Loiasis
Organism Loa loa, a nematode worm. The life cycle of the parasite is essentially the same as W. bancrofti, except that the vectors are Tabanid flies. Clinical features The disease is characterized by Calabar swellings named after a town in Eastern Nigeria , which are transient, itchy and found anywhere on the body. Fever and eosinophilia suggest they have an allergic aetiology. L. loa is often confusingly called the eye worm to be differentiated from O. volvulus , as the worm is sometimes seen...
Force of Infection
In a communicable disease, the number of new cases occurring in a period of time is dependent on the number of infectious persons within a susceptible population and the degree of contact between them. Persons, whether infectious or susceptible, and a period of time are all quantifiable factors, but the degree of contact can depend upon very many variables some of which have been covered above . The factors, such as proximity density of populations, carriers, reservoirs, climate and...
Strongyloides
Organism The nematode Strongyloides ster-coralis, which is morphologically similar to the hookworms. Far less common is S. fiilleborni. Clinical features There are several alternative cycles of development and it is the type of cycle which determines the nature and degree of pathological change and hence the clinical features. An infective filariform larva develops in warm moist soil, penetrates the skin, and follows the same internal route as the hookworms to the final resting site in the...
Enterobius Pin Worm
Organism A nematode worm Enterobius vermicularis. Clinical features The main symptom is intense pruritis ani. Heavy infections can rarely cause appendicitis or salpingitis in the female. Transmission The gravid female migrates out of the anus at night to lay her eggs on the perianal skin before dying. This activity of the female causes the patient to scratch so that eggs are transferred to the fingers where they are swallowed or passed on to someone else. Eggs are thrown into the air such as...
Nongonococcal Urethritis NGU
Organism A number of organisms have been found to be responsible for urethritis not caused by the gonococcus, including Chlamydia trachomatis, Ureaplasma urea-lyticum, Trichomonas vaginalis and Mycoplasma hominis. Clinical features A low-grade urethritis with mucoid rather than purulent discharge in the male, in which intracellular diplococci are not found in the smear, suggests NGU. Infection is a low-grade discharge in the female or is often asymptomatic so that a reservoir of infection can...
Period of communicability Up to years
Occurrence and distribution P. westermani disease is found mainly in China, other parts of Asia, Africa and the Americas. Closely related species are P. africanus and P. uterobilateralis in West Africa, P. pulmonalis in Japan, Korea and Taiwan, P. philippinensis in the Philippines, P. het-erotremus in Thailand and Laos, P. kellicotti, P. caliensis and P. mexicanus in Central and South America. In all, it has been calculated that some 30 million people suffer from the lung fluke. Control and...
Yaws Epidemiology
Organism Treponema pallidum subspecies pertenue. Clinical features Yaws is a non-venereal treponemal disease affecting both the skin and bone. It commences as a primary papule that starts to heal, but after a period, varying from a few weeks to several months, it is followed by generalized lesions, multiple rounded papules, scattered all over the body. These lesions exude serum, which is highly infectious. There is also a mild periostitis in focal bony sites, but these and the skin lesions...
Sanitation
With food and water supplies, the emphasis is on the prevention of contamination, but with sanitation, it is reducing the source of the contamination. Social habits concerned with excreta disposal are often strongly held and unless these are approached in a sensible manner, any new system will fail. Sanitation is not just the provision of latrines, but a complex and inter-related subject involving people, water supplies and all other aspects of environmental health. Health factors As shown in...
No meningitisx Relative humidity
Fig. 13.4. The seasonal variation of meningococcal meningitis in relation to relative humidity in the Sahel region of Africa. the northern boundary is the desert. Within this area, major epidemics, mainly of group A, occur at 7-14-year intervals, with lesser ones in between. In addition to the meningitis epidemic belt in Africa, there have also been epidemics of group A organisms in India and Nepal, and group B in the Americas, Europe and Pacific Island Nations. Epidemic meningitis is commonest...
Lymphatic Filariasis
Organism Wuchereria bancrofti, Brugia malayi and B. timori, nematode worms. Microfilariae, the larval form present in the peripheral blood, are taken into the mosquito's stomach when it feeds on humans or animal reservoir in B. malayi . The larva loses its sheath inside the mosquito, migrates through the stomach wall and burrows into the muscles of the thorax. It becomes shorter and fatter, commonly described as sausage-shaped. Developmental changes take place and it elongates to a third stage...
