Guinea Pigs (Laboratory) - Pathology
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Introduction
Guinea pigs or “cavy” originate from South America and in the wild are a type of burrowing animal. They are nocturnal herbivores and were originally domesticated by the Incas as a source of nutrition. Guinea pigs are widely used in experimentation for a number of reasons, including highly developed young at birth (due to a relatively lengthy gestation period), the ability of the young to eat solid food at age 2-3 days, being fully haired and completely mobile. These factors reduce many issues and complications regarding the use of experimentation animals. Guinea pigs are also among the easiest laboratory animals to handle and can be housed in open top cages due to their in ablility to climb or jump.
Guinea pigs have been used for a broad spectrum of scientific purposes including the production of antibodies, tumor studies, disease studies and nutritional studies. Guinea pigs have been used since the 17th century and in 1990 were even launched into space as scientific passengers by China. One of the most common experimental uses of Guinea Pigs in the past has been for the standardisation and assessment of vaccines.
Strains and Stocks
When compared with the continuous and long term breeding strains of mice such as the F344 and LEW, the breeding of Guinea pigs is not as well developed and there are few specific strains in use. The outbred strains available are mainly the Hartley, Dunkin-Hartley and Albino. Inbred strains available are strains “2” and “13”, although these are uncommon compared to the outbred strains and are only used for very specific research. There are relatively less guinea pig breeders available of the quality required for experimental use and there are likely to be considerable variations between strains from different breeders. The total number of experimental guinea pigs in use as of 2007 was 2% of the total animals used.
Other types used within experimental research are the hairless strains that were developed in the 1980s specifically for dermatological use. These strains are often a result of a spontaneous genetic mutation from the strains described above. The recently popularised “skinny pig” is a type of guinea pig originally developed for laboratory use.
Physiology
Please find details of Guinea Pig physiology within the WikiNormals section. Physiological information available includes “General”, “Biochemical”, “Haematological” and “Breed” parameters.
Anatomy and Histology
This section has been included to allow familiarisation with the peculiarities of Guinea Pig anatomy to provide a context for some of the disease and pathological headings found below. Therefore only anatomical areas with specific features warranting emphasis have been included below;
Reproductive System
In males the inguinal canal remains open throughout the guinea pig’s life and this can lead to an increased risk of abdominal organ prolapsed post-castration. Males also have coiled vesicular glands which extend into the abdomen approximately 10cm in a similar manner to a females uterine horns.
Urinary System
Guinea pig urine is opaque and creamy yellow with a normal pH 9. The urine may also contain crystals of ammonium phosphate and calcium carbonate.
Respiratory System
Although guinea pigs have a normal mammalian respiratory system, a healthy guinea pig will only breathe through its nose and mouth breathing is therefore a sign of respiratory distress.
Digestive System
Guinea pigs have a longer colon than other rodents, accounting for 60% of the length of its intestines. Guinea pigs also have a relatively large caecum for the digestion of cellulose and it is able to contain up to 65% of the gastrointestinal contents at any time. The stomach is non-glandular and the female spleen is significantly larger than a male spleen.
Diseases
For ease of use, the diseases of guinea pigs listed below will be by body system, or where this is not appropriate in an “Other” category displayed after the body system sections. Those diseases listed below are not exhaustive but rather highlight common diseases encountered with laboratory guinea pigs.
Integument System
Perhaps the most common finding in guinea pigs are skin problems related to diet, specifically a low vitamin C diet. Diet should always be considered when evaluating the integumentary system.
Ringworm
The causal agent of this mycotic infection is commonly either Trichophyton mentagrophytes or Microsporum gypseum. Areas of alopecia and seborrhoea can be found and hair can easily be plucked. Microscopic examination of hair plucked from areas of lesion is sufficient to confirm diagnosis.
Guinea pigs can suffer from a wide range of mycotic infections which can range in severity from mild skin changes to cystitis, pneumonia and reproductive disorders.
Mange
This mite infection is commonly caused by Trixacarus caviae, a sarcoptiform mite. Clinical signs usually occur 3-5 weeks after infection and lesions are usually visible around the head, shoulders and back. Hair is easily plucked and skin is usually seborrhoeic and pruritic. In some cases self trauma may also compound problems.
Microscopic examination of skin scrapings is usually sufficient to confirm diagnosis. Guinea pigs are also succeptible to Demodex infections with similar clinical signs and diagnosis.
Lice
Lice infestations can be caused by a number of different species with the most common being Gliricola porcelli. Gyropus ovalis and Trimenopen jenningsi are also common. The lice can often be seen in the fur and eggs can be seen as either black or white specks adhering to the hair shafts of the fur.
