Spread of pathogens by cockroaches

Like the flies mentioned in the previous section, cockroaches too have been implicated in the mechanical transmission of enteric disease. Fewer species of cockroach are involved as the vast majority are found free-living in forests and grasslands and are of no medical or public health importance. Only the peridomestic species that have successfully adapted to the human environment are considered here.

Fewer studies have been conducted with cockroaches to establish their role as vectors. Nevertheless, their role as mechanical vectors Is great.

The reasons for this are as follows:

Cockroaches do not have a seasonal incidence, therefore their ability to transmit disease remains undiminished throughout the year.

Their preference for a warm, humid and temperature-controlled environment, such as a kitchen or hospital, enhances their contact with pathogenic micro-organisms.

The immature stages of cockroaches share the same habitat, behaviour and food requirements as the adult and are of equal status as efficient vectors, whereas only adult flies have been implicated.
Being an indoor pest, cockroaches seldom venture outside and so share a close association with humans.
Cockroaches are long-lived insects. The period from first-stage nymph to adult may be in excess of twenty months, depending on the species involved.

The pest species involved are largely nocturnal and their presence often goes unnoticed.
In temperate climates cockroaches tend to crawl more than fly. Their contact time with contaminated material is then increased.

Studies using cockroaches have clearly established their ability to come into contact with and eat human faecal matter and infected material. Comparative studies on the micro-organisms found in the environment from which they were taken, the external surfaces of the insect and its gut contents have shown a marked similarity. Essentially, the cockroach gut contains precisely the same organisms as are found in its environment.
As with flies, the cockroach gut provides a hospitable and stable environment for the maintenance of pathogens, allowing transmission over a longer period as well as rapid multiplication. Bacteria isolated from the gut of cockroaches include Staphylococcus spp, E. coil, Shigella spp, Salmonella spp.
Cockroaches are extremely mobile animals and their proven ability to crawl from sewers and drains through broken traps into kitchen and hospital environments illustrates their massive potential as spreaders of pathogens. Numerous studies have shown that the incidence of gastro-enteric disease has been curtailed or reduced by control of the cockroach population.

Spread of pathogens by ticks

The increased amount of leisure time enjoyed by members of the public is often spent in pursuits where they may become exposed to ticks. The diseases that ticks carry and the disorders they cause can now come within the scope of the public health pest control professional.

Ticks are not true insects, being more closely related to spiders, mites and scorpions. The tick body is divided into just two sections as opposed to the three sections of the insect body.
For the most part ticks are parasitic, and it is this parasitising of vertebrates, feeding on their blood, that makes them transmitters of disease.

Most ticks are blood sucking parasites at all stages of their life and while the diseases they transmit can be a massive problem, the bites inflicted by them can often cause immense discomfort and some species can induce a general paralysis of the human host.

Ticks can transmit many small microorganisms during their feeding process. It is worth noting that in most cases feeding in ticks is a lengthy process when compared to many of the blood-feeding insects and, therefore, there is ample opportunity for the transference of pathogens.

Some examples are the Rickettsiae, the organisms causing typhus etc; the spirochetes, which may induce many of the relapsing fevers; and many viruses amongst which are the causative agents of various encephalitis-like infections, and finally many bacteria. The bacterial disease spread by ticks, which is increasingly being encountered, is Lyme disease, which is carried by the deer tick. The bacteria can infect dogs, horses, small mammals and humans. Humans can encounter and pick up the deer tick in moorland and on forest walks, hill climbs etc. Lyme disease is easily treatable with antibiotics but the diagnosis is often difficult since initial symptoms are often difficult to spot.


Miscellaneous insect-borne diseases

There are a huge number of diseases spread by biting insects throughout the world but these are not generally within the scope of public health pest control. However, because they can occasionally occur in urban settings where they have previously been unreported they should be mentioned.

The various types of encephalitis viruses are good examples of such diseases. St Louis encephalitis has, over the years, occurred in several large outbreaks. West Nile encephalitis is another that has spread into areas where it has never previously been encountered. These diseases can be alarming in the rapidity of their spread and their capability to cause extreme discomfort and even death.

In all these cases the vectors of the disease are mosquitoes. Mosquitoes are worldwide in their distribution and many are able to colonise diverse habitats. It is essential to be alert to habitats where mosquitoes may become established and to drain or alter them in such a way as to discourage the breeding of the mosquitoes.
The spread of disease by rodents.The main commensal rodent species, the brown rat. Rattus norvegicus. the black rat, Rattus rattus and the house mouse. Mus domesticus are all adapted extremely well to take advantage of the environments where food is grown, reared, stored, processed, prepared and sold.
In addition to the vast amount of damage done directly to that food through consumption, rodents have the capability to spread many human pathogens.