Malaria 1
Organism There are four human malaria parasites, Plasmodium falciparum, P. vivax, P. malariae and P. ovale. P. falciparum causes the most serious disease and is the commonest parasite in tropical regions, but differs from P. vivax and P. ovale in having no persistent stage the hypnozoite , from which repeat blood stage parasites are produced. P. vivax has the widest geographical range, being found in temperate and sub-tropical zones as well as the tropics. P. vivax infection will lead to...
Bacillary Dysentery Shigellosis
Organism Bacillary dysentery is due to Shigella invading the bowel. The species and strains of Shigella are numerous. There are four main groups S. dysenteriae with 12 serotypes S. flexneri with 14 serotypes S. boydi with 18 serotypes S. sonnei with one serotype. The most severe are S. dysenteriae and the least severe S. sonnei, with S. flexneri being the most common in endemic areas. Another form of bloody diarrhoea is due to enteroinvasive and enterohaemorrhagic E. coli, particularly serotype...
Chikungunya Onyongnyong West Nile Orungo Oropouche and Ross River
This group of infections are summarized in Table 15.1. They present as a denguelike disease see below with headache, fever, malaise, arthralgia or myalgia, lasting for a week or less. Rashes are common in Chikungunya, Onyong-nyong and West Nile. Chikungunya may present as a haem-orrhagic fever in India and Southeast Asia see below , and West Nile and Oro-pouche as encephalitides. Ross River predominantly presents as a polyarthritis and rash. There are many other arbovirus infections presenting...
Granuloma Inguinale
Organism Calymmatobacterium granulo-matis. Clinical features Granuloma inguinale is a chronic, progressive, ulcerating disease of the ano-genital area without regional lym-phadonopathy. An initial lesion on the genitalia becomes eroded and ulcerated with new nodules forming at the margins as the lesion extends. The lesions readily bleed on contact and ulceration can continue to produce extensive destruction. Carcinoma of the vulva has been reported to be associated with granuloma inguinale....
The Beef and Pork Tapeworms
Organism Taenia saginata, the beef tapeworm and T. solium the pork tapeworm. Clinical features The adult worm of both species can live in the intestines producing little pathology, being diagnosed often by accident. It does, however, share the food supply of its host so that when intake is inadequate, debility can occur. The serious problems are due to the Cysticercus cellulo-sae from T. solium . The cysts die and calcify, those in the brain being a common cause of epilepsy or mental disorder....
Anthrax
Organism Bacillus anthracis is a rod-shaped organism occurring in pairs or chains and staining positively with Gram's stain. In the vegetative state in the animal or where there is a low oxygen content, the bacillus is surrounded by a capsule. If the dead animal's tissues become exposed to the air or the organism is cultured aerobically, then spores develop, which appear as round-filling defects in the stained rods. The vegetative form is killed by heat at 55 C for 1 h, or if the carcass is not...
Onchocerciasis
Organism Onchocerca volvulus, a nematode worm that has a predilection for the skin and eye, is transmitted by Simulium flies. Microfilariae are taken up by the fly when it bites humans and then undergo larval changes within the thoracic muscles, migrating to the head of the fly as infective larvae. When the fly bites again, microfilar-iae break out on to the skin to enter via any abrasion, especially the bite wound. Clinical features The microfilariae as they migrate through the skin cause...
The Intestinal Fluke Fasciolopsis
Organism The large human fluke Fasciolop-sis buski. Clinical features The adult worm lives in the small intestines and produces damage by inflammatory reaction at the site of attach ment. This sometimes leads to abscess and haemorrhage, but as well as these local effects, the parasite produces toxins. These can lead to oedema, weakness and prostration, ending fatally in the debilitated child. Diagnosis is made by finding the egg in faeces, a giant among parasites Fig. 9.1 . The egg is...
The Sheep Liver Fluke Fasciola hepatica
Organism The sheep liver fluke Fasciola hepatica. Less commonly F. gigantica. Clinical features The parasite has a predilection for the liver, piercing the gut wall and migrating through the liver substance to lie in the biliary passages. This migration and residence in the liver causes extensive damage, leading to fibrosis and cirrhosis. Diagnosis is made by finding the very large egg in the stool, which is almost identical to that of Fasciolopsis Fig. 9.1 . Transmission The life cycle is...
