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Let us make an in-depth study of the arthropod-borne rickettsial and viral infections. The below given article will help you to learn about the following things:- 1. Diseases Arranged According to the Arthropods Vectors 2. Laboratory Diagnosis and 3. Application to Nursing.
Arthropod-Borne Rickettsial Infections:
Rickettsiae occupy a biological position which is intermediate between the smallest bacteria and the largest virus. Rickettsiae are small prokaryotic cells that have an obligate intracellular existence. They resemble bacteria as they are visible under light microscope and are known to divide by binary fission. They have cell walls containing the amino sugar (muramic acid) and are susceptible to the action of antibiotics.
Like viruses, they are obligate intracellular parasites except Rickettsia Quintana which grows extracellularly in the louse gut and has been cultivated on modified blood agar medium. Under natural conditions, they are present in the alimentary canal of blood sucking arthropods (lice, ticks, flea, and mite) but when they are transmitted to man they cause the disease “Typhus”—characterised by fever and rash.
Diseases Arranged According to the Arthropods Vectors:
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1. Louse borne typhus:
(a) Epidemic typhus Rickettsia prowazekii
(b) Trench fever R. Quintana
2. Flea borne typhus
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Murine endemic typhus R. mooseri
3. Tick borne typhus
Rocky Mountain spotted fever R. rickettsii
4. Mite borne typhus
(a) Scrub typhus R. tsutsugamuchi
Rickettsiae are coccobacillary. Though they are Gram-negative, they can be stained with Castaneda or Giemsa stain. In general, they are easily destroyed by heat, drying and chemical disinfectants. Rocky Mountain spotted fever, first recognised by Ricketts in America, is caused by R. rickettsii and is transmitted by a tick (Dermacenter andersoni). Fresh tick feces are infectious.
Louse borne typhus fever (epidemic typhus fever) is caused by R. prowazekii and is spread by the human body louse (Pediculus corporis).Though it is worldwide in distribution, it is confined to Middle East, Asia, Africa and Mexico. Lice become infected while biting infected patients or carriers, when the infected blood reaches the intestine of the louse, rickettsiae invade the epithelial cells until the host cells distend and rupture.
As a result, the feces of louse become heavily laden with the organisms and when they are discharged on the skin, they enter the blood stream through the abrasions or fresh bite wounds, causing rickettsiaemia. Typhus is characterised by an onset about two days during which there are nausea, headache, dizziness and high fever, then appears a rash which may cover the whole body. The patient is lethargic and delirious. The blood of the patient is infectious to the louse.
The organism cannot be demonstrated microscopically in the blood, but it can be cultivated in yolk sac. However, complement fixation test (CFT) is most accurate and specific as the antigen is derived from the rickettsiae cultivated in yolk sac.
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Q fever is an acute systemic infection usually characterised by an interstitial pneumonia clinically resembling Influenza; unlike other rickettsial infections, it has no rash. There is only high fever. The name of the disease is derived from the letter “Q” in Query as its aetiology was uncertain, though it was recognised in Queensland (Australia).
The causal agent, named as Coxiella burnetti, was first recognised by Cox and Burnett. Q fever is quite widespread in Europe and Egypt. It is an obligate intracellular parasite. It can be cultivated in animals and chick embryo. It remains viable for several days in water or milk or butter, Pasteurization at temperature 145°F or 62.7°C kills C. burnetti.
C. burnetti differs from other members of rickettsiae by the following properties:
it is filterable, highly resistant to heat and disinfectant, does not produce rash in infected patients and does not elicit the agglutination to the non-motile Proteus strains which is characteristic to typhus fever (Weil Felix test is negative for C. burnetti) hence it is not considered as the true member of rickettsiae and has been designated as a new member Coxiella.
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Q fever is spread throughout the world, mostly in animals. Infected animals may excrete coxiella in the milk and also during parturition; hence plenty of the organisms are present in the placenta and birth canal. Man may be infected by drinking contaminated milk or by inhalation of infected dust from the straw or dried placenta. C. burnetti is conveyed from animal to animal by the bite of tick. Ticks rarely transmit the infection to man. The risk of person to person transmission is small; so the strict isolation procedures are not needed.
Laboratory Diagnosis:
Isolation of causative agent is done in animals or chick embryo. Laboratory workers may easily get infected, so they should be careful.
Serology:
Some non-motile strain of proteus and rickettsiae are antigenically similar. Proteus organisms can be grown artificially in plenty. Therefore antigens from Proteus OX2, OXK OX19 are used in Weil-Felix test which is positive for Typhus Fever, except Q Fever. This test is useful but not reliable as it is strongly positive in patient with Proteus infection than Typhus fever. CFT is accurate and specific.
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Treatment:
Tetracyclines, Chloramphenicol and para amino benzoic acid exhibit a rickettsiostat effect; but in Q fever chloramphenicol and tetracycline are effective therapeutically.
Application to Nursing:
Nurse should take due care while collecting blood during febrile stage of Typhus fever, as the blood is highly infectious. The syringes and needles should be sterilised in autoclave. Steps should be taken to destroy all kinds of arthropods.
Arthropod-Borne Viral Infections:
The term arbor viruses is an abbreviation used to denote a large group of viruses transmitted by arthropods. The name “ARBOR Viruses” has been proposed for these viruses. Members of this group infect man and many other mammals and birds.
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Arthropods take in the viruses when they feed on the blood of an infected animals and transmit to a new host when they take next meal. The virus multiplies within the body of arthropods, without damaging their tissue or producing the disease.
The tropical heat and rain encourage the breeding of arthropods. So arbor viruses are most prevalent in tropical countries. In aqueous suspension arbor viruses are unstable at room temperature; at 20°C they survive better. In solid CO2 at -79°C, the viruses survive for several days.
Formaldehyde at low concentration inactivates the virus; heat at 60°C for 10 minutes or ultraviolet light for 10 minutes kills them. Sodium desoxycholate in 0.1 % concentration inactivates arbor viruses. After an incubation period of 4-21 days there is a sudden onset of fever with signs resembling those seen in aseptic meningitis. Marked drowsiness or stupor is characteristic and the mortality is 5-25% It is much high in elderly people.
