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A zoonosis is an infection acquired from an animal source.
- Infection can occur when humans enter the animal environment
(e.g. when camping).
- Transmission can be via vectors such as mosquitoes (e.g. Japanese
- Farming and pets are an important source of zoonotic
More than 100 animal viruses can cause human disease (e.g.
Herpes simiae, a monkey pathogen, which causes severe encephalitis,
or avian 'flu, both of which are associated with high mortality).
Other viral zoonoses are discussed in Tropical, exotic or arbovirus infections
Rat-bite fever is caused by either Streptobacillus moniliformis
Spirillum minus inoculated by the bite of a rat.
- The incubation period is 3 days to 3 weeks.
- Inflammation occurs at the site of the bite, with associated lymphangitis
and regional lymphadenopathy.
- Generalized maculopapular rash, fever, headache and malaise
- Polyarthritis occurs in 70% of cases.
- Endocarditis is the most serious complication.
- Diagnosis based on bacterial isolation or nucleic acid amplification
- Treatment is usually with penicillin.
See Listeria, Bacillus, Corynebacterium and environmental mycobacteria
Caused by Yersinia pestis
, infection is endemic in rodents in remote
rural areas. Rarely, epidemics may develop that spread worldwide
(e.g. the 'Black Death'). The organism is transmitted both between
rats and to humans by the rat flea, Xenopsylla cheopis
The incubation period is short; the disease has an abrupt onset
characterized by fevers and toxaemia. The regional lymph glands
draining the site of the bite become greatly enlarged (buboes) and
septicaemia is accompanied by generalized haemorrhage. Pneumonic
plague is a rapidly fatal pneumonitis that can be transmitted
by the respiratory route.
Plague is diagnosed clinically in areas where it is endemic. Direct
smear of a lymph gland aspirate, blood culture and NAAT are
used for diagnosis. Treatment is with tetracycline, chloramphenicol,
aminoglycosides or ciprofloxacin. The mortality rate of pneumonic
plague is high. There are concerns that it may be used as a
Borreliosis is transmitted from rodents or deer by ticks particularly
in open forest habitat (e.g. the New Forest) or by lice (see
The cat is the definitive host of Toxoplasma gondii
but the organism
infects a wide range of animals, including sheep, cattle and
humans. Infection is acquired by ingestion of oocysts from infected
cat faeces or from tissue cysts in infected meat (e.g. undercooked
Dermatophytes that are natural pathogens of animals can spread
to the human population by direct contact (see Filamentous fungi
is an ascarid parasite of dogs. The parasite eggs
are excreted in the faeces of infected dogs and mature in the soil.
Human ingestion occurs when food is contaminated by the soil or
when personal hygiene is poor (e.g. poor hand washing). The
larval stages hatch in the intestine, invade the host and migrate to
the liver and lungs. They are unable to develop into adults but
migrate throughout the body causing fever, hepatosplenomegaly,
lymphadenopathy and wheeze. If the larva migrates into the eye,
sight may be permanently damaged by the local inflammatory
response of the retina. The diagnosis is made serologically using a
specific enzyme immunoassay (EIA). The disease is usually selflimiting
but, if symptoms are severe, treatment with albendazole
may be beneficial. Ocular lesions should first be treated with steroids
to diminish the inflammatory response; the role of antihelminthic
treatment is less certain.
- Caused by infection with Bartonella henselae.
- There is a 10-day incubation period following a cat scratch or
- A papular lesion may develop at the site.
- Regional lymphadenopathy occurs.
- Symptoms resolve slowly over a period of 2 months, but a more
chronic course may ensue.
- Disseminated infection occurs more commonly in immunocompromised
- The diagnosis is usually made clinically but it can be confirmed
serologically by immunofluorescence or EIA. Culture requires
a prolonged incubation period; NAATs may also be used for
- Treatment with azithromycin, tetracyclines or rifampicin may be
Two species of parasite are responsible for human hydatid disease: Echinococcus granulosus
and Echinococcus multilocularis
. Dogs are
the definitive host of E. granulosus
, harbouring the tapeworm
stage. The eggs, which are passed in the faeces, are ingested by the
intermediate hosts (e.g. sheep or rodents) and multiple cysts
develop in the liver and lungs. The cycle is completed when dogs
eat infected tissues. Humans are accidental hosts. The disease is
common in sheep-farming areas.
is found in foxes, wolves and dogs;
rodents act as the intermediate hosts.
Pathogenesis and Clinical features
Cysts act as space-occupying lesions in the liver, lungs, abdominal
cavity or central nervous system. The cysts of E. multilocularis
a definite cyst wall and spread in the tissue.
Cysts may be demonstrated by ultrasound or CT. EIA for both
antibody and antigen is available.
If possible, hydatid cysts should be surgically removed. Albendazole
is given to kill the germinal layer of the cyst and praziquantel
to reduce the viability of protoscolices. Puncture, aspiration, injection
of chemicals and re-aspiration (PAIR) is seen as an alternative
to surgical excision. Cyst rupture can lead to multiple cysts in the
abdomen, or to anaphylaxis.