Population size
As mentioned above, the continuation of an epidemic is determined by the number of susceptibles remaining in the population. Fig. 2.6. Basic reproductive rate increasing - i.e. gt 1. Maximal transmission every infection produces a new case. Fig. 2.6. Basic reproductive rate increasing - i.e. gt 1. Maximal transmission every infection produces a new case. Fig. 2.7. Basic reproductive rate decreasing- i.e. lt 1. Unsustained transmission each transmission gives rise to less than one new case and...
Hepatitis A HAV
Organism Infectious hepatitis is a viral infection caused by a member of the Picorna-viridae, which includes both enteroviruses and rhinoviruses. Clinical features The main pathology is inflammation, infiltration and necrosis of the liver, resulting in biliary stasis and jaundice. The infection generally starts insidiously, the person feels lethargic, anorexic and depressed. Fever, vomiting, diarrhoea and abdominal discomfort ensue before the appearance of jaundice reveals the diagnosis. Once...
Investigation of propagated source epidemics
With a propagated source epidemic, phases of infection occur at regular intervals. The time-period between these phases is called the serial interval Fig. 2.5 . Features of the epidemic are measured in the same way as a common source epidemic, while an estimate of time of recurrence is given by the serial interval. After several propagated epidemics, cases remaining from the previous epidemic will merge with the next so that the regular serial pattern will be lost. Contagiousness or the...
The physical environment
Topography The nature of the physical surroundings can influence the diseases that are found there. In much of Asia, a complex interaction termed 'forest fringe malaria' describes the greater likelihood of developing malaria at the forest margin. The man enters the forest to fell timber, often illegally, while the woman goes there to collect firewood, bringing them into range of mosquitoes that live within the forest cover. A similar cycle of transmission occurs with yellow fever as illustrated...
Epidemic Theory
Epidemics can occur unexpectedly, as when a new disease enters a community, or can occur regularly at certain times of the year, as in epidemics of measles. Epidemic contrasts with endemic, which means the continuous presence of an infection in the community and is described by incidence and prevalence measurements. This section will cover epidemics and how they are measured. Epidemic means an excess of cases in the community from that normally expected, or the appearance of a new infection....
Endemicity
An endemic disease implies that there is a constant rate of infection occurring in the community. As new individuals are born, they become infected, are cured including self-cure , retain the infection for life or become immune. Prevalence rates will measure the level of endemicity as it applies to the community. Incidence rates will measure change in the level of infection over a period of time. While it is useful to compare prevalence from one community to another, on more Table 2.1....
mosquitoes
casual contact such as shaking hands, or lavatory seats through food, water or the respiratory route. control programmes The main method of control is health promotion and should involve community leaders, religious organizations and NGOs. This can be to the general public to supply them with the correct information or to specific groups. The most cost-effective health education will be to high-risk groups, such as commercial sex workers, homosexuals, single workers, etc. However, they are...
The cold chain
The cold chain is a descriptive term for the whole sequence of links that must be maintained in transporting the vaccine in a viable condition from the manufacturer to the person to be vaccinated. Vaccines will only survive when they are maintained at the correct temperature. There are certain limits when the vaccine can be allowed to depart from the optimal temperature, but the range and time are very short and vaccines rapidly lose their potency. To vaccinate with nonpotent vaccine is not...
The Fishtransmitted Liver Flukes
Organism The trematode fluke Opisthorchis sinensis previously called Clonorchis . Clinical features The adult fluke lives in the branches of the bile duct resulting in trauma and inflammation. Dilation of the biliary system causes a distortion of the liver architecture, which can lead to biliary stasis, hepatic engorgement, fatty infiltration and finally cirrhosis. O. sinensis is a risk factor for cholangiocarcinoma. Migration of the flukes up the pancreatic duct can damage the pancreas leading...
Meningococcal Meningitis
Organism Neisseria meningitidis. Many ser-ogroups and sub-groups have been identified, but of these A, B, C and Y are the most important in producing disease, while A and C predominate in epidemics. W135 has recently been responsible for some outbreaks. Clinical features Fever, headache, vomiting, neck stiffness and progressive loss of consciousness. A petechial rash, which does not blanch, is an important sign. Infants show floppiness and high-pitched crying, while children may present with...
