells.

M. tuberculosis divides every 15–20 hours, which is extremely slow compared to other bacteria, which tend to have division times measured in minutes (Escherichia coli can divide roughly every 20 minutes).

N. gonorrhoea was first described by Albert Neisser in 1879.

A. flavus is the major producer of carcinogenic aflatoxins in crops worldwide. It is also an opportunistic human and animal pathogen, causing aspergillosis in immunocompromised individuals.

Escherichia coli is a Gram-negative, facultatively anaerobic, rod-shaped bacterium of the genus Escherichia that is commonly found in the lower intestine of warm-blooded organisms (endotherms).

Some of the merozoite-infected blood cells leave the cycle of asexual multiplication. Instead of replicating, the merozoites in these cells develop into sexual forms of the parasite, called male and female gametocytes that circulate in the bloodstream.

Immunologically-mediated tissue damaging reactions often occur in persistent parasitic infections, as in this example.

Parasites like Trichinella frequently increase the IgE levels, usually polyclonally. This tends to inhibit the protective effect of IgE by diluting out the specific IgE on the mast cell. Inflammation induced by mast cell degranulation leads to influx of IgG which causes damage to the parasite.