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.
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 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.
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.
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.
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.