Incubation period months
Period of communicability is 12 months, but animals act as a permanent reservoir. Occurrence and distribution East Asia, especially China, Taiwan, Thailand, Borneo and Malaysia, in some 15 million people. Control and prevention is by the proper preparation and cooking of water plants. Much can be done to reduce transmission by regulating the use of human faeces as a fertilizer. Domestic animals should be kept away from water plant cultivation ponds. Fig. 9.1. Parasite eggs found in faeces,...
Protection of foods
Food-transmitted infections can spread either through contamination or by a specific intermediate host. Flies indirectly contaminate food. Protection of the food we eat can be by the following Fig. 3.4. Routes of transmission of the water- and sanitation-related diseases. Fig. 3.4. Routes of transmission of the water- and sanitation-related diseases. Table 3.1. A classification of water- and excreta-related diseases. Table 3.1. A classification of water- and excreta-related diseases. Modified...
Trichinosis
Organism Trichinella spiralis Fig. 9.7 , T. nelsoni, T. nativa, T. britovi and T. pseu-dospiralis, nematode worms. Clinical features The severity of the disease depends upon the dose of larvae that have encysted in the tissues. During the second week of infection, there is headache, insomnia, pain, dyspnoea and pyrexia with oedema of the orbit and eosinophilia. If the symptoms are sufficiently severe, death can result otherwise, once the attack is over, the cysts cause no further trouble,...
Lassa Fever
Organism Lassavirus is an arenavirus. Clinical features There is a gradual onset with fever, malaise, sore throat, cough, vomiting, diarrhoea and general aches and pains. By the second week, lymphadenop-athy, pharyngitis and a maculo-papular rash on the face or body develops. In severe cases, pleural effusion, encephalopathy, cardiac and renal failures can occur with a mortality of 15-20 . In endemic areas, 80 of cases are mild or asymptomatic so that serological investigation will find a large...
Hookworm
Consider a family of five people with four out of the five infected with hookworms, producing on average some 4000eggs g of faeces. Approximately 200 g of faeces are voided by the average person each day, so the four people are excreting 4 x 4000 x200 3.2 x 106 eggs day. If each of these eggs results in a viable larva, then the potential for infection would be astronomical. If the head of the household is now persuaded to install a latrine and he encourages his family to use it, then hopefully...
Lymphogranuloma Venereum
Clinical features Lymphogranuloma vener-eum is a chronic infection presenting as a small painless papule, vesicle or ulcer on the genitalia that often goes unnoticed, lymphadenitis being the clinical sign. The lymph nodes become grossly enlarged and generally suppurate with fistulas and fibro-sis developing, especially in the rectal area if treatment is delayed. Diagnosis is by finding the organism in lymph node aspirate with immunofluores-cence or DNA probe. Transmission Although sexual...
Ectoparasite Zoonoses
Ectoparasites are non-flying vectors of disease, such as fleas and lice. They are responsible for an important group of infections, which are often associated with animals in which the reservoir of infection is found. Because of the close inter-relation between the ectoparasite and the animals on which it feeds, focal zoonoses or exoanthropic zoonoses result. Humans are often the accidental victims of these zoonotic infections, and a knowledge of the biology and how to avoid these foci can...
Domestic and Synanthropic Zoonoses
A zoonosis is an infection that is naturally transmitted between vertebrate animals and humans. In the last chapter, a group of infections that were mainly zoonoses, but also involved a vector were covered, while this chapter includes infections in which a vector is not involved. Most of these infections are due to the close association humans have with their domestic animals, but there are also some zoonoses in which animals that live close to humans, but are not welcomed, such as rats, are...
Cholera 1
Organism. Vibrio cholerae. Classical cholera is caused by V. cholerae 01, while most of the recent epidemics have been due to the El Tor biotype. V. cholerae 0139, which appeared in 1992, is a more virulent ser-ogroup variant of the El Tor biotype. Clinical features A profound diarrhoea of rapid onset that leads to dehydration and death should be considered as a case of cholera until proved otherwise. The diarrhoea contains no faecal particles, but is watery and flecked with mucus not cells ,...
