Vector Borne Diseases


Vector Borne Diseases (VBDs) are caused by a range of infectious agents (including viruses, bacteria, protozoa and helminths) which are transmitted to animals via Insecta and Acari vectors.


Although these vectors are visible on dogs and cats, the diseases that they may be carrying are not, and the first signs of infection may not be until some time after transmission.


Some VBDs may be relatively well tolerated by pets, however, some may have serious pathogenic consequences or have zoonotic effects.



VBDs are important because:


  • They may be highly pathogenic in dogs and cats
  • Their transmission is often unpredictable
  • Their diagnosis and control are difficult
  • Variable clinical signs can develop after long incubation periods and these are rarely pathognomonic
  • Animals may have persistent infections and thus act as reservoirs
  • Several are important zoonoses such as leishmaniosis, borreliosis, rickettsiosis, bartonellosis and dirofilariosis


Epidemiology


Climatic and ecological changes, national regulations on management of stray dogs and cats together with the increase in pet travel and translocation of pet animals can influence the present epidemiological situation of vector-borne diseases in Europe. Rare diseases may increase in frequency in certain areas, either due to increased importation of infected animals or because the causative agents and their vectors spread to and establish in previously non-endemic areas. Such an expansion of endemic areas has been recorded for various parasitic diseases such as dirofilariosis, babesiosis and leishmaniosis. Babesiosis, for example, has been observed across central Europe in the past few years, emerging from previous endemic regions in Europe.


Vector-borne diseases can only be effectively controlled through knowledge of the infectious agents and their vectors.


For more information about VBDs please click the links below.


Transmitted by Insects


Transmitted by Ticks (TBDs)


Transmitted by insects:


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Bartonella henselae



  • Transmitted by fleas and ticks
  • Also known as Cat Scratch Disease in humans as the infection is often acquired following a scratch by a cat
  • Can cause disease, weight-loss, lassitude and anorexia in cats and dogs


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Thelazia callipaeda

(also detailed in helminths)



  • Transmitted by muscid flies
  • T. callipaeda is an eye worm which lives on the surface of the cornea in dogs and cats
  • It can cause cause epiphora, mucopurulent discharge, lacrimation, conjunctivitis, corneal opacity, corneal ulceration
  • Muscid flies (Tear feeding flies) ingest T. callipaeda eggs from the tears of dogs and cats and act as intermediate hosts allowing for larvae to develop into infective L3 larvae
  • L3 larvae migrate to the head of the intermediate host and are released into or near the eye when the fly feeds
  • L3 larvae develop into adult worms in the cat or dog's eyes and then lay eggs
  • Prepatent period is 3 weeks.


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Dirofilaria repens

(also detailed in helminths)



  • Transmitted by mosquitoes
  • D. repens causes subcutaneous dirofilariosis and is the most important species responsible for zoonotic infections in Europe
  • It is a filarial worm (nematode) which infects connective tissues and the vascular system of dogs and cats
  • D. repens microfilariae develop in the uterus of female worms and are excreted into the blood stream where they become available to blood-sucking mosquitoes
  • Microfilariae develop to the infective stages in the body of these vectors and are transmitted via their saliva during feeding
  • Adult worms are found between subcutaneous and deep connective tissue layers in most parts of the body, sometimes forming non-inflammatory nodules
  • Adults can live for several years.
  • Many mosquito species are competent intermediate hosts and allow the development of microfilariae to infective stages in the Malpighian tubules


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Leishmania spp.

(also detailed in protozoa)



  • Transmitted by phlebotomes (sandflies)
  • Leishmaniosis is a protozoan infection
  • In Europe, canine leishmaniosis is caused by Leishmania infantum which comprises various enzymatic types (zymodemes)
  • The dog is considered the main host of L. infantum infection, but cats can also be hosts of L. infantum transmitted by several phlebotome species
  • Zoonotic infection
  • Leishmania spp. are highly vector specific and are transmitted by the blood-sucking females of several Phlebotomus species while feeding on their hosts. Vector activity is highest during the night and at a minimum temperature of about 18- 22°C
  • The incubation period can vary between 2 months and 8 years and is dependent on a number of immunological mechanisms
  • The main risks in endemic areas are related to vector exposure and abundance of reservoir hosts which include dogs living outdoors, a high stray dog population, dogs adopted from animal shelters in endemic areas and hunting dogs.
  • Prevention of phlebotome bites by the application of repellents/insecticides to dogs in the form of impregnated collars, sprays and spot-on preparations is currently the most promising strategy for preventing leishmaniosis infection


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Transmitted by Ticks (Tick-Borne Diseases - TBDs):


Ixodes ricinus is the most important vector tick in the UK as it transmits the spirochaete, Borrelia burgdorferi sensu lato (Lyme disease), and Anaplasma phagocytophilum (anaplasmosis) and it is the most widespread tick.