Salmonella spp

In the UK there are around 20,000 to 30,000 cases of salmonellosis reported each year. Three main species of Salmonella are responsible for these: S. enteriditis, S. typhimurium and S. dublin. In a number of scientific investigations carried out between the 1950s and the 1990s, it has been shown that anything between 2 and 25% of the rat population and between 2 and 18% of the mouse population can be carrying Salmonella.
Salmonellae are found in many sites on farms and in 1993, 42% of farms sampled had significant rat populations.

Listeria spp

Listeria has been found in a large number of rodent populations with infection rates ranging from 10 to 75%. Listeria spp are common bacteria found in slaughterhouses frequented by rodents where the fur, feet and tail of the rodents could easily become infected with the pathogen.

Escherichia coil

Around 40% of the rat population is infected with E. coli and many of the hundreds of reported cases on E. coli infection could possibly be related to rodent transfers of this pathogen.
In a recent study on a farm 20% of the cattle faeces was infected with E. coil 0157:H7. Where such high incidences of the bacterium are found it is inevitable that the rodent population on the farm will become contaminated with bacteria and therefore be able to transfer it around the environment.

Cryptosporidium parvum

There are around 4000 reported cases of cryptosporidial infections reported in England and Wales each year. In two studies carried out on wild rat populations it was found that between 50 and 65% of the rats were infected with Cryptosporidium parvum.

Leptospira spp

Weil’s disease, caused by the bacterium Leptospira icterohaemorrhagiae, is worldwide in its distribution, with rodent urine being the prime source of human infection. The infection is picked up by humans either through contact of with urine or through contacting contaminated soil.


Hantaviruses are pathogens that have been prevalent for many years and there has been significant co-evolution of the hantaviruses and their rodent reservoirs. The term hantavirus may not be familiar but there are many local names associated with the condition. Wild rodent populations in addition to brown rats and house mice are frequent hosts for hantavirus. The brown rat is the only hantavirus reservoir that has a world-wide distribution, and the rat’s close association with humans has meant that many outbreaks of hantavirus can be attributed to the brown rat.


Campylohacter spp

Streptococcus spp

These bacteria have been found to be the causative agents of many acute gastro-enteritis type complaints.
Often found in skin and gut infections in humans, they too can be carried by flies. Chlamydia spp
These are parasites that can cause a variety of complaints in humans and there is increasing evidence that flies can transmit them.

Escherichia colt

In recent months there have been a number of publications showing evidence for the transfer of Escherichia coil 0157:H7 by flying insects. The most recent of these is where an outbreak of E. call has been reported in a school in Japan and the pathogens that were isolated from the human sufferers were also isolated from houseflies found at the time in the kitchen. A further finding of this study was that the E. coil seemed to be contaminating the labella folds of the housefly mouthparts and, in fact, actively proliferated there, leading to the conclusion that the flies may have a higher potential to disseminate the E. coif than had been previously suspected.
Further studies recently carried out in the UK show that E. coll, when fed to flies, is taken into the flies and distributed widely within the internal structures with significant numbers of bacteria being found in the foregut, ovaries, hindgut, and abdominal haemolymph. The contamination of the ovaries is intriguing and this may imply ovariole contamination and subsequent “infection” of the egg and possibly the larvae.
There are also many fungi that cause disease such as Candida spp, Mucor spp, Aspergillus spp, etc, and research shows that flies can carry all of them.
Food poisoning outbreaks can occur from a minute dose of bacteria. In these cases, the disease has could easily have been spread by flying insects, a fact which is rarely understood or appreciated.

Feeding behaviour

All true flies, to which this group belongs, can only ingest liquid food. Should they land on a solid food source they produce large quantities of saliva together with regurgitated gut contents. The mixture, rich with digestive enzymes and undigested matter from their last meal, is vomited onto the food together with any living bacteria, viruses and protozoa that it may contain. The mass is puddied together into a sticky bolus using the forelegs before being sucked back into the gut. The process may be repeated several times, during which time the insect may defecate to reduce the overall body weight in readiness for flight.
Evidence of this feeding behaviour may be seen on surfaces visited frequently by flies. Brown and white spots indicate the remains of dried faecal smears and gut contents, both of which have been shown to be rich in viable pathogens for a considerable period of time.The gut contents and the faeces have been shown to contain the same pathogens as are found on the external surfaces, but with an important difference. The more stable and moist environment of the gut allows not only a better-protected environment for the pathogens, enhancing their survival, but also permits multiplication. This means that a larger infective dose may be transmitted. This is a critical factor as the size of the inoculum has a direct bearing on the likelihood of successful transmission.