Rabies
Organism Rabies is caused by a rhabdovirus in the genus Lyssavirus. There are seven related viruses, including Mokola and Duvenhage found in Africa , which produce rabies-like illness. The virus withstands freezing temperatures for considerable periods of time, but is killed by boiling, sunlight and drying. It is not easily destroyed by disinfectants. Clinical features The disease starts quietly with malaise, fever, sore throat and lack of appetite paraesthesia develops and abnormal muscle...
Routine or passive surveillance
All health facilities collect data in their record keeping, at its simplest being the name, age and sex of the individual and the symptoms or diagnosis of their illness. Considerable use can be made of well-kept records and it is worth doing an analysis of the type of information collected to determine the best system to use with the resources available. Hospital records, while more detailed than in small clinics, will not be representative of the population. Additional categories can be added...
The Fish Tapeworm
Organism The large tapeworm Diphyllobo-thrium latum. Clinical features The presence of such a large worm 10 m or more in the intestines can consume a considerable quantity of nutrients, but the main pathology is due to its selective absorption of vitamin B12, resulting in a megaloblastic anaemia in the host. Diagnosis is made by finding the egg in the faeces Fig. 9.1 . Sometimes, worm segments proglotids are also passed. Transmission The adult worm is found in the intestines of humans, dogs,...
Superficial Fungal Infections Dermatophytosis
Organism Fungi of the genus Trichophyton, Microsporum, Epidermophyton and Scytali-dium. Clinical features Also called tinea, the fungi attack specific sites on the body, the moist skin in the feet or groin, the nails, the scalp or the body. Tinea corporis often called ring worm produces well-defined, circular lesions that spread out from the centre causing slight depigmentation as they proceed. Tinea capitis causes areas of baldness, hairs becoming brittle so that they break off. Tinea...
Human Papilloma Virus HPV
Organism Human papilloma virus HPV . Clinical features The main clinical presentation is genital warts on the external genitalia or within the vagina, but a large proportion of infected persons show no clinical signs. When cellular immunity is depressed condylomata acuminata, large fleshy growths in moist areas of the perineum develop. However, the most serious consequence of HPV infection is the development of carcinoma, particularly of the cervix, but the anus and penis can also be involved....
Western equine Eastern equine St Louis Venezuelan Japanese Murray Valley and
This group of diseases present with a high fever of acute onset, headache, meningeal Table 15.1. The important arbovirus infections of humans. Onyong-nyong West Nile Oropuche Orungo Ross River Australia, New Zealand, Pacific Islands Aedes aegypti, Ae. africanus, Ae. leuteocephalus Anopheles gambiae, A. modestus, C. univittatus Mosquitoes, possibly Culicoides Ae. dentatus, Anopheles spp. C. annulirostris, Ae. vigilax, Ae. polynesiensis Western Equine Americas Eastern Equine Americas, Caribbean...
The Lung Fluke
Organism Unique amongst all the helminths, the trematode Paragonimus wester-mani selectively inhabits the lung. Clinical features Foreign body reaction to the parasite in the lung results in fibrosis, compensatory dilation and abscess formation. Haemoptysis is often an important feature, mimicking tuberculosis. Symptoms include cough and chest pain. If the parasite migrates to a site other than the lung, it can cause CNS, liver, intestinal, genitourinary or subcutaneous disease. Diagnosis is by...
Economies of vaccination
Vaccination coverage is often poor because of constraints put on staff by the cost of vaccines. Vaccines should be supplied in small dose quantities so that a vial can be opened even if there is only one child to be vaccinated. Spare vaccine can often be used up on other children attending the health centre for other reasons. The cost of vaccination is not just the price of the actual vial of vaccine, but includes the whole cold chain and the salary of the vaccinator. To have a vaccinator...
Food poisoning due to organic or inorganic toxins
More generalized outbreaks involving large numbers of people not necessarily associated with each other and presenting with bizarre symptoms, such as paralysis, may be caused by an organic or inorganic poison contaminating the food. Examples are cyanide poisoning from poorly processed bitter cassava, eating unripe akees a fruit popular in the Caribbean or contaminants in cooking oil. Although very localized, such outbreaks can be serious with considerable morbidity and sometimes mortality,...