When ticks feed they can transfer pathogens they are carrying into the animal on which they are feeding. Details of the tick-borne diseases that travelling pets can acquire in Europe can be found by visiting the parasite maps information on the Travelling Pets page [link to relevant areas]. However, some of these infections are indigenous to the UK.


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Borrelia burgdorferi



  • Transmitted by Ixodes ricinus tick
  • Also known as Lyme disease
  • Lyme disease is so named as it was first identified as a cause of arthritis in children in Lyme, Connecticut, and it is an important disease in man
  • In dogs, the majority infected (>95%) remain clinically normal, however, weeks or months after the tick bite, a small proportion of dogs may show fever, anorexia, polyarthritis, and/or lymphadenopathy
  • A few develop "Lyme nephropathy" and do not respond well to treatment
  • While an important zoonosis, dogs will not transmit Lyme disease to humans
  • In cats disease is rarely documented but experimental infection with both organisms has produced relatively similar signs to dogs


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Anaplasma phagocytophilum



  • Transmitted by Ixodes ricinus tick
  • Previously known as Ehrlichia phagocytophilum
  • Is most important as cause of immunosuppression and tick-borne fever in sheep and cattle in the UK
  • It is a rare cause of granulocytic anaplasmosis in dogs, equines and humans
  • There is no evidence for transmission from dogs to humans
  • The few reports of disease in dogs, occurring one to two weeks after the tick-bite, describe acute onset fever, anorexia, lethargy, lameness, sometimes cough, pale mucous membranes, or other generalized sign, with thrombocytopaenia and anaemia and possibly immune-mediated disease
  • In cats disease is rarely documented but experimental infection with both organisms has produced relatively similar signs to dogs


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Ehrlichia canis



  • Rhipicephalus sanguineus is the vector of Ehrlichia canis
  • If R. sanguineus established in a building with its imported pet host it could transmit that pet's imported diseases to other pets (and man)
  • Male ticks take small meals moving to find females and it is known for a few males to move from one host to another to transfer disease
  • E. canis may overwinter in the infected tick, which may survive indoors even in countries with cold or temperate climates
  • During the incubation period of 8 to 20 days, the infectious agents of E. canis multiply by binary fission forming morulae within circulating mononuclear cells
  • E. canis spreads via the mononuclear phagocytic system to liver, spleen and lymph nodes
  • Circulating infected cells adhere to the vascular endothelium especially in the lungs, kidneys and meninges, and induce vasculitis and subendothelial tissue infection – this may lead to platelet damage, sequestration and destruction


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Babesia canis

(also detailed in protozoa)



  • Transmitted by Dermacentor reticulatus in mainland Europe and potentially could do so in the UK should a Babesia-infected dog from Europe enter a Dermacentor infested area or bring in a Dermacentor tick
  • Rhipicephalus sanguineus is also a vector of B. canis canis, B. canis vogeli and a range of other pathogens
  • If R. sanguineus established in a building with its imported pet host it could transmit that pet's imported diseases to other pets (and man)
  • A case of B. canis volgeli in an untravelled British dog in Ashford in Kent was presumed due to a tick dropping off a travelled pet, moulting and then feeding on the local dog
  • Babesiosis is a protozoan infection
  • Babesia are generally highly host-specific with regard to both the transmitting tick species and the mammalian host
  • After ingestion with a blood meal, Babesia stages penetrate the gut epithelium of the tick, multiply and migrate to different organs including the tick ovary and salivary gland
  • Transovarial transmission from infected adult females to their progeny occurs with large Babesia, and thus their larvae ("seed ticks") can be an important source of infection


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Hepatozoon canis

(also detailed in protozoa)



  • Transmitted by the ingestion of an infected tick
  • Hepatozoonosis is a protozoan infection
  • Clinical signs can be absent, mild or severe, including fever, emaciation, lethargy, anorexia, muscle pain, lymphadenopathy and anaemic signs


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Cytauxzoon felis

(also detailed in protozoa)



  • Likely transmitted by D. variabilis ticks
  • Cytauxzoonosis is a protozoan infection
  • There is currently no treatment and infection usually results in death


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