Microscopic examination of the hair and skin scrapings is sufficient for diagnosis.
Other Mite Infestations
Fur mites or Chirodiscoides caviae can be found in pairs within the fur but are commensal. Fur mites do not usually produce clinical signs, even during heavy infestations.
Rabbit fur mites or Psoroptes cuniculi can also be found within the fur and cause crusting and scaling dermatitis. This mite can also cause extreme pruritis
Fly Strike
Although this condition is uncommon in guinea pigs, the prescence of larvae of blowfly colonising a moist wound can lead toxic shock. Guinea pigs appear far less susceptible to fly strike than rabbits and in most cases prognosis for recovery is good.
Alopecia
Alopecia usually occurs hormonally, during pregnancy or post-natally. In most cases the hair loss will not be accompanied by pruritis. Bilateral symmetrical alopecia can be seen in older females suffering with ovarian cysts. Post-parturition hair loss is often bilaterally on the flanks and ventral abdomen.
Liver Disease
Hair loss is a common cause of hair loss once parasitic and fungal infections have been removed. During liver disease the overall fur covering is sparse and the skin is thickened. Often this general hair loss is accompanied with seborrhea.
Scurvy
Inadequate provision of vitamin C can cause hair loss and would usually be associated with weight loss, lameness, weakness and bleeding from the gums.
Abscesses
Abscessess are mainly caused by fighting and various types of bacteria have been isolated as the causal agent. These include Pseudomonas aeruginosa, Pasteurella multocida, Corynebacterium pyogenes, Staphylococcus aureus, Streptococcus spp. and other Enterobacteria. There will be localised swelling and most abcessess occur in the throat area.
Reproductive System
Pseudopregnancy
Although this condition is rare in guinea pigs, its duration is 17 days. Care must be taken that sows do not develop mastitis during pseudopregnancy.
Pregnancy toxaemia
Pregnancy toaemia is a metabolic disorder similar to twin lamb disease in sheep. Predisposing factors include obesity and stress leading to negative energy balance. The energy imbalance progresses to ketoacidosis together with aciduria, proteinuria and hyperlipaemia. There is also a fatty degeneration of the liver.
In cases of stress the sow stops eating, becomes depressed and will have a ruffled appearance. Some sows may also salivate profusely and the distinctive smell of ketones (pear drops) may be present. The condition is more prevalent in hot weather.
Miscarriage and Abortion
If abortion occurs early in gestation (up to 40 days) the sow will commonly not be systemically affected. Later than 40 days will result in systemic illness with severe blood loss with potentially fatal consequences.
Toxoplasmosis
A Toxoplasmosis infection is caused by a protozoan parasite called Toxoplasma gondii in which the definitive host is the cat. Guinea pig infection is caused by consumption of food contaminated with cat faeces, i.e. a faecal-oral route. Tissue cysts from the parasite can be found in muscle, heart, eye and brain. Toxoplasmosis can also cause vulval bleeding and abortion.
Mastitis
Predisposing factors for the development of this infection are poor standards of environmental cleanliness or a build up of faecal contamination. Various species of bacteria have been found to be involved including Pseudomonas aeruginosa, Pasturella multocida, Corynebacterium pyogenes, Staphylococcus aureus, Streptococcus spp and various types of Enterobacteriacae.
Affected mammary glands will be warm, hard and swollen and if the infection is heavy the gland may become ulcerated and necrotic. Mastitis in guinea pigs can also cause systemic illness.
Preputial infections
This type of infection is usually caused when a foreign body such as saw dust becomes trapped in the prepuce causing preputial dermatitis. This can lead to a swelling of the prepuce but also cause inability to copulate. Similarly it has been found that saw dust can also block the vagina of the female also leading to an inability to achieve copulation.
To decrease the risk of preputial infections and vaginal blockage wood shavings are superior to saw dust.
Urinary System
Polydipsia
Guinea pigs should consume approximately 10ml/100g but will vary dependant on the water content of the diet. Polydipsia often accompanies chronic renal failure and diabetes. Polydipsia is also associated with abdominal swelling and hydronephrosis of both kidneys.
Polyuria
Guinea pigs should have a urinary output of approximately 20-25ml per day. Polyuria is seen as a consequence of polydipsia. Excess urine output can lead to urine scalding and dermatitis.
Haematuria
“Bloody urine” can be a common symptom found in guinea pigs, both male and female. Cystitis, neoplasia of the bladder or uterus, urolithiasis and cystic endometritis are all causative factors for haematuria.