From the turn of the century to the present day there have been dozens of studies on the correlation between the seasonal fluctuations of fly populations and the rise and fall of gastro-enteric disease. The rise of one equates to a rise in the other and vice versa. Cases where fly populations were reduced by control measures have shown an immediate decrease in the incidence of disease.

The potential of flies as mechanical vectors should never be underestimated and, as studies continue, their role in the transmission of gastro-enteric disease gains in importance.


Feral pigeon Columba Livia var

History and development

The feral pigeon is found throughout Britain and also in most regions of the world.
Many people associate this bird with urban environments and as such it is sometimes called the “town pigeon”. However it is often found in rural situations such as on farms.Historically, these birds are descended from rock doves which explains why they often nest on buildings and other structures, usually on ledges, under eaves or on girders.Nests are constructed of grass and twigs but can also contain rubbish such as pieces of plastic. Life cycle
The feral breeding population is boosted by racing pigeons or escaped birds from domestic pigeon lofts.
The peak breeding season is between March and July but feral pigeons are capable of breeding all year round.

  • Size: Weight:
  • 300 – 350 mm
  • Plumage:
  • 275 – 550 g
  • Blue-greys, reds, blacks
  • Sexing:
  • Little visible difference

The brood usually consists of two off-white eggs laid on consecutive days. Incubation lasts for about 18 days and the hatched chicks are fledged after about 30 days.
Surprisingly, another clutch can be laid when the first young are only 20 days old. This means that up to nine broods can be produced per pair per year.

Diet and significance

Feral pigeons tend to scavenge food, often at food premises, docks and mills, and flocks of several hundred birds can be common where spillage is abundant.
In urban environments, they are encouraged by members of the public feeding them birdseed, bread etc. Control


Blowfly Calliphora erythrocephala

Key features

Adult flies are generally 9 – 13mm long with a wingspan of 18 – 20mm. The adults are large robust flies. with a stout abdomen. The thorax is black/blue and dusky, and the abdomen is likewise coloured.


Blowflies are primarily scavengers, depositing their eggs on fish, meat, decaying matter of animal origin and wounds of animals and man. In the absence of more favourable sites, they will lay their eggs on animal faeces and decaying vegetable matter. The females usually lay around 600 eggs – each egg being 1.0 – 1.5 mm long and creamy white in colour. The eggs hatch within 18-48 hours. The egg period varies according to the age of the egg when laid, and on temperature and humidity. However, when gravid females cannot find suitable laying sites, they retain eggs for as long as possible. In such circumstances, eggs hatch in a very short time, in fact, Calliphora spp have been seen to deposit living newly hatched larvae rather than eggs. Caliiphora spp usually oviposit at night. At temperatures below 41, eggs will not hatch.
From the eggs the larvae emerge and feed on the rotting meat, etc. Larvae grow rapidly and moult three times. Within 14 days, they have developed fully. Before pupation, larvae cease feeding and tend to migrate from the feeding medium. By choice, pupation takes place in loose soil. if this is not available. then cracks in walls, under sacks, etc will suffice. The length of time spent in each larval stage is variable – again being dependent on temperature, humidity and foodstuff.

Tolerance of low temperatures by eggs, larvae and pupae is exhibited and in such conditions, development periods are greatly extended. The following figures (for Calliphora vicina) are therefore only a guide.

Speed of development

(27°C on liver – time in hours):

  • Egg
  • 20 – 28
  • 1st stage larva
  • 18 – 34
  • 2nd stage larva
  • 16 – 28
  • 3rd stage larva
  • 30 – 68
  • Prepupa
  • 72 – 290
  • Pupa
  • 9 – 15 days

Worldwide – always closely associated with man and his dwellings. Significance
Blowflies are attracted to rotting animal remains on which to lay eggs. in their search they can mistake stored meat as a suitable “host”. The possibility of disease spread is similar to the housefly.


Treatment consists of finding the source of the infestation and, where possible, removing it.
The application of a residual insecticide may also be indicated, however attention to hygiene is the key to controlling this pest.

