FOCUS ON RODENT CONTROL: Rodents And Human Disease

The fame and fortunes of many rodents, rats and mice is inextricably bound up with man. Commensal rodents, and notably Rattus rattus, R. norvegicus and Mus. sp., share the same food sources (“commensal” literally means eating from the same table) and the same pathogenic microbes.

These rodents are not necessarily affected by diseases but they do act as disease reservoirs. Disease reservoirs function as a holding point and a staging post for pathogens between the environment in which they originate and the human body, where the disease is expressed.

It is important to remember that sylvatic (or field) rodents may form the initial reservoir of pathogen and disease. Most sylvatic rodents have little or no contact with man and his domestic animals, but they may act as an endemic carrier and disseminator, maintaining and circulating the infectious agent for a long period of time, as in the case of Chagas’ disease. When these field rodents mix with commensal species in rural areas, transmission begins and the end result is infection of humans by animal-carried diseases (zoonoses).

Disease is transmitted most efficiently by commensal rodents closely associated with man. Above all the pathogen status of a rodent is a reflection of its environment. On poultry farms where birds are free of salmonella, populations of Mus sp. (house mouse) are similarly free of the pathogen. Pathogens may be spread from rats to humans, either directly or through a physical or biological vector, in any situation where these two mammals interact.

PATHOGENS AND DISEASES. Rodents are reservoirs for the complete range of pathogenic agents and diseases (bacteria, [including rickettsial diseases], protozoa, viruses and nematodes/trematodes), called “zoonoses” because they are passed from animals to man. Many are globally distributed (e.g., murine typhus), while others (e.g., plague caused by the bacterium Yersinia pestis and Chagas’ disease caused by the protozoan Trypanosoma cruzi) are regionally or climatically restricted.

Some figure prominently in the history books but are now thankfully rare (e.g., bubonic plague), while others are a part of everyday life today (e.g., Salmonella sp.).

The following is a selection of zoonoses that illustrate the principles that underpin the role of rodents as disease reservoirs and vectors.

Bubonic plague. Bubonic plague is the best starting point, if for no other reason than history. A lethal combination of a bacterium carried by rodents and transmitted by fleas was responsible for virtually wiping out whole civilizations and thereby putting a brake on human population growth and the development of societies. Rodents are the natural hosts of the plague and man is just an accidental victim who was originally bitten by an infected rat or flea vector.

The disease is now relatively uncommon thanks to knowledge — based on control and eradication of the rodents, ectoparasitic fleas and the causal bacterium — but plague still occurs and its capacity for rejuvenation should not be underestimated.

Salmonellosis. It appears that the worst of Salmonellosis (a type of food poisoning) is yet to come. In the United Kingdom food poisoning increased six-fold between 1982 and 1999 with over half the cases accounted for by Salmonella bacteria. Transmission to humans is generally through the consumption of infected food or water that has been contaminated with faces from a human or animal carrier.

Rodents, primarily Rattus sp. and Mus sp., are an important reservoir of salmonella. Their easy access to contaminated food, garbage and sewage puts them right in the front line to become infected and transmit via excreta. It is amazing to think that cultures of Salmonella bacteria were once incorporated into rodent baits as a means of biological control. Rodents developed resistance to the specific Salmonella serotype in the bait and transmitted it to man with some known ghastly consequences.

Leptospirosis. Rattus norvegicus, the most common and widely distributed commensal rodent, is the main reservoir for the Leptospirosis bacterium. This together with transmission by contamination of water with infected urine combine to make Leptospirosis the most widespread zoonotic disease in the world, with particularly heavy concentrations in the wet tropics. Leptospirosis is particularly frequent in people whose work or leisure activity brings them into contact with infected water. The disease is caused by a pathogenic bacterium of many different serotypes and spread across a wide range of animal hosts.

Apodemus agrarius, A. speciosus and the water vole Clethrionomys glareolus are also known reservoirs.

Murine typhus. Murine typhus, found virtually worldwide, is caused by the Rickettsial pathogen Rickettsia typhi, which is common where there are dense populations of commensal rodents (R. rattus, R. norvegicus and B. bengalensis) in urban and periurban areas. Additionally called endemic or flea-borne typhus, the infection is transmitted from rats to man by infected fleas of the species Xenopsylla cheopis (the oriental rat flea) and to a lesser extent by the cat flea (Ctenocephalides felis). Fleas infected by R. typhi bite humans, which inadvertently rub infected feces into the wound during scratching in response to bite reaction. Fleas plus rodent lice are responsible for transmission between rats.

Chagas’ disease. Unlike the previously described infections, Chagas’ disease is caused by a protozoan parasite (Trypanosoma cruzi), with a regional distribution. It is found throughout Central and South America and called American trypanosomiasis because of its regional spread. This parasite naturally infects many different vertebrate animals and is passed between them (including man) by bugs of the family Triatominae. The disease has chronic and acute stages the latter mostly in children, with about 8 percent of those infected dying of myocarditis and meningoencephalitis. Chagas’ disease has been reported from just about every Latin American country and a small number of indigenous cases have been found in Texas. The infection has increasingly spread from its original rural base into urban areas with 20 million Latin Americans infected and up to a quarter of the region’s population at risk.

The author would like to acknowledge information from Sorex Fact File No. 6, Disease in Commensal Rodents, written by Adrian Meyer of the Acheta Partnership

The author is a consultant and journalist for pest control in public health, livestock and the food and feed industries. He specializes in rodent control, insect pest monitoring and the management of mycotoxins. He can be reached at tmabbett@pctonline.com