Acute Renal Failure
This condition is uncommon but where is does occur it is invariably fatal in guinea pigs. Oxalic acid poisoning can result from the consumption of large amounts of beetroot, spinach or dock leaves once the stems of these vegetables have turned woody. Clinical symptoms prior to death include depression, salivation and muscle tremors.
Chronic Renal Failure
Chronic renal disease may initially be indicated via polydipsia together with a period of anorexia, depression, diarrhoea, collapse and then death. Staphylococcol pododermatitis, chronic intersititial nephritis, diabetes and hydronephrosis have all been shown to lead to chronic renal failure in guinea pigs.
Diabetes
Diabetes is a common occurrence in guinea pigs together with polydipsia and weight loss despite a strong appetite. Some guinea pigs will develop cataracts and glycosuria.
Exogenous insulin is not required in treatment for this disease and guinea pigs often have spontaneous remissions.
Respiratory System
Epistaxis
Bleeding from the nose is often a symptom of vitamin K deficiency caused by a lack of greenstuffs in the diet.
Nasal Discharge
Mucopurulent discharges are often a symptom of upper or lower respiratory tract infections. Respiratory tract infections are also associated with “ruttling” which is a wheezy breathing sound. Predisposing factors for respiratory infections are a change in environmental temperature, humidity or ventilation. Ammonia build up as a consequence of dirty bedding/environment is known to weaken the resistance of the respiratory tract.
Any nasal discharge should not be confused with a milky type of fluid secreted from the eyes and nose as part of the normal guinea pig grooming process.
Pneumonia
The clinical symptoms of pneumonia include dyspnoea, ruttling (see above), sneezing, nasal discharge and coughing. If left untreated, guinea pigs can become depressed anorexic and it can become a fatal infection. In guinea pigs pneumonia can be caused be various bacterial and viral agents including Bordetella bronchiseptica, Streptococcus zooepidemicus, Streptococcus pneumonia, Klebsiella pneumonia, Pseudomonas aeruginosa and Pasturella spp.
Diagnosis is via isolation of the causal agent from a culture of the nasal discharge.
Adenovirus
Adenoviridae do not require co-infection with bacteria in order to become pathogenic within the respiratory system. The infection may have no clinical symptoms but in susceptible animals such as those that are stressed or immune-deficient it can be fatal.
Digestive System
Infectious Causes of Diarrhoea
Salmonella
Salmonella typhimurium and Salmonella enteriditis are the common causative agents and are often associated with contaminated food and bedding. Diarrhoea may be haemorrhagic, cause septicaemia and can also cause sudden death. Diagnosis is via isolation and culture of the Salmonella spp via faecal samples.
Chronic cases and those that recover from a Salmonella infection are likely to become carriers and should be destroyed to prevent further outbreaks.
Yersinia (pseudotuberculosis)
Acute forms of this disease can cause septicaemia and death within 48hrs. Chronic cases have more progressive clinical symptoms but will usually culminate in death after 3-4 weeks. During this 3-4 week period any young may become congenitally or neonatally affected. Diagnosis is via culture of Yersinia pseudotuberculosis from blood, lymph nodes or faecal samples.
Yersinia infections are transmitted via wild birds and other rodents contaminating their food, particularly green foods. Once the disease is endemic in a breeding population some infections can be passed vertically in the milk.
Clostridia
Normally this species are present in the large intestines as commensals but during certain circumstances such as antibiotic administration they are able to proliferate and cause fatal enterotoxaemia. Clostridium species can be cultured from the lumen of the infected intestine.
Parasitic Causes of Diarrhoea
Nematodes
The only nematode that affects guinea pigs is Paraspidodera uncinata and is usually restricted to outdoor housed animals. Heavy infestation can cause enteritis and the Paraspidodera resides in the caecum and can be 11-28mm long.
Protozoa
A wide range of commensal protozoa are carried and are not pathogenic except for Coccidia. Protozoan infections are generally the result of poor husbandry. Contact with reabbits can allow other rare protozoan infections in guinea pigs.
Eimeria caviae is the coccidial species contracted by guinea pigs eating contaminated food, however coccidiosis in guinea pigs is rare. Droppings will be slimy and contain blood. Oocysts will be present in the faeces and can be seen on microscopic examination.
Ascaridae
Ascaridae are not a natural parasite of guinea pigs but if housed outdoors in areas which has been contaminated with dog or cat faeces guinea pigs will suffer from weight loss, unthriftiness and diarrhoea.