Illustration: Calliphora vomitoria adult.


Flour mite, grain mite Acarus siro

Key features

Acarus siro is around 0.5 – 0.7 mm in length.
It is white in colour with a pearly iridescence, its legs often have a brown or pink colouration. Its body is divided into two clear sections with a distinct line between the two sections known as the proterosoma (the anterior end) and the hysterosoma (the posterior section).This mite has two pairs of long setae protruding from the posterior end of the hysterosoma.


The female can live for around 40 days as an adult and during that time she can lay as many as 500 eggs. The small white eggs are deposited in the food and within 3 – 4 days the first stage larvae emerge. These are around 0.15 mm long and have three pairs of legs. They feed for a few days (4 – 5 days normally) and then prepare for moulting to the next stage, the first nymphal stage, which has four pairs of legs. There is then a second nymphal stage which moults to produce the adult. Total nymphal span is around 15 – 20 days.
The total life cycle from egg to adult can vary from as little as 30 days at 10-15″C to 140 days at TC. Temperature and humidity significantly influence the mite life cycle & mites rarely exist in a relative humidity of less than 65%. Mite populations can increase seven-fold each week and thus very heavy infestations may develop


Worldwide. Significance

Acarus siro is probably the most serious and frequently encountered mite pest of stored products. It is a pest of a wide variety of different kinds of products such as flour, grain, linseed, cheese, dried fruits, etc. Its presence in commodities often imparts a taint known as “mintiness” and this effectively renders such contaminated product unusable. Animals fed on mite-infested feed may develop digestive problems. Mites of this genus have been implicated in causing dermatitis, rhinitis, respiratory tract irritation and intestinal upsets.


For treatment of the infestation, remove and destroy any infested commodities. Reduce humidity in the property.
Acaricides may be used on the fabric of the building, skirting boards, floorboards. etc.
Reducing the moisture content of a commodity below 12% prevents mite development. Detection of an infestation may be difficult as the mites appear dust-like.

Illustration: Acarus siro adult.


Common bedbug Cimex /ectularius

Key features

Adult bedbugs are 4-5 mm in length, wingless and uniformly mahogany brown in colour.
They have long well-developed walking legs with efficient tarsal claws for clinging on to the host during feeding.
Prominent antennae project from the head adjacent to the compound eyes.


Female bedbugs lay eggs throughout their life, an unusual feature in insects. They generally produce around 2 to 3 per day and since they can live for many weeks, indeed months, each female could produce around 400 – 500 eggs during her lifetime. The eggs are deposited all around the environment in which the bedbug is living and are small and white or whitish/yellow about 1 mm long.The nymph which emerges from the eggs after about 10 days at 22°C is a small version of the adult, feeding also on the blood of vertebrates.

The length of time spent in the five nymphal stages is greatly dependent upon the food resources available, temperature and relative humidity.

Bedbugs have well defined resting sites in which many individuals from all the different life stages are found. This harbourage is an essential part of the life cycle of the insect since it is In this area that the young bedbugs pick up the internal gut microorganisms which are essential to their survival.


This genus has representatives worldwide.


The close association of bedbugs with human beings means that they can cause substantial nuisance through their blood-feeding habit. They feed at night on the human hosts as they are sleeping.
If the infestation is high there can be a risk of anaemia being suffered by the human hosts, although this is rare.
The nuisance and itching caused by the bites and the possibility of secondary infection is more common. Bats, chickens and other domesticated animals may also be attacked.


A thorough inspection should be made to determine the extent and source of the infestation. Bedbugs may, for instance. have been introduced in second-hand furniture, where bugs may remain undetected for considerable periods until a suitable host appears.All harbourages should be treated with a residual insecticide.
A very thorough treatment is needed as harbourages are diverse and difficult to detect.

Illustration: Cimex lectulanus adult.


Carpet beetles Anthrenus spp

There are three common species of carpet beetle: Anthrenus verbasci, the varied carpet beetle, Anthrenus museorum, the museum beetle, and Anthrenus scrophulariae, the common carpet beetle. The varied carpet beetle and the museum beetle are the most commonly encountered by pest controllers.

The varied carpet beetle Anthrenus verbasci
Key features

The adult beetles, which are around 3 mm in length,have a speckled appearance which arises because of the many scales which cover the elytra and the prothorax. These scales are black, white and yellow / brown in colour on the upper surface and they give the beetle a variegated appearance, hence the common name, and a white appearance on the lower surface. As the beetles age these scales are often partially or totally rubbed off and the beetles then take on an even more mottled appearance. The antennae have eleven distinct segments with a three-segmented club.
The larvae of Anthrenus have a characteristic appearance, being covered in well developed hairs arranged in many tufts positioned at the intersegmental folds, giving rise to the common name for these larvae, “woolly bears”. At the posterior end of the larva there are three bundles of golden coloured hairs. The hairs, when inspected closely, can be seen to be segmented and have a sharp arrow-like head at the end.The larvae grow from around 1 mm when they first emerge from the egg to around 5 mm when fully grown.


The female beetles lay around 35 to 100 eggs in batches within the larval foodstuff. The eggs are small (0.2 to 0.5 mm long). white and are frequently “stuck” by a secretion from the female accessory glands to the fabric of the material in which they have been deposited. The larvae emerge from the eggs and start feeding. The larvae are repelled by the light and as a result as a result burrow deeply into their food. As they grow they moult and the cast skins are frequently found amongst the feeding larvae and this often gives the impression of a larger population than is, in fact, present.
Larval life is greatly dependent upon the quality of the food it is feeding upon. If the larvae have had periods in which they have fed well, they are capable of surviving prolonged periods without food – many months in fact.
Once fully grown the larvae pupate within the last larval skin and from the pupa emerges the adult.
There is often a gap between the development of the adult in the larval skin and the emergence of the adult; this can often extend to a month.

When the adults emerge they feed on the pollen of garden plants such as roses, viburnum and many other bushes. Indoors, adults may be found on windows from March to September.

Larvae may live in the nests of birds, insects and mammals.

In common with other insects, development times are influenced by temperature, relative humidity, moisture content, quantity and quality of food. The following figures are therefore only a guide.

Number of days spent as:
  • Egg
  • 14 – 31
  • Larva
  • 220 – 320
  • Pupa
  • 10 – 30
  • Adult male
  • 14 – 30
  • Adult female
  • 15 – 45

Egg to adult at room temperature averages 250 – 350 days.


German cockroach Blattella germanica

Key features

The adult is light brown in colour with two dark almost parallel longitudinal stripes on the pronotaf shield.
The female is darker than the male and with a broader abdomen.
Both male and female adults are fully winged.
Early instar nymphs have a pale area centrally on the dorsal thorax.
Later instars have two dark longitudinal stripes on the pronotum.


The ootheca is carried by the female until it is within 1 – 2 days of hatching.
Small 1st instar nymphs emerge from the ootheca and easily infest tiny cracks and crevices in the immediate area.
All the nymphal stages and the adults feed on the same type of food, making the establishment of an infestation extremely easy.
Prior to moulting, nymphs become immobile, remaining in harbourages. Significance

Adult size:

Number of moults:

German cockroaches are found throughout buildings but show a preference for warm humid areas.
13 – 16 mm 5 – 7

  • Development time: (Egg to adult)’
  • 1 – 3 months
  • Length of adult stage:
  • 3 – 6 months
  • No of oothecae produced
  • Average 5
  • in female lifetime:
  • Range 4 – 8

No of eggs produced Average 30 – 40
per ootheca: Range 18 – 50

They are good climbers, being able to climb vertical glass or tiled surfaces.
An infestation of these cockroaches can be quickly established once they have entered any premises.
This species is an extremely serious pest in many different types of premises ranging from hospitals to domestic houses.
Top: Blatiella germanica adult male. Bottom: Blattella germanica nymph.


Brown rat, common rat, Norway rat, sewer rat Rattus norvegicus


Originated from Asia and China.
First recorded in Europe at the beginning of the 18th Century.
They were referred to as Norway rats because they were thought to have travelled from the East on Norwegian timber ships.


R. norvegicus must drink water daily unless the food source is extremely moist.
Colour: This can vary but usually brown to grey-black with lighter underside.
Eyes: Small

Tail: Slightly shorter than head and body. The tail is dark above and lighter below.
Weight (adult): 400 – 550 g
Litters per year: 3 -6
Litter size: 8 – 10
Maturity: 2 – 3 months
Average life span: 12 months

They are considered omnivorous but if available, cereals seem preferred. They eat on an average one tenth of their body weight each day. R. norvegicus explores locations quite freely
However, it does have a fear of new objects. This is known as neophobia and this should be taken into account when baits are checked after an initial treatment.